How Far Can Polymer Solar Cells Go? In Need of a Synergistic

Perspective pubs.acs.org/JPCL

How Far Can Polymer Solar Cells Go? In Need of a Synergistic Approach Feng He and Luping Yu* Department of Chemistry and James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States ABSTRACT: The photovoltaic effect in organic materials is an interesting research area because it offers fundamental knowledge, waiting to be explored, and the potential to offer low-cost technology to replace traditional inorganic solar cells. Worldwide research effort in this area is largely motivated by the desire to develop a new technology platform to cost-effectively harvest solar energy. Currently, researchers from different disciplines are focusing on developing new materials, performing physical studies to gain basic understanding of charge separation and transport mechanisms in these disordered soft material systems, and formulating new device structures and processing conditions in order to push the solar energy conversion efficiency above threshold for commercialization. This Perspective reviews some of the work that has been done over the past 20 years and describes the efforts in materials development to move beyond certain milestones. We emphasize the importance of a synergistic approach in developing new materials to continuously enhance the performance of organic photovoltaic cells.

A

in field effect transistors,11 liquid-crystal displays,12 and lightemitting diodes.13 Most recently, the photovoltaic effect attracted attention after sizable power conversion efficiency (PCE) was achieved in organic semiconductor double layers 14 and bulk heterojunction (BHJ) solar cells.4 Solubility in common organic solvents makes reel-to-reel processing of organic materials a possibility and will drastically reduce the cost of electronic devices.

lthough solar energy is abundant, it is still the largest, almost untapped renewable source of energy available to this planet. One main reason that it is not fully utilized is the lack of effective approach to harvest it. Most recently, research effort on solar energy utilization heated up, as evidenced by more investment of the U.S. government via multiple energy frontier research centers (EFRCs) and solar energy hubs. Numerous approaches were proposed to harvest solar energy, including solar fuels, solar thermal energy, and solar cells.1−6 Among these technologies, solar cell converting solar energy into electricity is the most mature technology.7 The solar-panelbased inorganic semiconductors are capable of directly converting absorbed sunlight into electrical current with a range of efficiencies from extremely high (40%) in multilayer single-crystal devices to relatively low (8%) in thin-film devices.8 However, these technologies at the top end did not make a large impact on energy production due to the very high manufacturing cost. Modern commercial solar cells typically have crystalline silicon or another inorganic semiconductor as the active layer. These materials are expensive to produce and fragile, which prevented them from widespread and large-scale deployment. The thin-film technologies recently gained momentum in commercialization because of the reduction in the cost. However, they also face issues in the availability of materials.9 For example, the promising CIGS thin-film solar cells have to use relatively scarce indium as a component, which may impose high cost in the near feature especially when the ITO (indium tin oxide) is extensively used as a transparent electrode in electronic devices and the abundance of indium is limited.10 Organic semiconductors have seen significant developments over the past 20 years. Such materials have found applications © 2011 American Chemical Society

The development of organic solar cells with efficiencies comparable to inorganic cells would have a tremendous impact on energy production. The major advantage of organic materials is their low cost and ease in processing and the possibility for flexible devices. This field of research has been growing steadily for the past 2 decades and has seen an explosion of publications just within Received: November 8, 2011 Accepted: November 22, 2011 Published: November 22, 2011 3102

