Recent Advances in Morphology Optimization for Organic Photovoltaics

Transcription

1 REVIEW Hall of Fame Article Recent Advances in Morphology Optimization for Organic Photovoltaics Hansol Lee, Chaneui Park, Dong Hun Sin, Jong Hwan Park,* and Kilwon Cho* Organic photovoltaics are an important part of a next-generation energyharvesting technology that uses a practically infinite pollutant-free energy source. They have the advantages of light weight, solution processability, cheap materials, low production cost, and deformability. However, to date, the moderate photovoltaic efficiencies and poor stabilities of organic photovoltaics impede their use as replacements for inorganic photovoltaics. Recent developments in bulk-heterojunction organic photovoltaics mean that they have almost reached the lower efficiency limit for feasible commercialization. In this review article, the recent understanding of the ideal bulk-heterojunction morphology of the photoactive layer for efficient exciton dissociation and charge transport is described, and recent attempts as well as early-stage trials to realize this ideal morphology are discussed systematically from a morphological viewpoint. The various approaches to optimizing morphologies consisting of an interpenetrating bicontinuous network with appropriate domain sizes and mixed regions are categorized, and in each category, the recent trends in the morphology control on the multilength scale are highlighted and discussed in detail. This review article concludes by identifying the remaining challenges for the control of active layer morphologies and by providing perspectives toward real application and commercialization of organic photovoltaics. 1. Introduction Current human activities and industries require large amounts of energy, which have mainly been obtained thus far by burning fossil fuels. [1,2] However, the supplies of these fuels are finite, and their use has been confirmed to be the main reason for recent climate change, so sustainable and environmentally benign energy sources are being evaluated. In particular, solar energy has significant potential as a next-generation energy source because of its limitless supply, nonpolluting nature, H. Lee, C. Park, Dr. D. H. Sin, Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology Pohang 37673, South Korea kwcho@postech.ac.kr Dr. J. H. Park Nano Hybrid Technology Research Center Creative and Fundamental Research Division Korea Electrotechnology Research Institute (KERI) Changwon 51543, South Korea jhpark79@keri.re.kr DOI: /adma and near-worldwide availability. Organic photovolitaics (OPVs) have significant advantages over the diverse types of photovoltaics (PVs) in that they are light in weight and require only cheap materials, as well as their solution processability, low production costs, rapid energy-payback time, and deformability. [3 8] Research and development in the OPV field have in recent decades mainly focused on the synthesis of highly efficient organic semiconductors, [9 11] the engineering of charge-transferring interfaces, [12 14] and the control of the morphology of the photoactive layer, [15 18] which has resulted in the achievement of OPVs with power conversion efficiencies (PCEs) above 13%. [11] To overcome the large exciton binding energy of organic semiconductors, the photoactive layer should consist of a heterojunction of two organic semiconductors, one an electron donor and the other an electron acceptor. [3,5] This binary photoactive layer provides energy offset between the donor and the acceptor to separate the excitons, but as a result the morphology of the photoactive layer has a strong effect on the photovoltaic performance. Therefore, the morphology of the photoactive layer in OPVs has strong influences on light absorption, exciton dissociation, charge transport, and charge recombination, so it determines their overall photovoltaic performance. Detailed findings so far and theoretical background of the operational mechanisms of OPVs on molecular to macroscopic scales have led to identification of favorable morphologies of the photoactive layer. [15,19 27] Early-stage bulk-heterojunction (BHJ) layers were found to exhibit low exciton dissociation and charge transport efficiency, and thus much research was dedicated to optimizing the nanoscale BHJ morphology by controlling various morphological factors such as the phase separation and material crystallinity. [28 31] New organic semiconductors and processing solvents have been designed and various treatments have been adopted with the aim of optimizing BHJ morphologies. Moreover, for OPVs to be operationally stable, the optimized BHJ morphology must be maintained. The develop ment of methods for the fabrication of nanoscale interpenetrating bicontinuous and stable morphologies has been the subject of numerous studies, so we have collected and organized this research with the aim of providing researchers with comprehensive and multidisciplinary information that can be used to (1 of 39)

2 guide the optimization of the morphology of the photoactive layer. In this review, we discuss the recent understanding of the ideal morphology of the photoactive layer and systematically organize the research into the control of its nanoscale morphology into eight categories: material design, donor:acceptor composition, solvent engineering, morphology modifiers, post-treatment, molecular orientation engineering, ordered nanostructures, and morphological stability. Throughout this review, we try to include the complete range of morphological issues as well as recent remarkable findings. Each section provides an overall review of recent research as well as of early-stage research; the results of each approach are clearly explained with examples. Finally, this review provides perspectives for nanoscale morphology control, and suggests future directions for research into further morphology optimization and the fabrication of high performance OPVs with long-term stability, with particular emphasis on their ultimate commercialization. 2. Morphological Factors The process of photocurrent generation in OPV device can be divided into several steps (Figure 1). Incident photons are absorbed by photoactive materials consisting of an electron donor and an electron acceptor, which causes the formation of excitons. The excitons diffuse within the materials and dissociate by electron transfer when they encounter the donor acceptor interface. Electron transfer at the interface results in the formation of an interfacial electron hole pair that is still bound by Coulombic attraction (the charge transfer (CT) state). Some of these interfacial electron hole pairs separate into free charge carriers; the others are lost due to geminate recombination. The free electrons and holes are transported toward the device s cathode and anode, respectively: the electrons travel through the acceptor phase of the photoactive layer, and the holes travel through its donor phase. The free charge carriers are finally collected by the electrodes, and a photocurrent results. To make these processes efficient, the photoactive materials in the active layer should have appropriate microstructure at length scales from molecular to the mesoscale. [32] The morphology of the active layer is composed of these structural features, and the goal of research on morphology has been to find a general idea of the ideal morphology that maximizes the PCE of OPVs. A major feature that distinguishes OPVs from inorganic PVs is that in an organic material the excitons generated by optical excitation are strongly bound by their mutual Coulombic attraction due to the material s low dielectric constant (ε r ). [8,31,33 37] ε r is typically smaller in organic semiconductors (ε r 3) than in inorganic materials (ε r > 10), and therefore in organics there is a larger Coulombic barrier to the separation of excitons than in inorganics. [33,34] This Coulombic barrier (0.1 1 ev) is much larger than the thermal energy at room temperature, so the separation of excitons in organic semiconductors requires an additional driving force. [35] In OPVs, exciton separation is usually driven by an electron donor electron acceptor system, in which the energy offset at the donor acceptor interface separates Jong Hwan Park received his Ph.D. degree in chemical engineering from Pohang University of Science and Technology (POSTECH) in He was a research staff member of the Energy Material Laboratory at Samsung Advanced Institute of Technology (SAIT). In 2016, he became a senior researcher in the Nano Hybrid Technology Center, Korea Electrotechnology Research Institute (KERI). His research interests include applications of nanostructured materials to energy converting and storage devices, such as solar cells and Li ion batteries. Kilwon Cho is a professor in the Department of Chemical Engineering and director of the Polymer Research Institute at Pohang University of Science and Technology (POSTECH) in Korea. He is also a Director of the Global Frontier Research Center for Advanced Soft Electronics. He received his B.S. and M.S. degrees from the Seoul National University in applied chemistry and a Ph.D. degree from the University of Akron in polymer science in After working as a researcher at the IBM Research Center, he joined the faculty at POSTECH in His current research interests include polymer surface and thin film, and materials and interface engineering for organic electronics. the excitons by transferring electrons from the donor to the acceptor. A crucial development in the structure of the photoactive layers of OPVs has thus been to increase the donor acceptor interfacial area at which the excitons can be separated. Excitons have a limited lifetime, so they can be separated only if they reach the interface before they decay. The diffusion length of the exciton is as short as 10 nm, so only the excitons near the donor acceptor interface will reach the interface and contribute to photocurrent generation. [38 40] The earliest OPV devices had a bilayer structure and a single planar donor acceptor heterojunction, and therefore did not provide sufficient interfacial area for exciton separation. [41] To overcome this problem, the BHJ structure of the active layer was introduced. A BHJ is a solid-state mixture of donors and acceptors; its nanomorphology is the result of the spontaneous phase separation and self-assembly of the donor and acceptor components. [31] The intimate mixing of the donor and the acceptor obtained through BHJ formation greatly increases the interfacial area for exciton separation. [31] (2 of 39)

3 Figure 1. a) Band diagram for a typical donor acceptor heterojunction in organic solar cells. b) Photocurrent generation processes in bulk heterojunction organic solar cells: 1) Exciton generation via photon absorption, 2) exciton diffusion toward a donor acceptor interface, 3) charge separation and generation of free charge carriers, and 4) charge transport to each electrode. At the same time, efficient charge transport and collection in BHJ OPVs require that the donor and the acceptor also demix appropriately. Free electrons and holes can meet at the donor acceptor interface and recombine during their travel toward electrodes (nongeminate recombination). [42,43] If the finely mixed donor and acceptor phases have poor connectivity, free charge carriers can also become trapped in isolated domains and thus be unable to move to the electrode. [44] As a result, an excessively mixed donor:acceptor morphology can cause severe nongeminate recombination and charge trapping, which lead to current loss. [43] Therefore, to reduce nongeminate recombination losses and enable efficient charge transport, the BHJ should be phase-separated to some extent while also providing continuous pathways for the transport of electrons and holes. [45] Recent studies have explored the nanostructures of BHJs in more detail than possible with the classical two-phase viewpoint. BHJ morphologies can be regarded as having at least three phases: relatively pure donor and acceptor domains, and molecularly mixed regions between them. [17,46 50] In fact, optimized BHJs often have a multiphase morphology with a complex multilength scale (Figure 2) that sits between the two extremes of a morphology with a single completely mixed phase and a morphology with pure domains that are too large. [49] For example, an apparently large domain can have internal structure if it does not have sufficiently high domain purity; this internal structure can have a phase-separated morphology on small scales yet still yield a multiphase and multilength scale BHJ morphology. In such multilength scale morphologies, the domain size distribution, domain composition, and their volume fractions are important parameters that determine their characteristics. [49] The relationship between these characteristics and the efficiency of OPVs is a topic of current study. [32,49,51] The presence of mixed phases provides a large interfacial area, so some studies have suggested that it has an important influence on the processes of exciton separation and charge generation. The efficiency of charge generation increases when a certain proportion of mixed phase is present (Figure 3a). [47,52] However, complete mixing is not desirable; the coexistence of the mixed phase with relatively pure phases is necessary for efficient charge generation. [47,49,52] Indeed, geminate recombination can be severe in a completely mixed phase, [46,53,54] i.e., even though the donor and the acceptor are intermixed, they produce few free charge carriers if the mixed phase does not contain local aggregations of donor or acceptor molecules. [54] The effects of local aggregation or molecular order on charge separation have been reported in several papers (Figure 3b). [48,49,51,53 60] Local aggregation and molecular order can produce energy level shifts that create an energetic driving force for charge separation, [48,55,56,59] or result in sufficient local charge mobility to overcome the Coulombic barrier [57,59] or delocalize the electronic states and reduce the Coulombic barrier by increasing the effective initial separation distance between electrons and holes. [53,54,60] These effects can combine to improve current generation and to reduce charge recombination. Thus efficient charge generation requires a balance between the amounts of the mixed phase and the relatively pure phase, as well as the appropriate sizes and spatial distributions of the three phases. The molecular order of the donor and acceptor molecules is important not only for efficient charge separation in OPVs but also for efficient charge transport. The charge transport in an organic semiconductor is efficient when the conducting pathways are ordered and well connected. [45,61] Several studies have reported that increasing the material crystallinity in the BHJ yields improved charge transport and consequently improved device performance. [27,46,62 67] Furthermore, a recent study has shown that the electrical connectivity between domains could be the primary limitation on charge transport in BHJ structures. [45] The orientation of molecular crystals can also strongly affect charge transport in OPVs because the π π stacking direction perpendicular to the substrate is favorable for vertical charge transport within the active layer and for charge transfer to the interfacial electron-transport and hole-transport layers. [68 70] Therefore, to optimize charge transport, the crystal structures in BHJ must be precisely controlled. As discussed so far, optimization of BHJ morphologies requires fine control over various morphological factors, such as domain size, domain purity, donor acceptor mixing, (3 of 39)

4 strategies for BHJ structural modification, with detailed descriptions of the principles and results of each strategy. Figure 2. a) An illustration that describes the complex multiphase, multiscale morphology of bulk heterojunction of a PTB7:PC 70 BM organic solar cell. Reproduced with permission. [50] Copyright 2011, American Chemical Society. b) An example that classifies the photovoltaic characteristics of each phase in the multiphase morphology. Upper images: Composition maps of PTB7:PC 70 BM blend film prepared from CB without (left) and with (right) DIO. Lower images: Schematics of the detailed morphology that corresponds to the upper images. Region I (red): Pure PC 70 BM phase. Region II (blue): Mixed phase. The subscript E represents the regions where the photocurrent generation is efficient. The subscript D represents a region that is dead due to exciton relaxation. The subscript R represents the region where geminate recombination is significant. The use of DIO significantly reduces the inefficient regions by modifying the morphology. Reproduced with permission. [17] Copyright 2013, Wiley-VCH. material crystallinity, and molecular orientations. If the factors are controlled properly all together, the active layer morphology will approach the ideal morphology and yield maximized PCE. However, the factors are closely related to each other, so a change in one factor can simultaneously affect other factors. Therefore, such optimizations can be complex procedures. Researchers have developed several strategies for the fine control of BHJ morphologies. The following sections present these 3. Morphology Control 3.1. Material Design for Morphological Control The materials used in OPVs affect the nanoscale morphology of the photoactive layer, and as a consequence, its light absorption, exciton dissociation, and charge transport abilities. The properties of the materials depend on their structural components (i.e., monomers, side chains, end groups) and on their physical properties (e.g., molecular weight (M w ), polydispersity index, regioregularity, and planarity). [9,10,18,71 73] Several rules have emerged for the design of new PV materials to ensure high PV performance in the associated OPVs. The materials must have a low bandgap and a deep-lying highest occupied molecular orbital (HOMO) to broaden the light absorption range and the built-in potential. The carrier mobilities of PV materials must be controlled to reduce charge recombination during charge extraction and to increase fill factor (FF). Furthermore, morphology optimization should be considered during material design because the photo active layer is a mixture of donor and acceptor materials, and because the morphology of the photoactive layer has a strong influence on the PCE. Morphology is dominantly affected by material properties; the molecular structure of the organic semiconductor determines its interactions with other molecules such as the donor and acceptor molecules (miscibility) and the solvent molecules (solubility). The research in this area has focused on the control of the evolution of the BHJ morphology to achieve efficient exciton dissociation and charge transport. This section reviews the structural factors that affect the nanoscale morphology. To enhance the morphologies of blend films, PV materials require processability, i.e., solubility in organic solvents and miscibility with other organic semiconductors (Figure 4a). [71,72,74 76] These properties of PV materials can be varied by modulating their structures. Solubility can be achieved by modifying the functional side chains of the material, by breaking the planarity or other interactions in the molecular structure and by varying the M w of the polymer to impede self-aggregation. [71,72,74] The presence of long and bulky side chains hinders the molecular packing of the materials and makes them soluble in organic solvents. Materials with a highly planar structure or strong interactions are likely to (4 of 39)

