Investigation of Nanostructured Organic Solar Cells with Transmission Electron Microscopy

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1 Investigation of Nanostructured Organic Solar Cells with Transmission Electron Microscopy Golap Kalita, Koichi Wakita and Masayoshi Umeno Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai-shi , Japan Author address: Phone no.: Fax no.: Organic photovoltaic devices (OPVs) based on conjugated polymers (donor) and fullerene or fullerene derivatives (acceptor) is one of the most researched topics from the last decade. OPVs have the great potential as the future generation of solar cells with abundant materials, simple manufacturing process and flexibly. Bulk heterojuction OPVs are most efficient system with homogenous mixing of conjugated donor and acceptor. Solar cells performances are highly dependent on the solid state nanoscale morphology of the donor and acceptor in the photoactive layer. The proper understanding and characterization of nanostructure morphology is most important to optimize high device performance. Transmission electron microscope (TEM) is an important tool to investigate the nanoscale morphology which is discussed elaborately by giving suitable examples. Keywords: Organic solar cells, transmission electron microscopy, nanostructured morphology 1. Donor acceptor based organic solar cells The discovery of ultra fast photo induced charge transfer at donor- acceptor interface has brought lot of interest in OPVs based on conjugated polymer and fullerene (C 60 ) or C 60 derivatives. [1,2] Donor-acceptor based organic solar cells fabricated with either spin-coated or printed offer an alternative to the conventional inorganic solar cells with such potential advantages as low-cost, ease of manufacturing, non-toxicity, and lightness [3-5]. The idea behind a heterojunction of donor and acceptor is to use two materials with different electron affinities and ionization potentials. In heterojuction organic solar cells the donor acceptor interface is most important, the resulted potentials at the interface are strong enough to favor exciton dissociation. With separation of charges the electron will be accepted by the material with the larger electron affinity and the hole by the material with the lower ionization potential, provided that the differences in potential energy are larger than the exciton binding energy. [6,7] Separated charges are transported through the donor and acceptor material and collected at the electrode to obtain a photo-generated current completing the photovoltaic process. The following figure (1) presents a schematic view of donor acceptor based solar cells with corresponding energy level of donor, acceptor and electrodes as well as the charge transfer process. e - e - Light ev -3.7 ev Glass/plastic Cathode Anode Donor:Acceptor -4.2 ev -4.8 ev ev -6.1 ev h + Anode Donor Acceptor Cathode Figure 1 Schematic diagram of a donor-acceptor based organic solar cell and corresponding energy band diagram presenting the charge (electron and hole) transportation In a donor-acceptor based solar cell sunlight is absorbed and excites the donor molecule, leading to the creation of excitons. The created excitons start to diffuse (diffusion length 10 nm) within the donor phase and if they encounter the interface with the acceptor then a fast dissociation takes place leading to charge separation (hole and electron). [8,9] Subsequently, the separated free electrons and holes are transported with the aid of the internal electric field towards the cathode and anode, respectively where they are collected by the electrodes and driven into the external circuit. 354 FORMATEX 2010

2 Microscopy: Science, Technology, Applications and Education A. However, the excitons can decay, yielding e.g. luminescence, if they are generated far from the interface. Thus, the excitons should be formed within the diffusion length of the interface. Since the exciton diffusion lengths in organic materials are much shorter than the absorption depth of the film, this limits the width of effective light-harvesting layer. [10-13] In figure (2) the most widely investigated conjugated polymer as donor and fullerene derivative as acceptor is presented. (6,6)-phenyl-C61-butyric acid methyl ester Poly (3-hexylthiophene) Figure 2 Representation of conjugated polymer poly (3-hexylthiophene) fullerene derivative PCBM. A revolutionary development in OPVs came in the mid 1990 s with the introduction of the bulk heterojunction, where the donor and acceptor material are blended together [14]. If the length scale of the blend is similar to the exciton diffusion length, the exciton decay processes is dramatically reduced since in the proximity of every generated exciton there is an interface with an acceptor where fast dissociation takes place. Hence, charge generation takes place everywhere in the active layer. Now the important issue to carry this generated charges to the respective electrode so that no charges recombine again. With existence of continuous pathway in each material from the interface to the respective electrodes, the photon-to-electron conversion efficiency and, hence, the photosensitivity is dramatically increased. This determine by how the donor and acceptor material is mixed and the nanostrucutred morphology. The ordered polymer chain can collect hole to the anode and nonostructured fullerene percolation path can take the electron to the cathode. In the following figure (3) a sketch of an OPV device with nanostructured morphology of donor and acceptor in between the anode and cathode is presented. Figure 3 Sketch of OPV devices with nanostructured morphology of donor and acceptor in between the anode and cathode. 2. Transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique, where a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of FORMATEX

3 the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, photographic film, or to be detected by a sensor such as a charge coupled device (CCD) camera. TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de Broglie wavelength of electrons. This enables the instrument to be able to examine fine detail; even as small as a single column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope. TEM forms a major analysis method in different scientific field to investigate in nano scale order. The TEM consists of an emission source, which may be a tungsten filament, or a lanthanum hexaboride (LaB6) source. For tungsten, this will be of the form of either a hairpin-style filament, or a small spike-shaped filament. LaB6 sources utilize small single crystals. By connecting this gun to a high voltage source (typically ~ kv) the gun will, given sufficient current, begin to emit electrons either by thermionic or field electron emission into the vacuum. This extraction is usually aided by the use of a Wehnelt cylinder. Once extracted, the upper lenses of the TEM allow for the formation of the electron probe to the desired size and location for later interaction with the sample. Manipulation of the electron beam is performed using two physical effects. The interaction of electrons with a magnetic field will cause electrons to move according to the right hand rule, thus allowing for electromagnets to manipulate the electron beam. The use of magnetic fields allows for the formation of a magnetic lens of variable focusing power, the lens shape originating due to the distribution of magnetic flux. Additionally, electrostatic fields can cause the electrons to be deflected through a constant angle. Coupling of two deflections in opposing directions with a small intermediate gap allows for the formation of a shift in the beam path, this being used in TEM for beam shifting, subsequently this is extremely important to scanning TEM (STEM). From these two effects, as well as the use of an electron imaging system, sufficient control over the beam path is possible for TEM operation. The lenses of a TEM allow for beam convergence, with the angle of convergence as a variable parameter, giving the TEM the ability to change magnification simply by modifying the amount of current that flows through the coil, quadrupole or hexapole lenses. The quadrupole lens is an arrangement of electromagnetic coils at the vertices of the square, enabling the generation of a lensing magnetic fields. Typically a TEM consists of three stages of lensing. The stages are the condensor lenses, the objective lenses, and the projector lenses. The condensor lenses are responsible for primary beam formation, whilst the objective lenses focus the beam down onto the sample itself. The projector lenses are used to expand the beam onto the phosphor screen or other imaging device, such as film. Imaging systems in a TEM consist of a phosphor screen, which may be made of fine ( µm) particulate zinc sulphide, for direct observation by the operator. Optionally, an image recording system such as film based or doped YAG screen coupled CCDs. Typically these devices can be removed or inserted into the beam path by the operator as required. Detailed discussion on the TEM instrument, its operation and capabilities and its usage is out of scope of this chapter and the readers are requested to see some of the dedicated books on the transmission electron microscopy. [15-18] For investigation of naostrucutred organic solar cells, TEM is one of the important tools to have very good idea about the morphology. The formation of domain structure with acceptor and donor materials is most important in organic solar cells to obtain maximum device performance. TEM study can provide us in depth knowledge of the nanostructured morphology of the active material in solar cells, which enable to fabricate device with optimize device structures. 