Structural Factors That Affect the Performance of Organic Bulk

Perspective pubs.acs.org/Macromolecules

Structural Factors That Affect the Performance of Organic Bulk Heterojunction Solar Cells Koen Vandewal,* Scott Himmelberger, and Alberto Salleo* Department of Materials Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, California 94305, United States

ABSTRACT: The performance of polymer:fullerene solar cells is strongly affected by the active layer morphology and polymer microstructure. In this Perspective, we review ongoing research on how structural factors influence the photogeneration and collection of charge carriers as well as charge carrier recombination and the related open-circuit voltage. We aim to highlight unexplored research opportunities and provide some guidelines for the synthesis of new conjugated polymers for high-efficiency solar cells.

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al.5 Research efforts by several groups have led to the schematic picture of the morphology of polymer:PCBM solar cells shown in Figure 1.6−9 In addition to the nanoscale pure phases of aggregated PCBM and polymer, there is a mixed phase present, consisting of an amorphous polymer:PCBM solid solution. The length scales, relative amounts of the different phases, and the stoichiometric composition of the mixed phase are strongly dependent on the chemical structure and regioregularity of the polymer and can be tuned to a certain extent by altering thin film deposition techniques. In this Perspective, we review ongoing research on the relationship between the microstructure and the efficiency of the opto-electronic processes determining the photovoltaic action in polymer:fullerene bulk heterojunction devices. We start by discussing the processes responsible for the generation and collection of charge carriers upon illumination, and how they are affected by structural factors. We continue by discussing charge recombination and the resulting open-ciruit voltage (Voc). Even though Voc is considered to be mainly affected by the chemical structure of the polymers used, we show that it is also dependent on degree of polymer and fullerene aggregation and interfacial area. Finally, we pay special attention to the influence of the polymer molecular weight. Indeed, even for the same polymer, different molecular weights can yield vastly different photovoltaic performances. With this perspective article, it is our intention to indicate new research directions and provide some guidelines for the synthesis of new conjugated polymers for high efficiency solar cells.

INTRODUCTION The steady increase in power conversion efficiency (PCE) of organic solar cells in recent years has generated a major stimulus to the synthesis of new semiconducting polymers. A standard procedure for testing their photovoltaic performance is to blend them with the most popular electron accepting fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Photovoltaic cells using such blends as active layers typically yield PCEs varying between 1% and 9%. In addition to device architecture, absorption spectrum, and position of the energy levels with respect to vacuum, it is commonly accepted that a substantial part of this span of PCE values is related to differences in active layer microstructure. Indeed, this can be directly assessed by observing large differences in photovoltaic performance obtained when the microstructure of a given donor−acceptor system is tuned by the casting solvent,1 thermal or solvent vapor annealing,2 or by using solvent additives.3 An understanding of the relationship between the presence of various functional groups in the polymer repeat unit and the resulting microstructures under certain casting conditions would provide useful guidelines for the further development of new semiconducting polymers for photovoltaic applications. However, such relationships are extremely complex and difficult to predict a priori. Additionally, it is not fully clear which microstructural features are important for optimizing the range of optical and electrical processes necessary for efficient photovoltaic performance, which occur on a large distribution of length and time scales. Moreover, for commercial applications it is desirable that the optimum microstructure be easily obtained using high throughput printing techniques and remain sufficiently stable, ensuring a long lifetime of the organic photovoltaic cell. Morphological characterization of photovoltaic polymer:fullerene blend films has been one of the focus points in the field and was recently reviewed by Gomez et al.4 and Wuest et © XXXX American Chemical Society

Received: May 3, 2013 Revised: July 24, 2013

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dx.doi.org/10.1021/ma400924b | Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Schematic representations of the polymer:PCBM microstructure. (a) A representation of the three different phases present in P3HT:PCBM active layers: P3HT crystallites and PCBM clusters coexist with an amorphous mixture of P3HT and PCBM. Relevant length scales are indicated. Reproduced with permission from ref 6. (b) A similar microstructure was reported for new high performing conjugated polymer:fullerene mixtures. The use of solvent additives allows tuning of the relative amounts and sizes of the phases present in the blend. Reproduced with permission from ref 7. Copyright 2011 Wiley-VHC Verlag GmbH & Co. KGaA.