dx.doi.org/10.1021/jz201479b | J. Phys. Chem.Lett. 2011, 2, 3102−3113

The Journal of Physical Chemistry Letters

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the past 2 years. Although the PCE of organic solar cells is currently still not up to the level for vital commercial applications (>10%) in large areas, this value has been steadily climbing and demonstrates the great potential of organic solar cells as an alternative source of energy.15−22 Different applications are envisioned, ranging from an electricity charger for small electronic devices to decorative windows. Recently, polymer solar cells were employed in conjunction with white-light-emitting diodes, polymer lithium ion batteries, and flexible printed electronic circuits in lighting applications for the ‘‘Lighting Africa’’ initiative.23 Polymer solar cells have been shown to yield energy payback times of 1−2 years and very low CO2 equivalent emission figures of less than 40 g/kW h. As promising as organic materials may seem, they suffer from a few serious drawbacks compared to their inorganic counterparts. First, the hopping mechanism dominates charge carrier transport in these systems, which is hampered by the disordered nature of polymer chains. This necessitates the use of thin-film electroactive layers in the organic photovoltaic (OPV) devices architecture. Having thin films requires that the organic materials possess high absorption coefficients in the visible spectrum. Second, conjugated organic materials are commonly photosensitive and tend to decompose especially after exposure to sunlight for an extended period. Adequate studies to assess long-term stabilities of OPV devices are difficult to perform because each new material has a unique structure and hence its own reactivity upon photoexcitation. The time that it takes to test the stability of each new material would be considerable. Third, the blended active layer that is prepared by mixing the donor polymer and fullerene acceptor molecules is thermodynamically unstable and will undergo phase separation at elevated temperatures. The formation of large domains in this layer is one of the primary causes of poor solar cell performance.3,4 The scientific community is currently investigating these issues so that OPV devices can be commercialized into a mainstream source of electricity. Perspective for OPV Solar Cells with PCE > 10%. The PCEs of a solar cell are determined by the equation η = Pout/Pin = JscVocFF/Pin, where Jsc is the short-circuit current density, Voc is the open-circuit voltage, FF is the fill factor, and Pin is the input power. The Jsc in an OPV cell is strongly influenced by the band gap of the conjugated polymer and fullerene derivatives. The Voc is directly related to the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor molecules used. The FF is controlled by series resistance. Among all OPV cell architectures, the most widely implemented is the so-called bulk heterojunction device. This device employs a layer of mixed donor and acceptor materials, which gives a wide interfacial interaction. It can easily be deposited through simple processing techniques, such as spincoating, roll-to-roll printing, and solution spray to form a large surface area. This type of solar cell is advantageous because it enables the rapid testing of layers with different composition/ deposition conditions and is easier to implement in manufacturing industries.3 The blended active layer can also be prepared by a vacuum deposition technique for small molecular materials.5 The current status in polymer solar cells is that the PCE for a single cell device larger than the 8% was achieved.15−17,19 This indicates that breaching the 10% barrier is possible and may happen soon. It was predicted by Scharber et al. that for energy conversion efficiencies exceeding 10%, the

donor polymer must have a band gap < 1.74 eV and a LUMO level < −3.92 eV, assuming that the FF and the average external quantum efficiency (EQE) remain equal to 0.65. Current trend seems to indicate that new materials are closing in on that goal.24 Challenges that We Are Facing. These results are normally obtained in devices with small areas. It is usually observed that when the area of the devices increases, the efficiency will decrease dramatically. Thus, in reality, the challenge for commercialization is even bigger than one perceived based on small devices. In order to fabricate an organic solar cell device with suitable characteristics for commercialization, there are still major issues that need to be addressed. Among the challenges currently facing researchers include the design and preparation of new highly efficient materials that overcome the drawbacks mentioned above, comprehension of how light generates current in these systems, finding new ways of fabricating OPV devices to optimize the efficiency of the active layer, and developing a complete theoretical model that explains not only how the blend assembles in the active layer but also how current passes through it. Unique Features of Organic Solar Cells. Organic solar cells exhibit fundamental differences from inorganic counterparts based on either the p−n junction or the Schottky barriers. The major differences are in the formation of excitons after light absorption and the charge-transport process. Because organic molecules are discrete entities with weak intermolecular interaction, they do not form energy band structures as in inorganic semiconductors. The excited states reside in each individual molecule. Usually, the frontier orbitals, HOMO and LUMO, are singly occupied after ultrafast internal conversion. The second unique feature is that the charge-transport mechanism in organic semiconductors is via a hopping process, and charge carriers will hop intermolecularly to reach electrodes or to be recombined when encountering an opposite charge. Each hopping step will take a finite time to reoccur, which will lead to low mobility. These features have a pronounced effect in designing organic solar cells. The formation of an exciton using frontier orbitals allows fine-tuning of the energy band gap of the materials, which is beneficial to optimize the efficiency of light harvesting. However, the excitons have a limited lifetime, which calls for measures to ensure that the exciton can be effectively dissociated before it relaxes to the ground state. This issue has been solved in composite materials, namely, bulk heterojunction materials in which the domain structures of each component can be controlled to ensure that the exciton can migrate to the interface of the donor and acceptor domains. The second feature of low mobility, however, is a challenging and intrinsic issue in organic materials. It does not have a rational approach to solve that. One attempted to address that with enhanced π−π stacking between molecules. It worked to a certain degree but also faced issues of solubility. 25 This is an especially intriguing issue in conjugated polymers, which will be elaborated on more in the following discussion. Mechanism of Current Creation in the Active Layer. We need to discuss the operation mechanism of an organic solar cell before we can appropriately describe the properties of polymeric solar cells. In the active layer of a BHJ solar cell, donor polymer and acceptor molecules (either fullerenes or accepting polymers) are intimately mixed to form a film with a large D−A interfacial area (Figure 1).26 As shown in Figure 2, when light is absorbed by the active materials in solar cell devices, five main steps can 3103



