Effective choice of ZnO formation methodology for highly stable polymer solar cells under damp-heat (85 degrees C/85% RH) and light soaking conditions

The damp-heat stability at 85 degrees C/85% relative humidity (85/85) and photostability for 1000 h of polymer solar cells (PSCs) with various ZnO electron extraction layers (EELs) fabricated using different precursors and methodologies were comprehensively investigated. ZnO nanoparticles (ZnO NPs), ZnO fabricated by the sol-gel reaction, and ZnO formed from a pyrophoric precursor were used in this study. Although the initial performances of the PSCs depending on the type of ZnO EELs are similar, the damp-heat stability and photostability of the PSCs are completely different. 85/85 and light soaking stress generate localized states in the ZnO EELs and their interfaces, resulting in an increase in the recombination loss and a decrease in the collection efficiency and mobility of the PSCs. The most stable PSCs with a PTB7-th:EH-IDTBR-based photoactive layer were achieved using ZnO NPs, exhibiting the lowest initial disorder and highest initial pure ZnO/oxygen defect proportion among the ZnO EELs. The corresponding devices retained 85% and 81% of their initial power conversion efficiencies (PCEs) after damp-heat and photostability tests for 1000 h, respectively. Finally, the stability of state-of-the-art PM6:Y6-based PSCs (PCE similar to 14%) using ZnO EELs was also evaluated to prove their reproducibility with other active materials.