Light-Induced Degradation of Polymer: Fullerene Photovoltaic Devices: An Intrinsic or Material-Dependent Failure Mechanism?

Although degradation mechanisms in organic photovoltaic devices continue to receive increased attention, it is only recently that the initial light-induced failure, or so-called burn-in effect, has been considered. Both prototypical polythiophene: fullerene and polycarbazole: fullerene systems exhibit an exponential performance loss of approximate to 40% upon 150 h of continuous solar illumination. While the decrease in both the short-circuit current (J(SC)) and open-circuit voltage (V-OC) is the origin of performance loss in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT: PC 60 BM), in poly(N-9'-hepta-decanyl-2,7-carbazole-alt-5,5-(4', 7'-di-2-thienyl-2', 1', 3'-benzothiadiazole)):[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT: PC70 BM) the decline of the fill factor dominates. By systematic variation of the interface layers, active layer thickness, and acceptor in polythiophene: fullerene cells, the loss in J(SC) is ascribed to a degradation in the bulk of the P3HT: PC60 BM, while the drop in V-OC is reversible and arises from charge trapping at the contact interfaces. By replacing the C-60 fullerene derivative with a C-70 derivative, or by modifying the electron transport layer, the J(SC) or V-OC, respectively, are stabilized. These insights prove that the burn-in process stems from multiple concurrent failure mechanisms. Comparing the ageing and recovery processes in P3HT and PCDTBT blends results in the conclusion that their interface failures differ in nature and that burn-in is a material dependent, rather than an intrinsic, failure mechanism.