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energy loss for BHJ solar cells by Eg − eVoc = ∼0.6 eV, which includes the chemical potential needed for driving charge separation and energy loss brought about by the formation of the exciton and CT complex, as discussed below. This rule is roughly true for many polymeric systems, and the Voc value typically follows this relation for most of the existing polymer/ fullerene systems.17,20,29 Thus, it can be seen that the two parameters are in conflicting demand, and careful balance is needed to optimize the solar cell PCE. Once the exciton is formed, it must diffuse to the interface between the donor and acceptor in order for charge separation to occur. Because the exciton has a finite lifetime before thermal relaxation, the domain size of either the donor or acceptor must be smaller than twice the diffusion length of the exciton to allow it to efficiently reach the interface.30,31 This requires the control in the morphologies of materials. Ideally, the domain size should be large enough for the formation of a continuous chargetransport pathway and small enough for efficient exciton migration and dissociation. For the exciton to undergo dissociation, a proper driving force is needed to overcome the binding energy of the exciton while maintaining a minimum loss of free energy.32 This can be achieved by a suitable potential energy offset between LUMOs of the donor and acceptor.27,33 Once the charges are separated, they are initially bound by Coulombic attraction (V) across the D−A interface to form the CT exciton (CT state). This energy needs to be overcome before the electron−hole pair can break into free charge carriers

Figure 1. Bright field TEM image of the PTB7/PC61BM/DCB + DIO thin film (black lines are added to represent the heterojunctions between PTB7-rich and PC61BM-rich domains). Reprinted from ref 26.

(1)

where e is the charge of an electron, εr is the dielectric constant of the sounding medium, ε0 is the permittivity of vacuum, and r is the electron−hole separation distance. Because organic materials have a low dielectric constant (εr ≈ 2−4), the Coulomb attraction of these charges is very high and spatially very close.32 Zhu and co-workers investigated these CT excitons in crystalline pentacene thin films by using photoelectronic spectrscopy.34 They pointed out that a strong electronic coupling must exist between the molecular excitons in the donor and the CT excitons as both are involved in CT. For the CT state, the Coulombic attraction is represented by the binding energy EBCT.35 For most systems, this energy has been estimated to be in the range of 0.1−0.5 eV, which can only be overcome with sufficient LUMO offset between the donor and acceptor. With sufficient offset, the charges can fully dissociate into separate charge carriers. After the electrons and hole pairs are dissociated into free charge carriers, they will travel through the blend via a hopping mechanism.36 As mentioned above, the hopping events are slow and lead to a low charge carrier mobility. It is known that effective π−π stacking in conjugated polymers can facilitate charge transport. Careful control of the solid-state assembly can help to enhance the charge carrier’s mobility. Fullerene materials have relatively high electron mobility (10−3 cm2 V−1 s−1 measured in the space-charge-limited regime37 or ∼10−1 cm2 V−1 s−1 measured in field effect transistors38). The challenge is to produce a polymer/fullerene blend with balanced mobilities for both electrons and holes. Following this free charge carrier transportation, the charge injection into the corresponding electrodes will experience barriers. Proper modification in the electrode surface can help to enhance charge collection, which will be discussed in detail later.

Figure 2. Operating mechanism of polymer solar cells.

be observed in the process of converting light into current in a OPV device, (1) sun light absorption and the formation of the exciton, (2) exciton diffusion to the interface of the donor− acceptor, (3) electron transfer from the donor to the acceptor to form a charge-transfer (CT) complex, (4) CT complex dissociation into charge carriers (electrons and holes), and (5) the separated charges drifting away from each other until they reach their respective electrodes (normally the ITO anode and aluminum cathode). Each step needs to have a high efficiency to achieve a high PCE in the device. From this simplified mechanism, several requirements can be extracted for designing new semiconducting polymers with optimized solar cell PCE. The OPV material needs to absorb the most intense light in the solar spectrum. This is largely determined by the energy flux distribution in the solar spectrum and commonly accomplished by designing the polymer to have an absorbance that overlaps with the visible and near-IR wavelengths. The solar spectrum at the earth’s surface reaches its peak in the visible spectrum (400−700 nm) and gradually decreases in intensity at longer wavelengths (700−1400 nm). Careful modulation of the band gap (Eg) can tune the absorption of a polymer. According to theoretical calculation based on inorganic semiconductor solar cells, a narrow Eg (∼1.5 eV) produces the best solar spectral coverage. This prediction seems to hold well with PCE in a single-layer OPV.27 It has been demonstrated, however, that Eg directly influences the open-circuit voltage (Voc).28 It is related to the minimum 3104



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Scheme 1. Synthetic Scheme for Polymers PTB1-PTB7

Development of Low-Band-Gap Polymers: A Synergistic Approach. As discussed above, one must consider all of the major factors, such as energy band gaps, driving force, and morphology. A good example is the polymer system developed in our group, PTB series polymers. The polymer system is composed of thieno[3,4-b]thiophene and benzodithiophene alternating units, as shown in Scheme 1.

Because the BHJ OPV is a very complex system, a synergistic approach is needed to develop materials that will lead to high PCE.

