Photovoltaic Films

Organic photovoltaic films

by Jenny Nelson

Organic electronic materials are of interest for future The last two years have seen an unprecedented applications in solar cells. Although results for single growth of interest in solar cells made from organic layer organic materials have been disappointing, high electronic materials. This is partly due to the rapid growth of the photovoltaic market1, which has photocurrent quantum efficiencies can be achieved in stimulated research into longer term, more composite systems including both electron donating innovative photovoltaic technologies, and partly to and electron accepting components. Efficiencies of the development of organic electronic materials for over 2% have now been reported in four different display applications. The rapid progress in types of organic solar cell. Performance is limited by optoelectronic molecular materials has introduced a the low red absorption of organic materials, poor range of potential new photovoltaic materials, as well charge transport, and low stability. These problems as an improved understanding of the capabilities of 2 are being tackled by the synthesis of new materials, such materials and confidence in their application . the use of new materials combinations, and Organic materials are attractive for photovoltaics optimization of molecular design, self-assembly, and primarily because of the prospect of high throughput manufacture using reel-to-reel or spray deposition. processing conditions to control morphology. Power Additional attractions are the possibilities for ultra thin, conversion efficiencies of over 5% are within reach, flexible devices, which may be integrated into appliances or but the fundamental physics of organic donor- building materials, and tuning of color through chemical acceptor solar cells remains poorly understood. structure. The field has made impressive progress in the last five years. Solar power conversion efficiencies of over 2% have now been reported for four distinct classes of organic solar cell, a growing range of new photovoltaic materials have been studied, and an increasing number of academic research groups and companies have declared an interest in ‘soft’ solar cells3-5.

Centre for Electronic Materials and Devices, Department of Physics, Organic photovoltaic materials Imperial College of Science Technology and Medicine, Prince Consort Road, London SW7 2BZ, UK Organic electronic materials are conjugated solids where E-mail: [email protected] both optical absorption and charge transport are dominated Reproduced with kind permission of Current Opinions in Solid State and Materials Science (2002), 6 (1). by partly delocalized π and π* orbitals. Candidates for

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photovoltaic applications include crystalline or polycrystalline films of ‘small molecules’ (molecules of molecular weight of a few 100), amorphous films of small molecules prepared by vacuum deposition or solution processing, films of conjugated polymers or oligomers processed from solution, and combinations of any of these with either other organic solids or inorganic materials. A comprehensive discussion of the development of organic solids for photovoltaic applications is given by Halls5. Organic photovoltaic materials differ from inorganic semiconductors in the following important respects. - Photogenerated excitations (‘excitons’) are strongly bound and do not spontaneously dissociate into separate charges. (Dissociation requires an input of energy of ~100 meV compared to a few meV for a crystalline semiconductor.) This means that carrier generation does not necessarily result from the absorption of light. - Charge transport proceeds by hopping between localized states, rather than transport within a band, Fig. 1 Schematic energy-band diagram of a simple device consisting of a single organic which results in low mobilities. layer between two metal contacts. An electric field results from the difference in work functions of the contacts. Absorbed photons generate excitons that diffuse towards one or - The spectral range of optical absorption is relatively other contact where they may dissociate to yield charge pairs. Only the layer of organic narrow compared to the solar spectrum. material, which lies within an exciton diffusion length of a contact, can contribute to the photocurrent. - Absorption coefficients are high (~105 cm-1) so that high optical densities can be achieved, at peak wavelength, with films less than 100 nm thick. These properties impose some constraints on organic - Many materials are unstable in the presence of photovoltaic devices: oxygen or water. - A strong driving force should be present to break up the - As one-dimensional semiconductors, their electronic photogenerated excitons; and optical properties can be highly anisotropic. This - Low charge carrier mobilities limit the useful thickness of is potentially useful for device design. devices; The first two features arise from the fact that the - Limited light absorption across the solar spectrum limits intermolecular van der Waals forces in organic solids are the photocurrent; weak compared to bonds in inorganic crystals and much - Very thin devices mean interference effects can be weaker than the intramolecular bonds. As a consequence, important; all electronic states are localized on single molecules and - Photocurrent is sensitive to temperature through hopping do not form bands. Low mobility is aggravated by the transport. high degree of disorder present in many organic solids. The optical excitations accessible to visible photons are Principles of operation usually π to π* transitions. Most conjugated solids absorb Homojunctions in the blue or green; absorption in the red or infrared is The simplest device structure is a layer of organic material harder to achieve. However, the absorption bandwidth sandwiched between two different conducting contacts, depends on the degree of conjugation, and wider spectral typically indium tin oxide (ITO) and a low work function sensitivity can be achieved in highly conjugated dye metal such as Al, Ca, or Mg (Fig. 1). The difference in work molecules. function provides an electric field that drives separated

