Highly efficient organic solar cells with improved stability enabled by ternary copolymers with antioxidant side chains

Abstract The stability of organic solar cells (OSCs) remains a major concern for their ultimate industrialization due to the photo, oxygen, and water susceptibility of organic photoactive materials. Usually, antioxidant additives are blended as radical scavengers into the active layer. However, it will induce the intrinsic morphology instability and adversely affect the efficiency and long-term stability. Herein, the antioxidant dibutylhydroxytoluene (BHT) group has been covalently linked onto the side chain of benzothiadiazole (BT) unit, and a series of ternary copolymers D18-Cl-BTBHTx (x = 0, 0.05, 0.1, 0.2) with varied ratio of BHT-containing side chains have been synthesized. It was found that the introduction of BHT side chains would have a negligible effect on the photophysical properties and electronic levels, and the D18-Cl-BTBHT0.05: Y6-based OSC achieved the highest power conversion efficiency (PCE) of 17.6%, which is higher than those based active layer blended with BHT additives. More importantly, the unencapsulated device based on D18-Cl-BTBHTx (x = 0.05, 0.1, 0.2) retained approximately 50% of the initial PCE over 30 hours operation under ambient conditions, significantly outperforming the control device based on D18-Cl (90% degradation in PCE after 30 h). This work provides a new structural design strategy of copolymers for OSCs with simultaneously improved efficiency and stability.