Origin of the s-shaped JV curve and the light-soaking issue in inverted organic solar cells

Inverted organic solar cells generally exhibit a strong s-shaped kink in the current-voltage characteristics (JV curve) that may be removed by exposure to UV light (light-soaking) leading to a drastically improved performance. Using in-device characterization methods the origin of the light-soaking issue in inverted solar cells employing titanium dioxide (TiO2) as an electron selective layer is clarified. An injected hole reservoir accumulated at the TiO2/organic interface of the pristine device is observed from extraction current transients; the hole reservoir increases the recombination and results in an s-shape in the JV curve of pristine devices. The hole reservoir and the s-shape is a result of the energetics at the selective contact in the pristine device; the effect of UV exposure is to decrease the work function of the indium tin oxide/TiO2-contact, increasing the built-in potential. This hinders the build-up of the hole reservoir and the s-shape is removed. The proposed model is in excellent agreement with drift-diffusion simulations.