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charge carriers towards the respective contacts (in rough analogy to a p-i-n junction in amorphous silicon). This electric field is seldom sufficient to break up the photogenerated exciton. Instead the exciton diffuses within the organic layer until it reaches a contact, where it may be broken up to supply separate charges, or recombine. Since exciton diffusion lengths are short, typically 1-10 nm, exciton diffusion limits charge carrier generation in such a device. Photocarrier generation is, therefore, a function not only of bulk optical absorption, but also of available mechanisms for exciton dissociation. Other loss factors are non-radiative recombination at the interfaces and non-geminate recombination at impurities or trapped charges. Single layer solar cells of this type deliver quantum efficiencies (QE) of less than 1% and power conversion

Fig. 2 Schematic energy-band diagram of a donor-acceptor heterojunction. If both the efficiencies of less than 0.1%. QE is the ratio of electrons excited (LUMO) and ground (HOMO) state of the donor material lie at energies delivered to the external circuit per incident photon of a sufficiently higher than those of the acceptor material, then it is energetically favorable for an exciton reaching the interface to dissociate, leaving a negative polaron on the given wavelength, and is the figure of merit in organic acceptor and a positive polaron on the donor. For efficient photocurrent generation, charge separation (2) should compete successfully with geminate recombination (4) after photovoltaics. High QE is a necessary, though not sufficient, a photon absorption event (1), and transfer to contacts (3) should compete with condition for high photovoltaic efficiency. In organic devices, interfacial recombination (5). the value is still far from the values of 80-90% typical in inorganic solar cells. Heterojunctions and dispersed heterojunctions Most of the developments that have improved performance of organic photovoltaic devices are based on donor-acceptor heterojunctions. At the interface between two different materials, electrostatic forces result from the differences in electron affinity and ionization potential. If both electron affinity and ionization potential are greater in one material (the electron acceptor) than the other (the electron donor) then the interfacial electric field drives charge separation (Fig. 2). These local electric fields are strong and may break up photogenerated excitons, provided that the differences in potential energy are larger than the exciton binding energy. In a planar heterojunction, or 'bilayer' device, the organic donor-acceptor interface separates excitons much more efficiently than the organic-metal interfaces in a single layer device and with very high purity materials efficient photovoltaic devices may be made. A revolutionary development in organic photovoltaics (and Fig. 3 Donor and acceptor materials may be blended together to yield a dispersed photodetectors) came in the mid-1990s with the introduction heterojunction. If the length scale of the blend is similar to the exciton diffusion length, then the probability that an exciton will reach the interface and dissociate is high. For of a dispersed heterojunction, where an electron accepting efficient photocurrent collection, each material must provide a continuous path for the and an electron donating material are blended together. If transport of separated charge to the contacts. Isolated domains can trap charges, causing recombination. the length scale of the blend is similar to the exciton

