Synergistic cation engineering in >10 % efficient kesterite solar cells: Defect control and future prospects

Kesterite solar cells offer a rare convergence of earth-abundance, non-toxicity, low cost, and tunable optoelectronic properties, coupled with compatibility for scalable, industrial fabrication. Despite a favorable Shockley-Queisser limit near 30 %, device efficiencies plateaued at 12.6 % in 2014, with minimal progress for nearly a decade due to complex defect physics, severe voltage deficits, and non-ideal grain boundary behavior. Recent breakthroughs in cation engineering, culminating in a record 15.1 % efficiency, mark a critical turning point and have reignited efforts to fully realize the potential for commercial applications. This review provides a comprehensive analysis of synergistic cation engineering strategies, including isovalent and heterovalent substitution, alkali metal doping, and co-doping for mitigating intrinsic point defects, related defect complexes, and grain boundaries. These strategies influence the crystal lattice, modulate defect formation energies, enhance crystallinity, promote grain growth, and tailor the band gap and optoelectronic properties. Co-doping and multi-alloying approaches have emerged as particularly powerful tools for synergistically tuning defect chemistry, carrier dynamics, and band structure, often outperforming single-element doping in both efficiency and stability. Case studies are critically examined encompassing isovalent alloying (Cu+→Ag+; Zn2+→Cd2+, Ba2+, Mn2+; Sn4+→Ge4+, Si4+, Ti4+), alkali metal doping (Li+, Na+, K+), heterovalent substitution (Ga3+, La4+, In3+, Sb3+), co-alloying/co-doping (Ag+-Cd2+, Ag+-Li+, Ag+-Ge3+, Ag+-H+, Ag+-Pd2+, Li+-Na+, Na+-Cs+, Ge4+-Cd2+), and multinary alloying (Ag+–Cd2+–Ge3+), with emphasis on elucidating their underlying mechanisms for performance enhancement. Focusing primarily on devices exceeding 10 % efficiency, this review outlines key trends, challenges, and future opportunities, offering a roadmap for the rational design of next-generation high-efficiency kesterite photovoltaics.