3. Importance of TEM in investigation of organic solar cells Let us see how important TEM observation in nanostructured morphology of the donor acceptor based organic solar cells. As discussed previously, C 60 and its derivatives were used as electron acceptor material due to its high electron affinity in bulk heterojunction solar cells. The morphology of donor acceptor based solar cells are dependent on different parameters of device fabrication such as solvent, solubility, annealing and fabrication process etc. In the beginning of bulk heterojunction solar cells C 60 molecules directly blend with the polymer, but solubility of C 60 in some of the organic solvent is poor. How the poor solubility influence on the no scale morphology is discussed in the following. As C 60 shows poor solubility, C 60 derivatives with better solubility are used in blend. Improved in solubility of C 60 derivative (PCBM), allowed the incorporation of a larger fraction of C 60 into polymer blend films. The same holds for the conjugated polymers, which become only soluble due to side-chain addition. Thus it is clear that the part of the chemical structure that provides the solubility is an important parameter for reaching well-blended bulk heterojunctions of the constituents at the nano scale. Another factor is the chemical compatibility between the polymer and the C 60 is thermal annealing as required for better efficiency, which resulted in a phase separation of nano ordered between polymer and C 60. Now the nano scale phase separation of the polymer and C 60 in the blend is most influencing parameters which control the device performance. [19-23] Solvent processing and annealing of the donor-acceptor blend material determine the domain structure formation. In the blend film, PCBM molecules may diffuse and form larger aggregates with an at least partially crystalline structure. Thereby, the polymer domains also form a more ordered phase. For example: whilst poly[2-methoxy-5-(3,7 - dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) : PCBM blend films yielded generally the best results when spin cast from chlorobenzene, this solvent could not deliver similar good results for an MDMO-PPV : C 70 -PCBM blend. It has been shown that changing the fullerene to C 70 -PCBM required the use of ortho-dichlorobenzene instead of 356 FORMATEX 2010

4 Microscopy: Science, Technology, Applications and Education A. chlorobenzene to reach a small scale of phase separation. Thus chemical compatibility can be readily tuned by changing the solvent, and C60 blend system and its phase separation. It has been observed that with direct addition of C60 Figure 4 Transmission electron micrographs of P3OT/C60 and P3OT/plasticizer/C60 blends. Due to the plasticizer the average C60 domain decreased drastically in size (N. Camaioni et al.[24]). to P3OT solvent there are large aggregate of C60 due to less solubility, as shown in figure 4. To overcome the largescale phase separation in P3OT-C60 blends plasticizers were added. Figure 4 shows a TEM image of the P3OT-C60 blend with plasticizers to increase the compatibility between the two components, as reported by N. Camaioni et al. [24]. From image it is quite clear that the size of the C60 crystallites was reduced by a large extent and more homogeneous blends resulted. So the explanation on the nano scale morphology show the importance of the TEM study and how we can have the idea of phase separation and domain formation in OPVs blend film. [24,25] In the figure (5) a TEM image of well blend poly (3-hexylthiphene) and PCBM is presented. The dark spot in the image present PCBM domain which are formed through out the blend film. The other feature of the image is fiber like structures, which are crystalline P3HT. Now the TEM image is discussed in a pictorial presentation as shown in the figure to have a better perception. The PCBM in the polymer composite after the fabrication film with necessary processing thshows domain formation in nano scale order which are surrounded with polymer fibers. The formation of highly ordered region-regular P3HT fibers and nano scale PCBM domain help better photo exciton dissociation and charge transportation and thereby improve device performance can be achieved. [26,27] FORMATEX

5 Figure 5 TEM image of a polymer and PCBM composite film showing crystallization of the two components Schematic diagram of phase separated blend of polymer and PCBM. (X. Yang et al.et al. [24]). For better understanding of nanostructured order and domain formation let us discuss in a pictorial representation. Figure 6 presents a pictorial representation of large domain acceptor (dark red), with larger domain there will not be efficient charge separation. The diffusion length of exciton form in the donor materials is of the order of 10 nm; hence the exciton formed far away from the acceptor domain can not diffuse to interface of acceptor and for free charges. The acceptor domain large aggregates in certain are of the blend and not properly distributed to have efficacy charge transportation. As well as larges aggregate of the acceptor facilitate recombination of separated charges. Figure 6 presents a pictorial representation of small domain acceptor (dark red), well distributed through out the blend film. The small domains provide large surface area and well distributed within the donor to have efficient charge separation. The small domain can be within the diffusion length of the excition and can provide the interface for dissociation in separate charges. As well as, the nano scale acceptor domain can form percolation path for efficient electron transportation thought the blend,thereby achieving charge separation with out recombination. In the following, the nanostructured morphology of the acceptor in P3HT:PCBM solar cells and how electron percolation path formation can help in device performance is elaborately discussed. Figure 6 A pictorial presentation of domain size for the acceptor (dark red) in the blend film with large domain size and well distributed small domain size for efficient photo-excitation. 