Figure 2. Schematic representation of the processes responsible for photocurrent generation upon light absorption at donor:acceptor interfaces. Photon absorption is followed by the formation of interfacial CT states with a yield ηex, in competition with pure-phase exciton decay. The formed interfacial states dissociate with a yield ηdiss, determined by the competition between CT state decay (geminate recombination) and dissociation into free charge carriers. Once free carriers are formed, they can recombine by nongeminate recombination, re-forming an interfacial state, or be transported to the electrodes with a yield ηtrans.

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STRUCTURAL FACTORS INFLUENCING THE PHOTOCURRENT The photocurrent produced by a solar cell under solar illumination depends on the yield at which incoming photons are converted to electrons flowing in the external circuit, i.e., the external quantum efficiency (EQE). The EQE is in turn proportional to the fraction of incoming photons absorbed (A) and the yield at which the photogenerated excitations are converted to an electrical current. The latter is denoted internal quantum efficiency (IQE), which reaches 90−100% in the best organic solar cells.10−12 EQE = A ·IQE

(CT) state can occur. This process competes with the process whereby charge carriers move away from each other in the donor and acceptor material phases. Escaping this first, geminate recombination step occurs with a yield ηdiss. Once free carriers are generated and on their way to the electrodes, they can still encounter carriers of opposite charge, re-forming a CT state, and decay. The yield of free carriers that avoid this nongeminate recombination process and instead make it into the external circuit (ηtrans) will therefore depend on the probability of electrons and holes meeting. The efficiency of this process is influenced by the interfacial donor−acceptor area. The IQE can be written as

(1)

IQE = ηex ηdissηtrans

Several competing electron transfer processes determine the IQE. Excitations can decay or transfer an electron from the excited donor to the fullerene acceptor (or vice versa when exciting the acceptor), a process occurring with a yield ηex. After this initial electron transfer step, the electron and hole are in different materials, but their wave functions still overlap, and as a result the decayor recombinationof this charge transfer

(2)

Of the processes described above, the one that limits the total EQE depends on the polymer:fullerene material combination used. All of them are influenced by microstructure and will be described in detail below. Over the past several years, effort has been put into developing experimental B


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the aggregates and the fraction of aggregates and amorphous polymer as a function of the deposition solvent could be determined.35 It was shown that a morphology with a low degree of intra- and interchain disorder resulted in an increase in hole mobility and a concurrent overall improvement in charge carrier generation and extraction efficiency. Since such optical models were developed for the quite general case of H-aggregates comprising parallel-aligned, cofacially packed, weakly coupled conjugated chains, they have been shown to be applicable to other conjugated polymers.36,37 For the new, more complex push−pull donor polymers, which currently yield the highest photovoltaic performance, it is, however, not clear yet if a similar, simple analysis of the optical absorption spectrum can be used to infer information about the film microstructure. It should be noted that the fraction of aggregates determined optically does not necessarily correspond to the fraction of crystallites detected with the use of X-ray diffraction techniques.32,38−40 Only if an aggregate is composed of enough segments will it produce discernible diffraction peaks. There can, however, always exist aggregates that are too small to diffract X-rays but, based on their photophysical properties, would still be identified as being part of a nonamorphous region of the film.

techniques that can distinguish and quantify the yield for each of these processes individually. Measurements of optical parameters combined with optical modeling allow for the determination of A(E) as a function of wavelength or photon energy E.13,14 The competition between the electron transfer process and the luminescent decay of pure polymer excitations results in a quenching of the polymer photoluminescence and gives a measure of ηex. For most polymers, the addition of even a few percent of fullerene quenches their emission more than 95%, indicating that the electron transfer process is very efficient. The reason for such a high quenching efficiency becomes obvious when considering the presence of mixed phases and small domains of pure polymer (Figure 1). Techniques to distinguish the geminate recombination process from the nongeminate, free carrier recombination process have been developed in recent years, and the absolute yields ηdiss and ηtrans as a function of electric field can now be quite accurately determined.15−18 Sometimes, new donor polymer:fullerene combinations are limited by field-dependent geminate recombination, resulting in poor fill factors and a low photocurrent.19−21 The best organic polymer:fullerene solar cells, however, have a very efficient and field-independent free carrier generation efficiency. These devices are only limited by the competition between free carrier recombination and transport, i.e., nongeminate recombination. The microstructural factors determining why some donor−acceptor combinations have such an efficient free carrier generation,18,22 while others do not,15−20 are not yet fully clear. Moreover, it seems to be difficult to find systems that produce a high open-circuit voltage, while keeping a high ηdiss efficiency.23,24 Therefore, we provide a summary of the status of the research on each of the processes described in Figure 2, with a special focus on the role of microstructure.

