Sn2+-coordinated polyethylenimine as an electron transport layer for high-efficiency and stable inverted organic solar cells

Abstract

Polyethylenimine (PEI) is a widely used and effective electron transport layer (ETL) material for fabricating inverted organic solar cells (OSCs). However, detrimental chemical reactions between PEI and nonfullerene acceptors impede the efficiency and stability enhancement of non-fullerene-based OSCs. In this study, we present a method that utilizes tin(II) ions (Sn²⁺) to modify the properties of PEI. Sn²⁺ can coordinate with the amine groups in PEI, thereby preventing undesirable chemical reactions between PEI and non-fullerene acceptors and enhancing the chemical stability at the interface between the ETL and the active layer. Furthermore, PEI–Sn²⁺ also reduces interface impedance and suppresses trap-assisted recombination, further improving the device performance. As a result, compared to the PEI-based devices with a power conversion efficiency (PCE) of 15.7%, the PEI–Sn²⁺-based devices achieve a PCE of 17.5%, which is one of the highest reported efficiencies for all PEI-based devices. After 1600 hours of thermal stress aging at 65 °C and over 200 hours of continuous 1-sun illumination with maximum power point (MPP) tracking, the PEI–Sn²⁺-based devices retained 90.2% and 80.0% of their initial PCEs, respectively. The innovative Sn²⁺-coordinated PEI in this work significantly enhances both the efficiency and stability of OSCs, which is crucial for the large-scale production of high-efficiency and stable flexible OSCs.