- In all solar cells, and especially in extremely thin absorber (ETA) solar cells, proper energy band alignment is crucial for efficient photovoltaic conversion. However, available tabulated data usually do not agree with actual results, and in most cases, Voc values lower than expected are achieved. In fact, ETA cells suffer from a very low Voc/Egap ratio, such as in ZnO/CdS/CuSCN cells. Here, we investigate limiting factors of ZnO/CdS/CuSCN ETA cells, applying X-ray photoelectron spectroscopy (XPS), chemically resolved electrical measurement (CREM), Kelvin probe, and I–V characterization. We show that electric fields are gradually developed in the cell upon increased absorber thickness. Moreover, an accumulation layer, unfavorable for the solar cell function, has been revealed at the oxide–absorber interface An effective chemical treatment to prevent formation of this accumulation layer is demonstrated.