Mediating Solar Cell Performance by Controlling the Internal Dipole

Aug 2, 2012 - University Research Office, Intel Laboratories, Building RNB-6-61, 2200 Mission College Boulevard, Santa Clara, California 95054,...
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Mediating Solar Cell Performance by Controlling the Internal Dipole Change in Organic Photovoltaic Polymers Bridget Carsten,† Jodi M. Szarko,∥ Luyao Lu,† Hae Jung Son,† Feng He,† Youssry Y. Botros,⊥,# Lin X. Chen,*,‡,§,∥ and Luping Yu*,† †

Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States ‡ Department of Chemistry and §ANSER Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ∥ The Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ⊥ University Research Office, Intel Laboratories, Building RNB-6-61, 2200 Mission College Boulevard, Santa Clara, California 95054, United States # National Center for Nano Technology Research, King Abdulaziz City for Science and Technology (KACST), P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: We report synthesis and characterizations of two novel series of polymers, namely the PBTZ and PBIT series. The PBTZ1 polymer was synthesized as a copolymer of 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b′]dithiophene (BDT) along with 2,5-bis(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer of TzDPP and 2-(1-butylheptyl)thieno[3,4-d]thiazole (TTz). The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT), and BDT was also synthesized. The PBIT series of polymers showed enhanced ground and excited state dipole moments (μg and μe) when compared to the previously reported PBB3 polymer, while PBTZ1 showed the largest dipole change (1.52 D) from ground to excited state (Δμge) in respective single polymer units. It was found that the power conversion efficiencies of the polymer series were strongly correlated to Δμge. The results reported demonstrate the utility of the calculated parameter Δμge of single units of the polymers to predict the performance of donor− acceptor copolymers in photovoltaic devices. We rationalize this result based on the large degree of polarization in the excited state, which effectively lowers the Coulomb binding energy of the exciton in the excited state and leads to faster charge separation kinetics, thus facilitating the full separation of electron and hole.

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ances.13 Theoretical calculations were used to predict the ground (μg) and excited state (μe) dipole moments of proposed new monomers as shown in the previous result as well as the change in dipole moment on transition from the ground to excited states (Δμge). While it is well-known that the presence of a strong interfacial dipole moment14−16 (i.e., between “donor” polymer and “acceptor” PCBM) is important to drive charge separation in BHJ solar cells, a strong intramolecular local dipole was also shown to be important in assisting the separation of excitons along the polymer chain.17 Most recently, we have synthesized and investigated two new series of polymers, the PBIT series and the PBTZ series, to

INTRODUCTION It is known that bulk heterojunction (BHJ) solar cells are complex systems,1−4 the efficient operations of which are a function of a host of parameters, ranging from structures, energy levels and energy gaps of polymers,5,6 composition and morphology of composites,7,8 device structures,9−11 nature of electrodes and their interfacial properties, and processing conditions. Therefore, it is usually very hard to pin down a major parameter that determines the performances of BHJ OPV cells. However, if one controls the processing conditions so that the morphology of the film is similar, polymers with similar energy levels and crystallinity can be safely compared in their optimized performances.12 In our previous research, the comparison of several polymer systems revealed a strong effect of an internal dipole moment on charge separation in the conjugated polymer, thus also affecting solar cell perform© XXXX American Chemical Society

Received: May 31, 2012 Revised: July 22, 2012

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Figure 1. Structures of newly synthesized polymers (in red) PBIT1, PBIT3, PBTZ1, and PBTZ2 along with previously reported PTB2,21 PBT7,22 PTBF2,23 and PBB3.13

