Tert-butyl carbazole modified non-fused ring electron acceptor generating high triplet state energy level for efficient organic solar cell

Abstract

Two-dimensional side-chain plays a crucial role in the construction of efficient non-fused ring electron acceptors (NFREAs). Herein, we introduce a novel NFREA, PCT-4Cl, featuring a tert-butyl carbazole side-chain, and investigate its optoelectronic properties. Notably, PCT-4Cl exhibits a small singlet-triplet energy gap (ΔEST) of 0.25 eV. Blending PCT-4Cl with host eC9 or C8C8–4Cl acceptor extends the lifetime of singlet excitons and exciton diffusion length in the blend films. Moreover, the high energy level of the triplet state (ET1) in PCT-4Cl facilitates efficient transfer of triplet excitons to eC9 or C8C8–4Cl. The addition of PCT-4Cl to PM6:eC9 also leads to a longer crystallization time, restraining eC9 aggregation and enhancing crystallinity in the ternary blend films. Consequently, power conversion efficiencies (PCEs) of 18.84 % and 15.17 % are achieved based on the optimized ternary devices of PM6:eC9:PCT-4Cl (1:1:0.2, wt./wt./wt.) and PM6:C8C8–4Cl:PCT-4Cl (1:1:0.2, wt./wt./wt.), respectively, surpassing the performance of PM6:eC9 (17.34 %) and PM6:C8C8–4Cl (14.05 %). Furthermore, leveraging 2PACz as a hole transporting layer elevates the PCEs of the ternary OSCs to 19.25 % and 15.53 %. These findings underscore the significant role of the tert-butyl carbazole side-chain in elevating the ET1 of NFREA for efficient charge transfer, as well as its role in modulating the crystallinity, presenting a promising pathway for achieving high performance in both binary and ternary OSCs.