Figure 3. Current−voltage characteristics of polymer/PC61BM solar cells under AM 1.5 conditions (100 mW/cm2). Reprinted from ref 16.

exhibit high PCE in the range between 400 and 700 nm with EQE values of over 50%. A maximum EQE of 68.1% at 630 nm was obtained from solar cells based on PTB7/PC71BM (Figure 4a, chlorobenzene (CB)/1,8-diiodooctane (DIO) solvents). This device also exhibits a high internal quantum efficiency (IQE) value over 90% in a wide range of 420−660 nm, and the maximum IQE reaches 92% (Figure 4b, CB/DIO solvents). An interesting observation is that although these composite materials are complex system, they exhibited a rather accurate correlation between the energy level and Voc. For example, the alkyl-substituted PTB3 has an enhanced Voc compared to PTB2. The fluorinated polymer PTB4 devices showed an increase in Voc compared to PTB5. The changes in Voc are well correlated with the HOMO energy levels of polymers, which illustrates the importance of fine-tuning the energy level in order to maximize Voc by selecting an appropriate match between the donor and acceptor energy levels. An empirical equation describing the relationship between Voc and the energy levels of both the donor and acceptor was derived by Scharber and co-workers by examining several polymer systems. This derivation is based on a statistical analysis between Voc and ΔEDA (the energy offset between the donor and acceptor, eq 2).27 Here, e is the elementary charge, LUMO and EPCBM = −4.3 eV as determined by cyclic voltammetry. The value of 0.3 V in eq 2 is an empirical factor.

Photovoltaic properties of polymers PTB1-6 in solar cell structures of ITO/PEDOT/PSS/polymer/PC61BM(1:1, w/w)/ Ca/Al are summarized in Table 1 and Figure 3. The solar cells Table 1. Characteristic Properties of Polymers and Their Solar Cells in a PTBx/PCBM Composite polymers PTB1/ PC61BM PTB1/ PC71BM PTB2/ PC61BM PTB3/ PC61BM PTB4/ PC61BM PTB4/ PC71BM PTB5/ PC61BM PTB6/ PC61BM PTB7/ PC71BM

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

EHOMO (eV)

ELUMO (eV)

0.58

12.5

65.4

4.8

−4.90

−3.20

0.58

15.5

62.3

5.6

0.60

12.8

66.3

5.1

−4.94

−3.22

0.72

13.9

58.5

5.9

−5.04

−3.29

0.74

13.0

61.4

6.1

−5.12

−3.31

0.70

14.8

64.6

7.1

0.66

10.7

58.0

4.1

−5.01

−3.24

0.62

7.74

47.0

2.3

−5.01

−3.17

0.74

14.50

68.97

7.4

−5.15

−3.31

(2)

prepared from these PTB polymers are a clear function of composition of the active layers. Those prepared from polymer/PC61BM composite showed inferior properties to the ones prepared from polymer/PC71BM composite.17,39 A PCE of about 7.4% has been achieved from PTB7/ PC71BM solar cell devices, as listed in Table 1, which was the first polymer solar cell showing PCE over 7%. These solar cells

Obviously, this correlation is oversimplified as other important factors may need to be taken into consideration, such as molecular structures and morphology. A plot of the Voc versus the HOMO level of the donor with data (Table 2 and Figure 5) collected from the available HOMOs and Voc values for a broader array of polymers in the literature illustrates this point, which shows widely distributed data points. 3105



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Figure 4. (a) EQE spectra of champion cells from DCB with 3% DIO and CB with 3% DIO. (b) IQE derived from the EQE and absorption spectra in the same champion cells. Reprinted from ref 17 with permission. Copyright Wiley@VCH.

Table 2. HOMO, ΔEDA, and Voc Values Collected from Cited References polymer

EHOMO,D (eV)

ΔEDA (eV)

Voc (V)

ref

polymer

EHOMO,D (eV)

ΔEDA (eV)

Voc (V)

ref

1 2 3 4 5 6 7 8 9 10 11 12

5.0 4.93 4.8 5.7 5.45 5.05 5.36 5.5 5.5 5.36 4.9 4.94

0.70 0.63 0.5 1.4 1.15 0.75 1.06 1.2 1.2 1.06 0.6 0.64

0.64 0.72 0.52 0.97 0.89 0.68 0.57 0.68 0.87 0.85 0.58 0.60

40 41 42 43 45 46 47 49 51 51 16 15

13 14 15 16 17 18 19 20 21 22 23 24

5.01 5.01 5.15 5.56 5.48 5.57 5.17 5.2 5.35 5.4 5.7 5.23

0.71 0.71 0.85 1.26 1.18 1.27 0.87 0.9 1.05 1.1 1.4 0.93

0.68 0.62 0.75 0.85 0.87 0.81 0.68 0.61 0.79 0.76 0.86 0.91

15 15 15 44 44 44 48 50 52 53 54 55

δ = ΔHsol + (−e2/4πεε0rAD), representing two effects, namely, ion salvation and Coulombic attraction. If we remove the ΔE00 term, then eq 3 is identical to eq 2, but with more explicit explanation on the constant 0.3 in eq 2. Obviously, different polymers will have different matrix properties, mainly reflected in the dielectric constant, and the molecular geometry will affect Coulombic attraction between positive and negative charges due to the difference in distances. This seems to indicate the importance of the δ term in eq 3, which should not be treated as a constant. The relationship between Voc and Jsc in OPV solar cells can also be established starting from a generalized Shockley equation after a series of mathematical operations and equations.57−59

(4)

Figure 5. Plot of Voc versus the HOMO level of the donor.