investigate the effect of the net dipole moment on promoting higher charge separation efficiency and on improvement in PCE. The PBTZ1 polymer was synthesized as a copolymer of 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b′]dithiophene (BDT) along with 2,5-bis(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer of TzDPP as synthesized by modification of a procedure by Flores and co-workers.18 The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT) and BDT was also synthesized. Notably, during the preparation of this manuscript, the synthesis of the DPBT compound was independently reported by Cheng and co-workers.19 We compared the performance and dipolar properties of the polymers synthesized to the previously reported PTB2,20,21 PTB7,22 PTBF2,23 and PBB3.13 Unexpectedly, we found a strong correlation between PCE and the change in dipole moment on going from ground to excited state, Δμge, for all eight polymers despite the fact that their backbone structures varied. Given that the mechanism of charge separation in OPV polymers is complex and efficiency optimization usually incorporates synergistic optimization24,25 of a number of factors, the most important of which include morphology, bandgap, and energy level offset between donor and acceptor, we were initially struck by this seemingly strong correlation between the simple calculated dipole change parameter and the solar cell performance among polymers which exhibit significant difference in energy gaps and energy levels. Herein, we describe the properties of each polymer and propose a possible explanation for the strong correlation between ground to excited state dipole change and power conversion efficiency. We expect these results will attract the interest for theoretical investigation so that further understanding in rational design of high-performance solar polymers can be achieved.

Table 1. Summary of Molecular Weight Data for the Polymer Series (GPC Data, Polystyrene Used as Standard) polymer

Mn (kDa)

Mw (kDa)

PDI

PBIT1 PBIT3 PBTZ1 PBTZ2

50.5 7.3 73.0 39.0

114.9 13.0 158.0 92.8

2.27 1.77 2.16 2.38

and PBTZ2 are shown in Figure 2. The spectra were measured for thin films of each polymer spin-cast from a 10 mg/mL

Figure 2. Thin film optical absorption spectra for the polymer series spin-cast from 10 mg/mL chlorobenzene solution.

chlorobenzene solution, similar conditions to those used for solar cell device preparation. As shown in Figure 2, PBTZ2 showed a substantial red-shift in optical absorption relative to the remainder of the polymer series with significant absorption in the near-IR region of the spectrum. The optical properties of the series are summarized in Table 2. This polymer showed a λmax of 884 nm, and a second significant absorption maximum at 802 nm, with a broad spectral coverage throughout the UV− vis region from 550 to 1000 nm and an optical bandgap (Eg) of 1.24 eV as calculated from its absorption onset. PBTZ1 showed a fairly red-shifted λmax relative to the other polymers of 741 nm, with an energy gap as low as 1.55 eV as calculated from an absorption onset of 800 nm. PBIT1 showed a λmax of only 576 nm while the λmax of PBIT3 appears at 685 nm, corresponding to optical bandgaps of 1.95 and 1.50, respectively. The narrow optical bandgaps of the PBTZ series along with the broad

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RESULTS AND DISCUSSION Material Synthesis. In this study, we have investigated the physical properties of eight polymers, four of them newly synthesized (Figure 1 with red label). The synthetic procedures are described in the Supporting Information. These polymers all exhibit reasonable molecular weights as shown by GPC studies (see Table 1), and their chemical structures are confirmed by NMR spectra. Optical Absorption Spectroscopy. The optical absorption spectra for the four new polymers PBIT1, PBIT3, PBTZ1, B

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Table 2. Summary of Optical and Electrochemical Properties for the Polymer Series polymer

λmax (nm)

Egopt (eV)

λonset (nm)

HOMO (eV)

LUMO (eV)

Egelec (eV)

ΔELUMO

PBIT1 PBIT3 PBTZ1 PBTZ2

576 685 741 884, 802

1.95 1.50 1.55 1.24

635 825 800 1000

−5.15 −4.92 −5.22 −5.03

−2.97 −3.16 −3.47 −3.72

2.18 1.76 1.75 1.31

0.79 0.60 0.29 0.04

Figure 3. Transmission electron microscopy (TEM) images of PBIT1, PBTZ1, and PBTZ2 at magnification of 49K and accelerating voltage of 300 kV. Scale bar = 100 nm.

polymer showed the highest hole mobility of 4.03 × 10−4 cm2/ (V s) while PBIT1 showed a mobility of 3.43 × 10−4 cm2/(V s). Similarly, PBIT3 and PBTZ2 showed mobilities of 2.95 × 10−4 and 2.44 × 10−4 cm2/(V s), respectively. These values were all similar to the value of 4.1 × 10−4 cm2/(V s) previously reported for the high-performance polymer PTB7; thus, the charge transport characteristics of the series are clearly sufficient to enable adequate charge separation and transfer to electrodes. Significantly, the trend in charge-carrier mobility roughly tracks the trend in overall PCE of the polymer series. Morphology of Polymer/PCBM Blend Films. The blend film morphology is known to be critical in optimizing exciton diffusion and further charge separation.10 It is believed that the domain size should be on the order of the exciton diffusion length (∼10 nm) to enable the exciton to reach the donor/ acceptor domain boundary for its splitting before its decay to the ground state.3 The BHJ concept was developed to overcome the challenges of exciton diffusion and interfacial separation; thus, it is a key factor in determining the device performance. Figure 3 shows the transmission electron microscopy (TEM) images of polymer/PC71BM blend films prepared from the best performing solar cell devices based on PBIT1, PBTZ1, and PBTZ2. The TEM image of PBIT3 could not be obtained due to its moderately low molecular weight, which rendered the resulting polymer film too thin to allow adequate sample preparation. As shown, each polymer film possesses uniformly distributed fine features (approximately 10−20 nm scale), showing nanoscale phase separation. Additionally, the absence of large, spherical domains in the TEM image suggests the formation of a bicontinuous network in each case. Such morphology with an interpenetrating network in each polymer/PCBM film is expected to give a relatively high conductivity and high performance in devices. Thus, we can expect in the case of PBIT1, for example, that the morphology, energy levels, and bandgap are all adequate to produce a high-performance solar cell device. Additionally, PBTZ1 also showed optimal features for high performance as evidenced by morphology, energy levels, and bandgap. Solar Cell Device Performance of the Polymer Series. The solar cell performances of the PBIT and PBTZ polymers were studied in devices with the structure ITO/PEDOT:PSS/ polymer:PC71BM/Ca/Al (Table 3). The active polymer layer

spectral coverage and absorption in the near-IR range showed promise for these two polymers to exhibit high performance in solar cell devices. Electrochemical Properties. The electrochemical properties of the PBIT and PBTZ series were investigated using cyclic voltammetry (CV), and the resulting voltammograms are shown in the Supporting Information. The positions of the HOMO and LUMO energy levels were calculated as detailed in the Supporting Information from the oxidation and reduction onset potentials of each polymer, and the data are summarized in Table 2. The position of the LUMO of the donor material in organic photovoltaic devices is known to be important to provide sufficient driving force for charge separation.26 The band offset must be greater than the exciton binding energy; otherwise, the exciton will be the lowest energy excited state, and charge separation will not occur at the interface. It is reported that the offset must be between 0.1 and 0.5 eV in order to be sufficient to drive charge separation. The LUMO energy level of PC61BM is reported to be −3.70 eV.27 Our own CV measurements were consistent with this, showing the LUMO levels for PC61BM and PC71BM to be −3.75 and −3.76 eV, respectively. PBIT1 showed a LUMO energy level of −2.97 eV, a HOMO of −5.15 eV, and an electrochemical bandgap of 2.18. PBIT3 showed a reduced bandgap of 1.76 eV due to a slightly lowered LUMO energy level of −3.16 eV with a slightly higher HOMO energy level of −4.92 eV. PBTZ1 showed a similar bandgap of 1.75 eV, but with substantially lowered HOMO and LUMO energy levels of −5.22 and −3.47 eV. PBTZ2 showed an even lower LUMO energy level of −3.72 eV and a HOMO of −5.02 eV with a bandgap of 1.31 eV. The LUMO energy level offset between PBTZ1 and PCBM may still be sufficient (∼0.23 eV) to drive charge separation; however, in the case of PBTZ2, it is apparent that the LUMO energy level offset of 0.04 eV is too small to drive charge separation. On the other hand, PBIT1 shows a relatively high LUMO energy level which is clearly sufficient to drive the charge separated step. Charge-Carrier Mobility of the Polymer Series. The charge-carrier mobility of the polymer series was measured according to the space charge limited current model (SCLC) in the direction perpendicular to the electrodes. The PBTZ1 C

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Table 3. Summary of Best Solar Cell Performance Dataa polymer

PCE (%)

Voc (mV)

Jsc (mA/cm2)

FF (%)

