Elucidating the Chain-Extension Effect on the Exciton-Dissociation Mechanism through an Intra- or Interchain Polaron-Pair State in Push–Pull Conjugated Polymers

Abstract

We elucidated chain-extension effects of a benzodithiophene (BDT) and thienopyrroledione-based push−pull conjugated polymer (CP) on its exciton-dissociation mechanism within aggregate systems using transient absoption spectroscopy. The side-group extension CP with benzothiophene on the BDT unit induced H-type excitons with excess energy owing to decreased chain stiffness. This led to interchain polaron-pair (PP)-mediated exciton dissociation. The stiff side-group extended with thienothiophene on the BDT unit also induced H-type excitons, but the decreased energy and breadth of the density of states suppressed the interchain PP-mediated exciton dissociation. The main-chain-extension CP with two thiophenes on either side of the BDT unit has a curved structure disturbing the interchain packing. Thus, the driving force of exciton dissociation between the chains decreased, leading to intrachain PP-mediated exciton dissociation. Our findings can facilitate the development of novel CPs to further increase the efficiencies of polymer solar cells.

Fluorination-Induced Conversion of the Quasi- to the Normal Interchain Packing in Push–Pull Conjugated Polymer Aggregates

“…These findings suggest that the intrachain polaron or S 1 state is the predominant state compared to the interchain polaron state during the dissociation step of the entire exciton-dissociation process in the 40k PBDB-T disordered domains. Moreover, the P + states originating from the intrachain PP states primarily diffuse through the interchain packing channels, as reported for the localized interchain packing domains within CP aggregates with low-crystallinity chains, such as the BDT-based push–pull CP with a benzothiophene side group and the 3-hexylthiophene–thiophene random copolymer …”

“…These results indicate that, for PBDB-T aggregates, the intrachain rigidity is more pronounced in the ordered domains than in the disordered domains. At the same time, the H-type excitonic couplings are more suppressed in the ordered domains than the disordered domains. , …”

“…However, the ratio values of I PL0−0 /I PL0−1 at 70 K to I PL0−0 /I PL0−1 at 298 K were 2.4 under the 532 nm excitation and 1.8 under the 633 nm excitation, suggesting that H-type excitons were more induced in the ordered domains than in the disordered domains, resulting in a diminished J-like behavior signature of PM6 chains. 30,38 The ΔE 0−0 values for PM6 (0.097 for both 532 and 633 nm pump conditions, Table 1) increased compared to those for PBDB-T, with the σ value of PM6 (0.028) being higher than those of 40k (0.026) and 100k PBDB-T (0.025). In contrast, the ΔE 0−1 value decreased under the 633 nm excitation compared to the 532 nm excitation: 0.081 → 0.075 for 100k PM6 (Table 1).…”

“…38,39 The dominant chain aggregation pattern (J-or H-like) within the CP aggregate can be determined by analyzing the temperature-dependent PL peak intensity ratio of the 0−0 transition to the 0−1 transition (I PL0−0 /I PL0−1 ). 30,38 In Figure 3, the PL spectra of the CP thin films were deconvoluted using three Gaussian peak functions to obtain the I PL0−0 /I PL0−1 values, as listed in Table 1. Further details and additional data are provided in Figures S1 and S2/Tables S1 and S2 in the Supporting Information.…”

“…Raman spectra were measured using a WITec Alpha300 R Raman imaging microscope. The TA data of the CP thin films were obtained according to previously published protocols. , …”