Semi‐transparent NiO/ZnO UV photovoltaic cells

Abstract

Semi‐transparent pn‐heterojunctions were fabricated from pulsed laser deposited (PLD) n‐type ZnO and DC magnetron sputtered p‐type NiO, working as UV‐active solar cells. The complete cell stack has an average transmission of 46% in the visible spectral range and an optical absorption edge at 380 nm. The diodes exhibit high current rectification of up to eight orders of magnitude at 2 V. Upon illumination with a solar simulator, the devices show photovoltaic activity with open‐circuit voltages of up to 520 mV, short‐circuit current densities of 0.5 mAcm, and a maximum external quantum efficiency of 55%. However, we observed rather low fill factors of the current–voltage characteristics of around 40%, resulting in total power conversion efficiencies of around 0.1% and efficiencies in the UV range of 3.1%. To identify possible loss mechanisms, the voltage‐dependent efficiency of carrier collection was calculated. The data were fitted using a model that considers recombination losses at the NiO/ZnO interface as well as within the electric field region, yielding a high hole interface recombination velocity of 1cm s. We conclude that the carrier collection efficiency is strongly deteriorated by recombination losses at the NiO/ZnO interface, causing a strong bending of the jV characteristics under illumination and thereby low fill factors.