Built-in potential shift and Schottky-barrier narrowing in organic solar cells with UV-sensitive electron transport layers

The performance of organic solar cells incorporating solution-processed titanium suboxide (TiO_x) as electron-collecting layers can be improved by UV illumination. We study the mechanism of this improvement using electrical measurements and electroabsorption spectroscopy. We propose a model in which UV illumination modifies the effective work function of the oxide layer through a significant increase in its free electron density. This leads to a dramatic improvement in device power conversion efficiency through several mechanisms-increasing the built-in potential by 0.3 V, increasing the conductivity of the TiO_x layer and narrowing the interfacial Schottky barrier between the suboxide and the underlying transparent electrode. This work highlights the importance of considering Fermi-level equilibration when designing multi-layer transparent electrodes.