Significant Enhancement of Illumination Stability of Nonfullerene Organic Solar Cells via an Aqueous Polyethylenimine Modification.

Device stability under illumination is the main obstacle of nonfullerene (NF) organic solar cells for moving toward practical application. ZnO, a generally used electron-transporting layer in inverted cells, is prone to induce the decomposition of NF acceptors under illumination with air mass 1.5 (AM1.5) spectrum, resulting in poor device stability. Herein, we report an aqueous polyethylenimine (a-PEI) modification on the ZnO surface could significantly enhance the stability of the NF organic solar cells. After 1000 h of AM1.5 illumination, the efficiency of the cell without a-PEI modification degrades to 43% of its initial value, while the cell with a-PEI modification could maintain 75% of its initial efficiency. The a-PEI modification reduces the number of surface defects with reduced adsorbed oxygen ZnO surface, faster work function recovery kinetics after UV irradiation, and suppressed electron spin resonance response. The reduction of surface defects is beneficial to the stability of NF acceptors on ZnO and also device performance.