Silver-Alloyed Low-Bandgap CuInSe2 Solar Cells for Tandem Applications

Abstract

Photovoltaic conversion efficiency of CuInSe2 solar cells is limited by low fill factor (FF) and open-circuit-voltage (V OC) values compared to Si or perovskite solar cells. Herein, small quantities of Ag to alloy CuInSe2 to improve its properties are used, such as enhanced grain growth, higher crystal quality, and less detrimental defects, to overcome the device limitations of low-bandgap CuInSe2 absorbers. The impact of Ag on the electronic properties of the bulk material and the buffer–absorber interface is examined at different stoichiometric compositions. Ag alloying improves the morphology with larger grain sizes, extends carrier lifetimes, and elevates net doping densities. Ag alloying reduces Cu-depleted ordered-vacancy compounds at the buffer–absorber interface and causes a chalcopyrite phase of high-crystal-quality independent of the I/III ratio. It is suggested Ag affects the formation of the alkali-rich surface layer (Rb–In–Se). The best solar cell with an Ag-alloyed CuInSe2 absorber achieves a V OC over 600 mV and FF values of about 74%, which result in a power conversion efficiency of 18.7% for low-bandgap energy of 1.0 eV with short-circuit current above 42 mA cm−2. The advantage of Ag alloying on the device performance of CuInSe2 solar cells will impact future applications in tandem devices.