4. Nanostructured morphology of P3HT:PCBM solar cells In OPV research, P3HT has been most widely used as electron donor along with PCBM, as electron acceptor to form the photoactive layer of organic solar cells. Such type of organic solar cells performance is increasing with every passing years due to better understanding of working principle, device structure and nonostructured morphology of the photoactive layer. Here, we discuss about nanostructured morphology of the particular P3HT:PCBM system due to its immense interest in scientific community. The nanostructured morphology of the P3HT: PCBM composite layers have great influence on the device performances. In recent works, it has been reported about the relationship between the nanostructured morphology of the organic photoactive layer and the performance of the polymer solar cells. The morphology of this layer can be strongly affected by the processing conditions used such as the donor acceptor composition, the solvent and the thermal annealing of the organic layer as discussed earlier. In order to achieve high efficiency organic solar cells there should be increase in mesoscopic order and crystallinity in the P3HT:PCBM bulk heterojunction network. With enhanced crystallinity of the composite, there can be better percolation of charges by reducing internal resistance. Morphology of P3HT: PCBM blend films have been studied by using bright-field TEM images and selected-area electron diffraction (SAED) patterns. As well as, HRTEM and atomic force microscopic studies were done to explore the morphology of P3HT: PCBM composite layers. [28,29] In a study of P3HT:PCBM bulk heterojunction solar cells were fabricated with annealing or unanealing and varying the concentration ratio to observe the differences in morph local structures. Nanostructured morphology of the P3HT: PCBM composite films were analyzed by HRTEM studies to correlate the device performances. It was found that for annealed and unanealed films the morphology is quite homogenous even at high resolution. Whereas, it has been recently been shown that P3HT:PCBM films were composed of homogeneously distributed PCBM nanocrystals. [30,31] In the following figure (7) presents HRTEM image of unannealed and annealed films with different concentration ration of P3HT and PCBM. HRTEM image of the unannealed film with 1:1 weight ratio (Figure 7a) shows almost no crystallization of PCBM, although some atomic planes were observed. On the other hand, when these 358 FORMATEX 2010

6 Microscopy: Science, Technology, Applications and Education A. a c b 2PCBM nm nanocrystal d PCBM nanocrystal PCBM nanocrystal PCBM nanocrystal 2 nm 2 nm Figure 7 High resolution TEM images of P3HT: PCBM film with 1:1 concentration ratio before annealing (unannealed device) 1:0.5 concentration ratio with annealing (c) 1:1 concentration ratio with annealing, formation of PCBM nancrystals and its elongated structure (d) 1:2 concentration ratio with annealing (PCBM nanocrystals were denoted with arrow marks). films were annealed significant transformation in morphology was observed. As shown in figure 7 for annealed film with 1:0.5 concentration ratio of P3HT: PCBM, there are formation of small nanocrystals homogeneously distributed throughout the composite film. There are differences in the brightness in certain regions of the PCBM nanocrystals that are due to depth of the nanocrystals in the P3HT composites. Figure 7(c) presents nanostructure morphology of the P3HT:PCBM composite film with 1:1 concentration ratio. Elongated nanocrystals were clearly visible and these nanocrystals are homogeneously distributed in the composite film. The elongated nanostructures are composed of several nanocrystals having different plane orientations and d spacing was observed to be around 0.42 nm. The formation of elongated PCBM nanocrystals which is not observed in the unannealed and low PCBM concentrated film provides efficient percolation path for the electron. With better electron transportation in the composite film there are improved photocurrent and thereby better device performance. Figure 7(d) shows nanostructured morphology of the P3HT:PCBM composite film with 1:2 concentration ratio. PCBM nanocrystals were observed in these films as well, but in this case much larger nanocrystals were observed. The densities of