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Table 1 Key performance characteristics and peak QE for several device designs. For comparison, the characteristics of the best amorphous silicon solar cell are listed. -2 Material system JSC (mA cm ) VOC (V) Fill factor Efficiency (%) Peak QE & Reference wavelength (nm) replace Doped pentacene 7.7 0.90 0.66 4.5 - [15] heterojunction Doped pentacene 5.3 0.97 0.47 2.4 36% at 650 nm [14] homojunction Cu phthalocyanine 13 (?) 0.53 0.52 3.6 18% at 620 nm [17] /C60 bilayer cell 35% at 400 nm MDMO-PPV-PCBM 5.25 0.82 0.61 2.5 50% at 470 nm [18] Dye-sensitized solar 5 0.90 0.56 2.56 38% at 520 nm [21] cell with OMeTAD hole conductor Amorphous silicon 19.4 0.887 0.74 12.7 ~90% http://www.pv.unsw.edu.au/eff Monocrystalline silicon 42.2 0.706 0.83 24.7 >90% http://www.pv.unsw.edu.au/eff

diffusion length, then wherever an exciton is photogenerated studies of issues relating to production, field performance, in either material, it is likely to diffuse to an interface and and stability have been reported. Understanding of device break up. If continuous paths exist in each material from the function and the underlying physical processes remains interface to the respective electrodes, then the separated limited, but progress has been made with the understanding charge carriers may travel to the contacts and deliver of photocurrent generation and charge separation. current to the external circuit (Fig. 3). This effect was Breakthroughs in cell performance reported independently by several groups6-8 for a blend of Power conversion efficiencies of over 2% have now been two conjugated polymers. The blend improved QE to around achieved in the following four classes of device (Table 1): 6-8% from less than 1% for either polymer alone. Around the (1) Researchers at Bell Labs have made single layer and same time, Yu and co-workers reported a QE of 29% for a heterojunction solar cells using doped pentacene single blend of the hole transporter, polyphenylenevinylene (PPV), crystals14,15. These organic crystals have high anisotropic with a derivative of C60, where the C60 acts as the electron minority carrier (electron) mobilities that allow them to be transporting component9. This was followed by observations used in Schottky barrier and p-n bilayer structures, where of enhanced QE in heterojunctions made from conjugated collection is aided by the built-in electric field of a depletion polymers with inorganic nanocrystals10,11 and organic dye region, as in inorganic solar cells. In the single layer structure, crystals12. The demonstration of improved QE with dispersed a vacuum deposited pentacene crystal forms a Schottky heterojunctions represents a departure from the device barrier with an Al back contact and an Ohmic contact with physics of conventional solar cells and has led to new device an ITO window layer. In the bilayer case, the pentacene forms and materials designs. (Dye-sensitized solar cells, however, a heterojunction with a ZnO:Al window and an Ohmic function on similar principles13.) contact with Pt. In this structure, the field-bearing region is located closer to the window, which should assist collection. Recent Developments Doping with I or Br is essential for successful function. The The last two years have seen developments in the synthesis dopants are responsible for exciton dissociation, increasing of new photovoltaic materials; the combination of materials the QE, and improving absorption in the red and reducing into new device architectures; studies of the effect of series resistance. processing conditions and other factors on morphology and (2) Planar heterojunction devices made by vacuum deposition performance; and manipulation of materials at a molecular of thin films of small molecules have been studied by several level, exploiting molecular self-assembly and modification of groups, for application to LEDs as well as solar cells. An interfaces. During this period power conversion efficiencies of impressive result has been achieved at Princeton with a four over 2% have been achieved in four distinct classes of device layer heterojunction, containing wide band gap hole- and QEs of over 20% achieved in several others. Several transporting and electron-transporting ‘window’ layers16,17.