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INFLUENCE OF MICROSTRUCTURE ON THE FREE CARRIER GENERATION YIELD The exact mechanism of charge generation at donor−acceptor and specifically polymer−fullerene interfaces is a topic of active research and a source of debate. The process will strongly depend on the interfacial energetic landscape41 and the coupling between the involved electronic states. Both properties are strongly affected by the microstructure and molecular conformation at the interface. A direct probe for interfacial energetics and donor−acceptor interactions is provided by the radiative transitions between interfacial charge transfer (CT) states and the ground state occurring with a weak oscillator strength. Sensitive optical absorption and emission spectroscopy42−44 reveal the related spectral bands below the optical gap of donor and acceptor. The spectral position and energy of the CT state (ECT) in a particular donor−acceptor combination corresponds in a first approximation to the difference of the highest occupied molecular orbital (HOMO) of the donor molecule and lowest unoccupied molecular orbital (LUMO) of the fullerene acceptor. However, it has become clear that microstructural factors such as donor−acceptor interactions and distances as well as molecular orientation and packing can cause ECT to drastically deviate from this simple approximation. For example, increasing the degree of P3HT aggregation by annealing45 or preaggregating P3HT in solution using good solvent/bad solvent mixtures46 causes ECT to drop by ∼0.2 eV in P3HT:PCBM blends. This change in ECT is accompanied by a similar shift in the onset of oxidation, as evidenced by cyclic voltammetry measurements performed on P3HT:PCBM films directly.47 Additionally, it has been shown that when blending an amorphous PPV polymer with PCBM, ECT decreases with increasing PCBM content.48 This trend is followed by a change in the PCBM electron affinity of ∼0.1−0.2 eV49 but is not observed when PCBM is replaced with fullerene derivatives with a lower tendency to aggregate.48,49 This trend suggests that not only the aggregation of the polymer but also aggregation of the fullerene lowers the CT state energy. All

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INFLUENCE OF POLYMER MICROSTRUCTURE ON THE ABSORPTION SPECTRUM The functionalized fullerenes most commonly used for organic photovoltaics have an optical gap of ∼700 nm.25,26 The C60 derivates are only weakly absorbing at higher photon energies, while the C70 derivatives can absorb a large fraction of the light in ∼100 nm thick films.27,28 Harvesting of photons at wavelengths longer than 700 nm, however, can only be performed by polymers that can be tailored to be strongly absorbing, with an absorption onset tunable over a wide range of wavelengths. The class of so-called push−pull polymers has been shown to be very successful in this respect.29 Even though the absorption spectrum is usually considered to be determined by the chemical structure of the absorbing species, the microstructure can have a surprisingly large influence on the absorption spectrum, offering additional opportunities to control it and match it to the solar spectrum.30 Detailed studies on films and solutions of poly(3-hexylthiophene) (P3HT) in its regioregular form have shown that the aggregates and amorphous regions have different absorption spectra, with the absorption onset for the aggregates being redshifted by almost 0.5 eV as compared to the absorption onset of amorphous P3HT.31 A rigorous model for the absorption spectrum of the aggregates allows an easy and fast way to investigate the influence of the preparation and annealing conditions on the microstructure by simple UV−vis absorption measurements.32−34 Using this model in films of both pure P3HT and P3HT blended with PCBM, both the “quality” of C


