On the Morphology of Polymer-Based Photovoltaics

Transcription

1 On the Morphology of Polymer-Based Photovoltaics Feng Liu, 1 Yu Gu, 1 Jae Woong Jung, 2 Won Ho Jo, 2 Thomas P. Russell 1,3 1 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States 2 WCU Hybrid Materials Program and Department of Materials Science and Engineering, Seoul National University, San, 56 1, Shillim dong, Kwanak ku, Seoul , Korea 3 WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University Correspondence to: T. P. Russell ( russell@mail.pse.umass.edu) Received 18 January 2012; revised 21 February 2012; accepted 23 February 2012; published online 23 March 2012 DOI: /polb ABSTRACT: We review the morphologies of polymer-based solar cells and the parameters that govern the evolution of the morphologies and describe different approaches to achieve the optimum morphology for a BHJ OPV. While there are some distinct differences, there are also some commonalities. It is evident that morphology and the control of the morphology are important for device performance and, by controlling the thermodynamics, in particular, the interactions of the components, and by controlling kinetic parameters, like the rate of solvent evaporation, crystallization and phase separation, optimized morphologies for a given system can be achieved. While much research has focused on P3HT, it is evident that a clearer understanding of the morphology and the evolution of the morphology in low bad gap polymer systems will increase the efficiency further. While current OPVs are on the verge of breaking the 10% barrier, manipulating and controlling the morphology will still be key for device optimization and, equally important, for the fabrication of these devices in an industrial setting. VC 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: , 2012 KEYWORDS: conjugated polymers; diffusion; miscibility; morphology; neutron scattering; organic photovoltaics; phase separation; X-ray scattering INTRODUCTION In the past decade, there has been an unprecedented growth in interest of solar cells made of organic semiconductors, especially conjugated polymers. This is, in part, due to the rapid growth in the photovoltaic market and, in part, to the advances that have been made in our understanding and synthesis of organic electronic materials. Compared to conventional photovoltaic technologies, organic photovoltaics, OPVs, have a much broader selection of functional materials, for which the molecular structure and band gap can be turned through synthesis. OPV technologies are particularly attractive due to their potential in high throughput manufacture processes, like ink-jet printing and roll-to-roll processing, that are more energy efficient, in comparison to common silicon photovoltaic production processes. The packaging and encapsulation of OPVs can be realized by lamination technologies, which offer another advantage in reducing production costs. OPV cells are usually thin and, therefore flexible, making it easy to integrate them into appliances or building materials. Consequently, the range of applications is greatly broadened. A tremendous number of new OPV materials are being made each year that have pushed device efficiencies to the 10% barrier. However, a clear relationship between the material properties and device efficiencies to the morphology, which is key in understanding BHJ OPV device function, has not been realized and developing a generalized route to optimize device performance is still lacking. There has been a significant amount of research and development that have focused on the morphology of OPV BHJ. Here, we review some of the major developments, both in terms of the understanding of the morphology and in the new techniques that have emerged to investigate the morphology of these thin film materials. Different categories of materials are summarized, compared and discussed. Some commonalities emerge between these different materials and the routes by which device optimization can be achieved. BACKGROUND To get an OPV device to function, usually a donor material, which plays a major role in light absorption and generates excitons, and an acceptor material, which splits the excitons and creates electron pathways, are required. These donor acceptor materials are sandwiched between a pair of electrodes to complete the circuit. When light is absorbed, an electron is elevated to the unoccupied molecular orbital of the donor material, forming an exciton, a bound electron-hole pair, which diffuses to the interface between the donor and acceptor, where the exciton splits into positive and negative VC 2012 Wiley Periodicals, Inc PART B: POLYMER PHYSICS 2012, 50,

2 FIGURE 1 Polymeric solar cell heterojunctions. (a) Energy diagram of a heterojunction with an exciton in the polymer phase. The first heterojunction was a bilayer (b), but this architecture is limited as the active layer must be thin to utilize all excitons. To have every exciton separated, a heterojunction must exist within an exciton diffusion length. This can be achieved in a bulk heterojunction (c) or in an ordered heterojunction (d). Reproduced from Ref. 3, with permission from Elsevier. charge carriers due to the energy level offset at the interface. 1 The carriers are then transported to their respective electrodes, which requires continuous conducting pathways to the electrodes [see Fig. 1(a)]. 2 4 Thin film OPV technology was first established in a bilayer device structure designed by Tang in 1986, 5 which used a sequential thermal vacuum deposition of two small molecules. A significant advance was achieved by the UCSB and Cambridge groups in 1995, 6,7 with a new design of a BHJ device structure, where a conjugated polymer constituted the donor and modified fullerene (or n type polymer) was the acceptor. This fundamental breakthrough enabled the use of solution processing to fabricate BHJ OPVs. Another key advance was that the interfacial area between the donor and acceptor was markedly increased. The smaller the size of the domains in the BHJ, the larger is the interfacial area for exciton separation into electrons and holes. The smaller domain sizes are, also, commensurate with the diffusion distance or lifetime of the exciton. In an organic polymer the exciton will diffuse 10 nm before the bound electron-hole pair will recombine. Consequently, domains approaching several tens of nanometers are necessary to increase the probability of dissociation and charge extraction and, hence, device efficiency. Current research has focused on controlling the morphology of donor acceptor mixtures or block copolymers, attempting to balance charge transport and increase light absorption. 4,8 12 A maximum efficiency of 8.3% was obtained for a single active layer BHJ device. 13 Three different types of morphologies have been used in the active layer of OPVs, including bilayers, BHJs and ordered BHJs, which are schematized in Figure 1. 3 Polymer based bilayer devices were first fabricated by Sariciftci et al. by evaporating a thin layer of C 60 on top of spin-coated poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH- PPV) layer. 14 Although the device efficiency was low, it is still a good model system to study intrinsic physical properties of conjugated polymer interfaces and polymer-fullerene diffusion behavior. BHJ devices, especially polymer-fullerene blends, have been most intensively investigated due to their promising performance. A bicontinuous interpenetrating network of donor/ acceptor domains is obtained by processing the polymer-fullerene blends by different methods. This BHJ is nearly ideal for OPV devices, boasting of very large donor acceptor interfacial area and conducting channels for electron and hole carriers. Thus, large improvement of the short circuit current, J sc,is observed in BHJ devices. The ordered BHJ morphology is intrinsically the same as the normal BHJ, but the donor (n) and acceptor (p) channels have direct line-of-sight pathways to their respective electrodes and, hence, are expected to have good performance. However, to achieve polymer based ordered BHJ device, either nanoimprinting processes or the directed self-assembly of block copolymers are necessary, which require either specialized instrumentation or a more involved (and inevitably more expensive) synthesis and processing, making them difficult to implement, at present, in a production mode. The development of new polymers that can absorb more light and that better match the solar spectrum is another critical issue in OPV. New materials, together with optimized device fabrication, continuously increase device efficiency, simply by absorbing more photons. In Figure 2, a few typical polymers are shown. However, a detailed summary of material development is beyond the scope of this work and the reader is referred to several recent reviews on this topic. 15,16 In the current stage of OPV development, modified fullerenes PART B: POLYMER PHYSICS 2012, 50,

3 FIGURE 2 Typical conjugated polymers for OPV applications. are still the leading acceptor materials used in devices. For the development of new nonfullerene acceptors, please refer to recent reviews. 17 In this review, we will focus on the morphology of polymer-based OPVs and the parameters that dictate the morphology, such as casting solvents, blend composition, annealing conditions (either thermal or solvent), and the use of additives in the solvent casting process. Typical PV materials and their morphologies will also be summarized and compared. MORPHOLOGY OF POLYMER BASED OPVS Morphology Study Based on PPV Type Materials Poly(1,4-phenylene vinylene) (PPV)-based polymers were the first few conjugated polymers used in OPVs. It was also the first system where morphological studies were performed. 18 For poly(2-methoxy-5-{3 0,7 0 -dimethyloctyloxy}-p-phenylene vinylene) (MDMO-PPV), the power conversion efficiency reaches 3.0%, which was very high in the early stages of OPV research. 19 Research on PPV laid the foundation for morphological investigations, which clearly pointed out the importance of understanding the relationship between the morphology and device performance. Shaheen et al. first reported a 2.5% efficiency based on PPV in 2001 under AM 1.5 standard solar radiation. 20 By casting the PPV-PCBM blends from chlorobenzene rather than toluene, large scale phase separation was avoided and device efficiency was improved, as shown in Figure 3(a c). Rispens et al. processed the MDMO-PPV:PCBM blends from different solvents and observed a significant difference in the phase separation. 21 Martens et al. used TEM to investigate the morphology of MDMO-PPV:PCBM blends, and observed that with increasing of PCBM concentration, the domain size of the PCBM-rich phase (dark clusters in the image) increased. 22 They also observed that by reducing the drying time of device fabrication, the extent of phase separation was reduced. 23 A detailed characterization of the morphology was performed by Hoppe et al. using a high-resolution scanning electron microscopy (HR-SEM) on cross sections of toluene- and chlorobenzene-processed MDMO-PPV:PCBM blends. 24 For the case of toluene processing, large PCBM aggregates were observed, which limited the charge carrier generation efficiency [see Fig. 3(d g) for detail]. By using Kelvin force microscopy, they found that the PCBM aggregates were covered by a thin polymer skin [with scheme shown in Fig. 3(h,i)] which blocked the electron transport to the cathode. 25 They also found that by the increasing the amount of PCBM, the aggregate size increased. In the case of chlorobenzene-processing, large PCBM clusters were not observed. It was argued that the polymer formed nanospheres, approximately 1020 PART B: POLYMER PHYSICS 2012, 50,

