Polycrystalline Cu2O photovoltaic devices incorporating Zn(O,S) window layers

• ZnSO4 is identified as a current blocking layer in Cu2O/Zn(O,S) heterojunctions.
• Solar cell performance related to Cu2O/Zn(O,S) interface composition.
• Amount of interfacial ZnSO4 decreases with increasing substrate temperature.
• Optimal device performance achieved at 100 °C.

The tunability of the Zn(O,S) conduction band edge makes it an ideal, earth-abundant heterojunction partner for Cu2O, whose low electron affinity has limited photovoltaic performance with most other heterojunction candidates. However, to date Cu2O/Zn(O,S) solar cells have exhibited photocurrents well below the entitled short-circuit current in the detailed balance limit. In this work, we examine the sources of photocurrent loss in Cu2O/Zn(O,S) solar cells fabricated by sputter deposition of Zn(O,S) on polycrystalline Cu2O substrates grown by thermal oxidation of Cu foils. X-ray photoelectron spectra reveal that Zn(O,S) deposited at room temperature leads to a thin layer of ZnSO4 at the Zn(O,S)/Cu2O interface that impedes current collection and limits the short circuit current density to 2 mA/cm2. Deposition of Zn(O,S) at elevated temperatures decreases the presence of interfacial ZnSO4 and therefore the barrier to photocurrent collection. Optimal photovoltaic performance is achieved at a Zn(O,S) deposition temperature of 100 °C, which enables an increase in the short circuit current density to 5 mA/cm2, although a small ZnSO4 layer is still present. Deposition at temperatures above 100 °C leads to a reduction in photovoltaic performance. Spectral response measurements indicate the presence of a barrier to photocurrent and exhibit a strong dependence on voltage and light bias, likely due to the photodoping of Zn(O,S) layer.