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changes in ECT caused by such microstructural factors are largely followed by changes in open-circuit voltage (Voc), which is discussed in a paragraph below. Cyclic voltammetry (CV) and optical experiments suggest that the optically excited states and hole and electron states on the polymer and the fullerene have a lower energy when residing on the aggregates than on the amorphous phases. When considering a three-phase model as depicted in Figure 1, this observation implies that the CT excitons generated in a mixed amorphous polymer:amorphous PCBM phase have a higher energy than the interfacial states at the aggregated polymer:aggregated PCBM interface. It has been proposed that this energetic offset provides a driving force to enable spatial separation of electrons and holes.49,50 Such a tendency of PCBM to both finely intermix with polymers to quench excitons and form small aggregated domains with a higher electron affinity may be one major reason as to why PCBM is still one of the most successful fullerenes in organic photovoltaics. This effect of intermixing and aggregation on the energetic landscape of polymer:fullerene junctions is summarized in Figure 3.

For polymers, however, it remains unclear how to control their interfacial orientation and distances with respect to the fullerenes. Techniques for probing these issues are just beginning to emerge.54 It should be noted that determining the relative orientation of molecules at the interface is an extremely challenging characterization problem due to the similar nature of the donor and acceptor species (both carbonrich molecules) and the disordered nature of the blend.

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OVERCOMING NONGEMINATE RECOMBINATION Even though the highest efficiency polymer:fullerene solar cells achieve IQEs in excess of 90%,55 EQEs remain near 70%, indicating that a significant portion of the incident light is not being absorbed. Optically thick organic photovoltaic devices would increase A(E) and ease large-scale fabrication constraints. However, a high IQE can only be maintained if the charge carriers are still able to reach their respective electrodes without recombining. A compromise is required between the need to have high interfacial area for efficient exciton quenching and the presence of large domains for efficient transport and increased carrier lifetime. To date, only a few examples exist of polymer:fullerene solar cells that maintain high charge carrier collection efficiencies in thick devices (>250 nm).56−59 For example, high fill factors and photocurrents have been reported for thick fluorinated benzotriazole-containing polymers.56,57 These good device characteristics seem to be quite insensitive to large morphological changes and device thicknesses. The low miscibility of PCBM in this material is proposed to reduce the nongeminate recombination pathways. Because of the nanostructured nature of the polymer:fullerene blends, direct measurement of transport and recombination rates with a meaningful spatial resolution has so far not been possible. As a result, one has to resort to device models to obtain quantitative information on these parameters and their physical origin. Indeed, state-of-the-art electrical models are able to reproduce experimental I−V curves by considering transport through a density of states with a significant number of localized states in a band tail below the transport energy, with recombination occurring between a free carrier and a carrier trapped in the band tail (Figure 4a and 4b).60,61 The structural changes associated with various forms of processing have been shown to have a significant effect on the shape of this band tail and therefore the recombination rate.62 Additionally, disorder in polymer packing has been shown to introduce localized states into the band gap of the polymer.63 The paracrystalline disorder (g) can be measured by X-ray diffraction, and its broadening effect on the density of states in the band tail is shown in Figure 4c. In blends of the high-efficiency PCDTBT:PCBM system, thermal annealing reduced the π−π stacking order and broadened the band tail, resulting in the movement of hole traps deeper into the band gap.64 These changes worsened hole transport, resulting in increased recombination and a decrease in both the Voc and fill factor. Further work is needed in controlling structural order in polymer:fullerene blends and determining its precise effect on the density of states and resulting transport and recombination mechanisms. Most polymer:fullerene blends exhibit diffusion-controlled Langevin-type recombination, in which the recombination rate is limited by the probability of electrons and holes encountering each other. The Langevin recombination rate is proportional to the sum of the mobilities of the electron and hole. One welldocumented exception to this rule is the P3HT:PCBM blend,

Figure 3. Schematic representation of the proposed effect of PCBM aggregation on the energy landscape at the amorphous polymer− fullerene interface. The aggregated PCBM phase has a deeper LUMO energy level than the amorphous, intimately mixed polymer:PCBM phase. This provides a driving force for electrons to reside in the more ordered PCBM phase. Reproduced with permission from ref 50. Copyright 2013 Wiley-VHC Verlag GmbH & Co. KGaA.