4 FIGURE 3 AFM images showing the surface morphology of MDMOPPV:PCBM (1:4 by wt) blend films with a thickness of 100 nm and the corresponding cross sections: (a) Film spin coated from a toluene solution; (b) Film spin coated from a chlorobenzene solution. (c) Characteristics for devices with an active layer that is spin coated from a toluene solution (dashed line): J SC ¼ 2.33 ma/cm 2, V OC ¼ 0.82 V, FF ¼ 0.50, h AM1.5 ¼ 0.9%, and from a chlorobenzene solution (full line): J SC ¼ 5.25 ma/cm 2, V OC ¼ 0.82 V, FF ¼ 0.61, h AM1.5 ¼ 2.5%. Data are for devices illuminated with an intensity of 80 mw/cm 2 with an AM1.5 spectral mismatch factor of The temperature of the samples during measurement was 50 C. SEM cross-sections of chlorobenzene (d, e) and toluene-(f, g) based MDMO-PPV:PCBM blends. Whereas chlorobenzene-based blends are rather homogeneous, toluene cast blends reveal large PCBM clusters embedded in a polymer-rich matrix or skin-layer. Small features-referred to as nanospheres -are visible in all cases and can be attributed to the polymer in a coiled conformation. The blending ratio is depicted in the lower right corner. Schematic of (h) chlorobenzene and (i) toluene cast MDMO-PPV:PCBM blend layers as active layer in the solar cell. In (h) holes and electrons find percolated pathways to reach the respective electrode. In (i) electrons and holes suffer recombination due to missing percolation. Reprinted from Refs. 20, 24, and 25, with permission from American Physical Society, Wiley-VCH and Elsevier nm in radius, and percolated pathways for both electrons and holes were produced, leading to a threefold enhancement in the efficiency. 24 Janssen and coworkers performed a detailed study of the morphology of MDMO-PPV:PCBM blends with varying PCBM concentration processed from chlorobenzene. 26 No obvious phase separation was observed when the concentration of PCBM was less than 50 wt %. With PCBM concentrations higher than 67 wt %, domains of PCBM began to appear. It was shown that when the PCBM concentration was between 75 and 80 wt %, a fibrillar texture was observed in the blended thin film, where the fibrils had a width of a few tens of nanometers, which agrees well with the findings of Hoppe et al. [see Fig. 4(a)]. Devices with different compositions were fabricated and a maximum in efficiency was seen when the PCBM concentration was 80 wt %, mainly due to the significantly increased J sc and fill factor, FF, [see Fig. 4(b,c)]. In later work, Yang et al. and Bertho et al. showed that thermal annealing of the MDMO-PPV:PCBM induced crystallization of PCBM, which will affect the efficiency and stability of the devices. 27,28 These observations directly link the device efficiency to the morphology, clearly demonstrating the necessity of research on the morphology of BHJ OPVs. These initial findings, though, have bearing on the findings that are emerging on the more efficient low bandgap polymers of current interest. Morphology Study Based on P3HT Physical Properties of P3HT Poly(3-alkylthiophene)s (P3AT)s, in particular poly(3-hexyltiophene) (P3HT), are the most studied polymers in OPV research, due to their promising solar cell performance, high field effect mobility, and richness in morphology. 29,30 While the conjugated backbone is the origin of the optoelectronic properties and one would like to maximize the density of the main chain, the limited solubility requires the use of an alkyl side chain for ease of processing. The chemical PART B: POLYMER PHYSICS 2012, 50,

5 FIGURE 4 (a) TEM images of MDMO-PPV/PCBM blends with different weight percentages of PCBM as indicated in the upper right corner. (b) Short-circuit current density, power conversion efficiency, (c) fill factor and open-circuit voltage with vary PCBM concentrations. Reprinted from Ref. 26, with permission from Wiley-VCH. incompatibility of the conjugated main chain and alkyl side chains drives the polymer to self-assemble and crystallize with a p p stacking of adjacent thiophene main chains, that is, in the (010) direction. The alkyl side chains form a layer that separates adjacent thiophene chains in the (100) direction. While it only stands to reason that the stiffness of the thiophene chain would give rise to a correlated packing of adjacent thiophene chains in the (010) direction, the (010) reflection in most cases is rather weak. 31 X-ray diffraction studies have shown that the (010) reflection occurs at 0.38 nm, while the (100) reflection is found at 1.6 nm. 32 A schematic model of the packing of P3HT is shown in Figure 5(a). With respect to the orientation of the conjugated backbone, face-on and edge-on packing, as shown in Figure 5(b), are used in the literature. Also, based on different sequencing of thiophene repeat units, head-to-tail and head-to-head sequencing can occur. A polymer with only head-to-tail sequences is referred as regioregular P3HT (Fig. 2). The region-regularity of the polymer is the percentage of headto-tail sequences. The orientation of the P3HT crystals, which markedly influences the efficiency of light absorption and device performance, are clearly affected by the processing conditions and polymer properties. Sirringhaus et al. reported that the mobility and orientation of P3HT chains were strongly influenced by the regio-regularity and molecular weight [Fig. 5(c,d)]. 33 For spin-cast thin films with high region-regularity (>91%) and low molecular weight, the polymer chains adopt an edge-on orientation. In contrast, for samples with low region-regularity (81%) and high molecular weight, the crystals are preferentially oriented with a face-on structure. Also, the choice of solvent and processing conditions significantly affect the morphology and the orientation of the crystals within the film, 34 as shown by Bao et al. [Fig. 5(e,f)]. The nature of the substrate and its interactions with the main chain and side chain can significantly impact the orientation (face on vs. edge on) of the P3HT at the interface and the ordering of the P3HT. 35 When P3HT, the donor, is blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the acceptor, to make the active layer in solar cells, the morphology of this relatively simple two component system becomes much more complex. Using only one casting solvent, the solubility of both components will dictate the homogeneity of the cast film, to what extent the P3HT will crystallize and what growth habit it will assume. Many other factors, like region-regularity, blend ratio, annealing conditions and additives will strongly affect the final morphology of the thin film. Previous research has shown that high regio-regular (RR) P3HT performs better in BHJ solar cells, 36 due, in part, to the strong interchain interactions, which leads to better absorption and structural order, resulting in a higher hole-mobility. It was also observed that when the P3HT:PCBM blend ratio is 1:1 or 1:0.8, the blends gave the best device performance, 37,38 due, in part, to the ease in forming a cocontinuous morphology. Yet, processing conditions and postprocessing treatments will also largely affect the device performance. Thermal Annealing Approach Thermal annealing is the most commonly used and simplest method to enhance the device performance of P3HT:PCBM blends. Thermal annealing can increase the crystallinity of P3HT, which can be directly observed by the increased absorption at 605 nm [Fig. 6(a)]. At the same time, thermal annealing drives the separation of the P3HT and fullerenes and the formation of a cocontinuous morphology. Two different thermal annealing methods have been used: 1022 PART B: POLYMER PHYSICS 2012, 50,

6 FIGURE 5 (a) Schematic model showing the hierarchical organization of the semicrystalline structure of P3HT at three main length scales. The disordered P3HT chains in the amorphous zones, comprising chain ends, chain folds, and tie molecules, are represented as red segments. (b) Schematic model of edge-on and face-on orientation. (c, d), The Grazin-incidence-X-ray scattering (GIXD) images of two different P3HT thin film samples ( nm) with regioregularity of 96% (c) and 81% (d) on SiO 2 /Si substrates. The vertical (horizontal) axes correspond to scattering normal (parallel) to the plane of the film. The insets show schematically the different orientations of the microcrystalline grains with respect to the substrate. (e), (f) 2D GIXD patterns and AFM topographs of spin-cast RR P3HT films on the SiO 2 /Si substrates held at room temperature from CHCl 3 (e) and warm CH 2 Cl 2 (f). The insets in e illustrate schematic diagrams for face-on orientations of RR P3HT in the films. The inset in f represents 1D out-ofplane x-ray profiles extracted from 2D patterns of the spin-cast CHCl 3 and CH 2 Cl 2 films. Reprinted from Refs. 31, 33, and 34, with permission from Wiley-VCH, Nature Publishing Group and American Physical Society. preannealing, where a thin film of the mixture is annealed before the deposition of the cathode, and postannealing, where the cathode is evaporated onto the thin film mixture prior to thermal annealing. Heeger and coworkers systematically studied the postannealing of P3HT:PCBM blends, in which they found that the thermal annealing treatment increased both the J sc and FF [Fig. 6(b,c)]. 39 Blom and coworkers analyzed the single carrier mobility and photocurrent generation of postannealed devices and found that the increase in the hole-mobility of the P3HT was the singlemost important factor leading to the marked enhancement of the efficiency [Fig. 6(d)]. 40 Loos and coworkers studied the morphology of spin cast and postannealed P3HT:PCBM blends. 41 In their studies, it was shown that for the spincoated films, P3HT formed interconnecting fibrillar crystals and in a matrix of homogeneously mixed P3HT and PCBM. Upon thermal annealing, a cocontinuous network of P3HT and PCBM was produced. The increase in the crystallinity of the film and the controlled phase separation of the two components were the main reasons noted for the enhancement in the device efficiency [Fig. 6(e,f)]. A detailed morphological characterization on P3HT:PCBM blends was carried out by Russell and coworkers, wherein as-spun, preannealed and postannealed thin films were studied and compared. 42 For the as-spun thin films, small angle neutron scattering (SANS) showed that no obvious phase separation of P3HT and PCBM occurred [Fig. 7(a)], with the film consisting of a homogeneous mixture of P3HT and PCBM. This would be expected if the P3HT and PCBM were miscible, which was born out by the depression in the melting point of P3HT with the addition of PCBM. Within a few seconds of annealing at 150 C, a cocontinuous network morphology was formed with a characteristic length scale of PART B: POLYMER PHYSICS 2012, 50,

