Improvement in Performances of ZnO:B/i-ZnO/Cu(InGa)Se2 Solar Cells by Surface Treatments for Cu(InGa)Se2

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Abstract
Solar cells based on Cu(InGa)Se2 (CIGS) thin films fabricated by selenization/sulfurization were investigated. We have concentrated on studying the heterojunction quality to improve the efficiency and stability of n-ZnO/i-(atomic layer deposition :ALD)ZnO buffer-layer/p-CIGS structure devices. The effect of heat treatment for CIGS absorbers was studied. It is found that the heat-treatment can remove entities, e.g., excess InxSy, from the surface of CIGSS, which causes interdiffusion in the (ALD)ZnO buffer layer and decreases Voc and FF. We achieved 13.9% efficiency (Voc: 510 mV, FF: 0.736, Jsc: 36.9 mA/cm2) without the use of Cd-related material. Reversible light-soaking and current-injection effects were observed clearly in i-(ALD)ZnO/CIGS-based solar cells. These phenomena can be controlled by changing the CIGS surface conditions. Devices treated with NH4OH as well as deionized water in an ultrasonic bath prior to the growth of buffer layers are not sensitive to light illumination or application of bias voltage in the dark. The origin of the light-soaking/current-injection effect is assumed to be at/near the CIGS surface or grain boundaries.