An issue that has not been studied in great detail yet in polymer:fullerene solar cells is how molecular orientation and distance between the molecules at the heterojunction impacts interfacial energetics and charge separation kinetics. Rates of charge separation and recombination depend on the electronic coupling between states, which vary significantly depending on the interfacial molecular configurations.51 Several studies have been performed on small molecule systems, where effects of molecular orientation on the interfacial energy levels52 and photovoltaic properties have been demonstrated.53 While a face-to-face orientation was found to yield improved charge transfer over and edge-to-face orientation, it also results in a faster decay rate of the CT state, which can compete with the dissociation process into mobile charge carriers. D


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Figure 4. (a) Representation of the density of states occupation. Charge carriers are trapped in the conduction band tail (CBT) and valence band tail (CBT). Their occupation distribution is given by the position of the quasi-Fermi levels. (b) Recombination at the interface occurs via a free carrier and a carrier trapped in a band tail. (c) Influence of paracrystalline disorder parameter (g) on the density of tail states, calculated by a tight binding model. Reproduced with permission from refs 61 and 63. Copyright 2011−2012 American Physical Society.

required for a continuously percolating network of holetransporting material.69,70 This work has reopened the question of which morphologies are suitable for efficient charge collection and highlights the need for more research into how these cells work.

which switches from Langevin-type recombination behavior in unannealed blends to a significantly lower, non-Langevin, recombination rate in annealed films. This property allows for the fabrication of highly efficient P3HT:PCBM films having thicknesses in excess of 200 nm59 and has been attributed to the change in morphology which occurs during annealing, notably a well-defined phase separation and the formation of ordered crystalline domains in both polymer and fullerene, which decreases the probability for electrons and holes to meet and recombine. There has been some progress in developing new materials that exhibit reduced free carrier recombination; however, a clear morphological origin for the non-Langevin behavior cannot always be determined.65 Clearly there are opportunities in developing new non-Langevin materials and uncovering the structural mechanisms behind this behavior. Since domain purity and size are considered to have a major impact on transport,66 the miscibility of the fullerene with newly designed polymers for organic photovoltaics is of great importance. If the fullerene is too miscible, it will result in efficient exciton quenching but poor charge separation. However, if the fullerene is not miscible enough, domains may be too large, and excitons may not reach an interface before decaying. The tendency of the fullerene to form aggregates in the blend is determined not only by the chemical nature of the fullerene itself but also by the polymer with which it is mixed. For example, PCBM is significantly more miscible in poly(3-hexylselenophene) (P3HS) than it is in P3HT; however, by making small changes to the fullerene side chains, the fullerene miscibility can be tuned by several orders of magnitude in order to achieve a balance between the factors discussed above.67 Dramatic changes in BHJ miscrostructure and PCBM miscibility in a donor polymer can also be caused by subtle modifications to the polymer backbone. The addition of a fluorine atom to the backbone of a low-bandgap polymer increased the tendency for the polymer chains to aggregate, lowering PCBM miscibility and reducing bimolecular recombination.68 Even though a small amount of fullerene mixed into a polymer is considered to be detrimental for charge transport, leading to the trade-off described above between miscibility and transport/recombination, the opposite situation, i.e. a small amount of donor in the fullerene, does not seem to affect hole transport dramatically. It was recently shown that efficient devices with good charge extraction can be made even with very low donor concentrations (∼5%), below the threshold

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STRUCTURAL FACTORS INFLUENCING THE OPEN-CIRCUIT VOLTAGE The influence of polymer nanostructure on the photovoltage has not been considered in much detail until recently. The open-circuit voltage (Voc) has been shown to be mainly determined by molecular properties, i.e., the difference between the highest occupied molecular orbital of the donor (HOMO(D)) and the lowest unoccupied molecular orbital of the acceptor (LUMO(A)).71−73 Therefore, engineering of the molecular energy levels of polymer74,75 or fullerene76,77 is a valuable way to increase Voc and the overall power conversion efficiency of polymer:fullerene photovoltaic devices. A more detailed analysis clarifies the influence of microstructure on Voc: As Voc is determined by the density of charge carriers at open circuit,78 it will be maximized when the number of electron−hole recombination pathways is minimized. Since in polymer:fullerene solar cells the electrons move in the fullerene and the holes in the polymer, a recombination event will occur at the polymer−fullerene interface. The interfacial area79 and energy of the interfacial CT states ECT will thus determine Voc.80,81 In principle, the nanostructure influences both interfacial area and energetics at the interface. It has been shown that Voc under solar illumination relates almost linearly to ECT, and therefore, the influence of aggregation on the interfacial energetics discussed above is directly reflected in the Voc. This dependence implies that even though aggregation might have a positive influence on the photocurrent, by lowering the energy of the aggregated more delocalized sites as compared to the amorphous sites, it will also lower the photovoltage. The effect of interfacial area on Voc has been less explored. It has been proposed that decreasing the amount of interfaces or electron−hole recombination centers could increase the Voc by up to 200 meV.79 Other interfacial properties, such as the conformation of the molecules at the interface and the resulting coupling of the CT state to the ground state, have also been proposed to affect Voc. However, the extent to which coupling will have an effect on the overall recombination rate is strongly dependent on the recombination rate-limiting step. If the rate is E


