Visible Light–Induced Degradation of Inverted Polymer:Nonfullerene Acceptor Solar Cells: Initiated by the Light Absorption of ZnO Layer

Power conversion efficiencies (PCEs) of polymer solar cells (PSCs) have exceeded 18% in the last few years. Stability has therefore become the next most important issue before commercialization. Herein, the degradation behaviors of the inverted PM6:IT-4F (PBDB-T-2F:3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) solar cells with different ZnO layers are systematically investigated. The PCE decay rates of the cells and the photobleaching process of the IT-4F containing organic films on ZnO surface are directly correlated with the light-absorption ability of the ZnO layer in the visible light range, indicating that photochemical decomposition of IT-4F is initiated by the light absorption of ZnO layer. By analyzing the products of the aged ZnO/IT-4F films with matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), it is confirmed that photochemical reactions at the IT-4F/ZnO interface include de-electron-withdrawing units and dealkylation on the side-phenyl ring. Hydroxyl radicals generated by the photo-oxidation of dangling hydroxide by ZnO are confirmed by electron spin resonance (ESR) spectroscopy measurements, which is attributed as the main reason causing the decomposition of IT-4F. Surface treatment of ZnO with hydroxide and/or hydroxyl radical scavenger is found to be able to improve the stability of the PSCs, which further supports the proposed degradation mechanism.