7 FIGURE 6 (a) UV vis absorption-coefficient (a) spectra of P3HT:PCBM blend films, as-cast, and for different annealing temperatures (see legend). The annealing time was 4 min. For a better comparison, the absorption of a pristine P3HT film as cast is also shown (in arbitrary units). All films were measured on a glass/ito/pedot:pss substrate and corrected for substrate absorption. (b) Variation of fill factor (open squares) and short-circuit current (filled squares) with annealing temperature (AM 1.5, 80 mw cm 2 ). (c) Evolution of device efficiency (filled squares) with thermal annealing time at 150 C. (d, e) BF TEM images and the corresponding schematic representation (f) of the thermal annealed photoactive layer. The inset in Figure d is the corresponding SAED pattern. Outside ring is the (020) Debye-Scherrer ring from P3HT crystals Note: for Figure (f), the dash line bordered regions represent the extension of existing P3HT crystals in the pristine film, or newly developed PCBM-rich domains during the annealing step. The TEM images were recorded on a JEOL JEM-2000FX transmission electron microscope operated at 80 kv. Reprinted from Refs. 39, 40, and 41, with permission from Wiley-VCH, and American Chemical Society. FIGURE 7 (a) SANS profiles of P3HT/PCBM blend films. As spun, (black triangle); preannealed 30 min (blue triangle); postannealed 5 s (red triangle); postannealed 30 s (green triangle); postannealed 1 min (brown triangle); postannealed 5 min (aqua triangle); postannealed 30 min (purple triangle); and postannealed 1 h (orange triangle). The inset represents the correlation length (a) and the scattering invariant (Q) versus the postannealing time. (b) HRTEM of cross sections of P3HT/PCBM-based multilayered samples postannealed 30 min heating at 150 C. The insets represent high magnification. Reprinted from Ref. 42, with permission from American Chemical Society PART B: POLYMER PHYSICS 2012, 50,

8 approximately nm, as evidenced by SANS and high resolution TEM [Fig. 7(b)]. With further annealing, the phase separation remained relatively stable. It is also important to note that for the pre- and postannealed films, evolution of the morphologies in the films were essentially identical. The better performance of postannealed device should come from other factors that limit the device efficiency. SANS profiles for postannealed samples show more detailed process of phase separation kinetics. The correlation length increased from 0.5 to 4.6 nm after 5 s of annealing, and reached 5.3 nm after 30 min of annealing, which translates into average chord lengths for the P3HT-rich and PCBM-rich domains of 11 nm, in keeping with the HRTEM results. The crystal size of P3HT along (100) and (010) crystal planes in a direction normal to the film surface, determined from grazing incidence X-ray scattering, GISAXS, were 23 nm and 12 nm, respectively, which is consistent with SANS and TEM results. These results, along with electron energy loss spectroscopy, which is sensitive to the elements comprising each domain, indicated that one of the domains observed could be assigned to the P3HT crystals. It is tempting to argue that spinodal decomposition or phase separation of the P3HT and PCBM gives rise to the cocontinuous morphology. However, the miscibility of the P3HT and PCBM, the nanoscopic size of the domains and the sharpness of the interface between the domains formed are inconsistent with that typically seen in the phase separation of a polymer mixed with a small molecule. 43,44 Yet, we know that P3HT crystallizes and that the PCBM is not included within the crystalline regions. Consequently, a more likely explanation of the observed morphology is that the P3HT nucleates crystallization in the homogeneous mixture and, with time, the P3HT crystals grow, literally pushing the PCBM away from the growth front(s). From classic arguments of Kieth and Padden, 45 if the crystals can grow at a rate of G (in units of cm/s) and the PCBM can diffuse in the mixture with a diffusion coefficient D (in units of cm 2 /s), then the ratio D/G will yield a length scale, d, that is characteristic of an instability at the crystal growth front. It is known that P3HT crystallizes very rapidly at 150 C, in fact too rapidly to measure isothermally by X-ray scattering methods. It is also known, from dynamic secondary ion mass spectroscopy studies on bilayers, that PCBM diffuses in P3HT and its mixtures with PCBM with a diffusion coefficient in excess of cm 2 /s. 42,46 Therefore, it is reasonable to conclude that the characteristic size of the domain formed from the growth and impingement of ordered P3HT will be on the tens of nanometer size scale, as observed experimentally. An alternate origin of the small size of the crystalline P3HT and PCBM-rich domains is that there are numerous nucleation sites to initiate crystallization of P3HT and it is simply high nucleation density that gives rise to the small scale domains. In either case, spinodal phase separation cannot be the origin of the observed morphology and crystallization and exclusion of the PCBM to the regions between the crystalline P3HT domains gives rise to the observed morphology (Fig. 7). Similar conclusion was described by Kramer and coworkers. 47 With annealing, there is a slight purification of the P3HT domains, as evidenced by the slight FIGURE 8 DSIMS of the P3HT/deuterated PCBM blend films. a, as spun; b, preannealed 30 min; c, postannealed 5 s; d, postannealed 30 min; S signal (red line); D signal (black line). Reprinted from Ref. 42, with permission from American Chemical Society. increase in the SANS invariant, directly related to the product of the volume fraction of the components and the square of the difference in the scattering length densities of the domains [inset in Fig. 7(a)] It should be noted that the removal of PCBM from the P3HT domains also means that isolated PCBM, that can act as electron traps, are removed from the P3HT, which would give rise to an increase in the efficiency of the device, as is observed. The distribution of different components normal to the film surface is of critical importance for device efficiency. For an as-spun film, the competition between the lower surface energy of PCBM, in comparison to P3HT, and the preferential solubility of P3HT in the casting solvent, will dictate the distribution of the components in the initial as-spun film. Upon thermal annealing, the difference in the surface energies will influence the concentration of the components at the surface with the lower surface energy component preferentially segregating to surface. 48 This would describe the preannealed film where the electrode is evaporated onto the thin films after thermal annealing. However, for the postannealed film, it is the difference in interfacial energies of the components that is important. Consequently, even though the bulk morphologies of the preannealed and postannealed films may be the same, the concentration of components immediately at the cathode interface, the interface where carriers are transported across an organic inorganic interface, will be different in the two cases. These arguments are born out by the dynamic secondary ion mass spectroscopy, DSIMS, results in Figure 8 where, for the preannealed sample, a significant enhancement in the P3HT concentration, whereas for the postannealed sample, the opposite is the case. It is this slight difference in the concentration of the components at the cathode interface that can give rise to the observed differences in the efficiencies. The DSIMS results have, also, been PART B: POLYMER PHYSICS 2012, 50,