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molecular weight have been studied in some detail, new effects are being discovered, such as a significant effect of molecular weight on polymer texture.93 Additional research is needed to uncover more of these structural changes and their origins and to clarify their effects. Many materials properties have been found to be strongly molecular weight dependent, before reaching a plateau once the molecular weight is sufficiently high to ensure chain entanglement and reach “polymer”-like properties.85 It seems reasonable that since charge transport is one of these properties which plateaus, polymer packing, disorder, and band structure would stabilize as well.63,94

limited by the time for the electron and hole to meet, rather than the lifetime of the formed CT state, changes in donor− acceptor coupling will not have large effects on Voc. Microstructural effects on Voc have not been investigated extensively yet and present another opportunity for improving our understanding of the mechanisms governing the PCE of organic solar cells.

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INFLUENCE OF MOLECULAR WEIGHT The effect of molecular weight on polymer structure and photovoltaic blend morphology is a topic that deserves special attention. Polymers with the same chemical structure but different molecular weight can exhibit drastically different performance. Nonetheless, this parameter is often overlooked when testing new polymers for PV applications. For instance, the charge carrier mobility in P3HT as measured in field effect transistors increases by several orders of magnitude as molecular weight is increased.82 While low molecular weight polymers are typically found to have more ordered crystalline regions, this increase in mobility is widely attributed to an increase in intergrain connectivity in high molecular weight systems83 and results in improved photovoltaic efficiency due to better charge carrier collection.84 Increased molecular weights have many beneficial impacts on polymer structure, which have resulted in much-improved device efficiencies. Polymer absorption red-shifts with increased molecular weight due to the increase in conjugation along the chain backbone.85 While this effect sometimes results in a slightly lowered Voc due to a decrease in the electrical and optical gap of the polymer, such a drawback is often outweighed by a better matching of the polymer absorbance to the solar spectrum as well as increased collection efficiency due to improved charge transport within the heterojunction. Additionally, a reduction in chain ends upon increasing molecular weight will result in a reduction in traps, which is beneficial for both charge transport and exciton dissociation.85 The influence of molecular weight on blend morphology has also been explored. PCBM is more miscible in low molecular weight polymer fractions, resulting in a finely intermixed microstructure. While beneficial for exciton quenching, these domains are too intimately mixed for efficient charge separation and extraction, resulting in significantly higher recombination rates than higher molecular weight fractions, which take on a slightly more well-defined phase segregation.86,87 Molecular weight has also been demonstrated to affect the phase behavior of P3HT/PCBM blends, resulting in a less P3HT-rich eutectic composition as the molecular weight is increased and changing the P3HT:PCBM ratio at which optimal performance is achieved due to the resulting differences in blend microstucture.88 While higher molecular weights are most often linked to improved photovoltaic device performance, this trend is not always observed. In the case of very high molecular weights, polymer chains become entangled, inhibiting the ability of the material to order.89 Such disorder is accompanied by a drop in the hole mobility of the polymer and the formation of large PCBM aggregates which are unable to penetrate the entangled polymer microstructure. Further, higher molecular weight polymers are less soluble and may be more difficult to process.90 The aforementioned trends in molecular weight have been demonstrated to be important in new materials systems as well.86,91,92 While many morphological and structural effects of

