Semi‐transparent NiO/ZnO UV photovoltaic cells

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Abstract
Semi‐transparentpn‐heterojunctions were fabricated from pulsed laser deposited (PLD)n‐type ZnO and DC magnetron sputteredp‐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 at2 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 thejVcharacteristics under illumination and thereby low fill factors.

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