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material (HTM)13. The original design used a redox active liquid electrolyte for hole transport, but a non-volatile HTM is desirable for commercialization. The ideal material should regenerate the photo-oxidised dye quickly and transport holes with high mobility. Candidates for solid state HTMs include doped arylamine based small molecules (OMeTAD)19 and polythiophenes20. A power conversion efficiency of 2.5% using OMeTAD was recently reported21, greatly improving on earlier studies of that material. The improvement is attributed to the suppression of electron- hole recombination at the metal oxide surface through a dipole induced by adsorbed molecules. A number of other device types have achieved QEs comparable with the devices in Table 1, though lower power conversion efficiencies. These include donor-acceptor polymer blend devices with well-controlled morphology22,23, Fig. 4 Some approaches to improved charge separation and collection in organic solar 24 cells. In each figure the direction of photocurrent generation is from top to bottom and three layer donor-sensitizer-acceptor structures , inorganic- the electron and hole accepting components are shaded red and blue, respectively. (a) A organic heterojunctions25, and liquid crystal-crystalline dye compositionally graded blend improves the collection of positive and negative charges near the respective contacts8,22. (b) Elongated electron acceptors such as nano-rods or devices26, all discussed below. dye crystals can improve electron transport30,43, but for efficient collection they should be directed perpendicular to the contacts. (c) Self-organizing discotic liquid crystals are one way of achieving preferential charge transport perpendicular to the contacts26. (d) A light Current challenges absorbing dimer designed to drive positive and negative charges apart after photoexcitation can help to channel respective charges towards electron transporting and Table 1 shows that while organic solar cells produce quite 24 hole transporting layers . respectable open-circuit voltages, the short circuit photocurrent and fill factor are much lower than those These buffer layers function to block excitons from lossy available from inorganic devices. The lower photocurrent is metal contacts and to enhance optical field strength in caused by poorer light absorption, as well as photocurrent photoactive layers via interference effects. generation and transport; the fill factor is due to poor (3) Blends of PPV derivatives and methanofullerenes are a transport and recombination. Most current research is well studied combination and are under intense development therefore focused on the following goals: at Linz4. Photon absorption in the polymer is followed by - Improving light harvesting; electron transfer to the fullerene on a sub-picosecond time - Improving photocurrent generation; scale. Current collection depends on charge percolation - Improving charge transport; through the fullerene network and is, therefore, critically - Addressing manufacturing issues and stability; dependent upon the blend ratio and the degree of phase - Understanding device function and limits to performance. separation. MDMO-PPV and PCBM appear to be a promising Light harvesting materials combination. A recent breakthrough was achieved One strategy is to replace conducting polymers in devices by using chlorobenzene as a solvent in place of toluene, with others that absorb further into the red. In the polymer- leading to QE of over 50% and power conversion efficiency fullerene cell, possible lower band gap replacements for PPV of 2.5%18. The much improved performance is attributed to include polythiophene derivatives27, polypyrrole/thiazadole improved phase separation with chlorobenzene. copolymers28, and thiophene/naphthene copolymers29. (4) Solid state dye-sensitized solar cells (DSSC) are the most Synthesis of new materials with red absorbing moities is promising among organic-inorganic composite devices to underway. An alternative is to replace the electron date. In the DSSC, three active materials are used: an organic transporting polymer in a blend with conjugated dye as light absorber, a nanocrystalline metal oxide film as crystalline dyes, such as anthracene or perylene, with wider electron transporter, and liquid or solid hole transporting absorption bands30.