9 supported by variable-angle spectroscopy ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), near-edge X- ray absorption fine structure spectroscopy (NEXAFS) and neutron reflectivity (NR) studies in our laboratories and elsewhere. 42,47 51 It should also be noted that the distribution of components normal to the film surface is also dependent on the nature of the solvent used. 52 The interactions between the conjugated polymer, here P3HT, and PCBM, that is, their miscibility, and the solubility of the components in the solvent use for film preparation are important parameters that must be considered in describing and controlling the morphology of the resultant active layer. There is overwhelming evidence that P3HT and PCBM are highly miscible. 47,53 Russell and coworkers used a simple melting point depression study to demonstrate a significant decrease in the melting point of the P3HT with the addition of PCBM. From this an interaction parameter between P3HT and PCBM of was determined. These studies were augmented by studies where the molecular weight of P3HT was varied where it was found that the interaction parameter increased from 0.2 to 0.4, with the molecular weight increasing from 5 to 48 k. 54 This increase in the interaction parameter with molecular weight is consistent with the solubility of any polymer in a low molecular weight solvent where, as the molecular weight of the polymer increases, the solubility will decrease. Using NEXAFS, Ade and coworkers found that P3HT were miscible but the miscibility was temperature dependent. 53 Yin and Dadmun used SANS and found the miscibility limit of PCBM was 20%. 55 Both of these studies showed the existence of a crystalline P3HT phase, a pure PCBM phase and a mixed P3HT/PCBM phase [this is schematically shown in Fig. 9(a)]. Further evidence of the miscibility of P3HT and PCBM is found in the diffusion of PCBM into P3HT. These interdiffusion studies also point to an alternative route to prepare a BHJ morphology by allowing the PCBM to penetrate into the disordered phase in the P3HT layer resulting from the partial crystallization of the P3HT. Kramer and coworkers placed a layer of P3HT on top of a deuterated PCBM layer, annealed the bilayers at different temperatures and for different times and monitored the diffusion of the PCBM into the P3HT using DSIMS. 47 Here, the deuterium signal arising from the PCBM, provided a real-space concentration profile of the two components in the bilayer, as shown in Figure 9b. Several very surprising results emerged from these studies. First, in keeping with the studies discussed above, PCBM diffused rapidly into the P3HT layer, indicating that, at the temperatures investigated, P3HT and PCBM are miscible. Second, the diffusion profiles showed that the interface between the P3HT and PCBM remained sharp, indicating a resistance to a uniform diffusion of the PCBM across the interface. Thirdly, the concentration of the PCBM in the P3HT layer was constant and did not vary significantly. Finally, the concentration of PCBM in the P3HT layer reached a saturation value that depended on the temperature at which the interdiffusion was performed. Results of Russell and coworkers, also using DSIMS and deuterated PCBM, showed essentially identical results [Fig. 9(c)] with the conclusion that the P3HT rapidly orders and the PCBM diffuses into the P3HT between the ordered or crystallized region of the P3HT. Even though the diffusion of the PCBM into the P3HT layer is very rapid, with diffusion coefficients in excess of cm 2 /s, the crystallization of the P3HT is even more rapid. In fact, Russell and coworkers performed a second set of measurements where the P3HT was first ordered and then transferred onto the PCBM [Fig. 9(c,d)] with essentially the same result being obtained as when the as-cast sample was used. Not only do these studies demonstrate the miscibility of the P3HT and PCBM but they as show that PCBM can be introduced into the P3HT after the P3HT has been thermally treated to form an active layer [as schematically shown in Fig. 9(e)]. With TEM and GIXD experiments, they confirmed that the diffusion process did not disrupt the morphology established by P3HT and the interdiffusion driven morphology is essentially the same with homogenously mixed BHJ active layer. 56 In fact, the device performances achieved with this postdiffusion route were as good as when the P3HT and PCBM were initially uniformly mixed Solvent Annealing Approach Solvent annealing is another effective method that has been used to induce phase separation in P3HT:PCBM blends. When the solvent vapor enters the mixture, the mobility of both the P3HTand PCBM increases, enabling the P3HT to crystallize and drive the segregation of the PCBM. Wang et al. showed that a 1,2-dichlorobenzene (DCB) vapor treatment increased the ordering of the P3HT, enhanced its absorption, and increased hole mobility. 60 Cho et al. compared the quality of the solvents used in the vapor treatment of P3HT:PCBM blends and found that the blends annealed in a poor solvent yielded devices with better performance due, primarily, to an enhancement of the segregation of the PCBM. 61 Loos and coworkers used transmission electron microscopy tomography and found that solvent annealing resulted in an interpenetrating nanofibrillar morphology with fibrils of P3HT forming a crystalline network with a mixture of P3HT and PCBM in the interfibrillar regions. 62 Kim et al. used the ToF-SIMS to determine the composition of the components normal to the surface of the films and found that solvent annealing caused PCBM to migrate to the surface, 63 which is similar to the findings of Nelson et al. and is desirable for enhancing device efficiency. 48 Li et al. used a slow drying process to control the P3HT crystallization and phase separation. 64,65 They found that P3HT was better ordered, as evidenced by the enhanced absorption. By using time-of-flight mobility measurements, a more balanced charge transport was also obtained for the slow-dried samples. It was also seen that the surface roughness increased significantly due, more than likely, to the crystallization of the P3HT. In addition to the conventional solvent annealing, a solvent soaking treatment was also developed to induce phase separation of the P3HT:PCBM blends. 66 Yang et al. showed that by soaking the as-spun film in methanol/carbon disulfide mixture, the crystallinity was increased and an interpenetrating fibrillar network was formed. In addition, a 1026 PART B: POLYMER PHYSICS 2012, 50,

10 FIGURE 9 (a) Diagram showing the local structure of P3HT:PCBM mixtures. Straight lines indicate P3HT crystalline domains, while curved lines indicate P3HT amorphous domains. Note the miscibility of PCBM in the amorphous domains, as well as pure PCBM domains. (b) DSIMS profiles of 2 H in bilayer samples of P3HT and d-pcbm annealed from 5 minutes at different temperatures. The thicknesses of the films (which differed slightly across samples) were normalized such that 0 nm is the free surface of the film and 450 nm is the substrate. The deuterium concentrations were normalized such that the area under the counts versus distance for all profiles was the same after setting the normalized concentration in the d-pcbm part of the initial bilayer equal to 1. (c), (d): DSIMS of P3HT/dPCBM bilayer diffusion at 150 C at different times. (c) As spun P3HT/PCBM bilayer; (d) Preannealed P3HT/PCBM bilayer. Annealed at 150 C, 0 s (black), 5 s (red), 5 min (blue); S signal (solid star); D signal (solid line). (e) A scheme showing proposed P3HT/PCBM interdiffusion process: (1) As Spun P3HT/PCBM bilayer; (2) P3HT crystallizes after short time thermal annealing; (3) PCBM diffuses into P3HT film through the P3HT amorphous domains. Reprinted from Refs. 47, 55, and 56, with permission from Wiley-VCH and American Chemical Society. segregation of the P3HT to the anode interface and PCBM to the surface, or cathode interface was observed which resulted in a significant increase in the device performance. Solvent Additive Approach While the use of a solvent additive is commonly used in low band gap polymers to enhance device performance, it also been used for P3HT:PCBM mixtures to promote phase separation and polymer crystallization. Yang et al. studied 1,8- octanedithiol (OT) as an additive in P3HT:PCBM blends [Fig. 10(a d)]. 67 They found that with small amounts of OT added to the casting solvent, dichlorobenzene, the device performance was markedly enhanced with a strong increase in J sc and FF being found along with a slight decrease in the open circuit voltage, V oc. The presence of OT induced the crystallization of P3HT, which could be seen from the color of the film and the absorption profile. If too much OT was used, the device performance decreased due to a reduced FF, arising from a decrease in shunt resistance arising from a severe phase separation. In the GIXD characterization, the (100) d- spacing was nm for OT treated samples, much smaller than that found with the as spun (1.65 nm) and PART B: POLYMER PHYSICS 2012, 50,

11 FIGURE 10 (a) Current density-voltage (J-V) characteristics of devices made with different amounts of OT. Better device performance can be clearly seen upon increasing amount of OT when OT is less than 7.5 ll. Inset: spin-coated polymer films on ITO-covered glass with 0, 1, 5, 7.5, 15, or 20 ll of OT (per 250 ll of base solution) from left to right, respectively. (b e): V oc, J sc, FF, and PCE change with different amounts of OT. (f) 2D GIXD patterns of films with different amounts of OT. (g) 1D out-of-plane X-ray, Inset: calculated interlayer spacing in (100) direction with different amounts of OT. (h) azimuthal scan [at q (100)] profiles extracted from part f. Reprinted from Ref. 67, with permission from American Chemical Society. thermally annealed (1.64 nm) samples. The exact origins of these changes are not clear at this point. Additives other than OT could also be used effectively. 68 Lam et al. studied a series of alkanedithiols with different alky chain length as additives in BHJ blends. 69 They found that the extent of the additive-induced phase separation was related to the boiling point or vapor pressure of the additive and the interaction of the additive with PCBM. By using scanning transmission X-ray microscopy (STXM), they found that the additive affected the PCBM aggregation, with P3HT being more uniformly dispersed [Fig. 10(e,f)]. Nano-Fibrillar Devices P3HT is a unique polymer that tends to form fibrillar assemblies during solvent annealing or by casting films from a marginal solvent. Nanofibrils of P3HT are essentially crystalline nanorods of P3HT with good carrier mobility. The benefits of using a nanofibrillar motif of P3HT to fabricate solar cells are that the absorption at long wavelengths is increased; the width of P3HT nanofibrils is commensurate with exciton diffusion length and, thereby, enhances charge separation which will enhance light extraction; and the high aspect ratio of the nanofibrils enables them to readily form a continuous network and, when mixed with PCBM, a cocontinuous morphology. Several groups have successfully used this method to fabricate devices with good performance without subsequent annealing processes. Guillerez et al. produced P3HT nanofibers from p-xylene solutions and then blended them with PCBM. Devices made of this mixture showed an efficiency of 3.6%, which is comparable to solution cast, thermally annealed devices. 70 Cho et al. fabricated P3HT nanofibril devices using a marginal solution to age the solutions. 71,72 They first dissolved P3HT:PCBM blends in dichloromethane and allowed the solution to age. For specific aging times, nanofibrils of P3HT, of 10 nm in width and 5 10 lm in length, were obtained, as seen in AFM and TEM images [Fig. 11(a d)]. However, the device performance depended strongly upon the aging time. Longer aging times increased the order and crystallinity of the P3HT and led to a better balance of hole and electron mobilities, increasing device efficiency [Fig. 11(e)]. Another typical poly(3-alkylthiophene), poly(3-butylthiophene) (P3BT), is well-known to form nanofibrils. Jenekhe et al. fabricated nanofibrillar devices based on P3BT:PCBM blends [Fig. 11(f h)]. 73 P3BT was found to aggregate in cold 1,2-dichlorobenzene solution to form nanofibrils, for which the absorption and transport properties can be enhanced. The morphology of P3BT:PCBM nanowire blends are quite similar to P3HT:PCBM nanowire blends. Other Polythiophene Base Materials Besides P3HT, many other thiophene base polymers are also good candidate for OPVs. PQT12 is an interesting high 1028 PART B: POLYMER PHYSICS 2012, 50,

