  Solar cells with a heterojunction between colloidal CuInS$_2$ and ZnO
nanocrystals are an innovative concept in solution-processed photovoltaics, 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 CuInS$_2$/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 CuInS$_2$ 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 circa 40 nm near the CuInS$_2$/ZnO interface contribute to the
external photocurrent. The remaining part of the absorber can be considered as
"dead zone" for charge collection, which reasonably explains the limited device
performance and indicates a direction for future research. From the methodical
point of view, the optical modeling approach developed in the present work has
the advantage that no electrical input parameters are required and is believed
to be easily transferable to other material systems.
