Tuning terminal units to improve the photovoltaic performance of small molecules based on a large planar fused-ring core in solution-processed organic solar cells

In this report, two novel small molecules (SMs) with a D(A-Ar)(2) type molecular backbone based on a large planar carbazole (CZ) fused-ring central core (TBTDBC), grafting an electron-withdrawing diketopyrmlopyrrole (DPP) as bridged acceptor moieties and appending with or without a CZ as end-capping units, namely TBTDBC(DPP)(2) and TBTDBC(DPPCZ)(2), are rationally synthesized and characterized by H-1 NMR, MS, elemental, electrochemical and photophysical analysis. The influences of the optoelectronic, carrier mobilities, morphological properties, and likewise photovoltaic performances of the both materials were systematically investigated. The TBTDBC (DPPCZ)(2) presented higher hole mobility and better crystallinity than the TBTDBC(DPP)(2) analogs. After optimized, a power conversion efficiency (PCE) of 3.11%% is obtained for the device based on TBTDBC(DPP)(2)/PC71M, and a higher 3.47% achieved by the TBTDBC(DPPCZ)(2)/PCB71M-based device. These results indicate that an efficient strategy via inserting a large fused ring central core and grafting a suitable terminal unit can enhance the photovoltaic performance of the corresponding organic solar cells (OSCs).