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CONCLUSION AND OUTLOOK The importance of polymer microstructure and polymer:fullerene blend morphology has been recognized since the field of organic photovoltaics emerged. In this Perspective we discussed the microstructural effects on the optoelectronic processes responsible for production of photocurrent and photovoltage. Attaining the most suitable morphology is a challenging task as it involves optimizing processes at different length and time scales. The development of multiscale device models95−97 will therefore be of great importance, linking the functionality and performance of a certain device architecture with molecular scale processes, tunable by chemical structure modifications. For the best polymer:fullerene solar cells, there seems to be a consensus that the morphology consists of a significant fraction of mixed amorphous phase next to phases of pure material. It is not yet clear if the mixed phase is absolutely necessary for a high photovoltaic performance, but it has been demonstrated that it does not need to be detrimental. Furthermore, we summarized arguments based on energy level alignment that show that it may be beneficial to have an amorphous mixed phase. Even though the optimum miscibility of PCBM in the polymer is unknown and probably varies from polymer to polymer, it can be altered by tuning the polymer backbone and side chains, offering new opportunities for materials engineering. Because of the presence of the mixed phase, exciton quenching in polymer:fullerene blends is only very rarely a problem. In poorly performing devices, most of the photoexcitations are lost at the interface by geminate recombination and not in the donor phase. In the best organic solar cells, free carrier, nongeminate recombination is currently limiting the photocurrent. Therefore, a future task is to link the blend microstructure to the density of states and the related transport and recombination mechanisms. Identifying and reducing recombination pathways will allow for the production of thicker films which hold advantages for light absorption as well as large-scale fabrication techniques. The interfacial area available for electrons and holes to encounter each other is likely an important parameter in determining the rate of free carrier recombination. A reduced interfacial area can reduce this encounter probability and therefore decrease the total recombination rate. Techniques to quantify the interfacial area and compare it from device to device would be very useful for the characterization and further understanding of the competition between charge transport and recombination. Special attention must be paid to the molecular weight of the polymer, as it has been shown to have a large effect on the microstructure of blend films and resulting optical and electrical properties. For a sufficiently high molecular weight though, these properties seem to saturate. Therefore, in order to allow F


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for more accurate and meaningful comparisons across processing techniques and different polymer systems, it is best to choose molecular weights in this saturation regime, exhibiting the true “polymer” properties of the materials. We have highlighted a few research opportunities that can lead to further increases in the photovoltaic performance of organic solar cells, prepared using large-scale, high-throughput deposition techniques.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.V.); [email protected] (A.S.). Alberto Salleo received his Laurea degree in Chemistry from the University of Rome (Italy) in 1994. He received his M.S. (1998) and Ph.D. (2001) in Materials Science from UC Berkeley investigating optical breakdown in fused silica. He spent 5 years at the Palo Alto Research Center as a postdoc and then a member of the research staff in the Electronic Materials Laboratory before joining the Department of Materials Science and Engineering at Stanford University in 2005, where he is currently an Associate Professor. He is the recipient of the NSF Career Award, the 3M Untenured Faculty Award, and the SPIE Early Career Award.

Notes

The authors declare no competing financial interest. Biographies

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Foundation, the Center for Advanced Molecular Photovoltaics (Award No. KUS-C1-015-21), made by King Abdullah University of Science and Technology (KAUST) and the Department of Energy, Laboratory Directed Research and Development funding, under Contract DE-AC02-76SF00515. S.H. thanks the National Science Foundation for support in the form of a Graduate Research Fellowship.

Koen Vandewal received his PhD in Physics from Hasselt University, Belgium, in 2009 under the direction of J. V. Manca. He spent 2 years as a postdoctoral researcher in the Biomolecular and Organic Electronics group at Linkoping University, Sweden, led by O. Inganas. Currently, he is a postdoc at Stanford University in A. Salleo’s group. His research interests include charge carrier generation and recombination processes in organic optoelectronic devices.

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REFERENCES

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Scott Himmelberger graduated from UC Davis in 2010 with a B.S. in Chemical Engineering. He is currently working towards a Ph.D. in Materials Science & Engineering at Stanford University under the guidance of A. Salleo. His research interests include structure/property relationships in semiconducting polymers for use in large-area and flexible electronics. He is the recipient of a NSF Graduate Research Fellowship. G


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