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

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 (V-OC) and short-circuit current density (J(SC)), with V-OC decreasing and J(SC) increasing toward stoichiometric absorber composition. Increasing the ACGS deposition temperature leads to larger grains and improved J(SC), while V-OC 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 degrees C. Replacing CdS by a (Zn,Sn)O buffer with lower electron affinity strongly mitigates interface recombination. Moreover, the V-OC-J(SC) anticorrelation is not evident anymore and the highest efficiency of 11.2% (11.6% w/ARC, V-OC = 985 mV, J(SC) = 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 degrees C.