Examining the Effect of the Dipole Moment on Charge Separation in Donor–Acceptor Polymers for Organic Photovoltaic Applications

Abstract

A new low band gap copolymer PBB3 containing [6,6']bi[thieno[3,4-b]thiophenyl]-2,2'-dicarboxylic acid bis-(2-butyloctyl) ester (BTT) and 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b']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.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

“…Several groups performed Density Functional Theory (DFT) calculations in order to get a deeper understanding of the experimental observations [23,25,27,28,31,[38][39][40]57,60,61]. The necessarily limited number of monomers taken into account in the calculations does not, however, allow a quantitative comparison with the experiments.…”

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

“…Very recently, it has been reported that polaron pairs, formed within the polymer domains, have a significant role in photovoltaic action and are envisaged to be as important as excitons, provided their yield is substantial [16][17][18] . Although revealing important phenomena for the functioning of solar cells, these and other optical investigations have focussed on the dynamics of excitons 19 or charge-separated states when the polymer is mixed with the fullerene [20][21][22] . The question, of what the polaron pair generation yield in pristine donor-acceptor co-polymers actually is, remains largely unanswered while being of very large relevance for understanding and improving photovoltaics.…”

Incremental optimization in donor polymers for bulk heterojunction organic solar cells exhibiting high performance

“…PTB7, following the increase in the difference between the ground and excited state dipole moments of the polymers. 67 This result was supported by the long-lived excitonic state in PTBF2 and PBB3 relative to the PTB polymers, which led to a faster rate of recombination of the charges. 67 …”