Present Status of Solution-Processing Routes for Cu(In,Ga)(S,Se)2 Solar Cell Absorbers

Photovoltaic technologies offer a sustainable solution to the challenge of meeting increasing energy demands. Chalcopyrite Cu(In,Ga)(S,Se)(2), short CIGS-thin-film solar cells-having intrinsically p-type absorbers with a tunable direct bandgap-exhibits one of the highest stabilized power conversion efficiencies of 23.35%, utilizing absorbers typically fabricated via vacuum deposition methods. Research is increasingly devoted to absorbers deposited by solution processing techniques, which may inherently improve material usage, increase throughput, and lower financial barriers to commercialization. However, the performance of current devices with solution-processed absorbers is still falling short of their vacuum-processed counterparts with record power conversion efficiencies up to 18.7% reported to date. While hydrazine solvent-based routes offer reduced residual impurities, their toxicity poses hindrances to widespread adoption. Alternatively, less toxic and environmentally friendly routes based on protic and aprotic solvents are being researched and are showing promising device efficiencies well above 14%. This review describes the current status of CIGS solar cell absorber layers fabricated by pure solution-based deposition methods, provides a comparison of champion solution-processed devices (with and without hydrazine), and offers an outlook for future improvements.