It is clear from eq 4 that there is a linear correlation between Voc and ΔEDA, as indicated by the second term, and it agrees with eq 3. However, there is a logarithmic dependence between Voc and Jsc/Jso from the first term. Here, Jso is related to Jsc as a pre-exponential term and is determined by the reorganization energy and the intermolecular overlap at the D−A interface. For polymer/fullerene BHJ systems, Jso is considered largely dependent on the solubilizing side chains and is independent of ΔEDA.60 Clearly, the Voc will reach a maximum value when the Jso is minimized. However, Jso needs to be maximized in order to obtain the best Jsc. This apparent contradiction is characteristic of BHJ solar cells and highlights the compromises

Closer examination shows that this equation is actually very similar to that developed in photophysical chemistry to describe the free-energy change in exciplex formation56

(3)

where E(D/D+•) and E(A−•/A) refer to redox potentials of the donor and acceptor, ΔE00 is the excitation energy, and 3106



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Figure 6. GIWAXS images of (a) a polymer PTB1 film, and (b) a pristine PTB1/PC 61BM (1:1) film. Reprinted from ref 54.

that need to be made in polymer design. Thus, a delicate balance needs to be maintained in order to achieve both a high Voc and Jsc to maximize PCE. Our PTB system seems to approach that balance. The stabilization of the quinoidal structure from thieno[3,4b]thiophene renders the polymers with an optimal band gap of ∼1.6 eV and efficient absorption (around 700 nm) in the best photon flux region of the solar spectrum. These polymers possess preferred energy levels that match well with PCBM electron acceptors. The rigid backbone due to the quinoidal structure in the polymer chain enhances the planarity along the aromatic polymer backbone. The benzodithiophene unit has a more extended π system and enables the polymer to form an assembly with better π−π stacking and high hole mobility. Proper side chains on the polymer backbone ensure good solubility and miscibility with the fullerene acceptor, which enables the polymer blend systems to have a preferred morphology with interpenetrating networks that can benefit from not only the charge separation but also the charge transport, which leads to the high fill factor (see Figure 3 and Table 1). The polymer chain was found to be stacked on the substrate in the face-down conformation.54 This is very different from the polymer alignment in a well-studied P3HT solar cell system and favors charge transport. Grazing incidence wide-angle X-ray scattering (GIWAXS) revealed a unique lamellae packing structure with a π−π stacking distance of 3.7 Å for both a pristine PTB1 film (Figure 6a) and a PTB1/PC61BM blend film (1:1, w/w) (Figure 6b).54 For the PTB7 polymer, a similar π−π stacking distance has been observed, and a model had also been proposed by combining molecular orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered materials from diffraction, which indicated that there was only ∼20% of the polymer in the PTB7/PC71BM blend film that was ordered and preferred the face-down orientation. Furthermore, with the addition of the 1,8-diiodooctane additive, it would decrease the domain size of the interpenetrating BHJ structure and result in much higher PCE. It has also been pointed out that a high propensity to crystallize may not be necessary for an absorber polymer for highly efficient polymer solar cells.61 Efficient charge separation was observed in ultrafast spectroscopic studies of PTB1 and PTB1/PC61BM (1:1 in weight ratio) films in the NIR spectral region of 850− 1700 nm.54 A cation (or hole) peak centered at 1150 nm appears at 100 ps after the excitation in pure PTB1 polymer film due to the interchain CT.62 It was found that the average

