Silver Alloying in Highly Efficient CuGaSe2 Solar Cells with Different Buffer Layers

Abstract

This study evaluates the effect of silver alloying, stoichiometry, and deposition temperature of wide-gap (Ag,Cu)GaSe2 (ACGS) absorber films for solar cell applications. Devices using a standard CdS buffer exhibit a strong anticorrelation between the open-circuit voltage (VOC) and short-circuit current density (JSC), with VOC decreasing and JSC increasing toward stoichiometric absorber composition. Increasing the ACGS deposition temperature leads to larger grains and improved JSC, while VOC is not affected. By adding more silver to the absorber (maximum tested [Ag]/([Ag]+[Cu]) [AAC] = 0.4), the widening of the space charge region (SCR) significantly enhances carrier collection. Experimental quantum efficiency spectra can be accurately simulated when assuming a very low diffusion length and perfect collection in the SCR. The highest efficiency of 8.3% (without antireflection coating [ARC]) is reached for an absorber with AAC = 0.4 grown at 600 °C. Replacing CdS by a (Zn,Sn)O buffer with lower electron affinity strongly mitigates interface recombination. Moreover, the VOC–JSC anticorrelation is not evident anymore and the highest efficiency of 11.2% (11.6% w/ARC, VOC = 985 mV, JSC = 18.6 mA cm−2, fill factor = 61.0%) is reached for a close-stoichiometric ACGS solar cell with AAC = 0.4 processed at 650 °C.