Chloride side-chain engineered quinoxaline-based D-A copolymer enabling non-fullerene organic solar cells with over 16% efficiency

The simple halogenation strategy of side-chains has been proven an effective approach to boost the photovoltaic performance of organic solar cells (OSCs). Herein, two novel D-A copolymer donors, namely PBDTTS-2FQx and PBDTTS-2ClQx, comprising an alkylthiothiophene benzodithiophene (BDTTS) as donor unit, alkyl substituted thiophene as the pi-bridges and an identical molecular framework but alkoxy substituted fluorobenzene (or chlorobenzene) side chains on the quinoxaline (Qx) as acceptor units, are first developed and compared in parallel. The PBDTTS-2ClQx with chlorobenzene side chains on the Qx unit exhibits a distinct redshifted ab-sorption, suppressed energy levels, increased extinction coefficient and electron mobility compared with the counterpart PBDTTS-2FQx bearing fluorobenzene side-chains on the Qx unit. After blended with BTP-eC9 as non-fullerene acceptor (NFA), the blend film of PBDTTS-2ClQx:BTP-eC9 shows higher and balanced hole/electron mobilities, more favorable aggregation, as well as less charge carrier recombination and better molecular order. As a result, the OSCs based on PBDTTS-2ClQx:BTP-eC9 deliver an impressive power conversion efficiency (PCE) of 16.1% with simultaneously increased fundamental parameters, while the PBDTTS-2FQx-based OSCs exhibits only a PCE of 12.2%. The impressive PCE of 16.1% is by far one of the highest values for binary OSCs with the Qx-based copolymer as donors. This work reveals that chlorine side-chain engineering of the Qx-based copol-ymer donors is a simple and effective approach to further improve their photovoltaic performance.

Automated summary

The simple halogenation strategy of side-chains has been proven effective in boosting the photovoltaic performance of organic solar cells. In this study, two novel D-A copolymer donors, PBDTTS-2FQx and PBDTTS-2ClQx, were developed and compared. The PBDTTS-2ClQx with chlorobenzene side chains exhibited redshifted absorption, suppressed energy levels, increased extinction coefficient, and improved electron mobility compared to PBDTTS-2FQx with fluorobenzene side chains. After blending with BTP-eC9, the PBDTTS-2ClQx:BTP-eC9 blend film showed higher and balanced hole/electron mobilities, improved aggregation, reduced charge carrier recombination, and better molecular order. Consequently, the OSCs based on PBDTTS-2ClQx:BTP-eC9 achieved an impressive power conversion efficiency of 16.1%, while the PBDTTS-2FQx-based OSCs only reached 12.2%. Chlorine side-chain engineering of the Qx-based copolymer donors was identified as a simple and effective approach to further enhance their photovoltaic performance.