5 Figure 3. a) J V curves for organic solar cell devices fabricated from various active layer (pbttt:pc 60 BM) morphologies. (ii) One-phase morphology formed by fine mixing of pbttt and PC 60 BM. (iii) The morphology with significant fraction of phase-pure pbttt and PC60BM regions. (iv) The morphology with considerable amounts of pbttt:pc 60 BM finely mixed phase as well as pure pbttt and pure PC 60 BM phases. Reproduced with permission. [52] Copyright 2014, The Royal Society of Chemistry. b) Illustration of the effect of local aggregation or molecular order. Due to the smaller ionization potential (larger electron affinity) of crystalline donor (acceptor) domains, the interface between the mixed and crystalline domains provides an energetic driving force that assists the spatial separation of charge carriers (left: crystalline donor case; right: crystalline acceptor case). In the absence of the crystalline domain and energetic driving force, efficient photocurrent generation is inhibited by relatively rapid charge recombination (middle). Reproduced with permission. [56] Copyright 2013, Wiley-VCH. self-aggregate, so the introduction of an asymmetric structure can prevent molecular self-aggregation and enhance their solubility. The solubilities of PV polymers are also affected by their molecular weights. A long polymer chain reduces the freedom of chain movements, which results in decreased solubility. The extent of polymerization should be adjusted by modifying the molecular structure to increase solubility in organic solvents. The miscibility of materials is an important factor governing the phase separation in BHJ films. [77] Poor miscibility can cause severe phase separation with inefficient exciton dissociation, whereas good miscibility can result in a well-mixed blend morphology with an insufficient number of charge transport pathways. Therefore, to achieve the appropriate morphology for efficient exciton dissociation and charge transport, some intermediate level of miscibility of the active layer components is required. The Gibbs free energy change (ΔG mix ) for the mixing of materials 1 and 2 is determined by the composition of the mixture and their Flory Huggins mixing parameter (χ 12 ) [ ] G = RT n lnφ + n lnφ + n n χ mix (1) where R is the gas constant, T is the absolute temperature, n 1 and n 2 are the numbers of moles of the materials, and φ 1 and φ 2 are their volume fractions. The miscibility χ 12 of the materials can be controlled by molecular engineering of components such as backbones, side chains, and end groups. Leman et al. calculated the values of χ for poly(3-hexylthiophene) (P3HT) and poly[n-9 -heptadecanyl-2,7-carbazole-alt-5,5-(4,7 - di-2-thienyl-2,1,3 -benzothiadiazole)] (PCDTBT) with respect to [6,6]-phenyl C 61 butyric acid methyl ester (PC 60 BM), [6,6]- phenyl C 71 butyric acid methyl ester (PC 70 BM), [6,6]-diphenyl C 62 bis(butyric acid methyl ester) (bispc 60 BM), and indene-c 60 bisadduct (IC 60 BA) from their Hansen solubility parameters. [78] They found that the swelling of the polymer by fullerene-based acceptors above the glass transition temperature (T g ) and the corresponding morphologies are strongly dependent on χ. Ye et al. studied the mixing of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)- thiophen-2-yl)-benzo[1,2-b:4,5-b ]dithiophene))-alt-(5,5-(1,3 -di- 2-thienyl-5,7 -bis(2-ethylhexyl)benzo[1,2 -c:4,5 -c ]dithiophene- 4,8-dione)] (PBDB-T) with nonfullerene acceptors (NFAs), IT-M and IT-DM, and found that χ is higher for PBDB-T:IT-M ( 2.7) than for PBDB-T:IT-DM ( 2.0), so PBDB-T:IT-M system (5 of 39)

6 Figure 4. a) Schematic phase diagram of a polymer fullerene solvent system at constant temperature and pressure. The navy arrows indicate the increase in concentration; C P,i, C F,i, and C S,i are the initial concentrations of polymer, fullerene, solvent in the solution, respectively. The red arrow indicates rapid quenching of the solution toward a solid-state blend. b) χ parameters calculated from the Hansen solubility parameters at room temperature and inferred χ at the annealing temperature (160 C) in relation to spinodal and bimodal curves of the polymer:small molecule acceptor systems for a given weight ratio of 1:1 (the dashed lines indicate the composition in the amorphous mixed domain in the limit of complete phase separation). Reproduced with permission. [79] Copyright 2016, Wiley-VCH. c) Left: Chemical structure of PNTz4T and PBTz4T; middle: molecular structure of NTz2T- Me and BTz2T-Me from single-crystal X-ray analysis; right: optimized backbone structure of PNTz4T and PBTz4T. Reproduced with permission. [82] Copyright 2012, American Chemical Society. d) Side chain engineering of PBDT2FBT with alkyl, alkoxy, thienyl, and alkoxy thienyl groups and AFM and TEM images of PBDT2FBT polymer:pc 70 BM blend films. Reproduced with permission. [83] Copyright 2014, Wiley-VCH. exhibits a lower degree of mixing and thus higher average purity because the purity of the amorphous mixed domains and their composition are determined by the bimodal composition (Figure 4b). [79] Given the appropriate solubility and miscibility, some degree of self-aggregation must occur to form connected domains for the continuous charge transport pathways in BHJ films. [80,81] Therefore, to fabricate BHJ morphologies with interpenetrating bicontinuous pathways for efficient exciton dissociation and charge transport, the molecules must have self-aggregation properties in addition to solubility and miscibility. These properties can be controlled by appropriate molecular designs because structural components such as repeating unit, side chain, and end group of the materials critically affect molecular behaviors in the blend solution and BHJ film. Repeating units such as thiophene, benzene, carbazole, benzodithiophene (BDT), indacenodithiophene (IDT), diketopyrrolopyrrole (DPP), naphthalene diimide (NDI), thieno[3,4- b] thiophene (TT), benzothiadiazole (BTz), naphtho[1,2-c:5,6- c ]bis[1,2,5]thiadiazole (NTz), and thieno[3,4-c]pyrrole-4,6-dione (6 of 39)

7 (TPD) in the polymer backbone significantly affect its conformation, molecular ordering and optoelectric properties. [18,71,73] Osaka et al. synthesized polymers based on BTz and NTz, and characterized the differences between them (Figure 4c). [82] NTz, a doubly BTz-fused ring, has a highly π-extended structure, so the PNTz4T:PC 60 BM blend film exhibits higher crystalline ordering than PBTz4T:PC 60 BM; its orientation is mainly face-on with some edge-on orientation. The centrosymmetry of NTz results in an anti arrangement of the thiophene linkages, whereas the axisymmetry of BTz yields a syn arrangement of the thiophene linkages. PNTz4F has a more linear backbone shape with alternately upward-pointing and downwardpointing alkyl side chains attached to thiophene linkages, whereas PBTz4T has a wavy backbone shape; as a result, PNTz4T exhibits better molecular ordering than PBTz4T. Because of this superior crystalline ordering with a dominant face-on orientation as well as more suitable electronic properties, the OPV based on the PNTz4T:PC 60 BM blend achieved a PCE of 6.3%. OPVs based on PNTz4T were further optimized with an inverted structure; they achieved PCEs of 9.80% with PC 60 BM and 10.1% with PC 70 BM. [70] A pole figure analysis of PNTz4T:PC 70 BM blend films with various thicknesses ranging from 50 to 400 nm shows that the population of face-on crystallites increases with film thickness. The high hole mobility of PNTz4T and the favorable distribution of edge-on and face-on crystallites within the film yielded superior PV performance at a film thickness of 300 nm. Side chain engineering can be used to control the solubility, miscibility, molecular ordering, and optoelectric properties of these materials. Lee et al. synthesized PBDT2FBT polymers with and without alkoxy, thienyl, and alkoxy thienyl groups anchored to the BDT units (Figure 4d). [83] PBDT2FBT contains thienyl groups and has the finest film morphology due to its higher solubility and the similarity of its surface energy to that of PC 70 BM. The authors also enabled 2D π-conjugation by using oligothienyl side chains. [84] Various PBDT2FBT polymers were synthesized with alkylated thienyl, bithienyl, terthienyl, and quaterthienyl side chains. As the number of thiophenes in the side chain increases, the 2D π-conjugation in the polymer increases, and as a result the absorption spectrum is red shifted and the absorption between 350 and 500 nm is increased. The crystallinity of the polymer also increases as the number of thiophenes in the side chain increases. Furthermore, the blend films have increased numbers of fibrils that provide percolation pathways for charge transport, so the films had increased PCE. Saito et al. synthesized polymers based on thiophene, thiazolothiazole (TzTz), and NTz with alkyl chains of four different lengths attached to the thiophenes. [85] The alkyl side chains on the thiophenes attached to TzTz were 2-ethylhexyl (EH) and 2-butyloctyl (BO), and the alkyl side chains on the thiophenes attached to NTz were BO and 2-hexyldodecyl (HD), i.e., PTzNTz-EHBO, PTzNTz-EHHD, PTzNTz-BOBO, and PTzNTz-BOHD were prepared. PTzNTz-EHHD, PTzNTz- BOBO, and PTzNTz-BOHD were soluble in chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (DCB), whereas PTzNTz-EHBO was soluble only in hot CB or DCB due to its short alkyl side chains. PTzNTz-EHBO did not show the transition peaks at temperature <350 C whereas the others showed transition peaks at 250 C T 300 C; thus PTzNTz-EHBO has a more rigid structure and a stronger tendency to aggregate than the other polymers. Grazing incidence wide angle X-ray scattering (GI-WAXS) showed that the pure and blend films of PTzNTz-EHBO have the highest crystallinity with a favorable face-on orientation and the shortest π π stacking distance of 3.77 Å. Although PTzNTz-EHBO has the highest crystallinity, its blend with PC 70 BM has a good phase-separated morphology, and OPVs based on it have the highest PCE, 9.0%, whereas the blends of PTzNTz-BOBO and PTzNTz-BOHD with PC 70 BM contain large domains and have PCEs of 1.7% and 1.2% respectively. Fluorine is strongly electron-withdrawing, so fluorination of a polymer is an effective strategy to control its properties and blend film morphology. A fluorine anchored to the backbone withdraws electron density from the backbone, and thereby reduces the HOMO level of the polymer. Furthermore, fluorine interacts with other atoms in the polymer by hydrogen bonding, which affects the polymer conformation. Liu et al. synthesized fluorinated polymers based on thiophenes and BTz, i.e., PffBT4T-2OD with difluoro-btz (ffbt or 2FBT), two 2-octyldodecyl (2OD) thiophenes and two thiophenes, and PBTff4T-2OD with BTz, two 2OD thiophenes, and two fluorinated thiophenes. [86] Both polymers exhibit temperaturedependent aggregation behavior; they dissolve well in DCB at 85 C but aggregate strongly at 25 C. The film morphology was optimized by controlling the solution temperature and the spin rate. A warm solution and a slow spin rate yielded highly crystalline yet reasonably small polymer domains that are insensitive to combination with diverse fullerene derivatives, which produced a PCE of 10.8%. Shin et al. synthesized PBDT2FBT with conjugated phenyl side groups that had been fluorinated at the meta or ortho position. [87] Of the two polymers, the one with ortho fluorination had a lower HOMO and better miscibility with PC 70 BM, but the polymer with meta fluorination had a more-planar conformation and a higher crystallinity. The polymer with meta fluorination led poor solubility and strong intermolecular interactions, so the film based on it has a rough surface. During formation of blend films, the polymer with meta fluorination formed a crystalline structure, which increased the light absorption and ensured fast charge transport. Bifluorination on the meta position of the conjugated phenyl side group further reduced the HOMO of the polymer and increased the V OC of OPVs to >1 V, but bifluorination disturbed the effective molecular packing of the polymer, so its light absorption and charge transport ability are both inferior. [88] The elongation of the π-conjugation in the polymer backbone affects the optoelectric properties, phase separation, and molecular ordering of the associated BHJ film. Hwang et al. incorporated π-bridges into polymer backbones based on BDT and TPD to prepare PBT without π-bridge, PBT-OT with a thiophene π-bridge, and PBT-OTT with a thienothiophene π-bridge. [89] Incorporation of π-bridges extended π-conjugation along the polymer backbone; this change increased crystallinity and the efficiency of charge transport. PBT-OTT has the surface energy most similar to that of PC 70 BM, and therefore has the highest miscibility with PC 70 BM; this high miscibility increases interfacial area and yields efficient exciton dissociation in the PBT-OTT:PC 70 BM blend. The high miscibility of PBT-OTT with PC 70 BM was verified by GI-WAXS measurements. The face-on (7 of 39)

8 crystalline structure is dominant in pure PBT-OTT, but this crystalline structure disappears when PBT-OTT is blended with PC 70 BM; this change indicates that PBT-OTT mixes well with PC 70 BM. The phase separation and miscibility of polymer materials can be simply modified by performing end-group functionalization. Shim et al. synthesized end group functionalized PCDTBT polymers with H to give H-PCDTBT, then modified them with OH to prepare OH-PCDTBT and with CF 3 to yield CF 3 -PCDTBT. H-PCDTBT and OH-PCDTBT intermixed homogeneously with PC 70 BM to yield morphologies that resemble that of a typical PCDTBT:PC 70 BM blend film. [90] In contrast, CF 3 -PCDTBT contains fibrous structures and forms a bicontinuous network with PC 70 BM; this morphology exhibits increased charge separation and increased charge mobility. These changes are due to the various secondary bonds of fluorine with other atoms and molecules. Some BHJ films have unique nanostructures such as those of 1D polymer nanowire (PNW) materials and bimolecular crystals (cocrystals) with the appropriate structure and conformation. The use of 1D PNWs is a promising strategy to improve BHJ morphologies and the corresponding PCEs. [27,91] Crystalline PNWs have a higher absorption coefficient and higher charge mobility than polymers in a disordered domain. The mechanism of formation of PNWs has not yet been established, but the general consensus is that a polymer must have a planar structure to form PNWs. P3HT is a well-known polymer for the preparation of PNWs because of its simple structure and planarity. [27,92] Solvents in which P3HT has very low solubility are used to pack the P3HT backbones. Lee et al. first demonstrated PNWs consisting of a donor acceptor alternating copolymer, [93] PBDT2FBT-2EHO, which has high planarity due to the many noncovalent interactions in the polymer backbone and side chains. However, the fabrication of PBDT2FBT-2EHO PNWs does need a hot blend solution (100 C) in a solubility-controlled solvent mixture with CB and 1-chloronaphthalene (CN). Recently, readily solution-processable donor acceptor copolymers with low bandgaps and high carrier mobilities have been introduced. P4TNTz-2F has a symmetric conformation along the backbone and therefore has superior planarity. [94] The formation of PNWs consisting of P4TNTz-2F can be induced at 70 C by using an additive to control the solubility of the polymer in the mother solvent. More comprehensive understandings about the 1D PNW structures are discussed in Section 3.6. For some donor polymers that have enough free volume between side chains, the acceptor molecule can intercalate into the free volume to form a bimolecular crystal. [95 98] The d-spacing of poly(2,5-bis(3-alkyl-thiophene-2-yl)thieno[3,2-b]- thiophene) (pbttt) in a pbttt:pc 60 BM blend film is larger than the d-spacing of pure pbttt. [95] Pure pbttt is highly crystalline and has a (100) lamellar stacking distance of Å. However, the d-spacing of a pbttt:pc 60 BM blend film is 30.2 Å and its X-ray diffraction (XRD) peak positions are totally different from those of pure pbttt. These differences arise because PC 60 BM is intercalated into the free volume between the alkyl chains of the polymer to form a bimolecular crystal with the polymer. Conjugated polymer and fullerene derivative pairs to form bimolecular crystals have been widely studied. [97] C 60, PC 60 BM, PC 70 BM, PCBM-C84, LuPCBEH-C80, ICMA-C60, IC 60 BA, FrechetCl-C60, and FrechetAcetyl-C60 and F4-TCNQ intercalate in the free volume of pbttt. In contrast, bispc 60 BM, bicpc 70 BM and ICTA-C60 do not intercalate in pbttt. Moreover, PQT, PBTCT, PTT, POPTT, POPQT, PTBzT 2 - alpha, and PTBzT 2 -beta can form bimolecular crystal with fullerene derivatives. Recently developed materials that were synthesized following material design strategies have achieved PCE >10%. PBDTTTbased polymers had been investigated first. PTB7 initially provided PCE 6%; this value was increased to 10.02% by morphology optimization using solvent engineering and interface modification. [99] Furthermore, PTB7-Th (10.95%), [100] PTB7-DT (10.12%), [101] and PBDT-TS1 (10.5%) [102] provide PCEs >10% in studies that combined specific material design and the optimizations of their morphologies and interfaces. Apart from the PBDTTT-based polymers, various photovoltaic materials achieve PCEs >10% when blended with fullerene acceptors: PffBT4T-2OD (10.8%), [86] PBTff4T-2OD (10.4%), [86] PNTz4T (10.1%), [70] P4TNTz-2F (10.62%), [94] PNTT-H (11.3%), [103] PNT- BDT-H (10.0%), [103] and PThBDTP (10.15%). [104] A few poly mer donors are well matched with NFAs, and yield PCEs up to 13.1%: PBDTTPD-HT (10.2%), [105] PBDB-T (12.1%), [106,107] PB1-S (10.49%), [108] and PBDB-T-SF (13.1%). [11] Some polymers can be used to achieve PCEs >10% by fabricating ternary blend OPVs: PPBDTBT (10.41%), [109] PDOT (11.21%), [110] PDBT-T1 (10.2%), [111] PSTZ (11.1%), [112] and PBTZT-stat-BDTT-8 (11.03%). [113] 3.2. Donor:Acceptor Composition The ratio of the donor polymer to the acceptor significantly influences the electrical properties of the blend film, the morphology of the active layer, and the photovoltaic performance of the associated PV cell (Figure 5). For example, the carrier transport properties of blend films vary with the concentration of acceptor molecules. P3HT:PC 60 BM BHJ films exhibit ambipolar transport at PC 60 BM concentrations <90 wt%, and their mobility varies with the PC 60 BM concentration. [114,115] An optimized donor:acceptor ratio in the BHJ film guarantees the high crystallinity of components, bicontinuous percolation, and interconnected pathways for the charge carriers, and enables the optimization of carrier dissociation, transport, and extraction. [ ] van Duren et al. found that the morphology and cell efficiency of the MDMO-PPV:PC 60 BM BHJ blend system can be controlled by adjusting the concentration of PC 60 BM. [133] PC 60 BM concentrations <50 wt% yield homogeneous film morphologies, but PC 60 BM concentrations above 67 wt% yield nanoscale phase separation of PDMO-PPV and PC 60 BM; the result is a sharp increase in the photocurrent and FF. To ensure the percolation of PC 60 BM and the presence of interconnected pathways for charge dissociation and transport, the optimal PC 60 BM concentration in MDMO-PPV:PC 60 BM is 80 wt%. Müller et al. proposed a simple rationale for choosing the optimum composition of the P3HT:PC 60 BM BHJ system. [134] Using differential scanning calorimetry thermograms, they obtained the temperature/composition diagram of the P3HT:PC 60 BM system and found that the blend is a simple (8 of 39)

