Strategies for mitigating surface and interface defects in CIGS/perovskite tandem solar cells: a comprehensive review

Two-terminal (2T) monolithic CIGS/perovskite tandem solar cells exploit complementary light-absorption characteristics to approach the theoretical Shockley-Queisser (SQ) efficiency limit (~ 45%), with recently reported highest power conversion efficiency (PCE) of 31.09% and a certified PCE of 30.57%. Nevertheless, surface and interface defects remain the critical loss mechanism, inducing non-radiative recombination, reducing open-circuit voltage (V_OC), and accelerating degradation pathways. This review provides a systematic analysis of surface and interface defect chemistries and engineering strategies across 2T CIGS/perovskite architectures. We elucidate defect formation mechanisms at the CIGS/perovskite heterojunction and individual layer surfaces, encompassing point defects, grain boundaries, and structural imperfections, and their impact on device performance. Mitigation strategies are critically evaluated, including surface passivation, interface band-alignment engineering, compositional defect tolerance, and interconnection layer optimization. The coupled relationships between defect density, environmental degradation (moisture penetration, thermal cycling), and device reliability are analyzed systematically. Finally, we identify fundamental bottlenecks and research directions toward high efficiencies (≥ 32%), commercial scalability, and long-term operational stability in 2T CIGS/perovskite tandem solar cells.