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This paper describes the synthesis of low bandgap copolymers incorporating an artificial sweetener derivative, N-alkyl, 3-oxothieno[3,4- d]isothiazole 1,1-dioxide (TID). This new TID unit is identical to the well-known thieno[3,4- c]pyrrole-4,6-dione (TPD) unit except that one carbonyl has been replaced by a sulfonyl group. Semi-empirical calculations on the local dipole moment change between ground and excited states (Δμge) in the repeating units of the new polymer indicate that the replacement of the carbonyl by a sulfonyl group leads to larger Δμge values. The resulting polymers exhibit a diminished power-conversion efficiency (PCE) compared to a bulk heterojunction (BHJ) solar cells with PC71BM as an acceptor, which extends the correlation between PCE and Δμge of single repeating units in p-type polymers to a new regime. Detailed studies show that the strongly electron-withdrawing sulfonyl group is detrimental to charge separation in alternating copolymers containing a TID unit. [ABSTRACT FROM AUTHOR]

Exciton dissociation is a key step for the light energy conversion to electricity in organic photovoltaic (OPV) devices. Here, excitonic dissociation pathways in the high-performance, low bandgap 'in-chain donor-acceptor' polymer PTB7 by transient optical absorption (TA) spectroscopy in solutions, neat films, and bulk heterojunction (BHJ) PTB7:PC71BM (phenyl-C71-butyric acid methyl ester) films are investigated. The dynamics and energetics of the exciton and intra-/intermolecular charge separated states are characterized. A distinct, dynamic, spectral red-shift of the polymer cation is observed in the BHJ films in TA spectra following electron transfer from the polymer to PC71BM, which can be attributed to the time evolution of the hole-electron spatial separation after exciton splitting. Effects of film morphology are also investigated and compared to those of conjugated homopolymers. The enhanced charge separation along the PTB7 alternating donor-acceptor backbone is understood by intramolecular charge separation through polarized, delocalized excitons that lower the exciton binding energy. Consequently, ultrafast charge separation and transport along these polymer backbones reduce carrier recombination in these largely amorphous films. This charge separation mechanism explains why higher degrees of PCBM intercalation within BHJ matrices enhances exciton splitting and charge transport, and thus increase OPV performance. This study proposes new guidelines for OPV materials development. [ABSTRACT FROM AUTHOR]

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) πmax 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 PC71BM 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. [ABSTRACT FROM AUTHOR]

Exciton dissociation pathways within a polymer bulk heterojunction film in a solar cell device is investigated by L. Yu, L. X. Chen, and co‐workers on page 10. The blue fullerene derivative PCBM acts as an electron acceptor and the yellow ribbons represent the investigated low‐bandgap polymer PTB7. The polymer forms both ordered pi‐stacked domains and disordered domains within the film. This image was created by Nanjia Zhou from Northwestern University. [ABSTRACT FROM AUTHOR]