9 Figure 5. a) Plots of J SC, V OC, FF, and PCE for PTB7:PC 70 BM solar cells as a function of donor:acceptor weight ratio. b) AFM height images for PTB7:PC 70 BM blend films with different ratios. c) Schematic illustration describing the percolation of fullerene domains: hopping via polymer sites, hopping via polymer sites in the presence of a small amount of fullerene, hopping via both host and guest sites, and hopping via guest sites. a c) Reproduced with permission. [135] Copyright 2017, Wiley-VCH. eutectic system with an eutectic compositional point c e at 205 C and 65 wt% P3HT, and that the optimal composition with respect to the associated device performance is slightly hypoeutectic. The current density and PCE are maximized at the composition c c e. After thermal annealing, the maximized J SC arises at a concentration c of P3HT of wt%, which is consistent with a slightly hypoeutectic mixture. Ho et al. studied how the donor:acceptor composition affects the photovoltaic performance of solar cells with the PTB7:PC 70 BM blend system. [135] The addition of ultralow dosages of PC 70 BM to the blend was performed to reveal the early stages of the development of the electronic relationship between PTB7 and PC 70 BM (Figure 5a c). As the concentration of PC 70 BM increases, the electron mobility passes through three regimes. When a small amount of PC 70 BM is added, PC 70 BM molecules tend to dock with the thienothiophene unit in PTB7 instead of forming aggregates; the docked mole cules act as electron traps and impede electron transport. When enough PC 70 BM is added, aggregation and percolation of PC 70 BM occur; the result is a dramatic decrease in the electron trap density and an increase in the electron mobility. The FFs of the corresponding cells are anticorrelated with the trap density in the active films. Varying the composition of a blend system can change the crystalline features of the components. Chiu et al. systematically studied the morphologies of P3HT:PC 60 BM blend films by using grazing incidence small-angle X-ray scattering (GI-SAXS) and GI-WAXS. [136] Most pristine P3HT films have (100) plane crystallites along the z-direction. As the PC 60 BM concentration increases to 50 wt%, the in-plane correlation length of P3HT dramatically increases, but PC 60 BM in the films aggregates rather than crystallizes in the films. At PC 60 BM concentration >38 wt%, the presence of PC 60 BM aggregates increases the correlation length of the in-plane P3HT crystallites but disrupts the P3HT crystal ordering in the out-of-plane direction. The calculated sizes of the PC 60 BM aggregates are larger than those of the P3HT crystallites. At PC 60 BM concentration between 38 and 50 wt%, the PC 60 BM aggregates induce partitioning of the P3HT crystallites and confine them; the result is increased in-plane crystalline ordering and reduced out-of-plane ordering (9 of 39)

10 Tuning of the dimensions of PC 60 BM needle-like crystals in the P3HT:PC 60 BM BHJ system is performed by adjusting the donor:acceptor composition and the thermal annealing conditions. [137] Brief annealing at a low temperature causes P3HT to form crystallites with enhanced crystallinity, but long annealing at a high temperature induces the formation of needle-like PC 60 BM crystallites with lengths from a few micrometers to 100 µm. As the composition of P3HT:PC 60 BM is increased from 1:1 to 1:2, the average and variance of the length of the needles also increase; but further increasing the ratio to 1:4 reduces the variance of the needle length because an excessively high PC 60 BM concentration limits their growth. A highly crystalline P3HT thin layer forms around the PC 60 BM needles; this process is caused by the diffusion of PC 60 BM out of the matrix toward the needle-like PC 60 BM crystallites during thermal annealing. van Bavel et al. studied the effects on the morphology of varying the composition and performing thermal annealing. [138] By using the electron tomography technique to determine the 3D morphology of P3HT:PC 60 BM BHJ films, they found that P3HT films with wt% PC 60 BM undergo significant changes during thermal annealing. After thermal annealing, P3HT films with 40 wt% PC 60 BM crystallize and the resulting P3HT PNWs are almost homogeneously distributed in the vertical direction, whereas in P3HT films with 60 wt% PC 60 BM, there is a vertical gradient in the concentration of the P3HT crystallites, i.e., the density of P3HT crystallites is higher at the top of the film close to the Al electrode than at the bottom. This distribution is not favorable for high-efficiency devices. The optimal PC 60 BM composition is 40 wt%, for which a dense network of P3HT crystallites forms throughout the whole volume of the film and the degree of P3HT crystallinity is 45%. Some other polymer:acceptor blend systems require a higher acceptor content than those appropriate to P3HT:PC 60 BM and PTB7:PC 70 BM. [66] Park et al. observed that the nanoscale connectivity of PCDTBT is sensitive to the concentration of PC 70 BM. As the PC 70 BM concentration increases, phase separation of the PCDTBT nanofibrillar crystallites occurs; as a result, the optimum nanostructure occurs at a weight ratio of 1:4: this ratio yields an interconnecting PCDTBT network with long charge-carrier pathways. Blending composition affects the phase behavior and correlated optoelectronic properties of films containing P3HT and an NFA which is based on benzothiadiazole or dicyanovinyl units. [139] Both systems exhibited a simple eutectic phase behavior in a similar way to the P3AT:PCBM blends; however, the eutectic point and the optimal blending composition are different for both systems. These differences occur because the acceptors have different solubilizing units (di-n-propylated dithienylsilole and di-n-propylated dithienylsilole), which cause different microstructure formations during blend solidification. In a BHJ film with P3HT and the acceptor with di-n-propylated dithienylsilole unit, a persistent glassy solid forms over a range of hypoeutectic compositions with respect to P3HT, so this blend system requires a higher acceptor ratio than the other to form electron percolation paths in the crystalline acceptor network Solvent Engineering The morphology of an active layer varies dramatically with the solvent that is used for its deposition. The solubility and volatility of the solution system depend on the processing solvent; this dependence can be associated with dramatic changes in the solution state molecular conformation, and in the kinetics of the drying film. Mixtures of solvents can also be used to fine tune the active layer morphology. In this section, we report a solvent engineering strategy for the control of the morphology of active layers. Correlations between the properties of the solvent and the final active layer morphology are discussed, with an especial focus on the principle that the solvent system affects morphology development Effects of Solvent Selection The morphology of a solution-processed organic active layer depends critically on the solvent used (Figure 6a,b). [66,140] In particular, the degree of crystallization and the phase separation of the film are strongly affected by the choice of the solvent. For example, the evaporation rate of the solvent determines the time required for the film to dry. If the solvent resides for a long time in the film, then there is sufficient time for the donor and acceptor molecules to migrate to form a thermodynamically stable morphology; the result could be increased crystallinity or a phase-separated morphology. If evaporation is completed in a very short time, the fast removal of solvent can immobilize the donor and acceptor molecules, and the morphology can become kinetically frozen in some intermediate state. These phenomena are well illustrated by the case of P3HT. A film that is spin-coated from CF contains P3HT nanocrystallites with sizes <20 nm that are dispersed in a disordered phase and have poor interconnectivity. In contrast, a film spincoated from DCB develops P3HT crystals with typical sizes in the range nm that form through the growth and interconnection of the crystallites. These differences in the crystal growth of P3HT have been attributed to the different evaporation speeds of the two solvents, i.e., the rapid evaporation of CF limits the time for growth of the P3HT crystallites, whereas the slow evaporation of DCB does not. [141] This effect can be amplified by the use of a solvent additive with a low vapor pressure and a slow drying speed, which therefore resides for a long time in the drying film. The effects of solvent additives on the develop ment of the morphologies of active layers are discussed in detail in the following sections. The solubility of the active layer material in the solvent also affects its crystallization/aggregation and phase separation. Hoppe et al. found that the morphology of a MDMO- PPV:PC 60 BM blend film cast from toluene is very different from that obtained when it is cast from CB. In particular, PC 60 BM is less soluble in toluene than in CB, so PC 60 BM domains form larger clusters in the toluene-cast film than in the CBcast film. [123] Zhang et al. investigated the influence of solvent mixing on the morphology and performance of OPVs. For spin-coated films composed of a blend of poly(2,7-(9,9-dioctylfluorene)-alt-5,5-(4,7 -di-2-thienyl-2,1,3 -benzothiadiazole)) and PC 60 BM, their morphologies vary with the vapor pressure (10 of 39)

11 much larger and photocurrent generation is lower. The solubility of the blend in the casting solvent does not always affect the photovoltaic performance of the associated solar cell. [142] Machui et al. controlled the nature of the mixed solvent systematically by mixing CB with relatively poor solvents such as o-xylene, cyclohexane, and acetone, and by varying the mixing composition. The PCEs of the devices did not depend dramatically on the solubility. [143] The polymer crystallinity within the blend film can be improved by intentionally decreasing the solubility of the donor polymer. [65,67,92] The addition of cyclohexanone (CHN), which is a poor solvent for P3HT, to the blend solution of P3HT:PC 60 BM in CB promoted the selective formation of P3HT crystallites within the solution, which then seed the growth of crystalline P3HT NWs. The P3HT NW structure facilitates charge transport and balances the electron and hole mobilities, and thereby greatly increases the PCE of such solar cells. [67,92] A similar study has been conducted that used p-xylene as the poor solvent. [65] Variations in solution temperature can also produce large changes in blend morphology, especially when the aggregation properties of the dissolved material are strongly temperature dependent (Figure 6c). [86,144,145] Polymers with such aggregation behavior dissolve readily in solution at high temperatures, but aggregate strongly when the solution is cooled. The blend film morphology can then be finely tuned by using this property in combination with control of other processing conditions. In one study, a series of donor polymers with temperature-dependent aggregation behavior were designed, and their aggregation and crystallization was controlled during the film cooling and drying processes. The resulting films had near-ideal blend morphologies: small polymer domains with high crystallinity and preferential orientation. [86] Solvent Additives Figure 6. a,b) The effects of CF, CB, and DCB solvents on film morphology and device performance. a) TEM images of PCDTBT:PC 70 BM films spincast from CF, CB, and DCB solvents. b) J V curves of the devices fabricated with films cast from CF, CB, and DCB. c) Temperature-dependent aggregation property of donor polymers. Left: UV vis absorption spectra of a PffBT4T-2OD film and solution (0.02 mg ml 1 in DCB) at various temperatures. Right: Schematic illustration of the spin-coating and morphology forming process. Polymer chains aggregate during the cooling process. a,b) Reproduced with permission. [66] Copyright 2009, Nature Publishing Group. c) Reproduced with permission. [86,145] Copyright 2014, Nature Publishing Group. Copyright 2017, American Chemical Society. and solubility of the solvent mixture. When the main solvent CF is mixed with CB, the resulting film contains fine and uniform domains and yields significantly increased photocurrent generation during solar cell operation. In contrast, when CF is mixed with xylene or toluene, the resulting domains are The term solvent additive is used when the solvent added to produce a mixed solvent system has both a high boiling point and a selective solubility for the donor and acceptor molecules. The high boiling point or low volatility of the solvent additive means that the solvent additive persists for a long time in the film during the development of its morphology. As a result, the solvent additive has sufficient time to interact with the organic semiconductor molecules within the active layer film, and this extended interaction alters the film s nanostructures. The low volatility and long residence time also mean that the solvent additive can exert powerful effects even if only a small amount is used. However, the long residence time of solvent additives may reduce the operational stability of the device if the solvent additives are not completely removed from the completed device (Section 3.8). The term selective solubility means that the donor and acceptor components have different solubilities in the solvent, i.e., one component will aggregate while the other component remains free to move within the solution or within the film during drying. Together with the long residence time of the solvent additive within the drying film, this selective mobilization and immobilization of molecules imposed by the additives selective solubility can be exploited to effectively alter the fine nanostructure of the photovoltaic blend (11 of 39)

12 Figure 7. a) The effect of solvent additives on polymer crystallite sizes in PCPDTBT:fullerene blend films. b) The effect of additives on the relative crystallinity in both neat and blend films (corresponding device power conversion efficiencies are also shown). c) Time evolution of the orientational order parameters (S) calculated for the (100) and (100) crystallographic planes PCPDTBT, during the drying of a PCPDTBT:PC 70 BM blend film spincast from CB containing ODT. The parameter S is a quantity that varies between 0.5 and 1, where a value of 1 ( 0.5) indicates an edge-on (face-on) orientation of the crystallographic planes. d) TEM images of PTB7:PC 70 BM blend films prepared from CB without (upper image) and with (lower image) DIO. e,f) Resonant soft X-ray scattering analysis on PTB7:PC 70 BM blend films prepared from CB and CB:DIO. e) Azimuthally integrated scattering profiles with associated peak fits. f) Normalized histogram of scattering intensity versus fullerene domain size. g,h) The effect of solvent additives on the vertical phase separation of a blend film. g) Dynamic SIMS profiles of the S /C intensity ratio versus film depth for polymer:pc 60 BM blends processed with (DIO or CN) and without solvent additives. h) The effect of solvent additives on the degree of vertical composition fluctuation for various polymer:pc 60 BM blend films. a,b) Reproduced with permission. [147] Copyright 2011, Wiley-VCH. c) Reproduced with permission. [22] Copyright 2012, American Chemical Society. d) Reproduced with permission. [160] Copyright 2010, Wiley-VCH. e,f) Reproduced with permission. [17] Copyright 2013, Wiley-VCH. g,h) Reproduced with permission. [174] Copyright 2016, The Royal Society of Chemistry. The two major functions of solvent additives are to increase the crystallinity, and to modulate the domain sizes of the donor:acceptor phase-separated morphology. [146] An increase in the crystallinity of a donor polymer is mostly obtained by using an additive in which the donor polymer has relatively low solubility (Figure 7a,b). [93, ] The poor solubility in the solvent additive affects polymer aggregation and crystallization during the entire film formation process, from the solution state to the dynamic film drying process. The detailed mechanism by which the solvent additive improves crystallinity is discussed in Section However, the use of too much solvent additive can decrease π-stacking ordering and the crystallinity of the donor polymer; as a consequence, both the hole mobility and PCE decrease. [151] The presence of solvent additives can also facilitate the reorientation of the polymer crystallites in active layers. In a spin-coated poly[2,6-(4,4-bis(2-ethylhexyl)-4h-cyclopenta[2,1-b;3,4-b ]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):PC 70 BM blend film treated with 1,8-octanedithiol (ODT), the crystallographic orientation of the donor polymer changed over a long period of time after spin-coating (Figure 7c). After spin-coating, the edge-on orientation of the (100) crystallites becomes increasingly pronounced during the first 26 min and then remains constant, whereas the orientation of the (100) crystallites gradually rotates from preferentially edge-on to preferentially plane-on during the first 80 min. [22] In the blade-coated additive-free film of a P3HT:PC 60 BM blend, P3HT has a strong preference for the edge-on orientation. However, the addition of ODT broadens the distribution of orientations. The enhanced bulk crystallization of P3HT in the presence of ODT is attributed to this orientation distribution change. [152] Solvent additive induced changes in the molecular orientation distribution have also been observed in all-polymer (12 of 39)

