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A ZnO/CuO-based combinatorial heterojunction device library was successfully fabricated by a simple spray pyrolysis technique using ITO-coated glass as the substrate. The combinatorial approach was introduced to analyze the impact of the ZnO and CuO layer thicknesses on the performance of the solar cells. The thickness of the ZnO layer was varied from ∼50 to 320 nm, and the CuO layer was deposited orthogonal to the ZnO thickness gradient. In the case of CuO, the thickness varied from ∼200 to 800 nm. The photovoltaic performance of the cells is strongly dependent on the absorber layer thickness for a particular window layer thickness and reaches a maximum short-circuit current density of 3.9 mA/cm when the absorber layer thickness just crosses ∼700 nm. Reducing the thicknesses of the active layers leads to a sharp decrease in the device performance. It is shown that the entire built-in bias of the heterojunction is created in the absorber layer due to low carrier density. The poor performance of the devices having lower thicknesses is attributed to different interfacial phenomena such as optical losses due to the thin CuO layer, back-contact recombination of the carriers due to the low layer thickness because a minimum heterojunction thickness is required for the formation of the full built-in bias that slows down the recombination of the carriers, and other factors.

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