12 FIGURE 11 (a d) AFM and TEM images, respectively, of (a,c) P3HT nanowires and (b,d) P3HT nanowires/pcbm blend film. (e) The current-voltage characteristics for solar cells composed of P3HT nanowires/pcbm blend films with various ageing times, from 12 to 72 h. (f) Chemical structures of P3BT and PC 61 BM. (g) UV vis absorption spectra: (1) P3BT solution in ODCB; (2) P3BT-nw suspension in ODCB; (3) P3BT:C61-PCBM blend on ITO/PEDOT substrate; and (4) P3BT-nw/PC 61 BM nanocomposite on ITO/PEDOT substrate. (h) Schematic illustration of nanowire network of P3BT/PCBM composites. Reprinted from Refs. 72 and 73, with permission from Wiley-VCH and American Chemical Society. mobility polymer which has the same backbone of P3HT, yet its OPV device performs much worse compared to P3HT and in device fabrication, much more amount of PCBM is required. 74,75 Moule et al. suggested that the poor connectivity of crystalline nanoparticles could be a reason for its poor performance. PBTTT is another high mobility donor polymer which is similar to PQT12. McGehee and coworkers demonstrated a 2.3% efficiency could be obtained by 1:4 (wt) blend of PBTTT and PCBM. 76 The high loading ratio of PCBM inside these semicrystalline polymers is quite peculiar in polymer PVs, since if they behave as P3HT, the crystallization process of PQT12 or PBTTT would push out PCBM to form aggregated region to transport electrons. McGehee and coworkers found that PBTTT and PQT12 polymer chains could interact with PCBM molecules and form intercalated cocrystals since the long alky chains of PBTTT and PQT12 provide enough spacing to host PCBM molecules (see Fig. 12 for detail). 77,78 Once the PCBM intercalated in polymer side chains, the (100) d-spacing shifted to a larger size with higher orders of reflections observed, indicating the formation of cocrystals. This cocrystalization process takes large amount of PCBM thus a higher loading ratio is required to form interpenetrating acceptor phase. This effect is obvious seen in device work. In contrast to P3HT-PCBM blends, only mixing was observed, no cocrystallization process. Rance et al. studied the photoinduced carrier generation and decay physics of PBTTT-PCBM blends and their result shows that the intercalated PBTTT-PCBM crystals are not detrimental to the density of long-lived carriers, indicating these structures can be used to fabricate efficient organic photovoltaics in a bi-continuous network blends. 79 Although important progress has been made in understanding the physical properties and device function of the mixing phase in BHJ blends, this topic still needs much more effort to further explore. Low Band Gap Polymer Morphology The progress of the solar cells based on P3HT/PCBM reached the plateau, where the highest recorded PCE was 5%. It was obvious that P3HT was limited in light absorption, particularly in the near IR region. By taking advantage of the donor acceptor approach, by combining different repeat units, the HOMO and LUMO levels of the polymers PART B: POLYMER PHYSICS 2012, 50,

13 FIGURE 12 (a) molecular structures of pbttt, PC71BM, and bispc 71 BM; (b) schematics showing possible structures for pure and intercalated pbttt; (c) Specular X-ray diffraction patterns for pure pbttt (black), pbttt:bispc 71 BM (red), and pbttt:pc 71 BM (blue). The small peaks in the pure pbttt pattern are finite thickness fringes. Reprinted from Ref. 77, with permission from American Chemical Society. could be tuned and, thus, the band gap could be adjusted to cover a wider absorption range of the solar spectrum. It was also realized that controlling the morphology was also important for solar cells based on the low band gap polymers. In addition, due to the novel chemical structure, the physical properties of these new polymers were no longer similar to that of P3HT. They differed widely in morphologies in the active layer. Consequently, the optimized conditions for fabricating highly efficient solar cells based on P3HT/PCBM may not be suitable for the low band gap polymers. Here, we choose some typical low band gap polymers and discuss their morphologies in the blends with PCBM. Polyfluorene-Based Polymer Polyfluorenes (PF) are typical donor polymers with deep HOMO and high V oc. 80 Andersson, Inganas and coworkers prepared a series of alternating polyfluorene copolymers (APFOs), consisting of solubilizing fluorene units and donor acceptor donor (DAD) segments. 80 The typical polymers, APFO-Green 9 or APFO-3, are shown in Figure 2. The degree of phase separation between the polymer and PCBM had attracted a lot of attention. In the case of APFO-Green 9:PCBM blends, 81 a few processing parameters were taken into account, including the blend ratio of the polymer to PCBM, the nature of the substrates, solvents and acceptor materials, to optimize the morphology. Atomic force microscopy (AFM) and transmission electron microscopy tomography (TEMT) were used to examine the surfaces and bulk morphologies of thin films of the blend (Fig. 13). 81,82 A clear picture of phase separation governed by these parameters was obtained. TEM showed that different kinds of fullerene derivatives had a strong effect on the morphology, which was explained by the differences in their solubilities. For APFO-3:PCBM blends, that the mixed solvent (chloroform with chlorobenzene) approach enhanced the photocurrent density, due to the finer and more uniform distribution of the domains. 83 DSIMS was used to determine the distribution of APFO-3:PCBM blends normal to the film surface. 84,85 It could be seen that the size of the phase separated domains, the surface roughness and the enrichment of components at the surface of the film were markedly influenced by the processing conditions. The origin of the differences was attributed to the differences in the interactions between the components, the components and the solvent and/or the component and the substrate. The best PCE value was thought to result from a balance between a large interfacial D-A area and the generation of continuous paths in each phase. Interestingly, films of the APFO-3/PCBM blend, with a weight ratio of 1:4, that spin-coated from chloroform solutions showed the formation of a spontaneous multilayered structure, which was correlated with the higher PCE (reported to be 3.5%) than that from homogeneous composition. 86 Carbazole-Based Polymer In 2007, Leclerc and coworkers synthesized a low band gap conjugated polymer, PCDTBT, which yielded a PCE of 3.6% from BHJ device. 87 Subsequently, the carbazole moiety attracted much attention for OPV applications, due to its low HOMO level (ca. 5.5 ev), which results in a high V oc. In 2009, Heeger and coworkers reported 6.1% PCE from the mixture of PCDTBT and PC 70 BM. 88 In the device design, they inserted a thin TiO x film as a hole-blocking layer and an optical spacer [Fig. 14(a,b)], which increased the effective path length of light within the active layer without increasing the device thickness. The morphology of the device was optimized by using different solvents and adjusting the PCDTBT:PC 70 BM blend ratio. It was found that, when the blends was processed from chloroform or chlorobenzene solutions, large dark areas (>200 nm) were observed in the 1030 PART B: POLYMER PHYSICS 2012, 50,

14 FIGURE 13 (a d) AFM images in topography of APFO Green9:PC 70 BM spin-coated onto ITO/PEDOT:PSS substrate with (a) 1:1, (b) 1:2, (c) 1:3, and (d) 1:4 weight ratios. The Z range is 10 nm for (a c) and 15 nm for (d). (e h) Electron tomography reconstructions of APFO-Green9:PC 60 BM (1:3): (e) and APFO-Green9:PC 70 BM (1:3) (f) cast on PEDOT:PSS. The reconstructions are low-pass filtered at 10 nm; (g) and (h) show planar sections (ca. 16 nm thick) from the middle of the reconstructions. Reprinted from Ref. 81, with permission from American Chemical Society. TEM images [Fig. 14(c,d), respectively]. By using DCB as the processing solvent, a nanofibrillar network was observed [Fig. 14(e,j)]. When the amount of PCBM was progressively increased, the nanofibrillar morphology became more clear and better defined [Fig. 14(f i,k)]. These combined efforts yielded an internal quantum efficiency close to 100%, implying that at short circuit every photon absorbed resulted in charge dissociation and that all photogenerated charge carriers were efficiently collected. In the fabrication of highly efficiency PCDTBT devices, a thermal annealing at 70 Cwas used. Staniec et al. made a detailed characterization of the thermal treatment on the morphology of the PCDTBT:PCBM active layer. 89 By using neutron reflectivity to probe the depth-dependent concentration of PCBM, they found that the surface of a freshly cast thin film was enriched with PCBM, which is the opposite of what is observed with P3HT:PCBM. The thermal annealing at 70 C, did not significantly change the vertical distribution of the components and the overall crystallinity of the polymer remained relatively constant. The T g of PCDTBT is 130 C, 90 thus, the authors argued that the 70 C annealing brought the active layer above the T g of the PCDTBT due to the presence of residual solvent, and, with time, the trapped solvent diffused out of the active layer without significantly altering the overall morphology. 89 Cyclopentadithiophene-Based Polymer Cyclopentadithiophene-based polymers are another class of promising donor materials. Poly[2,6-(4,4-bis(2-ethylhexyl)- 4H-cyclopenta[2,1-b;3,4-b 0 ]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, is probably the most well-studied low band gap polymer. It has a band gap of 1.46 ev and a high hole-mobility. 91,92 However, the highest PCE was reported to be only 3%. 91 Initially, PCPDTBT was thought to be amorphous. It mixed well with PCBM and such intimate mixing between the donor and acceptor would cause significant charge recombination, and was thought to be the reason for the low efficiency. Thermal annealing did not improve the device performance. Heeger and coworkers added a processing additive to the primary solvent to cast films of the PCPDTBT/PCBM mixtures and found substantial improvement in the device efficiency to be more than 5%. 91 This was the first case where the use of a processing additive was found to be effective in enhancing the efficiency for low band gap polymers solar cells [Fig. 15(a)]. After testing a variety of additives and combinations of the additive with a host solvent, the requirements for a processing additive to be effective were summarized as follows: (i) there must be a selective solubility for PCBM and (ii) the additive must have a higher boiling point than the host solvent. 93 To determine the role of the additives and mechanism by which the additive influenced the morphology, the morphologies of the thin films processed with or without additives were characterized in great detail. At first, a phase separation between PCPDTBT and PCBM by use of the additive was thought to be the main factor for the increased efficiency as evidenced by AFM and TEM images [Fig. 15(b,c)]. 93 The role of the additive was schematically described by a surfactant-like effect where the PCBM was sequestered [Fig. 15(d)]. Subsequently, it was noticed that PCPDTBT crystallized under the influence of the additive. 94,95 GIXD showed the ordered packing of PCPDTBT chains and the chain packing assumed an edge-on orientation, similar to that seen in thin films of P3HT [Fig. 15(e)]. In situ UV-vis and GIXD were used to monitor the evolution of chain packing or crystallization process during the solvent evaporation. 96 Subsequently, Russell and coworkers used a combination of grazing incidence X-ray scattering and diffraction and neutron scattering, in combination with transmission electron microscopy, to show that a phase separated, multilength scale morphology was formed, comprised of nanofibrillar domains of crystalline PCPDTBT that formed a PART B: POLYMER PHYSICS 2012, 50,

