Semi-transparent NiO/ZnO UV photovoltaic cells

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 +/- 2V. 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(2), 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 1 x 10(5) cm s(-1). 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. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim