Published: November 12, 2011 r2011 American Chemical Society 20468 dx.doi.org/10.1021/ja208642b | J. Am. Chem. Soc. 2011, 133, 20468–20475 ARTICLE pubs.acs.org/JACS Examining the Effect of the Dipole Moment on Charge Separation in DonorAcceptor Polymers for Organic Photovoltaic Applications Bridget Carsten, † Jodi M. Szarko, || Hae Jung Son, † Wei Wang, † Luyao Lu, † Feng He, † Brian S. Rolczynski, ‡,§,|| Sylvia J. Lou, ‡,|| 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 b S Supporting Information ’ INTRODUCTION Organic photovoltaic (OPV) materials represent a highly abundant, low-cost alternative to their inorganic counterparts for conversion of solar energy into electricity in solar cells. Organic materials are flexible and can be easily processed via facile, low-cost processing techniques in solution. 1,2 Despite their numerous advantages, however, the field of organic photovoltaics is still in its early stages. The power conversion efficiency (PCE) of organic solar cells with large areas is still much lower than that of solar cells using inorganic materials, which limits their commercial viability. It is a consensus that organic materials providing PCE of greater than 10% are needed to make them commercially viable. Thus, extensive research effort is focused on the design of new materials with optimized structure and properties, and the develop- ment of device preparation conditions to achieve this goal. The conversion process of photons into electric energy in OPV devices consists of a series of steps. 3 First, when chromo- phores absorb photons possessing energy greater than their band gaps, an exciton is generated. The exciton then diffuses to the donor/acceptor interface where electron transfer to acceptor material, typically a fullerene 4 such as PCBM, may occur to form bound electronhole pairs (charge transfer complexes). Charge transfer complexes must then be further separated into free charge carriers to be transported to cathode and anode to generate the current through the device. One limitation in organic materials is that the exciton lifetime is typically very short; thus the maximum exciton diffusion length Received: September 13, 2011 ABSTRACT: A new low band gap copolymer PBB3 containing [6,6 0 ]bi[thieno[3,4-b]thiophenyl]-2,2 0 -dicarboxylic acid bis- (2-butyloctyl) ester (BTT) and 4,8-bis(2-butyloctyl)benzo[1,2- b:4,5-b 0 ]dithiophene (BDT) units was synthesized and tested for solar cell efficiency. PBB3 showed a broad absorbance in the near-IR region with a substantially red-shifted (by more than 100 nm) λ max at 790 nm as compared to the PTB series of polymers, which have been previously reported. The PBB3 polymer also showed both a favorable energy level match with PCBM (with a LUMO energy level of 3.29 eV) and a favorable film domain morphology as evidenced by TEM images. Despite these seemingly optimal parameters, a bulk heterojunction (BHJ) photovoltaic device fabricated from a blend of PBB3 and PC 71 BM showed an overall power conversion efficiency (PCE) of only 2.04% under AM 1.5G/100 mW cm 2 . The transient absorption spectra of PBB3 showed the absence of cationic and pseudo charge transfer states that were observed previously in the PTB series polymers, which were also composed of alternating thienothiophene (TT) and BDT units. We compared the spectral features and electronic density distribution of PBB3 with those of PTB2, PTB7, and PTBF2. While PTB2 and PTB7 have substantial charge transfer characteristics and also relatively large local internal dipoles through BDT to TT moieties, PTBF2 and PBB3 have minimized internal dipole moments due to the presence of two adjacent TT units (or two opposing fluorine atoms in PTBF2) with opposite orientations or internal dipoles. PBB3 showed a long-lived excitonic state and the slowest electron transfer dynamics of the series of polymers, as well as the fastest recombination rate of the charge-separated (CS) species, indicating that electrons and holes are more tightly bound in these species. Consequently, substantially lower degrees of charge separation were observed in both PBB3 and PTBF2. These results show that not only the energetics but also the internal dipole moment along the polymer chain may be critical in maintaining the pseudocharge transfer characteristics of these systems, which were shown to be partially responsible for the high PCE device made from the PTB series of low band gap copolymers.