13 donor:acceptor blends and in donor acceptor block copolymer systems. [ ] Solvent additive treatments induce dramatic changes in the donor:acceptor phase separation of blend films in various material systems. [ ] For example, in PTB7:PC 70 BM blends, the addition of 1,8-diiodooctane (DIO) dramatically shrinks the size of the pure fullerene domains that are embedded in the polymer-rich matrix. [17,160] This prevention of large-scale PC 70 BM aggregation leads to a change in the morphology to a multilength-scale structure and a concomitant increase in the size of the donor acceptor interface; as a consequence, the device efficiency is greatly increased (Figure 7d f). However, the effects of solvent additives on the degree of phase separation differ from blend to blend, even when the same solvent additive is used. [161] In some P3HT:PC 60 BM and PCPDTBT:PC 70 BM blends, the use of DIO or ODT additives increases the sizes of the donor and acceptor domains, [118, ] whereas in other blend systems such as poly[{2,5-bis(2-hexyldecyl)-2,3,5,6- tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}-alt-{[2,2 - (1,4-phenylene)bisthiophene]-5,5 -diyl}]:pc 60 BM and that of the alternating copolymer 4,4-bis(2-ethylhexyl)-dithieno[3,2- b:2,3 -d]silole and N-octylthieno[3,4-c]pyrrole-4,6-dione and PC 70 BM, the addition of DIO reduces the domain size, as is the case for the PTB7:PC 70 BM blend. [ ,165,166] These differing outcomes arise from the different intermolecular interactions between the participating molecules. Therefore, to enhance the tuning of morphology, the use of a combination of more than two different solvent additives is often tried. [31,167,168] General guidelines for the selection of the appropriate solvent additive with the aim of increasing the PCE have been suggested. Graham et al. calculated the Hansen solubility parameters of a wide range of solvent additives and correlated these parameters with morphological changes. The presence of additives in which both the donor and the acceptor have poor solubility leads to the development of small domains and thus increased hole mobility and PCE, whereas the use of additives in which both the donor and the acceptor have good solubility leads to the development of large donor and acceptor domains and thus decreased electron mobility and PCE. [161] Vongsaysy et al. proposed a more quantitative criterion for the selection of an effective solvent additive based on Hansen solubility para meters, and suggested that the solvent additive should have selective solubility with respect to the donor and the acceptor. [169] Li et al. showed that the effects of a solvent additive on the intermixing or phase separation of a morphology depend on the strength of the molecular interaction between the acceptor molecule and the selected solvent additive; solvent additives can be divided into type I solvents that interact weakly with the N,N -bis(1-ethylpropyl)- perylene-3,4,9,10-tetracarboxylic diimide acceptor, and type II solvent additives that interact strongly with it. Type I solvents include DIO and OTD. Type II solvents include CN, 1-benzothiophene, and 2,3-benzofuran. The use of type I additives in the 7,7 -(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b ]dithiophene- 2,6-diyl)bis(6-fluoro-4-(5 -hexyl-[2,2 -bithiophen]-5-yl) benzo-[c]- [1,2,5]thiadiazole) (p-dts(fbtth 2 ) 2 ):EP-PDI system, which when additive free has a finely mixed morphology, induces the formation of pure crystalline phases due to the unfavorable interaction between the additives and EP-PDI. The use of type II additives in the PTB7:EP-PDI system reduces the severe aggregation of EP-PDI molecules and increases the size of the donor:acceptor intermixed phase. These results show that a balance between the areas of the intermixed and pure phase can be achieved by selecting the solvent additive that is appropriate to the strengths of the intermolecular interactions in the donor:acceptor system. [170] Solvent additives can also be used to control the intercalation of fullerene into donor polymer side chains. [52,171,172] The effect of the solvent additive ODT on the inhibition of fullerene intercalation in poly(n-(2-ethylhexyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide) (PhBTEH)/PC 70 BM blend films was investigated. The blend film processed without ODT had a morphology characterized by large domains ( nm) with a fullerene-polymer intercalated structure; the PCE of the solar cell device based on this film is poor. However, when ODT is used as a solvent additive, the film morphology contains small (15 20 nm) domains that are completely free of intercalation; this morphological change means that the solar cell device based on this film has a significantly increased PCE. This morphological change was attributed to changes in the kinetics of phase separation and of the crystallization of the polymer and fullerene. The selective solubility of ODT induces the fast crystallization of PhBTEH, and thereby inhibits the intercalation of fullerene into the polymer side chains during phase separation. [172] A similar result was obtained in the pbttt:pc 60 BM system. In this case, fatty acid methyl ester was used as the solvent additive. In the absence of the additive, a fully intercalated 1:1 pbttt:pc 60 BM phase forms; it exhibits efficient exciton quenching but yields few free carriers. In contrast, the incorporation of an additive with the optimal chain length hinders the intercalation of PC 60 BM into the polymer and thus yields a partially intercalated morphology that provides sufficient donor acceptor interfaces and relatively pure domains of polymer and fullerene; this morphology was ideal for efficient free-charge generation and long-range charge transport. [52] A vertical gradient in the donor:acceptor composition can also be created by the use of solvent additives. [162, ] Moderate vertical phase separation with a donor-rich region near the anode and an acceptor-rich region near the cathode is favorable for efficient charge-carrier transport and collection. When the P3HT:PC 60 BM blend is spin-coated in the absence of the ODT additive, the film does not develop vertical phase separation, but the addition of ODT induces vertical phase separation consisting of a P3HT-rich region at the air surface and a PC 60 BMrich region at the bottom. [162,173] The prolonging of the solvent drying time that results from the addition of ODT probably provides sufficient time for the demixing of the P3HT:PC 60 BM phases. The preferential movement of P3HT to the top is attributed to the difference between the surface energies of P3HT and PC 60 BM: P3HT has a lower surface energy than PC 60 BM, so P3HT becomes segregated near the air surface as the solvent evaporates, whereas PC 60 BM wets the bottom interface between poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) and the active layer. It has been shown that DIO and CN solvent additives also strongly affect the vertical variation in the donor:acceptor compositions of active layers. [174] Once vertical phase separation is initiated by surface-directed spinodal decomposition, DIO and CN assist the development of this process over a large scale (Figure 7g,h) by inducing polymer (13 of 39)

14 aggregation and prolonging phase demixing. Similar phenomena have also been observed in PBDTTT-C-T:PC 70 BM and PBDTTT-C-T:PDI blends. [175,176] The Mechanisms of Solvent Additive Effects on Morphology The mechanism by which a mixed solvent affects morphology formation can be divided into two stages: the modification of the molecular configuration in the solution phase then the modification of nanostructure formation during deposition due to changes in the drying kinetics of the solution (Figure 8). The effects of solvent additives on solution-phase molecular configurations have been analyzed. [15, ] Lou et al. observed the effects of the solvent additive DIO on the solution-phase aggregation in blend solutions composed of PTB7 and PC 70 BM; the PTB7 and PC 70 BM molecules form aggregates within the solution, but their sizes are changed when DIO is added. DIO Figure 8. a) Solution SAXS profiles and schematic representation of the donor polymer conformation in the CB solution. The solution without the additive (bottom curve) and with 5 vol% CN (upper curve). The three highlighted areas indicate the overlapping polymer chains (low q regime), the development of stiff chain segments (intermediate q regime), and the onset of polymer ordering (high q regime). With CN, polymer chains aggregate and exhibit short stiff chain segments and short-range ordering within the aggregate. Without additives, polymer chains form aggregates; however, ordering of stiff segments is missing. b) Schematic illustration of the role of solvent additives in preventing the large-scale liquid liquid phase separation by inducing polymer aggregation before the liquid liquid phase separation starts. c) In situ GI-WAXS and GI-SAXS results during the drying of the pdpp:pc 70 BM blend processed from CF:DCB mixture. Filled black squares: domain size of scattering particles. Purple open squares: (100) d-spacing of pdpp. Blue open circles: pdpp (100) peak intensity. Green open squares: solvent weight. d) Proposed mechanism for film morphology evolution based on the data shown in (c). The numbers are corresponding to the stages obtained from (c). a) Reproduced with permission. [179] Copyright 2013, Wiley- VCH. b) Reproduced with permission. [180] Copyright 2015, Nature Publishing Group. c,d) Reproduced with permission. [182] Copyright 2012, Wiley-VCH (14 of 39)

15 selectively dissolves PC 70 BM, so the addition of DIO reduces the size and population of PC 70 BM aggregates. In contrast, the population of PTB7 aggregates is little affected, and the size of its aggregates increases slightly. Based on these observations, the authors proposed that the dissolution and size reduction of PC 70 BM aggregates within solution enables the formation of smaller domains, and as a result improves the donor:acceptor interpenetration within the dried films. [15] Schmidt et al. investigated the mechanism by which CN affects blend separation. In a blend solution of the DPP-based donor polymer and PC 70 BM, treatment with CN greatly affected polymer aggregation within the solution, as well as to increase the population of seed crystallites and increase the molecular ordering within the aggregates (Figure 8a). The addition of CN induces the development of stiff chain segments and short-range lamellar order within the polymer aggregates. The authors proposed that the seed crystallites in solution nucleate further crystallization of the polymer during film formation, and that the resulting increased number density of the nuclei leads to the formation of many small polymer domains with jagged donor acceptor interfaces. [179] Solvent additives can also affect the spinodal decomposition of blend films during deposition. A spin-coated blend solution can undergo liquid liquid phase separation when the solution composition enters the spinodal region as the solvent evaporates. This liquid liquid phase separation usually results in large-scale phase separation of the donor and the acceptor. However, van Franeker et al. found that the presence of solvent additives can prevent this large-scale liquid liquid phase separation by inducing polymer aggregation before such separation starts (Figure 8b). When a mixed solvent is used, the solvent quality for the polymer decreases as the film dries; this poor solvent quality induces polymer aggregation at a relatively high solvent content for which liquid liquid phase separation cannot occur. As a result of polymer aggregation, liquid liquid phase separation and the consequent large-scale phase separation are inhibited, so the domain size in the final blend film decreases. [180] Solvent additives can also affect morphology development during the drying of blend films. [150,152, ] Gu et al. monitored the drying processes of such films by using in situ GI-WAXS. Blend solutions of PCPDTBT and PC 60 BM dissolved in a mixture of CB and DIO were drop cast onto a substrate. In the absence of DIO, the diffraction peak of PCPDTBT is absent for the entire drying process; thus mixing of PCPDTBT with PC 60 BM fully suppresses the crystallization of PCPDTBT. When DIO is present, a broad diffuse peak is observed that is due to aggregates of PCPDTBT in solution. This peak occurs due to the insolubility of PCPDTBT in DIO. CB has a higher vapor pressure than DIO, so CB evaporates faster than DIO; as a result of the increase in the proportion of DIO, the solvent becomes poorer for PCPDTBT. Consequently, the PCPDTBT component crystallizes into fibrils that form a network structure. PC 60 BM remains dissolved in DIO and is gradually excluded from PCPDTBT as it crystallizes. During the final stage of drying, PC 60 BM and the noncrystalline PCPDTBT fill the interfibrillar regions of the crystalline PCPDTBT network as the remaining DIO evaporates. [181] A similar study was performed on a mixture of the donor polymer pdpp and PC 60 BM in a mixed solvent system consisting of DCB and CF (Figure 8c,d). In this case, pdpp has good solubility in CF but poor solubility in DCB, whereas PC 70 BM dissolves well in both DCB and CF. As in the previous case, the fast-drying component CF largely evaporates during the initial stage of drying, then the remaining DCB-rich solvent dries. As the solvent evaporates, the solubility of pdpp decreases and it begins to crystallize to form a fibrillary network, whereas PC 70 BM remains dissolved. The domain size grows as the crystallization proceeds, which results in a phase-separated morphology during the intermediate stage. Upon further evaporation, the remaining pddp and PC 70 BM further crystallize and segregate within the interfibrillar region to yield a film with a multilength scale morphology. [182] The prolonging of film drying because of the slow evaporation of solvent additives increases the crystallinity of the resultant films. [152,183,184] In blade-coated active-layer films, the presence of solvent additives such as ODT and CN can significantly prolong the time required for complete drying of the films. [183] In films without an additive, the whole solidification process occurs over a relatively short time while the main solvent (e.g., CB) evaporates, but after the main solvent has dissipated the remaining polymer and fullerene cannot undergo further crystallization and phase separation, and therefore form mixed amorphous regions in the final film. In contrast, in a film treated with solvent additives, the solidification of the film continues even after the removal of the main solvent because a considerable amount of the solvent additive remains in the film and evaporates slowly. The significantly delayed completion of solidification enables the extended evolution of the nanostructure, in particular an increase in the overall crystallinity due to the further crystallization of the remaining polymer. [152] 3.4. Nonsolvent Morphology Modifier A third component can be incorporated into binary blend system to modify the morphology of the photoactive layer. [ ] This third component remains in the BHJ film and affects the morphology in contrast to the solvent additive which is removed after it modifies morphology. Third-component materials include organic semiconductor, organic insulator, and block copolymer, and can be a second donor or acceptor, an energy cascade linker, or an alloy of a donor and an acceptor depending on the optoelectronic properties and the location of the third component in the blend. In this section, we introduce and categorize the function of the third component as a morphological modifier, we exclude optoelectronics and focus on the morphological viewpoint. Proper use of morphological modifier controls the domain size, crystalline ordering or defect sites in the blend film. The domain size of the blend film must be controlled to ensure charge separation because it is limited by exciton diffusion. Therefore, the domain size should be less than the exciton diffusion length but also offer appropriate charge transport pathways. Incorporating crystalline molecules into the host binary blend is an efficient strategy to adjust the domain size. Gasparini et al. incorporated highly crystalline poly[(4,40-bis(2- ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(4,7-bis(2- thienyl)-2,1,3-benzothiadiazole)-5,50-diyl] (Si-PCPDTBT) into (15 of 39)

16 an amorphous PTB7 blend with PC 70 BM (Figure 9a). [199] The host amorphous PTB7 provides highly efficient charge separation and the guest Si-PCPDTBT facilitates charge transport; overall, this system mimics the behavior of an efficient threephase morphology. The domain size was increased to 200 nm by the addition of 25% Si-PCPDTBT, but the contrast variation in TEM images on the scale of nm indicates that the small domains are not affected by Si-PCPDTBT addition. Charge transfer from the disordered PTB7 regions to the highly ordered Si-PCPDTBT domains avoids most morphological traps and, remarkably, almost completely prevents charge recombination: FF = 77% when 15% Si-PCPDTBT is added. Zhang et al. incorporated a nematic liquid crystalline small molecule, benzodithiophene terthiophene rhodamine (BTR), into a PTB7- Th:PC 70 BM blend. [200] BTR is highly miscible with PTB7-Th because of their BDT groups, so addition of BTR improves the morphology of the ternary blend film by decreasing the π π stacking distance, extending the coherence length, increasing domain purity, and producing finely dispersed fibrils. As a result, ternary blend OPVs with 25% BTR had PCE of 11.40% for a 250 nm thick active layer, and PCE of 8.37% for a 400 nm thick active layer. Chang et al. incorporated crystalline P3HT as a morphology control agent into a PCPDTBT:PC 60 BM blend. [201] The addition of 1% P3HT and the solvent additive DIO modulated the ternary blend morphology by increasing the domain size and length to produce favorable bicontinuous phase separation and fast charge transport, and improved PCE improved by 20%. Zhang and co-workers incorporated a highly crystalline small molecule, BDT-3T-CNCOO, into a PBDTTPD-HT:PC 70 BM blend. [202] The addition of 40% BDT- 3T-CNCOO increased the domain size and promote polymer crystallization. BDT-3T-CNCOO exhibits an edge-on orientation when blended with PC 70 BM, but a face-on orientation when blended with both PBDTTPD-HT and PC 70 BM. The resulting changes in film morphology, i.e., high crystallinity and appropriate phase separation, boosted the PCE from 6.85% to 8.40%. Similar phenomena were observed when BDT-3T-CNCOO was added to a PBDTTT-C-T:PC 70 BM blend: the domain size and purity are increased when 50% BDT-3T-CNCOO is incorporated; moreover, the presence of BDT-3T-CNCOO induces the formation of nanofibrous polymer domains that facilitate charge transport. [203] Huang et al. incorporated 2,4-bis[4-(N,Ndiisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) into a P3HT:PC 60 BM blend and found that it broadens the spectral absorption and induces the formation of a nanomorphology. [204] The smooth P3HT:PC 60 BM blend film morphology is transformed by SQ addition into a highly ordered fibrillar structure with a feature size of 20 nm which is approximately twice the Forster radius of 8 nm. Highly efficient Forster resonance energy transfer (FRET) from P3HT to SQ ( 96%) occurred on a picosecond timescale due to the large spectral overlap between the emission of P3HT and the absorption of SQ; this high FRET improves exciton migration over long distances. Some copolymers can be used as additives to increase the miscibility of donor polymers and acceptors, to reduce phase segregation, and to suppress excessive phase aggregation upon thermal annealing. [ ] Sivula et al. designed and synthesized an amphiphilic diblock copolymer as a compatibilizer. [206] Addition of this copolymer to a P3HT:PC 60 BM blend reduced the interfacial energy between the donor poly mer and the fullerene acceptor, and ultimately changed the morphology of the blend film. The phase segregation in the film significantly decreases, and it maintains its intimate BHJ structure even after thermal annealing; as a result, the device efficiency is high. Yang et al. synthesized a rod-coil diblock copolymer that contains C 60 then added it to a P3HT:PC 60 BM blend as a surfactant. [209] The copolymer forms a nanofibrillar structure even though the bulky fullerene group is present in its structure. The addition of 5% of the copolymer to the blend increases the miscibility between P3HT and PCBM and results in a 35% increase in the PCE of the associated solar cell. In some cases, aggregation arises in the blend film, and this tendency can be disrupted by adding a third component to induce the formation of a nanoscale bicontinuous morphology. For instance, the incorporation of amorphous RRa-P3HT into an RR-P3HT:PC 60 BM blend reduces the sizes of the RR-P3HT crystals and the PC 60 BM domains, but increases the overall crystallinity of the film. [211] Liu et al. incorporated the wide-bandgap PDBT-T1 into a PTB7-Th:PC 70 BM blend. [212] A ternary blend film with 20% PDBT-T1 had smaller domain sizes with fibrous features and smoother surfaces than did PTB7-Th:PC 70 BM binary blend film, and attained PCE of 10.2%. Adding a monomer of conjugated polymers to a BHJ blend system can modify the morphology of the active film and thus achieve a high efficiency. [213,214] 3HT, the monomer of P3HT, was added to the P3HT:PC 60 BM blend; the resulting film underwent clear phase separation to form an interpenetration network without requiring thermal annealing. [214] Addition of the monomer also decreased the interlayer distance of the conjugated backbone of P3HT; this change reduced the resistance to carrier hopping, and yielded a high J SC and a small series resistance R S. Thermal annealing with the aim of increasing the crystallization of P3HT and PC 60 BM only slightly increased the PCE. Therefore, this method of adding 3HT is simple and annealing-free. Lu et al. studied ternary blend OPVs composed of a donor, fullerene, and the ITIC (Figure 9b). [109] These blends retained the high electron mobility and FF of PC 70 BM, and the presence of ITIC widen the range of light absorption. However, blend films that contain ITIC have aggregated morphologies, which lead to poor exciton dissociation and severe charge recombination. The morphology can be controlled by varying the ITIC:PC 70 BM ratio and using DIO as a solvent additive to control film-drying kinetics and phase separation. OPVs based on a poly{4,8-bis(4- (2-ethylhexylthio)phen-1-yl)benzo[1,2-b:4,5-b ]dithiophen-2,6- yl}-alt-{5,5 -(5-(2-decyltetradecyloxy)-6-fluorobenzo[c] [1,2,5]- thiadiazole-4,7-diyl)di(thiophen-2-yl)} (PPBDTBT):ITIC binary blend had PCE of 7.72%, with an especially high V OC of 0.92 V, but severe ITIC aggregation limited FF to 0.63, which is less than the FF (0.75) of an OPV based on a PPBDTBT:PC 70 BM binary blend. In these ternary-blend OPVs, PC 70 BM disrupts the molecular stacking of ITIC and reduces its aggregation, thereby increasing the donor acceptor interfacial area, while retaining high electron mobility. The highest PCE of 10.35% was achieved for an ITIC:PC 70 BM ratio of 60:40; in this system, J SC is increased by the complementary light absorption, V OC is increased by the high lying LUMO level of ITIC, and FF is slightly decreased by ITIC aggregation. Fan et al. incorporated (16 of 39)

17 Figure 9. a) Schematics of a three-phase morphology modified by adding a crystalline Si-PCPDTBT (green) into amorphous PTB7 (red) and PC 70 BM (black) blend films and TEM images of PTB7:PC 70 BM, PTB7:Si-PCPDTBT:PC 70 BM (0.85:0.15:1.5), and PTB7:Si-PCPDTBT:PC 70 BM (0.75:0.25:1.5) blend films. b) TEM images of PPBDTBT:ITIC and PPBDTBT:ITIC:PC 70 BM (1:1.2:0.8) blend films. c) GI-WAXS patterns of pure PTB7 and PTB7:PDTP-DFBT (0.8:0.2) blend films. d) DSIMS profile of p-dts(fbtth 2 ) 2 :PC 70 BM film with 2.5 wt% d-ps. 2 H signal comes from d-ps and 34 S signal comes from the donor polymer. The total area under the counts versus distance of all profiles was normalized to correspond to the weight concentration of each component. e) J V plots for p-dts(fbtth 2 ) 2 :PC 70 BM solar cells without or with DIO or PS. f i) Microscope images of thickness-corrected elemental mappings of carbon in red, associated predominantly with PS and PC 70 BM-rich phases, and sulfur in green, corresponding to the polymer-rich phase. i) The addition of both PS and DIO does not interfere with the BHJ formation, but induces the formation of the polymer fibrillary crystals. a) Reproduced with permission. [199] Copyright 2016, Nature Publishing Group. b) Reproduced with permission. [109] Copyright 2016, Wiley-VCH. c) Reproduced with permission. [222] Copyright 2016, The Royal Society of Chemistry. d,e) Reproduced with permission. [227] Copyright 2014, Wiley-VCH. f i) Reproduced with permission. [228] Copyright 2015, Wiley-VCH (17 of 39)

18 (5Z,5 Z)-5,5 -((7,7 -(4,4,9,9-tetrakis(4-hexylphenyl)-4,9- dihydro-s-indaceno[1,2-b:5,6-b ]dithiophene-2,7-diyl)bis(6- fluorobenzo[c] [1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))- bis(3-ethyl-2-thioxothiazolidin-4-one) (IFBR) into a benzodithiophene-alt-difluorobenzo[1,2,3]triazole (PBTA-BO):PC 60 BM blend. [215] The PBTA-BO:PC 60 BM blend film had typical clump features with sizes >100 nm that confine the charge carriers and induce charge recombination. In the ternary blend film with IFBR, the clump features are reduced in size (30 40 nm) and distributed in a fibrillary network; these changes are beneficial to efficient exciton dissociation and charge transport, and resulted in an increase in PCE from 5.70% to 8.11%. The authors also fluorinated IFBR to synthesize IffBR, and incorporated it into the PBDT-BO:PC 70 BM blend. [216] PBTA-BO:PC 70 BM binary blend films exhibit severe aggregation due to the poor miscibility of PBTA-BO with PC 70 BM, but the ternary blend containing IffBR exhibits reduced aggregation and a smooth and uniform morphology. Further optimization of the ternary blend film by using diphenyl ether as a solvent additive reduced the phase separation and the π π stacking distance, with the results of enhanced exciton dissociation and charge transport, as well as an increase in the PCE from 4.74% to 9.06%. Materials with high crystallinity possess the regular molecular arrangements that are required for a continuous morphology and high charge mobility. [217] The addition of a third component can increase the crystallinity of the host donor or acceptor materials. Incorporation of a thieno[3,2-b]thiophene-alt-pentathiophene copolymer (DHP 3 ), [218] oligo(benzothiadiazole-alt-3,3 -dihexyl-2,2 :5,2 :5,2 -quaterthiophene) (BT4T) [219] or P3BT PNWs [220] into a P3HT:PC 60 BM blend increases the P3HT crystallinity by nucleating P3HT crystallization. DHP 3 and BT4T induce P3HT crystallization and increase the charge mobility and light absorption of P3HT in the blend. The P3BT PNWs facilitate aggregation of PC 60 BM into larger clusters and induce P3HT crystallization. The hole and electron mobilities are increased by the formation of interconnected pathways in the ternary blend. Gu et al. incorporated amorphous PCPDTBT into P3HT:PC 60 BM, and the resulting phase separation between PCPDTBT and P3HT induced formation of P3HT fibrils with a unique multilength-scale morphology. [221] The bundles of well-defined P3HT fibrils embedded in the amorphous mixture of the components form a network. Zhang et al. incorporated poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5h-dithieno[3,2- b:20,30-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothiadiazole) (PDTP-DFBT) into a low crystallinity PTB7 blend with PC 70 BM. [222] The pristine PTB7 film has low crystallinity with a preferential face-on orientation, and the crystallinity of PTB7 increases with a strong lamellar stacking and faceon order when 20% PDTP-DFBT is introduced (Figure 9c); these changes might be a consequence of the different solubilities of PTB7 and PTDP-DFBT in the solvent mixture. The improved crystallinity and preferred molecular orientation increased the PCE from 8.63% to 9.06%. Lu et al. chose two similar donor polymers with different bandgaps, i.e., PTB7 and poly(3-oxothieno[3,4-d]isothiazole-1,1-dioxide/benzodithiophene) (PID2). [223] The ternary blend film with 10% PID2 had improved film crystallinity and evenly distributed fibrils; these traits led to improved charge separation and transport with reduced charge recombination. This approach increased the PCE from 7.25% to 8.22%. Zhang et al. incorporated the highly crystalline small molecule, p-dts(fbtth 2 ) 2, into a PTB7- Th:PC 70 BM blend. [224] p-dts(fbtth 2 ) 2 forms an alloy with PTB7-Th, improves the crystallinity of PTB7-Th, and induces a strong face-on tendency; the phase separation of the PTB7- Th:PC 70 BM blend with domain sizes of nm is retained upon the incorporation of up to 15% p-dts(fbtth 2 ) 2. Upon the addition of p-dts(fbtth 2 ) 2, the correlation length of the π π stacking increases from 8 to 23 Å, and the purity of the PTB7-Th domains is improved. As a result, charge separation and transport were enhanced and charge recombination was reduced; PCE increased from 9.2% to 10.5% with FF >75%. The FF indicates the balance between charge extraction and recombination; this balance is strongly affected by film morphology. After optimization of the morphology of the PTB7-Th:PC 70 BM blend film by adjusting its p-dts(fbtth 2 ) 2 content, the associated ternary blend solar cell had a larger FF value than the binary system solar cells. Kumari et al. incorporated a BDTbased small molecule, DR3TSBDT, into a PTB7-Th:PC 70 BM blend; this addition improves the nanoscale nanomorphology, crystallinity, and orientation of the ternary film. [225] The ternary blend film with 25% DR3TSBDT has coexisting face-on and edge-on orientations with strengthened lamellar stacking and increased crystallite size, whereas the PTB7-Th:PC 70 BM and DR3TSBDT:PC 70 BM binary films have face-on- and edge-ondominant orientations respectively. These 3D textures contained finely dispersed fibrils with lengths of nm and PCE of 12.10%. Some polymers act as nucleating agent. The copolymer thieno[3,2-b]thiophene-alt-pentathiophene acts as a crystallization nucleating agent in P3HT:PC 60 BM blends, and yields high structural ordering of their donor phase. [218] This improved structural order increases charge-carrier transport and broadens the absorption spectrum of the active film. Addition of an insulating polymer to a polymer:acceptor BHJ blend sometimes improves the efficiency of the associated solar cell. Doping the P3HT:PC 60 BM BHJ system with an insulating polymer affects its photovoltaic properties. [226] Doping with 0 16 wt% of the insulating polymer poly(methyl methacrylate) (PMMA) increased the homogeneity of the distribution of P3HT and elongated the P3HT crystals in the blend film. Unintentional doping and vacancies can also arise in typical BHJ films; these vacancies act as trap sites that hinder charge transport and thereby cause current leakage. Doping with PMMA prevents such vacancies from forming, and thereby suppresses trapping of charge carriers. Theoretical calculations suggest that doping will improve both V OC and FF. The fabrication of polymer:acceptor BHJ solar cells can be impeded by problems such as dewetting or difficulties in controlling the thickness of the active layer. Addition of insulating polymers such as polystyrene (PS) can solve these problems. Addition of 1 5 wt% of high M w PS to p-dts(fbtth 2 ) 2 :PC 70 BM blend increases the solution viscosity and film thickness without any drawbacks. [227] This addition also improves the microscopic film uniformity and absorption, and reduces dewetting from the substrate. Furthermore, the PS distributed homogeneously throughout the film without accumulation at interfaces (Figure 9d), and did not reduce the crystallinity (18 of 39)

19 of the p-dts(fbtth 2 ) 2 polymer nor change the roughness of the film. PS slows the solidification of the film and increases the crystallinity of the donor polymer; as a result, the PCE of the associated cell increased from 7.1% to 8.2% (Figure 9e). These effects are similar to those of adding DIO. PS and DIO additives have synergetic effects on the morphology and film formation mechanism in the p-dts(fbtth 2 ) 2 :PC 70 BM blend system (Figure 9f i). [228] The insulating PS polymer disperses in the BHJ film and does not interfere with charge transport or charge extraction. By using temporally resolved in situ absorption spectroscopy and in situ GI-WAXS, they concluded that the morphological evolution during the film-casting process is divided into two stages by the effects of PS and DIO. Initially, PS retains CB solvent for longer than the film without PS does, so the solvent evaporation time extends, and phase separation is promoted in the film. However, the retention of PS is insufficient to obtain the highest PCE of the associated solar cell. Due to the high boiling point of DIO, its addition induces further morphological evolution during the second stage. In this stage, the donor polymer crystallites are reconstructed (Section 3.3.2). The complementary synergetic effects of the PS and DIO additives occur at different stages, and yield better BHJ organization than when only one (or neither) of the additives is used. Crosslinkable or polymerizable additives can improve the morphological stability of the active layer. [213,214] Cheng et al. synthesized two styryl-functionalized fullerene derivatives ([6,6]-phenyl-C 60 -butyric acid styryl dendron ester (PCBSD) and [6,6]-phenyl-C 60 -butyric acid styryl ester (PCBS)) that can be polymerized by using heat treatment. [213] Addition of these molecules improved the thermal and long-term stability of P3HT:PC 60 BM BHJ solar cells (Section 3.8) Post-Treatment Post-treatments can be usefully performed on deposited active films. In this section, we discuss thermal annealing, solvent annealing, and polar solvent post-treatment methods (Figure 10). These methods can be used to optimize the morphologies of polymer/acceptor BHJ structures. Typically, they induce the formation of a highly crystalline domain consisting of the donor polymer, interconnections between the donor and the acceptor, and vertical phase separation that improves exciton dissociation, charge transport, and charge collection; as a result, the associated solar cell has a high PCE Thermal Annealing Thermal annealing can be used to modify the morphology of the active layer. The applied heat energy induces phase separation and changes in properties such as the nanomorphology and crystallinity of the donor:acceptor blend; these properties are closely related to the optoelectronic properties of the associated PV cell (Figure 10a,b). For the P3HT:PC 60 BM blend system, Padinger et al. reported that heating of the cells to 75 C for 4 min increases J SC from 2.5 to 7.5 ma cm 2, V OC from 0.3 to 0.5 V, and FF from 0.4 to 0.57; as a result, the PCE increased from 0.4% to 2.5%. [229] The authors concluded that the increase in J SC was a result of improved charge-carrier mobility, and that the increases in V OC and FF resulted from a reduction in the number of shunts. Reyes-Reyes et al. [231] and Ma et al. [230] reported the achievement of PCEs >5% by carrying out thermal annealing at temperatures greater than 150 C. Cho et al. improved the field effect mobility of a P3HT film to 0.3 cm 2 s 1 V 1 by performing thermal annealing at 150 C for 10 min. [232] From the morphological viewpoint, thermal annealing increases the crystallinity of P3HT and promotes the diffusion of PC 60 BM molecules that are located in the amorphous P3HT region. By using transmission electron microscopy (TEM), it has been demonstrated that P3HT forms fibrillar crystals in as-cast P3HT:PC 60 BM films, and that PC 60 BM forms nanocrystals in the matrix. [233] Thermal annealing at 120 C extends the length of the P3HT nanocrystals and PC 60 BM-rich domains develop in the film. Thermal annealing results in nanoscale interconnecting networks with crystalline order for both P3HT and PC 60 BM. Ma et al. showed that a well-interconnected network developed in a P3HT:PC 60 BM film after thermal annealing at 150 C for 30 min. [230] Further annealing for up to 2 h increased the connectivity of the donor:acceptor network. The thermal annealing increases the intensities of the XRD peaks due to interchain spacing in P3HT by producing interdigitated alkyl chains and face-to-face packing. Erb et al. demonstrated that thermal annealing increases the number of P3HT nanodomains in a P3HT:PC 60 BM film, presumably due to the diffusion of PC 60 BM molecules to form PC 60 BM aggregates and the conversion of amorphous P3HT to P3HT nanocrystals (Figure 12a). [234] Verploegen et al. used atomic force microscopy (AFM) and in situ GI-WAXS with a custom-built heating chamber to investigate the morphological rearrangements of P3HT and PC 60 BM films during thermal annealing (Figure 12b); the roughness of 1:1 P3HT:PC 60 BM films increases as the thermal annealing temperature increases until the temperature reaches the melting temperature of P3HT. [235] As-cast P3HT crystallites grow with π stacking primarily perpendicular to the substrate. The thermal annealing of the films increases both the P3HT layer spacing and the coherence length of the crystallites. Agostinelli et al. demonstrated that the evolution of P3HT:PC 60 BM morphologies passes through two important stages during thermal annealing; these changes in morphology are correlated with the photovoltaic performance of the associated PV cells. [236] Initially, most P3HT lamellae have an edge-on orientation; during thermal annealing, the proportion of lamellae with an edge-on orientation P3HT with alkylstacking direction increases, while that of lamellae with face-on P3HT crystallites decreases. The crystallization of P3HT upon thermal annealing occurs in 5 min, and is correlated with a major increase in photocurrent. In contrast, PC 60 BM diffuses and aggregates on the relatively longer time scale of 30 min, which results in an improvement in FF. Thermal annealing is also a strong tool for the modification of the morphology of the active layer in other donor:acceptor blend systems, [ ] including recently reported polymer:nfa blend systems. [ ] For example, the thermal annealing up to 140 C of PCDTBT:PC 70 BM films results in the development of (19 of 39)