15 FIGURE 14 (a) Device structure: the bulk heterojunction (BHJ) film is a phase separated blend of PCDTBT and PC 70 BM. The inset shows the transfer of photogenerated electrons from PCDTBT to PC 70 BM. The titanium oxide (TiOx) layer is introduced as an optical spacer on top of the BHJ layer. (b) Energy level diagram of the components of the device. (c e) TEM images of PCDTBT:PC 70 BM films spin-cast from CF (c), CB (d), and DCB (e) solvents. The insets show the surface phase images measured by atomic force microscopy (AFM). (f i) TEM images of the PCDTBT:PC 70 BM blend films spin-cast from DCB with increasing amounts of PC 70 BM: blending ratios 1:1 (f), 1:2 (g), 1:3 (h), and 1:4 (i). (j) J V characteristics of the devices fabricated with films cast from CF, CB, and DCB. (k) J V characteristics of the devices fabricated using BHJ films with blend ratios of 1:1, 1:2, 1:3, and 1:4. Reprinted from Ref. 88, with permission from Nature Publishing Group. network of fibrils with a mesh size on the hundreds of nanometer level which was imbedded in a matrix comprised of PCBM-rich aggregated domains of PCPDTBT on the tens of nanometer size scale. 97 Consequently, a multilength scale morphology was formed due, primarily, to the preferential solubility of the PCBM and insolubility of the PCPDTBT in the additive. Further efforts were made by Yang et al. 98 with a silole-containing polymer, poly{[4,40-bis(2-ethylhexyl)dithieno(3,2- b;20,30-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (PSBTBT). By replacing the 5-position carbon of PCPDTBT with a silicon atom, the stacking of the polymer chain is improved [Fig. 16(a)] and, thus, the crystallinity of the polymer was enhanced. GIXD clearly showed strong (h00) peaks in the out-of-plane direction, corresponding to the edge-on orientation [Fig. 16(b,c)]. 99,100 The higher crystallinity of the polymer is thought to be responsible for the better charge transport and reduced bimolecular recombination. Thermal annealing was demonstrated to be effective to increase PCE from 3.8 to 5.6%. Russell et al. used a broad range of techniques to investigate the morphology of the PSBTBT/PCBM blends and the evolution of the morphology by thermal annealing. 101 The results showed that PSBTBT was enriched at the cathode interface in the films immediately after spin coating and thermally annealing the films led to an increase in the concentration of PSBTBT at the surface. However, if an electrode was evaporated onto the surface of the film initially, thermal annealing was found to increase the concentration of PCBM at the electrode (cathode) interface [Fig. 16(d)]. GIXD and SANS showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating and that thermal annealing increased the ordering of PSBTBT and caused a segregation of the PCBM [Fig. 16(e)], forming domains 10 nm in size, leading to an improvement in photo voltaic performance. Studies on PCBM diffusion within PSBTBT/PCBM bilayer films showed that PCBM can only diffuse between the crystalline domains of PSBTBT with thermal annealing, similar to that seen with P3HT. More recently, Leclerc and coworkers synthesized a new polymer by combining dithieno[3,2-b:2 0,3 0 -d]silole and thieno[3,4-c] pyrrole-4,6-dione units with a PCE of 7.3% obtained PART B: POLYMER PHYSICS 2012, 50,

16 FIGURE 15 (a) IPCE spectra of polymer bulk heterojunction solar cells composed of P3HT:C61-PCBM before (dotted red line) and after (solid red line) annealing, and PCPDTBT:C71-PCBM with (solid green line) and without (dotted green line) the use of 1,8-octanedithiol. The AM 1.5G reference spectrum is shown for reference17 (black line). TEM image of films cast from PCPCTBT/C71- PCBM with additives: (b) none, (c) 1,8-octanedithiol. (d) Schematic depiction of the role of the processing additive in the self-assembly of bulk heterojunction blend materials. (e) GIWAXS data of neat PCPDTBT films cast from chlorobenzene with 3% DIO in sector plot form with the positions of the two different alkyl chain stacking peaks highlighted. Reprinted from Refs. 91, 93, and 95, with permission from Nature Publishing Group, American Physical Society and Wiley-VCH. Reynolds and coworkers further explored the dithienogermole (DTG)-containing conjugated polymer. A 7.3% efficiency was obtained in an inverted device. 103,104 Diketopyrrolopyrrole-Based Polymer Recently, conjugated polymers based on diketopyrrolopyrrole (DPP) have been studied extensively for the optoelectronic applications including both organic field effect transistors (OFETs) and OPVs. Owing to the planar conjugated bicyclic structure, which leads to strong p p interactions and high crystallinity, DPP-based conjugated polymers exhibit high hole-mobilities and high power conversion efficiencies. The DPP unit is also a strong electron withdrawing group, thus DPP-based polymers usually have a low band gap, which is of significant interest for OPV applications. Janssen et al. systematically studied the DPP-based polymers and their device performance. They first reported a DPP-quaterthiophene polymer. When processed from a chloroform/dichlorobenzene solvent mixture, a 4% PCE was obtained. 105 The low band gap property of this polymer effectively extended the EQE to around 900 nm [Fig. 17(a)], arising from its better absorption. They also synthesized polymers composed of DPP and terthiophene and thiophene-benzene-thiophene (TPT) units (Fig. 2). 106,107 The device performance was improved to 5.5%. For the PDPP-TPT polymer, when its blends (with PCBM) were spin coated from chloroform, large (>200 nm wide) fullerene clusters were seen. This macroscopic phase separation led to poorer efficiencies, 2%. However, when a small amount of DIO was added, the phases became more uniform and nanofibrillar structures developed [Fig. 17(b d)]. Interestingly, when the concentration of DIO exceeded 25 mg/ml the efficiency decreased. Woo et al. studied the effect of bridged molecules of DPPbased polymer. 108 When furan was incorporated into the conjugated polymer as a bridge molecule, the solubility of polymer was dramatically increased. They also used processing additives to develop the BHJ morphology. When a mixture of the polymer and PCBM was spin coated with chlorobenzene, macroscopic phase separation, similar to that seen with DPP-TPT, was observed. However, with the addition of 9% of 1-chloronaphthalene to the chlorobenzene, a nanoscale phase separation was observed and 5% efficiency was achieved [Fig. 17(e,f)] Many other DPP-based polymers have since been developed. For example, Huo et al. developed a series of DPP polymers with various electron donating moieties. 109 A 4% efficiency was obtained without additive processing. Janssen et al. developed thienothiophene based DPP polymer, the power conversion efficiency was enhanced to 5.4%. 110 It is worthwhile to mention that DPP-based polymers are also promising candidates for OFETs. It is common PART B: POLYMER PHYSICS 2012, 50,