charge separation (CS) rate for the pristine PTB1/PC 61BM film was 1.5 ps, which is more than twice as fast as that of 4 ps observed for the annealed P3HT/PC61BM film.63 Studies of the magnetic field effect on photoconductivity of PTB/PCBM films showed that the CT complex in these materials can be easily dissociated, as indicated by its insensitivity to the magnetic field, consistent with spectroscopic studies. This faster CS rate, combined with higher charge carrier mobility, contributes to the high device efficiency of the PTB1/PC61BM solar cell. All of these features indicated that the polymer system exhibits a host of properties that are indeed synergistically combined, leading to the enhancement in the solar cell performances. In addition to our work, many other researchers have attempted to lower the HOMO of the donor while retaining enough driving force between the D/A LUMO levels by integrating conjugated side chains,41,64 replacing the C atom with a Si atom,43,65 substituting the benzene ring with pyridine,66 introducing electron-withdrawing groups, and so forth. These efforts have led to a varied degree of success in enhancing solar cell PCE. For example, as shown in Scheme 2, Jen, Leclerc, and Fréchet et al. synthesized low-band-gap polymers based on BDT and the thieno[3,4-c]pyrrole-4,6 dione unit.20,67,68 After optimizing the molecular weights, Fréchet et al. were able to prepare solar cell device P1/PC61BM blends, which showed a PCE of 6.8% (Jsc = 11.5 mA cm−2, Voc = 0.85 V, FF = 0.70).68 When dithienosilole was introduced to replace the benzodithiophene, Leclerc et al. synthesized the copolymers with thieno[3,4-c]pyrrole-4,6 dione, which gave a low-band-gap (1.73 eV) polymer (P2) with a deep HOMO energy level, while keeping enough driving force for photoinduced electron transfer to the fulleride in BHJ solar cells. The solar cell device prepared using P2/PC71BM (1:2 w/w) solution and CB with 3% DIO achieved a PCE of 7.3%, with a Jsc of 12.2 mA cm−2, a Voc value of 0.88 V, and a FF of 0.68.69 Using a similar strategy of fluorination, You et al. introduced two fluorine atoms to the benzothiadiazole unit and synthesized polymer P3, which lead to the decrease in both LUMO and HOMO energy levels.29 As a result, the Voc (0.91 V) was noticeably enhanced, and a BHJ solar cell prepared using a P3/ PC61BM (1:1 w/w) blend yielded a PCE of 7.2% with a Jsc of 12.9 mA cm−2 and a FF of 0.61. Similarly, a new polymer (P4) was synthesized by changing a fluorinated benzothiadiazole in P3 to fluorinated 2-(2-butyloctyl)benzo[d][1,2,3]triazoles (FTAZ).70 When blended with PC61BM in a BHJ solar cell, P4 exhibited a PCE above 7.1%. Reynold et al. modified polymers developed by Leclerc by introducing Ge to replace Si to 3107



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Scheme 2. Polymer Structures

form new copolymers of dithienogermole and 1,3-dibromo-Noctylthienopyrrolodione. The resulting polymer showed a low band gap at 1.69 eV. An inverted bulk heterojunction solar cell prepared from this polymer and PC71BM blends yielded an average PCE of 7.3%.71 Morphological Effect on FF, Jsc, Voc. Another major factor that influences the solar cell PCE is the morphology of the composite materials. Numerous examples have shown the importance in controlling phase separation and domain size. Different approaches were used to achieve optimal morphology, such as the thermal annealing process,72 the solvent annealing process,73 and use of cosolvent.74 The ultimate goal of these efforts is to control the domain size so that it is large enough for the formation of an interpenetrating network for effective charge transport small enough to prevent exciton relaxation to the ground state before it reaches the donor− acceptor interface. The effectiveness of these approaches largely depends on the nature of donor polymers. The thermal annealing process is effective in the P3HT/PCBM system but fails completely in the PTBx polymer system, leading to a dramatic decrease in device performance.54 The cosolvent approach works the same way and must be judiciously applied to different polymer systems. Bazan and Heeger et al. found that adding small amounts of 1,8-octanethiol to solutions of the donor polymer and fullerene acceptor led to dramatic increases in device performance; PCE increased from 3.2% without the additive to 5.5%.74 The additive approach worked well with our PTB family of polymers.16 For example, the fluorinated PTB4 suffers from the nonoptimized morphology, and large features (over 100 nm) can be observed in the TEM image of PTB4/ PC61BM blend film (Figure 7a). Although PTB4 showed the lowest HOMO energy level and the largest hole mobility, its photovoltaic performance in simple polymer/PC 61BM solar cells is modest (3.10%). The PTB4/PC 61BM blend film prepared by using dichlorobenzene (DCB)/DIO (97/3, v/v) as the solvent exhibited improved morphology (Figure 7b). There are no large features in the TEM image, and it shows a

Figure 7. TEM images of polymer/PC61BM blend films; (a) PTB4 and (b) PTB4 blend films prepared from mixed solvents DCB/DIO (97/3, v/v). (The scale bar is 200 nm.) Reprinted from ref 16.

similar morphology as PTB1 or the PTB2 blend film in TEM images. Dramatic performance enhancement can be observed in PTB4/PC61BM solar cells. Besides the increase of the Voc, the PTB4 solar cell showed larger Jsc, and the PCE reached 6.1%. It has been shown that the side chain on the conjugated polymer strongly influences the balance between the fullerene miscibility and crystallinity of the polymer to achieve optimal morphology. We used the GIWAXS to investigate the effect of different side chains on the PTB polymer series on solar cell properties. The results showed that the film morphology is very sensitive to the structure of the PTB side groups attached to the TT or BDT subunits. As the nature of side chain changes, the π−π stacking distance also changes. However, there is no clear correlation between the π−π stacking distance and solar cell performances except for the solar cell FFs. A striking linear relationship between the OPV device FF and the π−π stacking distances of the seven PTB polymers was observed, in which the closer π−π stacking distance in the polymer film gave the larger FF in its corresponding BHJ device (Figure 8).55 This result implies that short π−π distance favors charge transport 3108