20 Figure 10. a) A schematic illustration for the effects of thermal annealing on P3HT:PC 60 BM BHJ films. b) GI-WAXS images of 1:1 wt% P3HT:PC 60 BM blend film (top) without thermal annealing, and (bottom) annealed at 220 C. c) A schematic illustration for the morphology evolution upon solvent soaking in P3HT:PC 60 BM blend. Orange wires: P3HT chains; big black dots: PC 61 BM; blue and green dots: CS 2 and methanol, respectively. d) XRD for P3HT:PC 60 BM film before and after solvent annealing: DCB, CB, CF, methylene chloride, acetone, as prepared from up to down, respectively. e) A schematic illustration of morphology evolution upon polar solvent fluxing. f) J V plots for (top) PBDTTT-C-T:PC 70 BM and (bottom) PCDTBT:PC 70 BM solar cells. Black lines: vacuum-dried; red lines: polar solvent fluxed. a) Reproduced with permission. [234] Copyright 2005, Wiley-VCH. b) Reproduced with permission. [235] Copyright 2010, Wiley-VCH. c) Reproduced with permission. [287] Copyright 2011, The Royal Society of Chemistry. d) Reproduced with permission. [25] Copyright 2009, American Chemical Society. e f) Reproduced with permission. [281] Copyright 2014, Wiley-VCH (20 of 39)

21 phase-separated polymer:pc 70 BM films and PCDTBT polymer regions with a fibrous structure. [249] Bin et al. synthesized medium-bandgap conjugated bithienyl-benzodithiophene-alt-fluorobenzotriazole copolymers with three different side chains and blended the copolymers with ITIC. [246] By using GI-WAXS analysis, the authors found that the thermal annealing of the blend films results in significant increases in the intensities of the peaks with narrow widths due to polymer components, and decreases in their facial π π stacking distances. These results indicate that thermal annealing improves the crystallinities of these blend films, enhances the proportion with the face-on orientation, and reduces the π π stacking distances of the donor polymers, which are favorable for high photovoltaic performances. The timing of thermal annealing during the fabrication of polymer PV cells can affect the morphology of the active layer and the efficiency of the cell. Generally, preannealing refers to annealing performed before the deposition of the top electrode, whereas postannealing refers to annealing carried out after that process. Our discussion thus far has only considered preannealing. Now we consider postannealing and compare its effects to those of preannealing. In some BHJ systems, postannealing achieves a higher photocurrent, superior light harvesting in the active layer, and reduced Rs in comparison to preannealing. [250] This difference arises because postannealing enlarges the interfacial area and increases the roughness of the interface between the metal electrode and the active layer. This increase in interfacial area improves the charge-collection efficiency at the metal polymer interface, and the increased roughness enhances internal reflection and light collection, which increases the efficiency of PV cells over that of their preannealed counterparts. [251] Postannealing can also induce metal diffusion and chemical reactions between the metal electrode and the active layer to form bonds such as C Al or C O Al. [231,252,253] Chen et al. investigated the differences between the morphologies resulting from preannealing and postannealing by using GI-WAXS and near-edge X-ray absorption fine structure. [254] Preannealed and postannealed P3HT:PC 60 BM films were both found to contain markedly improved P3HT crystal ordering. In the preannealed film, edge-on P3HT crystal packing is dominant with a reduced concentration of PC 60 BM at the surface and within 10 nm of it. In the postannealed film, face-on P3HT packing is much more common than edge-on packing, and PC 60 BM is segregated at the interface between the Al electrode and the active layer. Thermal annealing can also be used to modify the morphologies of planar heterojunction films. Here, changes in the morphology at the donor acceptor interface are focused. Huang et al. showed that pre- and postannealing have different effects on PCPDTBT:fullerene bilayer solar cells. [255] Preannealing at 200 C induces the formation of a fibrous morphology that increases the area of the interface between the donor and the acceptor. Postannealing induces the reorganization of the interface between the donor and acceptor phases so as to improve contact, and was found in this system to yield a PCE = 2.85%. In many studies, thermal annealing induced the diffusion of PCBM molecules in donor:pcbm bilayer films into the donor phase, which results in intermixing at the donor acceptor interface. [ ] Chen et al. revealed that thermal annealing at 150 C rapidly results in P3HT crystallization, and drives PC 60 BM diffusion into the P3HT film through the P3HT amorphous domains. [16] Thermal annealing for only a few seconds results in the development of a BHJ-like morphology through the diffusion of PC 60 BM molecules into the P3HT region. However, Gevaerts et al. reported that the thermal annealing of an intermixed P3HT:PC 60 BM bilayer does not produce the same morphology as that of BHJ P3HT:PC 60 BM films cast from a single solution. [263] However, thermal annealing is not always effective. Some active films can be damaged by thermal annealing, even at low temperatures, and those that already have high crystallinity before thermal annealing are affected little by the process. [240,264] Therefore, thorough optimization should be conducted for each polymer to achieve the active layer morphology appropriate for high-efficiency cells. In addition, the operating temperature of cells under illumination should be considered because it can cause similar effects with thermal annealing. The continuous and excessive thermal stress under illumination leads to thermal degradation of the active morphology. We discuss this stability issue in Section 3.8 in detail Solvent Vapor Annealing Solvent vapor annealing is also known as solvent annealing and is achieved by confining a photoactive film and a processing solvent together in a closed system. During spin-casting, the solvent evaporation rate is relatively fast, so relatively little time is available for the donor:acceptor blend to form a BHJ film. Solvent vapor annealing can be a good method for the preparation of a well-aligned BHJ structure because the solvent vapor in the closed system slows the drying process and thus provides enough time for control of the morphology of the active film (Figure 10c). Mihailetchi et al. reported that this process increases the hole mobility in P3HT:PC 60 BM films by a factor of 33. [265] Li et al. reported that extending the solvent evaporation time was used to achieve a P3HT:PC 60 BM cell with PCE = 4.4%. [266] Solvent vapor annealing also improves the crystallinity, nanoscale phase separation, and extent of the interconnected donor:acceptor network. These changes can increase the hole mobility, current density, and PCE of solar cells. [ ] The effects of solvent vapor depend on the solvent vapor used and the annealing time. [267,277,278] The importance of solvent selection for solvent vapor annealing has been demonstrated by Park et al. (Figure 10d), who used various solvents such as acetone, methylene chloride, CF, CB, and DCB, which exhibit different interactions with the P3HT:PC 60 BM active layer. [25] The use of good solvents resulted in increased current densities and PCEs, but the increases in these parameters were limited by exciton loss due to excessive phase separation. The use of the poor solvents produced phase separation such that the P3HT and PCBM domain sizes do not exceed the exciton diffusion length, and even to result in higher current densities and PCEs than were obtained with the good solvents. By using time-of-flight secondary ion mass spectroscopy (TOF-SIMS) measurements to determine the 3D BHJ morphologies, Jo et al. revealed that (21 of 39)

22 solvent annealing forces PC 60 BM molecules in the BHJ film to diffuse toward the top surface near the Al electrode. [279] This movement results in the development of a vertical concentration gradient of the components; its extent is affected by the solvent annealing time Polar Solvent Treatment Polar solvent treatments use a polar solvent such as ethanol or methanol. The morphology of the donor:acceptor active layer can be tailored by spin-casting the polar solvent onto the active layer, or by soaking it with the polar solvent. [280] Xiao et al. observed that methanol treatment after active layer deposition induces vertical phase separation of the components and increases the PCE of the associated solar cell (Figure 10e,f). [281] Ye et al. reported that methanol treatment after film deposition removes solvent additives with a high boiling point from the active layer. [282] The slow evaporation of residual solvent additives from the dried film even after the deposition of the active layer induces undesirable morphologies. For example, the presence of residual DIO in films can mediate the oxidization of fullerene to form fullerene-iodide radical species. [283] Methanol treatment efficiently removes such solvent additives and ensures the formation of a smoother and optimal component distribution; these changes increase the PCE and the reproducibility of fabrication. Liu et al. revealed that the improvement in the overall device efficiency after ethanol treatment is caused by the penetration of ethanol into the active layer and the resulting modifications of the contact properties of the interface between the PEDOT:PSS and BHJ layers. [284] By using AFM and time-of-flight grazing incidence small angle neutron scattering, Guo et al. studied the effects of commonly used polar solvents such as methanol, ethanol, 2-propanol, and 1-butanol on the morphology of the PTB7:PC 70 BM BHJ system. [285] All four alcohols changed its surface structure and to induce phase separation and inner film reconstruction; these solvents all also reduce the domain and structure sizes, but treatment with methanol results in the smallest domain size, and thus provides well-optimized phase separation for efficient charge separation, transportation, and extraction. As a result, the methanol treatment increased the PCE in this system by 25%. Methanol has the smallest molecular volume of these alcohols, so can easily penetrate the blend film and induce morphological changes. In addition, its high volatility means that it is easily removed from films during evaporation. The immersion of a polymer:acceptor active layer in a polar solvent is generally known as the solvent soaking method. This process leads to morphological effects that are similar to those of polar solvent treatments. [286] Li et al. reported that the solvent soaking of a P3HT:PC 60 BM BHJ blend film in methanol/cs 2 for one minute induces the formation of an interpenetrating network composed of highly crystalline P3HT and PC 60 BM aggregates and vertical phase separation in which P3HT is enriched at the bottom anode and PC 60 BM diffuses to the top cathode. [287] These changes improve exciton dissociation, and provide continuous pathways for charge-carrier transport Ordered Nanostructures within the Photoactive Layer This section reviews approaches that intentionally introduce well-ordered nanostructures within the active layer to improve the photovoltaic performance of OPVs. The main purpose of the introduction of such nanostructures is to build well-organized pathways for charge transport so that charge carriers can be collected at electrodes without severe losses. Fabrication of the ordered nanostructures has usually been achieved by exploiting molecular self-assembly and nanostructured templates. [288,289] In the following, we discuss the two most widely studied ordered nanostructures: ordered BHJ structure and 1D PNW Ordered Bulk-Heterojunction Structure Although the conventional BHJ structure can provide a large interfacial area for exciton separation, its complex pathways for electron and hole transport hinder the efficient extraction of free charge carriers. Free electrons and holes meet each other and recombine more frequently if they have to pass along complex paths. Furthermore, some free charges can become spatially trapped if they are formed in isolated domains. [290] Therefore, there have been several attempts to fabricate ordered BHJ solar cells, even though the results have not surpassed the efficiencies of optimized conventional BHJs. A typical ordered BHJ has a periodic phase-separated structure in which the columnar donor and acceptor domains interpenetrate (Figure 11a). These columnar domains are vertically aligned and form straight pathways for charge transport. The sizes of the donor and acceptor columns should be comparable with the exciton diffusion length for efficient exciton separation. Such straight charge-transport pathways are expected to reduce charge recombination, decrease the travel lengths of carriers to electrodes, and finally, increase the PCE. [ ] An early successful study exploited the needle-like crystalline structure of CuPc. The organic vapor phase deposition of CuPc films occurs through island-plus-layer growth that generates needle-shaped crystals on a continuous layer. These needles had an average diameter of 30 nm and a height of 35 nm, and thus to be suitable for the fabrication of ordered BHJ devices. These needles are composed of single CuPc microcrystalline domain, which ensures the efficient transport of holes. The device was completed by the sequential deposition of the acceptor, 3,4,9,10-perylenetetracarboxylic bis-benzimidazole, and the top electrode onto the CuPc film and found to exhibit a significantly increased PCE when compared to planar heterojunction devices or conventional BHJ devices. [291] The fabrication of an ordered BHJ has also been demonstrated by using the glancing angle deposition method, in which C 60 is deposited at an oblique flux incidence onto a rotating substrate. This process produces a C 60 film with vertically oriented C 60 columns. A donor material is then solution-cast onto the C 60 film by using an orthogonal solvent to fabricate an ordered BHJ. A resultant device exhibited a higher PCE than planar or conventional BHJ devices. [292] Another approach to the fabrication of arrays of vertical crystalline organic semiconductors uses a modified thermal evaporation method and graphene (22 of 39)

23 Figure 11. a) Schematic representation of an ordered BHJ organic solar cell based on the P3HT nanorods and C 60. b) J V curves of devices. Open symbol: planar P3HT film/c 60 solar cell. Filled symbol: P3HT nanorod/c 60 solar cell. c,d) Scanning electron microscope images of the AAO template used for P3HT nanorod fabrication: c) top view and d) side view. e) A schematic illustration of the ordered BHJ with mutual diffusion of donor and acceptor by thermal annealing, and corresponding scanning force microscope images. f) J V curves of the ordered BHJ devices with mutual diffusion prepared from various thermal annealing conditions. a d) Reproduced with permission. [294] Copyright 2010, Wiley-VCH. e,f) Reproduced with permission. [303] Copyright 2016, Wiley-VCH. substrates. This technique, in which the donor molecule, tetraphenyldibenzoperiflanthene, is deposited on top of a vertically oriented C 70 layer, was used to fabricate ordered BHJ solar cells. This approach also improves the PCE. [293] Kim et al. successfully demonstrated the fabrication of an ordered BHJ structure by preparing vertically oriented P3HT nanorods on anodized aluminum oxide (AAO) templates (Figure 11a d). The template had a pore size of 50 nm, an interpore size of 100 nm and a height of 150 nm (Figure 11c,d). Molten P3HT is infused into the pores of the AAO template under vacuum. After removing the template, P3HT nanorods with dimensions identical to those of the AAO pores are fabricated. The resulting P3HT chains in the nanorods had a face-on orientation, in contrast to P3HT bulk films, which have an edge-on orientation with respect to the substrate plane. This orientation of the P3HT nanorods increases the electrical conductivity in the vertical direction when compared to that of the planar P3HT film. A solar cell device constructed using a P3HT nanorods/c 60 ordered BHJ structure had a PCE (1.12%) six times higher than planar P3HT/C 60 heterojunction devices. [294] Chang et al. fabricated crosslinked fullerene nanorods. The nanorods were fabricated by spin-coating a solution of crosslinkable PCBSD on top of an AAO template, then pressing with a glass/ito/zno/ultrathin PCBSD layer under heat. The pressing induces the infiltration of PCBSD into the AAO pores. Further heat was applied to crosslink the PCBSD, and then the AAO template was removed to leave vertically aligned, crosslinked PCBSD nanorod arrays. An inverted solar cell device was fabricated by spin-casting a solution of P3HT:IC 60 BA on top of a glass/ito/zno/pcbsd nanorods template. The resulting device yielded a PCE up to 7.3%; this high value was mainly attributed to the increased donor acceptor interfacial area and the increased electron mobility that result from the introduction of PCBSD nanorods. [295] Wang et al. fabricated ordered BHJ solar cells based on P3HT/ PC 60 BM core-shell nanorods by spin-coating a blend solution of P3HT and PC 60 BM onto an ITO substrate, then placing an AAO template on top of the film. The P3HT:PC 60 BM/AAO (23 of 39)