17 FIGURE 16 (a) Structures of the head-to-head dimers. Binding energies (green) are given in kcal mol 1. Distances are shown in A. (b) Out-of-plane and (c) azimuthal scan (at q(100)) X-ray profiles of PSBTBT and PSBTBT/PC70BM films. These X-ray profiles were extracted 2D GIXD patterns measured at an incident beam angle of (d) SANS profiles of the PSBTBT:PCBM blend films. (e) DSIMS profiles for PSBTBT:PCBM blend systems: (A) As-spun, (B) Preannealed at 150 C for 1 min, and (C) Postannealed at 150 C for 1 min followed by removal of Al electrodes. Reprinted from Refs. 99 and 101, with permission from Wiley-VCH. to see these polymers with hole-mobilities of 1 cm 2 /V s, due to their high crystallinity. 111,112 Thienothiophene-Benzodithiophene-Based Polymer Yu et al. synthesized a series of polymers with thieno[3,4- b]thiophene and benzodithiophene alternating units, termed PTB1 to PTB7. 113,114 A PCE of 7.4% was achieved for BHJ solar cells based on PTB7/PC 71 BM blends. 115,116 The PTBx series have the unique property that the chains order on the substrate in the face-on orientation. GIXD images showed a strong out-of-plane (OOP) (010) reflections for the neat polymer, which markedly contrasts the (010) orientation seen for P3HT and other conjugated polymers used in the solar cells. 117,118 This unique property arises from the rigidity and planarity of the PTBx backbones. Recently, the molecular orientation distribution of PTB7 was determined by polarizing light absorption spectroscopy and GIXD data (DeLongchamp and coworkers.), which showed that the crystallinity of PTB7 in the blend with PC 71 BM is lower than the neat PTB7; only 20% of the polymer was ordered in the blend. 119 Yet, this face-on orientation still dominated in the out-of-plane direction with the interchain p p stacking of the backbones orienting normal to the substrates, which resulted in the high efficiencies. The device performance significantly increased with the addition of DIO to the casting solution [Fig. 18(a)]. The crystallinity and orientation of PTB7 did not change significantly, but the morphologies of the thin films differed if processed with or without DIO. AFM and TEM images showed more smooth surfaces and finer mixing after using DIO, corresponding to a decreased domain size [Fig. 18(b,c)] PART B: POLYMER PHYSICS 2012, 50,

18 FIGURE 17 (a) Spectral response of pbbtdpp2:[60]pcbm solar cells processed from different media. (b d) TEM images of PDPPTPT:[60] PCBM layers processed with 0, 25, and 200 mg ml 1 DIO added to CHCl 3. (e f) AFM phase images of 1:3 PDPP2FT:PC71BM blend films spincoated (e) from chlorobenzene only and (f) from chlorobenzene þ 9 vol % CN. Inset: Height images of the same films. The data scale is 0 60 nm. Reprinted from Refs. 105, 107, and 108, with permission from Wiley-VCH and American Chemical Society. X-ray scattering showed a complex hierarchical morphology, composed of domains of PTB7 crystals, crystalline aggregates, and amorphous mixtures of PTB7 and PCBM mixture domains, which was thought to give rise to the enhanced device performance [Fig. 18(d f)]. A variety of chemical structures of novel donor polymers, including backbone and alkyl chain architectures, can be used to tune the band gap of the materials. While each polymer shows its own unique properties and optimization conditions will vary from polymer to polymer, there are still FIGURE 18 (a) J-V curves of PTB7/PC71BM devices using DCB only, DCB with 3% DIO, CB, and CB with 3% DIO as solvents. The structure of PTB7 is shown in inset. (b, c) TEM images of PTB7/PC71BM blend film prepared from CB without (b) and with (c) DIO (the scale bar is 200 nm). (d, e) The 2D GIWAXS patterns of the thin films of PTB7/PC61BM/CB, PTB7/PC61BM/CBþDIO. (f) RSoXS profiles (open symbols), the calculated scattering intensities I(q) (solid lines). Reprinted from Refs. 114, 115, and 118, with permission from American Chemical Society and Wiley-VCH. PART B: POLYMER PHYSICS 2012, 50,

19 FIGURE 19 (a) Topographical AFM image of a spun PFB:F8BT film, using xylene as the solvent. (b) 15 lm 15 lm F8BT composition map of the 150 nm thick film. Highlighted are the wider F8BT rich areas that show little variation in surface features yet a marked decrease in F8BT concentration. The scale bar is 5 lm. (c) Energy level diagrams of various donor and acceptor polymers. (d) I-V curves and detailed photovoltaic data of all-pscs based on PT1/PC-PDIs spin-coated from different solvents. (e) AFM height images of the surface of PT1/PC-PDIs spin-coated from different solvents (dimensions: 3 mm 3 mm). Reprinted from Refs. 123, 124, and 128, with permission from American Chemical Society and Wiley-VCH. several common parameters that emerge, including the miscibility of the polymer and PCBM, the solubility of polymer or PCBM in each solvent component, and the interactions between polymer, PCBM and the substrate. The degree of crystallinity, the growth habit of the crystal, the orientation of the crystal, and the segregation of the PCBM determine the details of the thin film morphology and significantly influence device performance. All Polymer BHJ Solar Cells All-polymer OPVs, where an n-type semiconducting polymer, instead of fullerene derivatives, is used as the electron acceptor, is another interesting BHJ system. Compared to polymer-fullerene blends, all-polymer OPVs are less efficient. Yet, this approach has some intrinsic advantages. First, conjugated polymers have a much high absorption coefficient in comparison to modified fullerens. Second, synthetic polymers are much easier to obtain with well-tuned energy levels. A main drawback of all-polymer systems is the tendency to form large scale domains, microns in size, due to the size of the polymer chains [Fig. 19(a)]. 120,121 Although several promising high electron mobility polymers have been synthesized, 122 all-polymer solar cells still lags behind in efficiency. The morphology of PFB:F8BT blends and TFB:F8BT blends are well-studied model systems, in which large scale of phase separation has been shown. 123,124 McNeill et al. characterized the TFB:P8BT using STXM and gave a detailed description of the phase composition, which showed an effective way to study all-polymer solar cell morphology [Fig. 19(b)]. 124 For a device standpoint, Holcombe et al. reported that the OPV device using POPT as an electron donor and CNPPV as an electron acceptor in bilayer structure exhibited a 2% PCE. 125 Although the bilayer device yielded a low J sc 1036 PART B: POLYMER PHYSICS 2012, 50,

20 FIGURE 20 (a) Chemical structure and AFM phase images of the fullerene-attached diblock copolymers. (b) TEM images and SAED patterns (inset) of F3T4-HP (1,4), F4T6-HP (2,5), and F5T8-HP (3,6) thin films after thermal annealing (1-3) or solvent-vapor annealing (4-6). Reprinted from Refs. 133 and 134, with permission from Royal Society of Chemistry and American Chemical Society. due to the limited active area in the bilayer structure, a high V oc of 1 V indicated that the use of suitable n-type semiconducting polymers could further enhance the performance of OPV devices. Koetse et al. reported MDMO-PPV-based all-polymer solar cells with a PCE of 1.5%. 126 McNeill et al. fabricated P3HT:F8TBT BHJ devices with efficiencies of 1.8%. 127 More recently Tajima and coworkers optimized the performance of all-polymer BHJ devices using perylenedimide (PDI)-based polymers as the electron acceptor. 128 A large number of PDIbased acceptor polymers were synthesized, among which they found carbazole-containing PDI-based polymer yielded superior performance (>1% PCE). This work clearly demonstrated the ability to tune the polymer acceptor electronic structure [Fig. 19(c)]. To further optimize the device performance, the solvent was optimized and a solvent mixture of toluene/chloroform (9/1) was found to be effective, with a PCE of up to 2.3%. This enhancement was mostly ascribed to the optimized morphology of the blends [Fig. 19(d,e)]. Besides polymer blends, block copolymer (BCP) architecture were also investigated as the active layer in OPVs. Block copolymers can microphase separate into microdomains, tens of nanometers in size, where the size of the microdomains is dictated by the molecular weight of the BCP. An ideal morphology of the active layer of an OPV device would be a nanostructured bicontinuous morphology with the size of the electron donor and electron acceptor domains being comparable to the exciton diffusion length (10 nm). 120 By varying the volume fraction of the components, morphologies including hexagonally packed cylindrical microdomains, alternating lamellar microdomains, and Gyro(ig) morphologies can be obtained. These morphologies would be ideal for the active layer in OPVs, provided the orientation of the microdomains and the time required for microphase separation and orientation of the microdomains can be made compatible with processing strategies. Unlike BCPs comprised of flexible polymer chains, the polymers comprising the blocks, that is, the donor and acceptor blocks, tend to be much more rigid and gaining control of the microdomains orientation can be challenging. Hadziioannou and coworkers studied fullerene-oligophenylenevinylene, which is the early stage of BCP photovoltaics. 129 Thelakkat and coworkers studied the P3HT-poly(perylene bisimide acrylate) diblock copolymer and its PV response. 130,131 Emrick and coworkers also visited P3HT-perylene bisimide diblock copolymers. 132 A typical example of the use of BCPs in the active layer of OPVs can be found in the work of Tajima and coworkers. They developed a Diblock copolymers of P3HT with one block having pendant fullerene [Fig. 20(a)]. 133 Single component solar cells were fabricated. After thermal annealing at 130 C, the block copolymer film showed a lamellar microdomain morphology with lamellae 20 nm in width. The OPV device made from this single block copolymer had a 1.5% PCE and was thermally stable for more than 80 h. The block copolymer approach can be extended to better-defined multifunctional oligomers. Bu et al. described a block-oligomer single molecule OPV. 134 After solvent-vapor treatment of F5T8-HP, a lamellar microdomain morphology was observed with a center-to-center distance between the microdomains of 10 PART B: POLYMER PHYSICS 2012, 50,