the nanocrystals are much higher then that of the previous composite films, this due to the fact that PCBM concentration is much higher in this film. Though the density and size of the nanocrystals increase with increase in PCBM concentration, device performance significantly reduced then that of device fabricated with 1:1 weight concentration of P3HT:PCBM. The good performance of the device with 1:1 weight concentration of P3HT:PCBM is attributed to an optimized morphology that enables close intermolecular packing of P3HT chains. Inferior performance for the 1:0.5 compositions is attributed to poorer intermolecular packing with increased PCBM content. These results show clearly that it is very much important to consider the morphology of the PCBM nanocrystals in order to enhance the efficiency of P3HT:PCBM bulk heterojunction solar cells.[32] FORMATEX

7 5. Concluding remarks Importance of transmission electron microscopy (TEM) in investigation of nanostructured organic solar cells is discussed elaborately with different examples. Solar cells performances are highly dependent on the solid state nano scale morphology of the donor and acceptor in the photoactive layer. TEM gives direct insight into nano scale morphology of the donor acceptor composite film. TEM study revel how important the phase separation study in bulk heterojunction solar cells. Phase separation of the donor acceptor materials is analyzed with pictorial representation to have a clear overview on nanostructured morphology. It is the one of the most important and most reliable technique for correctly identifying the nature and formation of blend of different materials in organic solar cells. It can be used for imaging, electron diffraction and chemical analysis of different material in organic solar cells. With different advantages of TEM, it has become a versatile and comprehensive analysis tool for characterizing the chemical and electronic structure at nano scale material systems. References [1] S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258, 1474 (1992). [2] J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Maeseflia, R. H. Friend, S. C. Oratti, A. B. Holmes, Nature 376, 498 (1995). [3] C.J. Brabec, J.C. Hummelen, N.S. Sariciftci, Adv. Funct. Mater. 11, 15 (2001). [4] M. Reyes-Reyes, K. Kim, D.L. Carroll, Appl. Phys. Lett. 87, (2005). [5] Y. Kim, S. Cook, S. M. Tulandhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. McCulloch, C.-S. Ha, M. Ree, Nat. Mater. 5, 194 (2006). [6] G. Kalita, fabrication and characterization of organic and hybrid solar cells, Ph.D. thesis (2010) [7] G. Kalita, S. Adhikari H.R. Aryal, M. Umeno, R. Afre, T. Soga and M. Sharon, Appl. Phys. Lett. 92, (2008). [8] G. Yu, J. hummelen, F. Wudl, A.J. Heeger, Science 270, 1789 (1995). [9] M. Granstrom, K. Petrisch, A. C. Arias, A. Lux, M.R. Andersoon, R.H. Friend, Nature 395, 257 (1998). [10] C. J. Brabec, G. Zerza, G. Cerullo, S. De Silvestri, S. Luzzati, J. C. Hummelen, S. Sariciftci, Chem. Phys. Lett (2001). [11] T. E. Goliber, J. H. Perlstein, Jour. of Chem Phys. 80, 4162 (1984). [12] K. Petritsch, Organic solar cell architectures, PhD. Thesis (2000). [13] B. Kraabel, et al., Phys. Rev. B 50, (1994). [14] G. Yu, J. hummelen, F. Wudl, A.J. Heeger, Science 270, 1789 (1995). [15] D. B. Williams, C. B. Carter, Transmission Electron Microscopy, Plenum Press, New York [16] Z. L. Wang, Elastic and Inelastic Scattering in Electron Diffraction and Imaging, Plenum Press, New York, [17] L. Reimer, Transmission Electron Microscopy, 3rd Edition, Springer Verlag, New York, [18] Z. L. Wang, C. Hui, Electron Microscopy of Nanotubes, Kluwer Academic Publisher, New York [19] H. Hoppe, M. Niggemann, C. Winder, J. Kraut, R. Hiesgen, A. Hinsch, D. Meissner and N. S. Sariciftci, Adv. Funct. Mater. 14, 1005 (2004). [20] C. R. McNeill, H. Frohne, J. L. Holdsworth and P. C. Dastoor, Synth. Met. 147, 101 (2004). [21] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15, 1617 (2005). [22] M. Reyes-Reyes, R.L. Sandoval, A. Alatorre, R.G. Alonso, D.L. Carroll, A. L. Martnez, Thin Solid films 516, (2007). [23] Gag Li, Vishal Shrotriya, Jinsong Huang, Yan Yao, and Yang Yang, SPIE newsroom, (2006). [24] N. Camaioni, M. Catellani, S. Luzzati and A. Migliori, Thin Solid Films 489, (2002). [25] H. Hoppe and N. S. Sariciftci J. Mater. Chem.16, (2005). [26] X. Yang et al., Nanoletters 5, 579 (2005). [27] M.S. Kim Understanding Organic Photovoltaic Cells: Electrode, Nanostructure, Reliability, and Performance, Ph.D. thesis, [28] Z. Uladzimir; E. Tobias, H. Harald, G. Gerhard, N. S. Sariciftci, Thin solid films, 496, (2006). [29] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15, 1617 (2005). [30] E. Kymakis, I. Alexandrou and G.A.J. Amaratunga, J. Appl. Phys. 93, (2003). [31] D. M. Guldi, G. M. A. Rahaman, F. Zerbeto and M. Prato, Acc. Chem. Res. 38, 871 (2005). [32] G. Kalita, K. Wakita and M. Umeno, J. of Nanoscience Nanotechnology 10, , (2010). [33] G. Kalita, M. Masahiro, W. Koichi and M. Umeno, Solid State Electronics 54, , (2010). 360 FORMATEX 2010