PBIT1 PBIT3 PBTZ1 PBTZ2

1.96 0.47 3.46 0.29

646 421 839 559

4.96 4.24 7.27 2.01

61.4 26.4 56.7 25.9

External Quantum Efficiency. The external quantum efficiency (EQE) of the polymer series was measured from the best performing devices for each polymer (Figure 4). Both

a

All devices shown fabricated in the configuration ITO/PEDOT:PSS/ polymer:PC71BM/Ca(20 nm)/Al(80 nm) from 10 mg/mL concentration of polymer/PCBM in chlorobenzene with 3% DIO additive.

was spin-coated from a 10 mg/mL chlorobenzene solution with a small amount of 1,8-diiodooctane (DIO) additive. The optimal morphology was found in the presence of this additive; thus, the performance of these devices was expected to be higher than in its absence. The devices were also optimized for weight ratio of polymer:PC71BM (1:1−1:2) and spin-coating rate, and the best results are summarized in Table 3. The current density versus voltage (J−V) curves were measured under AM 1.5G irradiation at 100 mW/cm2. As shown, PBTZ1 showed the highest performance (PCE) of the series at 3.46% with a relatively high Voc (open-circuit voltage) of 839 mV, a Jsc of 7.27 mA/cm2, and a FF of 56.7%. Because the Voc is known to be proportional to the difference between the HOMO of the donor and the LUMO of the PCBM acceptor (Table 2), the relatively high Voc is attributed to the low-lying HOMO energy level of PBTZ1 (−5.22 eV). PBTZ1 showed the highest shortcircuit current density (Jsc) at 7.27 mA/cm2. This was an interesting result because the driving force for charge separation as estimated from the LUMODONOR−LUMOACCEPTOR offset is relatively small (∼0.23 eV), while this polymer still showed a substantial degree of charge transfer as evidenced by the relatively high Jsc and PCE (power conversion efficiency) among the polymer series. This suggests that other factors beyond the LUMODONOR−LUMOACCEPTOR offset may be involved in charge separation. The solar cell performance of PBIT1 was moderate with the best PCE of 1.96%, accompanied by a Voc of 646 mV, a Jsc of 4.96 mA/cm2, and a FF of 61.4%. The low PCE is puzzling because the LUMODONOR−LUMOACCEPTOR offset is more than 0.7 eV (Table 2), which is clearly sufficient to drive charge separation. Additionally, the Voc is also relatively high due to the sufficiently low-lying HOMO energy level of −5.15 eV. Thus, the major contributor to the low efficiency is the low Jsc. This suggests either inadequate morphology leading to exciton decay without charge separation or additional assistance to the LUMODONOR−LUMOACCEPTOR offset is required for effective charge separation/diffusion/collection. Similarly, PBIT3 showed inferior performance to both PBIT1 and PBTZ1 with the best PCE of 0.47% (Voc of 421 mV, Jsc of 4.24 mA/cm2, and FF of 26.4%). Based solely on energy levels of HOMO and LUMOs (Table 2), PBIT3 would be expected to show high performance in solar cell devices, with its LUMODONOR− LUMOACCEPTOR offset of almost 0.5 eV that is expected to be sufficient to drive charge separation and its relatively high HOMODONOR−LUMOACCEPTOR offset for a higher Voc. However, the PCE for the device made from this polymer is only 0.47%. The PBTZ2 polymer showed the lowest PCE of 0.29% in the series (Voc of 559 mV, Jsc of 2.01 mA/cm2, and FF of 25.9%) as expected because LUMODONOR−LUMOACCEPTOR offset is only 0.04 eV, too low to drive the charge separation, resulting in the low PCE.

Figure 4. External quantum efficiency data for PBIT1 and PBTZ1 based on best device preparation conditions for each polymer.

PBIT3 and PBTZ2 showed negligible responses, and their EQE curves are not shown. PBTZ1 and PBIT1, however, showed significant contributions to overall quantum efficiency throughout their absorption ranges. As shown, PBTZ1 showed relatively high photoconversion efficiencies of 25−30% throughout the visible and near-IR range of 300−850 nm, while PBIT1 showed values of 30−35% from 300 to 650 nm. Dipole Calculations. The calculated dipole moments for a single polymer repeat unit of PBIT1, PBIT3, PBTZ1 and PBTZ2 are shown in Table 4 along with data for the previously Table 4. Calculated Dipole Moments for PBB3, PBIT1, PBIT3, PBTZ1, PBTZ2, PTB2, PTB7, and PTBF2 polymer a

