Abstract
The near-infrared (NIR) absorbing fused-ring electron acceptor, COi8DFIC, has demonstrated very good photovoltaic performance when combined with PTB7-Th as a donor in binary organic solar cells (OSCs). In this work, the NIR acceptor was added to state-of-the-art PBDBT-2F:IT4F-based solar cells as a third component, leading to (i) an efficiency increase of the ternary devices compared to the binary solar cells in the presence of the highly crystalline COi8DFIC acceptor and (ii) much-improved photostability under 1-sun illumination. The electron transport properties were investigated and revealed the origin of the enhanced device performance. Compared to the binary cells, the optimized ternary PBDBT-2F:COi8DFIC:IT4F blends exhibit improved electron transport properties in the presence of 10% COi8DFIC, which is attributed to improved COi8DFIC molecular packing. Furthermore, transient absorption spectroscopy revealed a slow recombination of charge carriers in the ternary blend. The improved electron transport properties were preserved in the ternary OSC upon aging, while in the binary devices they seriously deteriorated after simulated 1-sun illumination of 240 h. Our work demonstrates a simple approach to enhance both OSC efficiency and photostability.