Recent Advances in Wide Bandgap Perovskite Solar Cells: Focus on Lead-Free Materials for Tandem Structures

A tandem solar cell, which is composed of a wide bandgap (WBG) top sub-cell and a narrow bandgap (NBG) bottom subcell, harnesses maximum photons in the wide spectral range, resulting in higher efficiency than single-junction solar cells. WBG (>1.6 eV) perovskites are currently being studied a lot based on lead mixed-halide perovskites, and the power conversion efficiency of lead mixed-halide WBG perovskite solar cells (PSCs) reaches 21.1%. Despite the excellent device performance of lead WBG PSCs, their commercialization is hampered by their Pb toxicity and low stability. Hence, lead-free, less toxic WBG perovskite absorbers are needed for constructing lead-free perovskite tandem solar cells. In this review, various strategies for achieving high-efficiency WBG lead-free PSCs are discussed, drawing inspiration from prior research on WBG lead-based PSCs. The existing issues of WBG perovskites such as V_OC loss are discussed, and toxicity issues associated with lead-based perovskites are also addressed. Subsequently, the natures of lead-free WBG perovskites are reviewed, and recently emerged strategies to enhance device performance are proposed. Finally, their applications in lead-free all perovskite tandem solar cells are introduced. This review presents helpful guidelines for eco-friendly and high-efficiency lead-free all perovskite tandem solar cells.