PBB3 PBIT1 PBIT3 PBTZ1 PBTZ2 PTB2a PTB7a PTBF2a

μg (D)

μe (D)

Δμge (D)

0.61 4.46 6.99 0.88 1.92 3.60 3.76 3.35

0.82 4.80 6.83 2.41 1.48 6.37 7.13 5.45

0.47 0.34 −0.16 1.52 −0.44 2.96 3.92 2.41

Values from ref 13. μg = ground-state dipole moment, μe = excitedstate dipole moment, and Δμge = change in dipole moment between ground and excited state. a

reported other four polymers for comparison.13 The dipole analyses for both the ground and excited states were determined using the Austin model (AM1) in Hyperchem. Calculations for PBIT3 were based on placing the ester groups in 2 main positions, then averaging the dipole of the 2 positions with the ester groups oriented 180 degrees relative to one another. To simulate the randomization of the ester positions, the averaged dipole moments for each polymer were determined and used for the analysis of the polymer backbone dipole dependence. Both the ground and excited state dipole moments were calculated for each polymer in the series. The overall change in the ground and excited state dipole Δμge was determined by accounting for the changes of the dipole along each coordinate axis as follows: D

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Δμge = [(μgx − μex )2 + (μgy − μey )2 + (μgz − μez )2 ]1/2

force and other effects), and further charge separation (affected by hole−electron binding energy, dielectric constant, and mobility). When the BHJ films exhibit similar morphology of interpenetrating networks in favor of charge transport, we believe the charge transfer process becomes the bottleneck for the photovoltaic effect. By enhancing the polarizability of the excited state, excitons “preseparate” along each polymer chain, effectively lowering the Coulomb binding energy of the electron and hole, which makes electron transfer from donor polymer to acceptor easier. A detailed study in a series of isolated polymers including PTB7 and PTBF2 in solution from our recent study indicated that a high local Δμge promotes the formation of a polarized exciton with intrinsic hole and electron separation. Consequently, such a polarized exciton with a large Δμge promotes an instantaneous intramolecular charge separation, resulting in effective displacement of hole and electron which, reduces germinate recombination due to the strong Coulombic interactions and high device PCEs.17 Hence, a possible explanation for this phenomenon is shown in Figure 6.

Using this relationship, Δμge values are shown in Table 4. As shown in Table 4, the highest values of ground (μg) and excited state (μe) dipole moments were 6.99 and 6.83 D, respectively, as calculated for PBIT3. PBIT1 also showed a significant ground state dipole moment of 4.46 D. Thus, we predicted that, all other factors held equal, these polymers would show promising performance in solar cell devices, consistent with our previous results.13 As shown in Table 4, PBTZ1 showed a μg value of 0.88 D but showed the highest Δμge value of 1.52 D. Because PBTZ1 showed a ground state dipole similar to that previously observed for PBB3, we expected only slight improvement in PCE. Dependence of Power Conversion Efficiency on Δμge. The PCE values shown in Table 3 represent the optimized performance values achieved for each polymer as previously reported, and the values for Δμge in Table 4 were calculated for the ground to excited state dipole change for a single repeat unit of each polymer. The results indicate that PCE and μg do not exhibit any clear correlation. However, as shown in Figure 5, a linear correlation exists between Δμge and PCE, where the

Figure 5. Plot of linear fit of power conversion efficiency vs Δμge for PBIT1, PBIT3, PBTZ1, PBTZ2, PTB2,21 PTB7,22 PTBF2,23 and PBB3.13 PCE values based on best performing devices as previously reported in refs 20, 21, 22, and13, respectively.