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Dye sensitization is a different strategy where a monolayer been studied. Meanwhile, organic electron transporters with of a third material, usually an organic dye, is introduced improved mobility and stability are being developed46,47. between donor and acceptor to function as light absorber. Control of morphology Since light absorption and charge transport are carried out by In a dispersed heterojunction device, both photocurrent different materials, the light absorber does not need to be a generation and charge transport are functions of good bulk transporter of charge. Efforts to improve light morphology. Photocurrent generation requires uniform harvesting in DSSCs include development of alternative dyes blending on the scale of the exciton diffusion length, while and combinations of dyes. A similar concept, of an all-organic transport requires continuous paths from interface to donor-absorber-acceptor structure was proposed by Yoshino8. contacts. In polymer-nanocrystal or polymer-fullerene blends, A different approach is to improve the utilization of the concentration of the particulate component should be absorbed photons with light trapping structures. This sufficient for charge percolation11,48. One attractive improves the capture of photons where absorption is weak hypothetical configuration is a set of intersecting electron and allows thinner photovoltaic films to be used, which has and hole transporting channels, directed perpendicular to the advantages for transport. The possibilities for light trapping contacts (this is one motivation for the study of rod-like for organic solar cells are discussed by Inganas31. Some of nanocrystals). Another is a compositionally-graded blend these have been investigated using embossed polymer layers with an excess of donor-type material on one side and for light trapping32 and exploiting interference effects inside acceptor-type on the other. This has been demonstrated by a cavity made from photoactive and transmitting organic Yoshino using vacuum deposited layers8 and by Granstrom layers16,33. Embossed polymer light trapping structures are for a laminated assembly22 (Fig. 4). The concept was taken already used in thin film silicon solar cells. Interference further by Takahashi and co-workers, who reported a QE of effects need to be included in the calculation of light 49% for a three layer structure where electron and hole absorption in organic thin films, and theoretical tools have transporting layers are separated by a heterodimer light been developed for planar structures34,35. absorbing layer24. The absorbing layer is polarized upon light Improving charge transport absorption to drive the charges towards the appropriate Charge transport is limited by the low intrinsic mobilities of transport layers. This is essentially an all-organic version of organic solids, and by the charge trapping effects of the dye-sensitized solar cell. impurities and defects. In several recent studies, high mobility In practice, many materials tend to segregate when polymers such as fluorene-triarylamine and thiophene blended, and much attention has focused on ways of copolymers have been used to replace PPV in blend controlling the morphology of blends. Routes include: devices36,37. Materials with ordered phases offer high, though - Control of blend morphology by processing conditions. anisotropic, mobilities and in this respect liquid crystals26,38 Choice of solvent, atmosphere, and substrate temperature and polymers with ordered phases, such as polythiophenes39, strongly influence the morphology of polymer blends36,37. are interesting. Choice of solvent appears to influence segregation of Because organic electron mobilities are generally very fullerenes in PPV18. poor, an inorganic electron transporting component may be - Self-organization. Self-assembly by discotic liquid 25,40 10 26 preferred. Thin film and dispersed nanocrystalline TiO2 crystals , and by ionically or electrostatically interacting has been used in several approaches. Efficient charge transfer monolayers49,50, has been used to construct structured 10,41 to TiO2 from various polymers has been reported (PPV , heterojunctions. Self-assembled monolayers can also be 25 20 phenyl-amino-PPV , polythiophene ). TiO2 is the most used to modify substrate surfaces to control the widely studied material in such structures, but the segregation of blend components23. sensitization of other oxides such as SnO2 with polymers has - Synthesis of donor (D)-acceptor (A) copolymers (such as a been demonstrated42. Elongated crystalline components are polymer with pendant fullerene groups51) and block attractive as electron transporters if crystal size and copolymers. Positioning D and A groups on the same orientation can be controlled. Blend devices using crystalline polymer backbone can ensure effective photoinduced dyes30, CdSe nano-rods43, and carbon nanotubes44,45 have D→A electron transfer under all conditions and avoids the