21 FIGURE 21 (a) A TEM of a cross section of a 100-nm-thick film consisting of 40 wt % 7 nm by 60 nm CdSe nanorods in P3HT reveals that most nanorods are partially aligned perpendicular to the substrate plane. (b) TEM micrograph of hyperbranched nanocrystal-p3ht blend, scale bar, 20 nm. (c) TEM image of CdSe tetrapods. Scale bar, 50 nm. (d) Absorption spectra of films of pure CdSe tetrapods (dot-dashed, blue), PCPDTBT (dotted, red), and their blend (solid, black). (e) TEM images of P3HT/CdS hybrid films synthesized using grafting process by solvent exchange. The inset images show schematic representations. (f) The photovoltaic performance summary (J SC and V OC ) of P3HT/CdS after the chemical grafting and ligand exchange, as a function of the CdS weight concentration. Reprinted from refs. 135, 137, 146, and 147, with permission from American Association for the Advancement of Science and American Chemical Society. nm was formed. OPV devices based on oligo(fluorene-altbithiophene)-perylene diimide (F5T8-HP) had a 1.5% PCE [Fig. 20(b)]. Polymer-Inorganic Hybrid Solar Cells Polymer-inorganic hybrid solar cells are another important area of polymer based-opvs. Many inorganic materials, for example CdSe, ZnO, and TiO 2 are good acceptor materials with high mobility that can be naturally used to replace PCBM in BHJ blends. Alivisatos et al. demonstrated that CdSe nanorods could be used as the acceptor in BHJ OPVs, which significantly increased the choice of acceptor materials. For CdSe and other related semiconductors, the band gap can be turned by controlling the nanorod radius, which can be used to optimize the absorption of the material. The proof-of-concept device showed a 1.7% efficiency under A.M. 1.5 solar radiation. 135 For CdSe to form an interconnecting network, usually a large amount of CdSe is required. For this purpose, branched and hyperbranched CdSe nanocrystals were developed and were found to improve device efficiency. 136,137 A 1038 PART B: POLYMER PHYSICS 2012, 50,

22 FIGURE 22 (a) Schematic process flow for the fabrication sequence for bulk heterojunction solar cells with surface-relief gratings. (b) Schematic comparison of conventional flat bulk-heterojunction cell and AAO-assisted nanoimprinted cells. (c) Raman spectra of P3HT:PCBM-based solar cells imprinted at different pressures. (d f) GIXD spectra of P3HT/PCBM photoactive layer films: reference photoactive layer film before thermal annealing (d), thermally annealed reference photoactive layer film (e), a nanoimprinted photoactive layer film with thermal annealing (f). Reprinted from refs. 148, 149, and 150, with permission from American Physical Society and Wiley-VCH. few nanocrystal:polymer blend morphologies are shown in Figure 21(a c). The morphology of these hybrid blends, actually nanocomposites, can be optimized by mixed solvent casting processes or by vapor annealing. Huynh et al. found that the use of a binary solvent mixture in which one solvent is a ligand to nanocrystals could help the dispersion of the nanocrystals inside polymer matrix. 138 By using a benzene- 1,3-dithiol chemical vapor annealing, Wu et al. found that ligand exchange and phase separation occurred, which was responsible for efficiency improvement. 139 Other nanocrystals were also used to fabricate hybrid devices, for example silicon nanocrystals, CdS, ZnO, GaAs, and PbS Conjugated polymers other than P3HT were used to enhance the absorption. For example, the low band gap polymer PCPDTBT:CdSe tetrapods blends generated a power conversion efficiency of 3.2% [Fig. 21(c,d)]. 146 The low band gap polymer largely enhanced the absorption properties of the blends, thus leading to an efficiency increase. More recently, Gradecak et al. combined P3HT nanowires and CdS quantum dots and obtained an efficiency of 4.1%. 147 This improvement was done by binding CdS onto P3HT nanowires by grafting and ligand exchange. This also avoided a macroscopic phase separation, as shown in Figure 21(e,f) which improved electronic interaction between donor and acceptor was achieved, leading to a critical enhancement of short circuit current. Nanofabricated Solar Cells It is evident that controlling the morphology of the active layer is critical for the optimization of solar cells performance. Ideally, donors and acceptors would form two distinct cocontinuous phases where the average size of the domains will be tens of nanometers in size. It is highly desirable for the materials to spontaneously form the domains of the right size with external intervention, that is, a bottom up type approach. The desired small size of the domains generates a large interface area, but there are some challenges that must be met, including the inherent disordering, the unfavorable domain size and dead ends of the donor and acceptor domains. Therefore, top down strategies have also been considered where a BHJ morphology is produced by use of a template. The ideal structure of the BHJ morphology is shown in Figure 1(d). The donor and acceptor domains are aligned normal to the electrodes. Yet, there is still a thin layer of donor material covering the anode interface and a layer of acceptor material contacting the cathode interface. By use of a template, the donor and acceptor materials can be forced to separate into domains of the right size; the charges have straight pathways to the electrodes; the dimension of the domains can be designed to ideally suited for exciton diffusion and charge separation; the polymer chains can be aligned and oriented to favor the light absorption and charge transport. Nanoimprinting lithography (NIL) was first applied on the solar cells to produce periodic line-patterned structures or surface relief gratings (SRGs) in the active layer. Kim et al. used a damage-free soft lithography and a PDMS mold to stamp gratings on the P3HT/PCBM layer [scheme show in Fig. 22(a)]. 148 The gratings with a period and height of PART B: POLYMER PHYSICS 2012, 50,

23 FIGURE 23 (a) Schematic procedure of imprinted PV device fabrication. (1) Patterning of P3HT film spin-cast on a ITO/glass substrate coated with PEDOT/PSS as anode by SANIL using a Si mold. (2) Using the patterned P3HT as a mold to imprint a F8TBT film spin-cast on a Al cathode on a Si wafer or Kapton substrate, resulting in a double-imprinted PV device. (b) A photograph of flexible double-imprinted P3HT/F8TBT PV devices with six 2 4 mm pixels based on PET and Kapton polyimide films. (c) Three-dimensional and cross-sectional view of the imprinted PV device configuration. (d) AFM images and lines traces of 100 nm wide posts in P3HT film and (e) 100 nm wide holes in F8TBT. Reprinted from Ref. 155, with permission from American Chemical Society. and 20 nm, respectively, were successfully transferred, so more light could be scattered and absorbed within the active layer without increasing its thickness. The efficiency was increased from 3.6% for the reference cell to 4.1% by introducing the periodic structures. Other molds, like Si pyramids. 149 and anodic aluminum oxide (AAO) membranes have been used as the molds [Fig. 22(b)], 150 which are harder, have well-defined structures and smaller feature size. Improved efficiencies can be achieved by ordering and orienting P3HT chains during the imprinting process, as demonstrated by Raman spectroscopy and GIXD [Fig. 22(c f)]. NIL changes the thin film morphology of the blend and it affects the packing and orientation of P3HT chains due to the confinement imposed by the mold and by flow of the polymer into the mold. Detailed discussions have been provided by Hu et al. and Ocko et al. 151,152 P3HT gratings or pillars were fabricated by NIL using Si molds. The out-of-plane and in-plane GIXD results were scribed in great detail. It is evident that the imprinted P3HT thin films assumed different chain alignment and orientation from the flat thin films. Hu claimed that the P3HT chains assumed a vertical alignment within the nanopillars and nanogratings, and p p stacking of the chains was found along the grating direction. However, Ocko et al. claimed that the face-on orientation was enhanced by the imprinting in the nanogratings and the P3HT backbones were aligned along the grating direction. Both authors claimed that the interfacial energy, that is, the interactions of P3HT with mold walls, is one of the driving forces to induce the chain alignment. Nanoimprinting techniques was also used to fabricate bilayer structured solar cells. Schmidt-Mende et al. used the AAO membranes to pattern the surface of a P3HT layer and then spin-coated a PCBM layer on top of this using orthogonal solvent, that is, a solvent that dissolved the PCBM but not the P3HT. 153 The roughness of the interface increased the interfacial area between the donor and the acceptor, which led to increased device efficiency. Carter et al. used a PDMS stamp and solvent-assisted soft stamp-based NIL (SASSNIL) to prepare P3HT/PCBM bilayer solar cells and showed a device efficiency almost twice that of the planar bilayer devices. 154 A double nanoimprinting process was developed by Huck and Friend and coworkers. 155,156 Here, a prepatterned Si mold was used to imprint the first layer of conjugated polymers (usually P3HT) and then the resultant layer was used as mold to imprint the second layer to form nano-pattern (Fig. 23). This method worked well for polymer/polymer and polymer/pcbm systems. The feature size of the interpenetrating columns can be as small as 25 nm with an areal density of columns as high as mm 2. For polymer polymer devices, the efficiency was 50% higher than the blend control cell. The authors attributed it to the small domain size and large interfacial area produced by the double imprinting process. Template-based methods are alternate routes to pattern the BHJ of solar cells with the ordered structure. Anodic aluminum oxide (AAO) membrane is the most frequently used template for this purpose. To fabricate polymer nanorods by AAO template, first the polymer thin film is contacted with AAO membrane; then by annealing above the melting 1040 PART B: POLYMER PHYSICS 2012, 50,