Relationship between Charge-transfer State Electroluminescence and the Degradation of Organic Photovoltaics

Previous studies of organic photovoltaics have related open-circuit voltage to charge transfer state emission efficiency using a theory based on detailed balance. We modify this theory by distinguishing between electroluminescent external quantum efficiency and external charge transfer emission efficiency, and use it to quantify the degradation of archetype vacuum- and solution-processed devices. The presence of aging behavior is included using an empirical factor, m, whose value varies for degradation occurring within, m = 1, and outside, m > 1, of the photoactive heterojunction. It is observed that m = 1.02 ± 0.01 for vacuum-processed devices, and m = 2.93 ± 0.09 for solution-processed devices. We conclude that vacuum-processed devices exhibit degradation within the heterojunction while solution-processed devices exhibit degradation in regions peripheral to the heterojunction. This characterization suggests that the reliability of solution-processed devices can be extended by improving the stability of layers and interfaces external to the heterojunction.