Investigation of the Spatially Dependent Charge Collection Probability in CuInS₂/ZnO Colloidal Nanocrystal Solar Cells

Solar cells with a heterojunction between colloidal CuInS₂ and ZnO nanocrystals are an innovative concept in solution-processed photovoltaics (Appl. Phys. Lett. 2013, 103, 133902), but the conversion efficiency cannot compete yet with devices employing lead chalcogenide quantum dots. Here we present a detailed study on the charge collection in CuInS₂/ZnO solar cells. An inverted device architecture was utilized, in which the ZnO played an additional role as optical spacer layer. Variations of the ZnO thickness were exploited to create different charge generation profiles within the light-harvesting CuInS₂ layer, which strongly affected both the external and internal quantum efficiency. By the reconstruction of these experimental findings with the help of a purely optical model, we were able to draw conclusions on the spatial dependency of the charge collection probability. We provide evidence that only carriers generated within a narrow zone of ∼40 nm near the CuIn_S/ZnO interface contribute to the external photocurrent. The remaining part of the absorber can be considered as a “dead zone” for charge collection, which reasonably explains the limited device performance and indicates a direction for future research. From the methodological point of view, the optical modeling approach developed in the present work has the advantage that no electrical input parameters are required, and the approach is believed to be easily transferable to other material systems.