Analysis of loss mechanisms in Ag2ZnSnSe4 Schottky barrier photovoltaics
Recently, a new type of photovoltaic device based on an n-type Ag2ZnSnSe4 (AZTSe) absorber was demonstrated with an efficiency of over 5%. This work examines in detail several critical loss mechanisms in FTO/AZTSe/MoO3/ITO Schottky barrier devices. It is shown that the Schottky barrier height in the present devices under illumination is only ∼0.82 eV. With a barrier height of this magnitude, the device has an upper-bound of only 6.4% to the efficiency. Second, the AZTSe thin films used in the present devices exhibit exceptionally short minority carrier lifetimes (≤225 ps), as probed using two-photon spectroscopy. Third, hysteresis is observed in the J-V response of these devices. Pulsed electrical measurements reveal that reverse-bias voltage pulses create residual shunts in the device proportional to the pulse amplitude and duration. Reversible Ag-doping of the MoO3 buffer is proposed as an explanation of the observed shunting upon reverse bias. Therefore, improving the contact materials and the minority carrier lifetime is identified to be critically important for improving AZTSe-based photovoltaics. Finally, AZTSe is found to be photo-conductive with a Hall carrier density that increases by two orders of magnitude upon illumination. The origin of this effect is still unknown.