Silver alloying in Cu(In,Ga)Se 2 photovoltaics: a review of growth, structure, and optoelectronic properties

This review presents a comprehensive analysis of silver-alloyed copper indium gallium selenide (ACIGS) thin-film photovoltaic (PV) technology, focusing on its structural, optoelectronic, technological, and economic implications for next-generation solar cells. The global transition toward low-carbon electricity and the increasing demand for high-efficiency, flexible, and resource-efficient PV systems motivate the exploration of advanced absorber materials beyond conventional crystalline silicon. ACIGS emerges as a promising evolution of CIGS, offering improved grain growth, reduced structural and grain-boundary defect densities, enhanced open-circuit voltage, and lower processing temperatures through partial substitution of copper with silver. The review examines vacuum and non-vacuum growth methods, including co-evaporation, sputtering, solution processing, and hybrid approaches, highlighting their influence on film quality, scalability, and industrial relevance. Structural and thermodynamic modifications induced by silver incorporation are discussed in detail, including lattice expansion, phase stability, and metastable behaviour. The impact of Ag alloying on bandgap engineering, defect passivation, carrier recombination, and band alignment is critically analysed, alongside the role of alkali post-deposition treatments in performance optimisation. Special attention is given to ultrathin absorbers, flexible substrates, bifacial and semi-transparent architectures, and tandem integration with perovskites, demonstrating the versatility of ACIGS across emerging PV applications. Finally, techno-economic and sustainability aspects, including material criticality, cost trade-offs, energy payback, and environmental impact, are evaluated. This review identifies both the opportunities and remaining challenges for ACIGS, providing guidance for future research aimed at enabling its broader commercial deployment.