Small molecule semiconductors for organic photovoltaics: a truncation approach

Abstract

Solution-processable small molecule (SM) organic semiconductors for organic photovoltaics have been in the spotlight for several years. In particular, SM semiconductors have been developed in an attempt to control their film morphology, study their intermolecular interactions, and synthesize new electron donor and electron acceptor subunits. SMs have many advantages over polymers including well-defined molecular structures, monodispersity, and no batch-to-batch dependence. Although SM semiconductors can be designed by truncation from polymers, such examples have rarely been reported. Herein we designed SM semiconductors by truncating a representative polymer, Poly[4-(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b’]dithiophen-2-yl)-alt-benzo[c][1,2,5]thiadiazole] (PBDTBT). Based on density functional theory (DFT) calculations, 2,1,3-benzothiadiazole (BT) was chosen as an electron acceptor subunit instead of thieno[3,4-c]pyrrole-4,6-dione (TPD). The SM semiconductors were end-capped with pyridine derivatives. Thermal, optical and electrochemical properties of these materials were examined to confirm the degradation temperature, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) energy levels, and the band gaps. In particular, the benzyloxyl pyridine-capped semiconductor (SM1) exhibited power conversion efficiency (PCE) of 1.92% which is higher than those shown by the corresponding polymer PBDTBT (0.90% and 1.71%).