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problems of phase segregation. D-A copolymers may be effects of heterojunctions and interfaces on materials, and designed to absorb longer wavelength photons than single the effects of light, bias, and ageing. Further understanding polymers, so improving light harvesting, but charge and the development of appropriate models requires extraction may be more difficult. fundamental experimental studies on model systems. - Use of porous organic or inorganic films as templates (for Here we mention some of the characterization techniques example, see52). relevant to heterojunction solar cells. (Techniques for - Cosublimation of small molecules to form graded D-A characterizing the optical, electrochemical, and transport heterostructures53,54. properties of single polymers are amply described elsewhere.) Performance and production issues Photoinduced absorption is useful for studying the kinetics Several studies of performance and stability of organic solar of electron transfer, i.e. of exciton dissociation and polaron cells have been carried out on polymer-fullerene devices. recombination. Ultrafast spectroscopy has been used to These devices exhibit improved performance with increasing study charge separation in polymer-fullerene structures4, temperature, an advantage over inorganic devices that is polymer nanocrystal blends59, and in donor-acceptor attributed to temperature dependent mobility55. However, copolymers. Nanosecond-microsecond spectroscopy has stability is poor56. Stability is a common problem with proved useful for monitoring the rate of charge conjugated polymers and will need to be addressed by recombination in dye-sensitized devices60 and may be encapsulating cells and by using more stable materials applied elsewhere. Electron spin resonance can be used to (organic dyes, liquid crystals, metal oxide nanocrystals, detect the unpaired spin of polaron states and is useful for and siloles are promising in this respect). Studies of the monitoring the formation and lifetime of charge-separated effects of increasing cell area57 and deposition by screen states61. printing58 show that polymer-fullerene cells can be scaled- Photocurrent generation in planar structures is reasonably up without large losses in performance. In the case of doped well understood and can be interpreted in terms of the filter pentacene solar cells, replacing the single crystal with a effect62, although interference must be accounted for34. In thin film of pentacene degrades efficiencies slightly to 2%, blends, photocurrent generation is a function of morphology, indicating that this technology could be used with flexible as discussed above. Novel characterization techniques have substrates15. been developed to study blend morphology, including One frequent problem with new device designs is spatially resolved fluorescence37 and confocal Raman degradation of QE with increasing light intensity, so that cells spectroscopy63, as well as acoustic force and scanning perform well only under low illumination. Such behavior is a electron microscopy imaging techniques . signature of recombination mechanisms, which may be due The influences on photovoltage are much less well to impurities, and must be eliminated for the cells to be understood. In single layer devices voltage appears to be useful in solar conditions. limited by the difference in work function of the electrode The search for efficient ways to generate interpenetrating materials. In blends, however, the photovoltage can be larger networks, discussed above, is also a manufacturing issue. For than that difference, and appears, instead, to be related to large-scale production, complicated procedures such as the difference in electron affinity of the donor and ionization lamination are not feasible, while self-assembly from a single potential of the acceptor4. This indicates that photocurrent solution is attractive. collection does not require a macroscopic electric field, a Understanding function and experimental techniques situation that is largely agreed to apply to dye-sensitized Understanding of the device physics of organic solar cells is solar cells64. Electroabsorption is useful for measuring still at a primitive stage, compared to inorganic solar cells. electric fields inside organic devices65 and may assist in Important differences are that light absorption is not answering such questions. AC admittance measurements equivalent to photocarrier generation, and that relating device performance to charge accumulation and fundamentally different recombination mechanisms may Fermi level profile66 and optical measurements of dominate in the light and the dark. The subject is recombination kinetics may be equally useful. These complicated by uncertainties in material parameters, the questions remain of active interest.

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Conclusions testing of new photovoltaic materials in established device The progress in organic photovoltaic materials and structures and the development of new structures, where devices in the last two years has been impressive. Power morphology is controlled through self-assembly and conversion efficiencies over 2% have been achieved in four processing conditions. Experience with commercial-scale different device structures, varying from high-quality, devices is still limited, as is the theoretical understanding of vacuum-deposited, multilayer molecular films to dispersed device function. Based on current trends, efficiencies of 5% heterojunctions in spin-cast soluble polymers. All are based appear to be within reach, although stability remains an on the concept of a donor-acceptor system, where obstacle. MT photogenerated excitons are split by forces at the donor- acceptor interface. Higher efficiency requires improvements Acknowledgments The author is grateful to the Engineering and Physical Sciences Research Council and the in absorption of red light, charge transport, and material Greenpeace Environmental Trust for financial support. stability. Recent research has focused on the synthesis and

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