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efficiency. So far, this issue has not been fully solved. Significant progress in solution-processed tandem solar cells was achieved by Kim and Heeger et al. in 2006. An efficient stacked cell with two active layers of different band gaps enables absorption over a broad range of photon energies within the solar emission spectrum, which showed about 38% higher PCE compared to its single cells.79 The device integrated poly[2,6-(4,4-bis-(2ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT)/PC61BM for the bottom layer and P3HT/PC71BM for the top layer. A transparent TiOx layer was used to separate and connect the two layers. The performance of this polymer tandem solar cell reached a high PCE of 6.5% due to a substantially increased Voc (1.24 V) close to the sum of the open-circuit voltages of the two sublayers. Recently, Yang et al. reported a tandem solar cell with PCE up to 7.0% by using a robust interconnecting layer (ICL), whose device structure is ITO (150 nm)/PEDOT (40 nm)/P3HT/ICBA(150 nm)/TiO2 (20 nm)/m-PEDOT (90 nm)/PSBTBT/PC71BM(100 nm)/Ca (20 nm)/Al (100 nm).80 Overall, the organic tandem solar cells have not shown competitive edge in both efficiency and cost. OPV Solar Cell Device Lifetime. Highly conjugated organic materials are prone to oxidation by O2 or even H2O under atmospheric conditions. This renders PSCs in need of strict encapsulation under an inert atmosphere before they can be used. We recently investigated the photochemical stability of several PTB polymers with different degrees of fluorination on the backbone.81 It was found that monofluorination on the thienothiophene unit is effective for improving the PCE of the solar cell with improved Voc without sacrificing Jsc and FF. Perfluorination of the polymer backbone on the benzodithiophene unit can be detrimental to the solar cell performance. It was found that the PTBF2 and PTBF3 polymers are photochemically much less stable than PTBF0 and PTBF1 under ambient conditions (Figure 9). With UV light exposure to the spin-coated polymer films under air conditions, the thienyl ring directly linked to the polymer main chain underwent a [2 + 4] cycloaddition reaction with the singlet oxygen (1Δg state: 0.9 eV) generated via photosensitization by the polymer. However, even under inert conditions, several factors may contribute to device degradation. Prolonged heating under sun may cause phase separation to form large domains. This could reduce the interaction between the donor and acceptor material, leading to an overall loss of performance. There are only limited studies on the durability of an encapsulated PSC device, and most of them are done in companies. Studies by

Figure 8. Correlation between the OPV FF and the π−π stacking distance for the PTB polymers. PTB7 is indicated in blue because this polymer was fabricated using PC71BM while the other polymers were fabricated using PC61BM in this investigation. The line is a visual aid. Reprinted from ref 55 with permission Copyright Wiley@VCH.

from the active layer to the electrode. The GISAXS data show that the PCBM domains with tens of nanometers dimensions, typically present in most BHJ OPV films, are not present in these PTB-based materials. For example, by using the energyfiltered TEM technique, a homogeneous phase separation distance of only about 20−40 nm had been observed in the PTB7/PC71BM blend film prepared from DCB/DIO solvents, which is very close the ideal domain size, allowing the excitons to efficiently reach the interface of the polymer and PC71BM acceptor.61 Together, these results indicate that the materials architecture of PTB-based OPV devices has a completely new structural foundation, which in turn creates a very different BHJ morphology/microstructure that favors enhancement of solar cell performances. Development of New Device Technology. It was estimated that the PCE limit for the single BHJ solar cell is around 10%.27 A tandem concept could be an effective approach to further enhance this value.24 When polymers or organic molecules with different absorption windows are used, it has a good chance to enhance the PCE of the resulting solar cells. Over the last 20 years, extensive efforts had been focused on the organic tandem solar cells, including both the vacuum-deposited low-molecularweight solar cell and fully solution processed tandem solar cell.75−78 Research on the organic tandem solar cell was started in 1990 by Hiramoto et al. by evaporation of small molecules. Kawano et al. reported the first solution-processed organic solar cell by using polyphenlyenevinlyene (PPV)-type material (MDMO-PPV) blended with PCBM as the active layer for both top and bottom cells. Soon, it was realized that impedance match in the interlayer of the solar cell is crucial for high

Figure 9. (a) Absorption spectra of the PTBF3 film recorded as a function of irradiation time under air. (b) Decrease in the optical density (λ max) of polymer films. Reprinted from ref 81. 3109



