Synthesis and photovoltaic properties of solution-processable star-shaped small molecules with triphenylamine as the core and alkyl cyanoacetate or 3-ethylrhodanine as the end-group

Two novel star-shaped donor-acceptor small molecules, TPA-TBT-CN and TPA-TBT-R, with triphenylamine (TPA) as the core, benzothiadiazole (BT) as the arm, and alkyl cyanoacetate or 3-ethylrhodanine as the end-group are synthesized for application as donor materials in OSCs. The two small molecule films show broad absorption bands from 300 nm to 850 nm, narrow optical band gaps (1.5-1.7 eV), deep HOMO energy levels (-5.0 to -5.1 eV) and moderate hole mobilities. OSCs based on blends of the two donors and PC₇₀BM acceptors exhibit power conversion efficiencies of 1.34% and 1.79%, respectively. Notably, TPA-TBT-R with 3-ethylrhodanine as the end-group displays a broader solar spectral coverage, a lower HOMO level, a higher hole mobility and higher photovoltaic properties. Our results indicate that 3-ethylrhodanine as the acceptor and end-group is a promising linker in constructing donor materials for high efficiency OSCs.