24 template samples were then thermally annealed under vacuum. The removal of the AAO template left an array of vertically aligned nanorods. The different diffusion rates into the AAO pores mean that the nanorods have a PC 60 BM-rich core and a P3HT-rich shell. Solar cells based on these core-shell nanorods exhibited a PCE 3.2%, which is higher than achieved with conventional BHJ solar cells. [296,297] Nanoimprint lithography can also be used to fabricate ordered BHJ structures. Chen et al. fabricated P3HT nanorod arrays by using an AAO template as a mold for nanoimprint lithography. The AAO templates were treated with low-m w polydimethylsiloxane as a mold-release agent to reduce their surface energies. Ordered heterojunction solar cell devices fabricated with the P3HT nanorod arrays had a PCE 2.4%. [298] As the diameter of the P3HT nanorods is decreased, the proportion of P3HT chains in the face-on orientation with respect to the substrate plane increases and the conductivity along the axis of the nanorods increases. [299,300] These ordered heterojunction devices have lower energetic disorder and a lower density of trap states than conventional BHJ or planar heterojunction devices, and therefore exhibit less charge recombination loss. [301] Ding et al. used a patterned silk fibroin film as a nanoimprinting mold for the fabrication of P3HT nanogratings. PC 60 BM was spin-coated on top of the P3HT nanogratings to fabricate ordered BHJ solar cells, which yielded an optimal PCE of 1.55%. [302] Ko et al. investigated the effects of thermal annealing on the morphology and performance of ordered BHJ solar cells obtained with nanoimprint lithography (Figure 11e,f). Thermal annealing of the P3HT nanorods/ PC 60 BM ordered structure led to mutual diffusion between P3HT and PC 60 BM at their interface, and the intensity of the diffusion depended on the annealing temperature and duration. As the mutual diffusion proceeds, the P3HT domain size decreases and a hierarchical nanostructure forms around the P3HT nanorods. The fraction of the preferred face-on orientation of P3HT increases as the size of the P3HT crystallites decreases. However, thermal annealing at a very high temperature caused mutual diffusion that destroys the nanorod structure and decreases the P3HT crystal size. The PCE of this ordered BHJ solar cell with optimal mutual diffusion was twice that of the as-prepared ordered heterojunction solar cell. [303] Block copolymers (BCPs) can also be used to fabricate ordered BHJ solar cells, by exploiting self-assembly of BCPs into periodic nanostructures. [ ] Especially, self-assembly of BCPs into lamellar or cylinder morphologies is preferred for ordered BHJ solar cells if the microdomains are oriented perpendicularly with respect to the substrate because they provide efficient charge transport pathways. One strategy for the application of BCP in ordered BHJ solar cells is to use a BCP as a template with ordered nanostructure. This can be achieved when the BCP contains sacrificial block that can be removed after the formation of periodic nanostructure. [305] A representative work was done by Botiz et al. They used a poly(3- hexylthiophene)-block-poly(l-lactide) (P3HT-b-PLLA) BCP to pattern active material into ordered structures. [308,309] P3HT-b- PLLA formed lamellar structure with the alternating domains oriented perpendicular to the substrate. Then the PLLA block was removed using NaOH solution to yield a nanostructured P3HT template. The empty spaces of the nanostructured P3HT template were filled with C 60 by dip-coating the template in an aqueous solution of C 60. This method gave an ordered BHJ of P3HT and C 60. Another strategy to fabricate ordered BHJ by using BCPs is to use a donor acceptor BCP as an active material. Guo et al. reported the highest PCE of BCP-based OPVs by using poly(3-hexylthiophene)-block-poly-((9,9-dioctylfluorene)- 2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2,2 - diyl) (P3HT-b-PFTBT) as an active material. [155,310] P3HT-b- PFTBT self-assembled and phase-separated to form lamellar morphology with alternating donor and acceptor domains, oriented normal to the substrate. The width of the lamella was 9 nm, which is comparable to the typical exciton diffusion length in organic semiconductors, and is therefore beneficial for efficient exciton dissociation. Furthermore, the P3HT chains in the crystalline P3HT block adopted dominant faceon orientation that further facilitated charge transport in the P3HT domains. The P3HT-b-PFTBT device had maximum PCE = 3.1%, whereas the device with the homopolymer blend had only PCE = 1% D Polymer Nanowires In this section, we discuss 1D nanostructures of conjugated polymers, the so-called 1D PNWs. Well-constructed nanostructures of conjugated polymers provide favorable phase separation for OPVs, and an efficient carrier transport pathway for BHJ solar devices (Figures 12 and 13). [311,312] For example, Kim et al. fabricated lateral cells with an Al/P3HT:PC 60 BM/ MoO 3 /Au structure with and without P3HT NWs in the active layer. [311] The NWs provide an efficient hole transporting network with a hole mobility 100 times higher than that of samples without P3HT NWs. Fractional precipitation is a commonly used method for the preparation of preformed PNWs in blend systems. [311, ] Kim et al. used dichloromethane (DCM) as the marginal solvent for P3HT in the fabrication of PNWs in a P3HT:PC 60 BM blend solution. [27] The P3HT:PC 60 BM solution was prepared in DCM at 42 C and cooled to room temperature to produce the PNWs. Subsequently, the solution was stirred for ageing times in the range h. As the solution ageing time increases, the amount of amorphous P3HT decreases and that of crystalline P3HT increases. The P3HT NWs in the resulting blend films were 10 nm wide and 5 10 µm long. The P3HT NWs form interconnected networks surrounded by PC 60 BM phases, i.e., a bicontinuous morphology appropriate for charge separation and transport. Upon the addition of a pure donor phase to the hole transport layer and the active layer to fabricate ITO/ PEDOT:PSS/P3HT/P3HT NW:PC 60 BM/Ca/Al, the optimal ageing time that maximizes solar cell efficiency was 60 h; after this ageing period, light absorption is maximized and the charge-carrier mobility is balanced. The same group fabricated P3HT NWs by mixing a marginal solvent (CHN) and a good solvent (CB), [92] and found that small nanocrystals without P3HT seeds in P3HT:PC 60 BM blend can be fabricated by using only the good solvent (Figure 12a,b). The addition of 50 vol% CHN to P3HT:PC 60 BM blend in CB formed P3HT seeds in the solution. Subsequent stirring for 2 h induced development of the P3HT (24 of 39)

25 Figure 12. a) Schematic model for the formation process of P3HT NW in the P3HT:PC 60 BM blend solution. Red wires represent P3HT NWs and gray spheres represent PC 60 BM domains. b) Cross-sectional electron tomography image close to the top surface of a P3HT:PC 60 BM BHJ film with P3HT NWs. c) J V curve obtained from photoactive layers with various film thicknesses containing P3HT NWs. d) A schematic illustration describing vertical pathway for charge transport with different NW dimensions. (a) Reproduced with permission. [92] Copyright 2010, The Royal Society of Chemistry. b d) Reproduced with permission. [67] Copyright 2014, American Chemical Society. seeds into ordered P3HT NWs. This structure consisting of P3HT NWs in a P3HT:IC 60 BA active layer produced a high PCE of 5.42%. Furthermore, even when the thickness of the active layer was increased to 600 nm, 70% of the maximum PCE was maintained, whereas the PCEs of the films without P3HT NWs decreased significantly as the thickness increases (Figure 12c). [67,92] Considering that the optimal thickness of the active layer is determined by light absorption and the charge transport dynamics, the PNWs must increase hole transport and reduce carrier recombination; these changes lead to the formation of efficient carrier pathways through the interpenetrating donor:acceptor network despite the thickness of the active film (Figure 12d). Xin et al. investigated the relationships between the characteristics of poly(3-butylthiophene) (P3BT) nanostructures and their PV properties. [316] P3BT NWs were fabricated in DCB by ageing the solution for 72 h to allow the polymer to self-assemble. The density of P3BT NWs in the active layer can be controlled by varying the drying time of the film after deposition prior to thermal annealing. After a short drying time, the film retains residual solvent, so the morphology develops further. The high density and long bridges of the PNWs provide a high current density but low FF and V OC values; in contrast, devices without NWs have low current densities but high FF and V OC values. The major loss mechanisms are inefficient carrier collection after short ageing times and the development of shunt pathways due to the presence of the NWs after long ageing times. Later, the same group reported an all-nw BHJ solar cell based on P3HT NWs as the donor and oligothiophene-functionalized NDI (NDI-nTH and NDI-nT) NWs as the acceptor. [317] The P3HT NWs were prepared with the marginal solvent method, in which P3HT is dissolved in DCM at a high temperature of 80 C then cooled to room temperature. The NDI nanostructures were prepared with the solvent exchange method by adding methanol, which is a nonsolvent for NDI, into the NDI solution drop by drop. The morphology of the NDI NWs was controlled by varying the solution concentration and the amount of nonsolvent added. Combining P3HT with NDI-3TH in an all-nw BHJ solar device yielded the highest PCE for this system of 1.11%, which is 54% higher than that achieved with the conventional P3HT:NDI-3TH BHJ film (PCE = 0.72%). This increase is attributed to the formation of a well-defined nanomorphology consisting of bicontinuous and percolated NW networks, and the increased surface areas of the donor and acceptor materials. Some conjugated polymer 1D nanostructures can be generated during active film deposition. The introduction of solvent additives to the blend solution, or introduction of solvent mixtures is typical method. The addition of solvent components that have a higher boiling point than the processing solvent extends the solvent evaporation time; and thereby allows time for the polymer crystals to assemble into nanostructures. Lee et al. firstly reported organic BHJ solar cells based on PNWs comprising the donor acceptor conjugated polymer PBDT2FBT- 2EHO (Figure 13a,b). [93] This donor acceptor conjugated copolymer was blended with PC 60 BM in a 1:4 polymer:acceptor weight ratio at >100 C. The addition of CN to the blend solution to create a mixed solvent induces the formation of the NWs. The density of the NWs gradually increases as the concentration of CN is increased to >50%. At a CN concentration of 30%, the resulting NWs were 22 ± 7 nm wide and a few hundred nanometers long. The formation of the PNWs is caused by the increased critical concentration of the crystalline formation, and by the prolonging of the crystals evolution time by the incorporation of CN, which has a higher polarizability ( cm 3 ) than common organic solvents. Due to the enhanced charge transport in the PNW structure, the PCE of the associated solar cell reached 8.18%, which is nearly 60% larger than that of an equivalent cell prepared without an NW structure. Recently, the preparation of PNWs in the P4TNTz-2F:PC 70 BM blend system (Section 3.1) was reported by the same group (Figure 13c,d). [94] P4TNTz-2F:PC 70 BM films prepared from a blend solution with DIO additive at 70 C contained PNWs (25 of 39)

26 Figure 13. Molecular structure of a) PBDT2FBT-2EHO and c) P4TNTz-2F. b) GI-WAXS images and TEM images for PBDT2FBT-2EHO:PC 70 BM blend films fabricated with a CB:CN mixed solvent ratio of (top) 0 vol% and (bottom) 30 vol%. d) TM-AFM topography (left row) and phase images (right row) for P4TNTz-2F:PC 70 BM blend films prepared from (top) CB without DIO and (bottom) CB with 3 vol% of DIO. a,b) Reproduced with permission. [93] Copyright 2014, Wiley-VCH. c,d) Reproduced with permission. [94] Copyright 2017, The Royal Society of Chemistry. with widths of 6 nm and lengths of a few hundred nanometers, whereas films processed without DIO had NWs with widths of 14 nm. The formation of these PNWs was attributed to the prolonging of the crystals evolution time that results from the addition of DIO, which has a higher boiling point than the processing solvent. The authors inferred that by selectively dissolving PC 70 BM, DIO induces the integration of PC 70 BM into the polymer aggregates during film formation, and that this process leads to the development of a nanoscale interpenetrating network with widths that are favorable for efficient exciton dissociation and charge generation because the typical exciton diffusion length is a few nanometers. The optimal thickness of the active layer was 350 nm; the associated solar cell had a high PCE = 10.62% as well as efficient photoluminescence quenching ( 98%) and hole transport (µ h = cm 2 V 1 s 1 ); these results are attributed to the continuous and evenly distributed PNW network with its longrange p-connectivity of a few hundred nanometers. [318] Kim et al. investigated the different nanomorphologies that were obtained in isoindigo-bithiophene-based donor polymer/ PC 70 BM BHJ blend films prepared by using sequential solution processes with various selective solvents. [319] In this method, the donor polymer is deposited on the PEDOT:PSS substrate, then PC 70 BM in a selected solvent (CF, CB, DCB, DCM, or toluene) is drop cast onto the polymer film. DCM is a good solvent for PC 70 BM but an orthogonal solvent for the polymer. DCB is a good solvent for PC 70 BM and a partially good solvent for the polymer (but not as good as CF and CB). Drop-casting PC 70 BM in different solvents induces the formation of selfassembled polymer NWs with different dimensions and densities. The films fabricated by drop-casting PC 70 BM in DCB contained densely packed polymer NWs with diameters in the range nm, and to produce the best PCE for this system of 5.02%. In contrast, films processed from CF contained inhomogeneous PNWs with diameters in the range nm. The high PCE arising from the densely packed NWs was attributed to efficient photon harvesting and to a vertical donor:acceptor distribution that facilitates carrier extraction Molecular Orientation Engineering at the Donor acceptor Interface The molecular orientation of the organic semiconductors in electronic devices critically affects their characteristics (26 of 39)

27 Figure 14. a d) Effects of molecular orientation on the optical and charge transport properties of pentacene/c 60 bilayer organic solar cells. a) Schematic illustrations of orientation-controlled pentacene/c 60 devices. The pentacene orientation was controlled by inserting a monolayer graphene at the anode interface. b) UV vis absorption spectra of pentacene films with different pentacene orientations and thicknesses. Blue curves: pentacene films with lying-down orientation. Black curves: pentacene films with standing-up orientation. c) Orientation-dependent exciton diffusion length (L d ) of the pentacene film measured by using spectrally resolved photoluminescence quenching method. Blue: L d = 83 nm for lying-down pentacene film. Black: L d = 43 nm for standing-up pentacene film. d) Charge-carrier lifetimes under 1 Sun illumination measured by transient photovoltage decay. Blue: 51 µs for lying-down pentacene device. Black: 32 µs for standing-up pentacene device. e h) Effects of donor acceptor interfacial orientation studied from p-sidt(fbtth 2 ) 2 /C 60 bilayer organic solar cells. e) Molecular structures of p-sidt(fbtth 2 ) 2 and C 60. f) GI-WAXS images of face-on (left) and edge-on (right) oriented p-sidt(fbtth 2 ) 2 films. g) J V curves of p-sidt(fbtth 2 ) 2 (face-on or edge-on oriented)/c 60 bilayer devices under 1 Sun illumination. h) External (dashed lines) and internal (solid lines) quantum efficiencies of the bilayer solar cells with varying p-sidt(fbtth 2 ) 2 orientation. The absorption spectrum of each material is also shown in the background. a d) Reproduced with permission. [69] Copyright 2015, American Chemical Society. e h) Reproduced with permission. [328] Copyright 2017, Nature Publishing Group. (Figure 14). The face-on orientation of organic semiconductor molecules with respect to the substrate is favorable in general for charge transport in OPVs because the π π stacking of faceon oriented crystals facilitates the vertical transport of charge carriers. [68 70] Furthermore, the optical properties of organic semiconductors such as optical transition and exciton diffusion can be highly anisotropic and strongly dependent on molecular orientation. [69,320] Jo et al. demonstrated these orientation effects by constructing bilayer OPVs with orientation-controlled pentacene layers (Figure 14a d). The authors inserted monolayer graphene at the anode interface as an epitaxial template for the growth of highly oriented pentacene crystals in the (27 of 39)

28 lying-down orientation. This orientation significantly increases light absorption and the exciton diffusion length, and thereby increases the efficiency of photon harvesting by the pentacene layer over that with a layer in the standing-up orientation. The transport of free charge carriers is also facilitated in devices with lying-down pentacene; furthermore, the carrier lifetime is increased and nongeminate recombination is reduced. As a consequence, the device with lying-down pentacene had a PCE five times higher than that of the device with standing-up pentacene. [69] More recently, it has been found that the orientation of molecules at the donor acceptor interface also has a strong influence on the photocurrent generation and interfacial energetics of OPVs. In this section, we focus on recent progress in the unraveling of the effects of donor acceptor interfacial orientation on OPV device operation Effects on Organic Photovoltaics of Interfacial Molecular Orientation Many studies of the effects of interfacial molecular orientation have been performed. [ ] Most of the early studies used planar heterojunction solar cells with controlled donor orientation. Ojala et al. found that interfacial orientation in a merocyanine dye/c 60 planar heterojunction structure affects exciton dissociation. In the as-deposited film, the orientation of the dye molecule is preferentially edge-on with its long axis perpendicular to the substrate, but when the film is thermally annealed at a temperature >T g, the angle with the substrate tilts to 45. This change in dye orientation doubles the FF; an analytic model that describes electric-field-dependent exciton dissociation was created that suggests that this change is a result of reduced geminate recombination. Computations of E CT and the exciton dissociation rate also suggest that the tilting of the interface speeds up exciton dissociation due to the absence of energetic traps and results in reduced stabilization of excitons at the interface. [324] Rand et al. also reported orientation-dependent exciton dissociation in zinc phthalocyanine (ZnPc)/C 60 planar heterojunction devices. The orientation of ZnPc can be controlled to either edge-on or face-on depending on the presence of a CuI layer underneath the ZnPc layer. The device with a face-on ZnPc/C 60 interface exhibits improved exciton dissociation and photocurrent generation, as indicated by an increased J SC and a higher internal quantum efficiency, when compared to the device with an edge-on ZnPc/C 60 interface. This improved exciton dissociation was explained in terms of the relative strength of the electronic coupling between the states: the strengths of the electronic couplings for both charge separation from the excited states of the molecules and charge recombination into the ground state (GS) were calculated as functions of the angle between ZnPc and C 60. This result suggests that the overall charge separation, which is determined by the ratio of charge separation to charge recombination, is more efficient at the angle that corresponds to the face-on configuration than at the angle that corresponds to the edge-on configuration. [325] The electron transfer rate is affected by the orientation of the donor acceptor interface and was directly measured by Ayzner et al. for the copper phthalocyanine (CuPc)/C 60 bilayer model system. [326] Electron transfer from the CuPc donor to the C 60 acceptor was four times faster at the face-on CuPc/C 60 interface than at the edge-on interface. This difference was attributed to the stronger interfacial electronic coupling at the face-on interface, which arises as a result of increased molecular orbital overlap in this orientation between the donor and the acceptor. Electron transfer was not observed at the donor acceptor interface, where π-orbital overlap is negligible. [327] For N,N -dioctyl- 3,4,9,10-perylene tetracarboxylicdiimide (PTCDI-C 8 )/diindenoperylene (DIP) heterojunctions, the excitons in both DIP and PTCDI-C 8 are not quenched at the interface when the interface has an edge-on/edge-on configuration because CT across such interfaces is inhibited by unfavorable molecular orientations and the presence of the insulating alkyl chains of PTCDI-C 8. However, the opposite effect has also been reported, in which the edge-on donor acceptor interface exhibits exciton dissociation and photocurrent generation that are superior to those of the face-on interface. [328] Ran et al. fabricated a planar heterojunction solar cell with a sharp, well-defined interface by using benzo[1,2-b:4,5-b]bis(4,4 -dihexyl-4h-silolo[3,2-b]- thiophene-2,2 -diyl)bis(6-fluoro-4-(5 -hexyl-[2,2 -bithiophene]- 5-yl)benzo[c][1,2,5]thiadiazole (p-sidt(tftth 2 ) 2 ) and C 60 (Figure 14e h). p-sidt(tftth) 2 had a face-on:edge-on orientation ratio of 99.5:0.5 when the film was cast from CB, and an edge-on:face-on orientation ratio of 94:6 when the film was cast from a mixed solvent consisting of CB and 0.4% DIO. Deposition of C 60 onto the p-sidt(tftth 2 ) 2 film showed negligible miscibility of C 60 into the p-sidt(tftth 2 ) 2 film. The fabrication of this precisely controlled donor acceptor planar heterojunction enabled direct observation of the effects of interfacial molecular orientation. A higher internal quantum efficiency is obtained from edge-on oriented devices than from face-on oriented devices; this difference indicates that charge generation is more efficient in this system at the edge-on donor acceptor interface than at the face-on interface. In addition, charge generation is more temperature dependent in face-on oriented devices than in edge-on oriented devices; this difference might indicate that the energy barrier to charge generation is higher in this system at the face-on interface than at the edge-on interface. Calculations of the electronic structures were performed, and it was concluded that the more efficient charge generation at the edge-on interface can be attributed to the much weaker electronic coupling between its CT state and the GS, which is the electronic coupling that corresponds to charge recombination to the GS. The lower CT state/gs electronic coupling at the edge-on interface is expected to decrease the geminate recombination rate, thereby making charge generation more efficient there than at the face-on interface. [328] Molecular orientation also affects spontaneous interfacial mixing. This effect is attributed to interfacial-orientationdependent charge generation. Ngongang Ndjawa et al. found that the molecular orientation of ZnPc films can significantly affect the mixing behavior at the ZnPc/C 60 interface. Spontaneous interfacial mixing is more favorable in the face-on terminated ZnPc film than in the edge-on terminated film. The authors suggested that this interfacial mixing increases the interfacial area for charge generation, and promotes charge separation by changing the local energetic landscape at the donor acceptor interface. [329] (28 of 39)

