The synergistic effect of fluorine atom and alkyl chain positions in enhancing organic photovoltaic open-circuit voltage and morphology miscibility

Abstract

A key to improving the power conversion efficiency (PCE) of Y6-based non-fullerene based organic solar cells (OSCs) is the development of a high open-circuit voltage (V_oc) polymer. In recent years, side-chain engineering has become a promising strategy for researchers to modulate and realize high V_oc. In this study, we adopted a new side-chain to coordinate the molecular crystallinity and miscibility, affording a copolymer L19, in which the benzodithiophene (BDT) and dithienobenzothiadiazole (DTBT) units were used as the electron-sufficient and -defective units, respectively. As a comparison, a non-fluorinated analogue L18 was also synthesized. As a result, the OSCs based on L19:Y6 exhibited an impressive V_oc than L18-based devices (0.93 V vs. 0.89 V) due to further fluorination. L19 had the higher hole mobility of 1.6 × 10⁻³ cm² V⁻¹ s⁻¹ than L18 of 7.13 × 10⁻⁴ cm² V⁻¹ s⁻¹, and better miscibility, thereby achieving a higher PCE of 11.73% for L19 than that of L18 (6.44%). Moreover, the high V_oc of 0.93 V is a good reference for Y-series small molecular acceptor systems. These results indicate that the new side-chains of L19 can synergistically regulate the HOMO level and miscibility between the donor and acceptor, providing more options for the development of a high open-circuit voltage polymer for Y-series based non-fullerene systems.