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McGehee and co-workers show a simulated operating lifetime of 7 years with a 20% loss linearly within the operating time. 82 Ef fect of Processing Conditions on OPV Cell Performances. Processing conditions play an important role in determining the device performances. Many strategies are explored to improve solar cells’ performance. A thin layer of poly(3,4ethylenedioxylenethiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) has been widely used in organic solar cells to increase the work function of ITO for effective hole collection 83 and to smooth the relative rough ITO surface for better connection with the top layer. Besides PEDOT/PSS, some other materials or techniques, such as carbon nanotubes (CNTs),84 transition-metal oxides (such as MoO2),85 and selfassembled monolayers (SAMs)86 have also been employed to modify the ITO surface for better hole-transporting or electronblocking properties. Different cathode materials such as LiF/Al and Ca/Al are used to enhance the electron injection. Semiconducting titanium suboxide (TiOx)87 or zinc oxide nanoparticles (ZnO NPs) assisted with a SAM layer88 are also used to serve as an optical spacer and hole blocker to significantly improve charge collection and therefore improve PCEs of the polymer solar cells. Methods are sought to enhance the absorption efficiency of the active layer toward the coming sunlight in those devices for high EQE. The surface plasmon resonance (SPR) appears to be attractive for light-trapping techniques for its enhanced optical field associated with metallic nanostructures. Wang et al. reported that the PCE of the PCDTBT/PC71BM-based solar cell could be enhanced up to 7.1% by directly mixing silver nanoparticle clusters in the active layer.89 From the lessons learned from OLED (organic light emission diodes) research, thin layers of conjugated polyelectrolytes (CPEs) were found to enhance the device efficiency. By using the ultrathin layer of an alcohol-soluble ionic conjugated diblock copolymer poly(9,9-bis(2-ethylhexyl)fluorene]-b-poly[3-(6-rimethylammoniumhexyl)-thiophene] (PF2/6-b-P3TMAHT) between the active layer and the metal cathode, Seo et al. achieved a PCE for the BHJ solar cell of about 6.5%, up from about 5% before the addition of the PF2/ 6-b-P3TMAHT layer.90 Recently, He et al. reported using this method of incorporating another alcohol/water-soluble conjugated polymer, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfl uorene)] (PFN) as a cathode interlayer in polymer/fullerene bulk heterojunction solar cells based on our PTB7/PC71BM system, in which the PCE can be increased to 8.37%.21 The improvement in device performance and the simplicity of fabrication by solution processing suggest a promising and practical pathway for improving polymer solar cells with high efficiencies. Issues and Challenges. The importance of solar technology has never been more apparent than it has been at this moment. Our consumption of fossil fuels as well as our reliance on nuclear power will only increase as our society grows and other societies continue to modernize. The only way to meet the growing demand for energy is to expand our portfolio of energy sources with renewable ones. The largest renewable source of energy on this planet is solar energy; without finding an efficient means of harnessing this power, we will face an uncertain future. Polymer solar cells represent one method of harnessing solar power. Current polymeric materials are continually improving toward the goal of 10% PCE. What has traditionally been an academic pursuit has now piqued the interest of industry as recent reports by corporations claim efficiencies higher than 9%. It is only a matter of time now

before such technology is put into large-scale production, but still more fundamental issues need to be addressed so as to increase the efficiency beyond 10% as well as extend the operational lifetime. The realistic view is that this technology will find niche applications but will not able to challenge inorganic solar cells for long-term deployment on rooftops.

One must be cautious in predicting the future of polymer solar cells, neither overhyping the technology nor entirely dismissing it.

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

BIOGRAPHIES Feng He received his B.E. (2002) and Ph.D. (2007) degrees in polymer chemistry from Jilin University under the supervision of Prof. Yuguang Ma. Then, he moved to the University of Toronto, Canada, to work with Prof. Mitchell A. Winnik on the design and synthesis of fluorescent block copolymers as well as their supramolecular selfassembly. In 2009, he joined Prof. Luping Yu’s group at the University of Chicago, where his research interests are in the synthesis of lowband-gap copolymers for organic solar cell applications. Luping Yu was born in Zhejiang Province, People’s Republic of China. He received his B.S. (1982) and M.S. (1984) degrees in polymer chemistry from Zhejiang University and his Ph.D. degree (1989) from the University of Southern California. He is currently a Professor of Chemistry at the University of Chicago. His current research focuses on polymer chemistry, surface chemistry, and supramolecular chemistry. Typical examples of current projects include (1) polymerization methodology, (2) conjugated diblock copolymers for the formation of self-assembled, nanosized electroactive materials, (3) ladder polymers and oligomers and molecular electronics, (4) photoinduced electron transfer and photovoltaic materials, and (5) nanoporous hydrogen storage polymeric materials. http://lupingyu.uchicago.edu/research.html

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ACKNOWLEDGMENTS

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REFERENCES

We would like to acknowledge the support from NSF, AFOSR, DOE, NSF-MRSEC (the University of Chicago), Intel, and Solarmer Energy Inc. for the preparation of this Perspective.

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