29 The V OC values of OPVs are also strongly affected by the interfacial molecular orientation. [328,330,331] The V OC values of solar cell devices are linearly correlated with E CT at the donor acceptor interface, where the difference between V OC and E CT can be separated into voltage losses from radiative and nonradiative recombination. E CT is primarily determined by the energy gap between the HOMO energy level of the donor and the LUMO energy level of the acceptor, once the interfacial Coulombic binding energy of the electron hole pair has been subtracted. A change in the interfacial molecular orientation can alter the interfacial energetics as well as the recombination processes; these changes can affect V OC. [328, ] This effect was demonstrated in the study of Kitchen et al., in which the molecular orientation distributions of P3HT thin films were controlled by varying the processing conditions. In P3HT/PC 60 BM planar heterojunction solar cells fabricated by using the stamping transfer method, the orientation of the P3HT molecules has a significant influence on V OC ; V OC is linearly correlated with the HOMO level of oriented P3HT. Due to the intrinsic dipole of the P3HT molecule, face-on-oriented P3HT has a deeper HOMO level than edge-on-oriented P3HT, so the difference between P3HT s HOMO level and PC 60 BM s LUMO level is larger for face-on-oriented P3HT than for edgeon-oriented P3HT. [331] Ran et al. investigated the influence of interfacial molecular orientation on voltage losses due to recombination. In the orientation-controlled p-sidt(tftth) 2 / C 60 system, the voltage loss due to nonradiative recombination is smaller at the face-on interface than at the edge-on interface. In contrast, the voltage loss due to radiative recombination is nearly independent of molecular orientation. The face-on interface also has a higher E CT, and therefore a much higher V OC than the edge-on interface. [328] Engineering of Interfacial Orientation in Polymeric Bulk- Heterojunction Solar Cells The orientation of the donor acceptor interface also affects the electrical characteristics of BHJ OPVs. Variations in the molecular structure of a material and in the film-processing conditions have a strong influence on the interfacial orientation in the BHJ and on the performance of the associated device. Tumbleston et al. found that the substitution of fluorine onto the polymer backbone affects the donor acceptor interfacial orientation of polymer:fullerene blends (Figure 15). The fluorinated polymers tend to have a face-on orientation with respect to the polymer-fullerene interface. This tendency is stronger when the film is processed with DCB than with CB. The J SC and FF values of devices are strongly correlated with the Figure 15. Effects of donor acceptor interfacial orientation in BHJ organic solar cells. a) Molecular structures of PC 60 BM and polymer (PNDT-DTBT) where fluorine is substituted for hydrogen on the polymer backbone (red circles). Depending on whether or not the polymer is fluorinated, the polymer orientation with respect to the polymer PC 60 BM interface changes. b) J V curves for the PNDT-DTBT (fluorinated or nonfluorinated):pc 60 BM BHJ devices. The extent of performance enhancement also depends on the choice of processing solvent, either CB or DCB. c) FF (upper) and J SC (lower) as a function of degree of molecular orientation (DMO). The DMO strongly correlates with FF and J SC, where the positive and negative values of DMO correspond to preferential faceon and edge-on polymer orientation at the polymer PC 60 BM interface. a c) Reproduced with permission. [334] Copyright 2014, Nature Publishing Group (29 of 39)

30 interfacial molecular orientation, and increase with increases in the proportion of the face-on interfacial orientation. [334] Jung et al. observed that the orientation of the NDI-based polymeric acceptor (poly{[n,n -bis(2-octyldodecyl)-naphthalene- 1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5 -(2,2 -bithiophene)}, P(NDI2HD-T2)) varies with its M w. The preferential orientation of P(NDI2HD-T2) with respect to the substrate changes from edge-on to face-on as the polymer M w is increased. When combined with the donor polymer PTB7-Th, which has a preferential face-on orientation, the PCE and J SC resulting from the use of the PTB7-Th:P(NDI2HD-T2) blend both increase with increases in the M w of the acceptor. This increase in J SC with M w is attributed to the increased efficiency of exciton dissociation into free charges at the donor acceptor interface, which arises as a result of the face-to-face orientation of the donor and acceptor molecules. [335] The molecular orientations of donor polymers can be affected by the proportions and natures of their constituents. Jo et al. prepared donor polymers through the random copolymerization of 5-fluoro-2,1,3-benzothiadiazole with 2,2 -bithiophene (2T) and dithieno[3,2-b;2,3 -d]thiophene (DTT) in 2T:DTT ratios from 1:0 to 0:1. The preferential molecular orientation of the donor polymers changed with the 2T:DTT ratio. When blended with the NDI-based polymeric acceptor, which has a face-on orientation in the blend films, the chargegeneration efficiency of the donor polymer increases with the proportion of the face-on orientation. This efficient charge generation is attributed to the face-to-face orientation of the donor acceptor interface; this orientation is more favorable for both exciton quenching and charge separation than the face-to-edge orientation. [336] The donor acceptor interfacial molecular orientation can also be highly dependent on processing conditions. Ma et al. found that the interfacial molecular orientation at a polymerfullerene interface can be changed significantly by varying the spin speed of spin-coating. The proportion of the donor polymer in the face-on orientation with respect to the polymer-fullerene interface increases as the spin speed increases. [337] The use of solvent additives can also strongly affect interfacial orientation. Schubert et al. and Zhou et al. found that the use of CN as a solvent additive effectively changes the orientation of the NDIbased polymeric acceptors while not affecting the orientation of donor polymers such as P3HT and PTB7-Th. The use of CN additive to induce the face-to-face interfacial orientation of the donor and the acceptor greatly increases charge-carrier generation in these all-polymer blends. [153,154] Güldal et al. found that the use of DIO as a solvent additive increases the face-on orientation of the donor polymer at the polymer-fullerene interface in a polymer:pc 60 BM blend. [159] 3.8. Morphological Stability To be useful in practice, solar cells must be morphologically stable. Several critical factors influence the stability of OPVs, including thermal degradation, diffusion of components, chemical reactions between components, and photodegradation. Here, we review some factors affecting the morphological stability of active layers and typical efforts, such as use of crosslinkable donors and acceptors, side chain modification, incorporation of metal nanoparticles into the active layer, and removal of residual solvents and additives, to improve the morphological stability of the associated OPVs. Thermal degradation is a major cause of morphological instability. The operation temperature of cells under 1 Sun reaches 65 C, so solar cells should be stable at that temperature for long periods. [338,339] However, our presentation of the effects of postannealing in Section demonstrated that excessive thermal energy induces immoderate crystallization and phase separation; these processes can cause phase segregation, which reduces exciton dissociation and hinders charge transport and collection. These morphological degradations especially occur around the T g of the blend system. For example, Sachs-Quintana et al. found that a 1:1 P3HT:PC 60 BM blend has T g = 56 C and that its V OC begins to degrade at T = 57 C; in the PCDTBT:PCBM blend system, which has a T g of 136 C, V OC begins to degrade at T = 127 C. [339] Thus, many attempts have been made to improve the thermal stabilities of polymer solar cells. The use of nonsolvent additives can improve the thermal stabilities of BHJ films. [206,208,210,213,295, ] Cheng et al. added two crosslinkable fullerene derivatives PCBSD and PCBS to a P3HT:PC 60 BM blend; PC 60 BM diffuses out of this polymer matrix and aggregates to form large PC 60 BM clusters or single crystals at high temperatures. [213] The addition of the styrylfunctionalized polymerizable fullerene derivative, PCBSD, to the P3HT:PC 60 BM blend protected its initial morphology and device characteristics during extended heating at 150 C. Furthermore, PCBS is obtained by removing one styrene insulating group from PCBSD; it has a higher C 60 concentration and better electron transport properties than PCBSD. Chang et al. reported that a PCBSD nanorod structure fabricated with an AAO template as a scaffold can be used to extend the thermal stability of ITO/ZnO/crosslinked-PCBSD nanorods/ P3HT:IC 60 BA/PEDOT:PSS/Ag solar cells. [295] In inert air at 100 C, the time elapsed before the PCE deteriorated by 50% was 689 h in cells containing the PCBSD nanorods, but 220 h in cells with a planar PCBSD layer instead of PCBSD nanorods. The thermal stability is enhanced because the PCBSD nanorods confine the P3HT:IC 60 BA matrix by suppressing its thermal diffusion and the consequent excessive crystallization of P3HT and IC 60 BA. The addition of fullerene-end-capped polyethylene glycol (PCBPEG) was reported by Tai et al. to increase the homogeneity and thermal stability of a P3HT:PC 60 BM active layer. [343] PCBPEG provides an immobile nucleation center for PC 60 BM and as a result reduces the diffusion of PC 60 BM molecules and the consequent formation of large PC 60 BM aggregates at high temperatures. This reduction ensures the stability after thermal annealing at 150 C for 2 h of a P3HT:PC 60 BM BHJ film with crystallites of 3 nm, whereas under the same conditions the film without PCBPEG formed PC 60 BM crystallites with sizes up to 10 nm. The modification of the molecular structures of donor polymers can also suppress the thermal degradation of their associated BHJ films. [ ] The presence of crosslinkable components in BHJ films can prevent other components from diffusing from the original matrix (Figure 16). [341,352, ] Miyanishi et al. synthesized a crosslinkable RR (30 of 39)

31 Figure 16. a) A schematic illustration of the morphological degradation by thermal aging and morphology locking effect of photo-crosslinking in polymer polysilaindacenodithiophenebenzothiadiazole:pc 70 BM blend. Photo-crosslinking prevents thermal diffusion and aggregation of the components. b) The addition of a small crosslinker, 1,6-diazidohexane (DAZH), increases thermal stability of the active film, preventing the formation of PCBM aggregates and the polymer skin near the cathode after thermal aging. c) J V plots for as-cast cells with various amounts of DAZH bis-azide crosslinker. d) Normalized PCEs of cells with various amounts of the crosslinkers after UV curing as a function of thermal ageing time. a d) Reproduced with permission. [341] Copyright 2015, Wiley-VCH. poly(3-(5-hexenyl)thiophene) (P3HNT) and reported that its use results in an initial PCE similar to that obtained with the P3HT:PC 60 BM BHJ system. [363] After thermal annealing, the P3HNT:PC 60 BM blend contains few PC 60 BM aggregates and little Al electrode damage, whereas many PC 60 BM aggregates form in the P3HT:PC 60 BM blend film. Griffini et al. have developed a photo-crosslinkable polymer containing TPD with a terminal. [362] When a primary bromide functionality is appended to the octyl solubilizing group (TPD-Br), the roughness of the crosslinked TPD-Br containing 16% Br increases slightly from 2.1 to 3 nm after thermal annealing at 150 C for 72 h, whereas that of the non-crosslinked TPD-Br dramatically increases from 2.5 to 34.9 nm. The significant increase in the roughness of the non-crosslinked TPD-Br impedes charge separation and transport, and therefore causes a large decrease in the PCE of a TPD-Br16:PC 70 BM BHJ solar cell after thermal annealing. The PCE of the crosslinked TPD-Br16:PC 70 BM BHJ solar cell is well retained after thermal annealing; however, its initial PCE is lower than that of the non-crosslinked blend. This decrease is due to the crosslinking of the p-stacking polymer backbones, which degrades their electronic properties. Modifying the side chains of the components of BHJ films changes their morphologies and can alter their degradation kinetics. [ ] Kesters et al. synthesized PCPDTBT polymers with CPDT side chains modified by the insertion of an ester or alcohol moiety. [355] Due to the high T g of the copolymers, the incorporation of functional moieties into the polymer suppresses excessive crystallization and the large-phase demixing of the components. As a result, the films were thermally stable at 85 C for 650 h. Morse et al. correlated the (31 of 39)

32 effects of side chain modification with the morphologies and morphological degradations of poly(bdt-alt-dpp):pc 60 BM BHJ films. [353] BDT was either unmodified or one of three side chains was added: linear alkyl, linear alkoxy, or branched alkylthienyl. The linear alkyl side chain provided the most efficient morphological stabilization. The polymer with no side chains has significant PC 60 BM miscibility and permits rapid PC 60 BM diffusion due to the voids in the polymer backbones. The polymer with the linear alkyl chain is densely packed and has the shortest lamellar spacing, so this poly mer reduces the rate of PC 60 BM diffusion. The polymer with linear alkoxy chains has a high PC 60 BM miscibility and develops a large lamellar packing distance and increased disorder of the polymer chains; as a result, the rate of PC 60 BM diffusion is high. The polymer with the branched alkylthienyl chains exhibits disordered packing and has a lower PC 60 BM miscibility than the other derivatives due to its bulky and branched side chain; as a result, the morphology of the films deteriorates rapidly. The selection of the proper cell structure can improve the device stability. Kettle et al. reported that PCPDTBT:PC 70 BM BHJ films undergo vertical phase separation with ageing. [366] The diffusion of components results in the formation of a PCP- DTBT-enriched region close to the top electrode and a PC 70 BMenriched region at the bottom electrode. Therefore, the use of an inverted cell structure could significantly improve film stability with respect to ageing. Introducing metal particles into the donor:acceptor blend can improve morphological stability of the blend system. [ ] Incorporation of silver (Ag) nanoparticles (NPs) in P3HT:PC 60 BM improved photovoltaic performance of the cells due to localized surface plasmon resonance, and also improved the structural and morphological stability of the active layer. [367] Incorporation of Ag NPs reduced the roughness change caused by reorientation of the blend after illumination, compared to the sample without Ag NPs. Incorporated Ag NPs hinder the segmental motions of the donor polymer to increase the effective glass transition temperature, thereby increasing / stability. Solvents or solvent additives with a high boiling point can remain even after the film has dried. Such residues can cause the formation of inhomogeneous morphologies and the diffusion of components, as well as undergo undesirable reactions with other components. Thus, the improvement of the morphological stability of BHJ structures requires the removal from dried films of residual solvent and processing additives. [282,283,340, ] Chang et al. demonstrated that CB and DCB residues affect active layer films after they have dried (Figure 17a). [157] The residual solvents in BHJ films induce the diffusion of PC 60 BM within the polymer matrix and increase the size of PC 60 BM Figure 17. a) A schematic illustration of the effect of the environment on removal of residual solvent. Air flow during thermal annealing prevents evaporated solvent molecules from diffusing back into the film again. b) Microscope images of P3HT/PC 60 BM films without and with DIO, after thermal annealing at 150 C for 30 min in a glove box; scale bar: 50 µm. c) A schematic illustration, electron paramagnetic resonance, and X-ray photoemission spectroscopy data describing the chemical reaction between iodine ion from DIO and PC 70 BM to form an oxidized PC 70 BM. a,b) Reproduced with permission. [157] Copyright 2011, Wiley-VCH. c) Reproduced with permission. [283] Copyright 2016, Wiley-VCH (32 of 39)