Silver Alloying in Highly Efficient CuGaSe<sub>2</sub>Solar Cells with Different Buffer Layers
Jan Keller, Lars Stolt, Tobias Törndahl, Marika Edoff
Abstract
This study evaluates the effect of silver alloying, stoichiometry, and deposition temperature of wide‐gap (Ag,Cu)GaSe 2 (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 °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 °C.