Device Performance of Small-Molecule Azomethine-Based Bulk Heterojunction Solar Cells

Recently, we demonstrated that small-molecule azomethines are promising candidates as electron-donating materials for organic photovoltaic (OPV) devices. Azomethines can be prepared via well-known condensation chemistry, with water being the sole byproduct. Here we present a record power conversion efficiency for azomethine-based small-molecule OPV devices of 2.2%. To understand the underlying physics limiting device performance, devices of the small-molecule azomethine TPA–TBT–TPA were characterized using a range of spectroscopic and electro-optical techniques. Light-intensity-dependent current-density measurement showed the presence of nongeminate charge recombination, which is most likely the result of poor charge mobility. In addition, transient absorption measurements showed a relatively short lifetime for the exciton (120 ps). However, due to the very fast charge dissociation (<300 fs), charge separation is relatively efficient. This knowledge presents a guideline for preparing subsequent generations of compounds with improved device performance.