highest values of PCE (7.4%) were obtained for the polymer PTB7, which also showed the highest Δμge (3.92 D). In the case of PBTZ1, a substantial improvement in PCE was obtained, leading to the highest value of 3.46% even though the ground-state dipole moment was the lowest for this polymer (0.88 D). The results from optimized solar cell devices correlated with this parameter clearly implicate the dipole change Δμge as the key factor in determining PCE. This is an unexpected result, considering the complexity of the BHJ solar cell as discussed in the Introduction. How can we explain this result? Asbury and co-workers15 emphasized the importance of a strong interfacial dipole between the “donor” polymer and “acceptor” PCBM, which was observed to lower the Coulomb binding energy because the negative region of the dipole was found to repel electrons at the interface. In the current context, this does not explains the reason for the enhancement in PCE observed because of the lack of a clear correlation between PCE and μg. It is known that the photovoltaic effect in BHJ solar cells involves several crucial processes: exciton migration (mainly controlled by morphology), charge transfer (affected by driving

Figure 6. Graphical representation of the proposed explanation of the dipolar effect as shown for the excited state of PBTZ1 polymer. Δμge is the ground to excited state dipole change of the polymer, Coulomb binding energy of the exciton is given by V, r is the separation distance between electron and hole pair, e is the fundamental charge on the electron, ε0 is the permittivity of free space, and εr is the dielectric constant of the polymer.

The large value of Δμge correlates to a large degree of polarizability, which effectively stabilizes the charge separated state or cationic state after electron transfer to PCBM (Figure 6). As shown in the figure, in the case of PBTZ1, the lifetime of the charge-separated state is enhanced due to its greater stability, similar to our previous observations of PTB7. In the case of PBIT1, however, the degree of polarization of the excited state is relatively small; thus, electron transfer is difficult and recombination is made easier. This is because the DPBT unit remains largely electron rich as we previously saw in the case of PBB3. PBIT1 showed a Δμge value of 0.34 D, while PBB3 showed a similar value of 0.47 D; thus, after electron E

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transfer to PCBM, the “acceptor” unit of the copolymer is more compatible with positive charge, which enhances binding energy between electron and holes. The distance between the hole and electron decreases, corresponding to a decreased value of r and thus facilitating recombination even after electron transfer to PCBM. This underscores the strength of Δμge as a predictor of polymer performance and shows that, even when all other factors such as morphology, energy levels, and bandgap are taken into consideration, the effect of the ground to excited state dipole change plays a significant role in the process of charge separation, ultimately leading to enhanced device performance. However, a completely clear explanation of the linearity of PCE vs Δμge values requires further investigation, which is in progress to sample an even wider range of polymers with comprehensive investigations on their intra- and intermolecular exciton splitting, and correlation of branching ratios of initial populations of the charge separated state versus charge transfer state, as well as theoretical calculations on the binding energies of excitons in various polymers. Perhaps more experimental evidence, especially new polymers with even larger Δμge values, can further validate or overturn this seemingly linear relationship. Detailed theoretical studies will certainly shed light on the validity of this relationship.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of this work from NSF (NSF DMR-1004195), AFOSR, NSF MRSEC program at the University of Chicago, Intel Corporation, DOE via the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001059.

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CONCLUSION Eight low-bandgap polymers with different structures, energy bandgaps, and levels were used to prepare BHJ solar cells. It was found that optimized PCE values of these solar cells exhibit a seemingly clear linear relationship with the change in dipole moment in donor polymers on transition from ground to excited state, Δμge. It is postulated that when the BHJ films show similar morphology, the electron transfer process become a rate-determining step. Large polarizibility of the excited state increases the degree of separation between electron and hole in the exciton, thus lowering the overall Coulomb binding energy and enhancing the electron transfer rate. Even though the PV effect in BHJ cell is a complex phenomenon, we demonstrated that the parameter Δμge may be used to screen possible new monomers and polymers for high-performance solar cells. The verification of these proposed explanations remains the subject of future study in which transient absorption spectral analyses of the exciton and charge transfer states will be used to confirm the effect of internal dipole on charge separation in the polymers reported herein. Further applications of this effect to design high-performance solar cell polymers are the subject of ongoing research. ASSOCIATED CONTENT

* Supporting Information S

Experimental procedures, H NMR and MALDI-TOF characterization for all new compounds as well as descriptions of instrumentation, device preparation, and measurement. This material is available free of charge via the Internet at http:// pubs.acs.org.

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*E-mail: [email protected] (L.Y.); [email protected] (L.X.C.). Notes

The authors declare no competing financial interest. F

dx.doi.org/10.1021/ma3011119 | Macromolecules XXXX, XXX, XXX−XXX