Work

Thickness-engineered cascaded (FA)2BiCuI6/Cs2SnBr6 perovskite solar cells with ∼ 30% efficiency in Pb-Free photovoltaics

Year: 2026

Type: article

Abstract: We propose and numerically investigate a cascaded dual-absorber architecture that combines the formamidinium-stabilized double perovskite (FA) 2 BiCuI 6 and the vacancy-ordered perovskite Cs 2 SnBr 6 to achieve broad, complementary light harvesting while preserving lead-free chemistry. Device structures were modelled in SCAPS-1D under AM1.5G (1000 W·m −2 ) and 300 K, using P3HT as the hole transport layer, TiO 2 as the electron transport layer, and FTO/Ag contacts. The baseline single-absorber device FTO/TiO 2 /(FA) 2 BiCuI 6 /P3HT/Ag reproduces the simulated performance (V OC = 0.95 V, J SC = 11.37 mA·cm −2 , FF = 87.62%, PCE = 9.49%). Inserting Cs 2 SnBr 6 , which acts as both a complementary absorber and a band-alignment interlayer, between TiO 2 and (FA) 2 BiCuI 6 produces a cascaded (FTO/TiO 2 /Cs 2 SnBr 6 /(FA) 2 BiCuI 6 /P3HT/Ag) with markedly improved photon collection and reduced recombination, raising simulated PCE to 25.97% (V OC = 1.05 V, J SC = 28.20 mA·cm −2 , FF = 87.13%). Systematic thickness optimization reveals strong sensitivity of device metrics to absorber geometries: a global optimum is predicted at (FA) 2 BiCuI 6 = 200 nm and Cs 2 SnBr 6 = 2500 nm, where PCE reaches 29.43% alongside V OC = 1.07 V, J SC = 31.12 mA·cm −2, and FF = 87.74%. The optimized device incorporates a 200 nm (FA) 2 BiCuI 6 absorber with a donor density of 1 × 10 13 cm −3 and a 2500 nm Cs 2 SnBr 6 absorber with an acceptor density of 1 × 10 18 cm −3 , while maintaining low interface defect densities at both the TiO 2 /Cs 2 SnBr 6 and Cs 2 SnBr 6 /(FA) 2 BiCuI 6 interfaces. Mechanistically, the performance gain arises from spectral complementarity between the two absorbers, improved internal quantum efficiency, and mitigation of interfacial recombination through energetic and physical buffering by Cs 2 SnBr 6 . These findings establish cascaded, lead-free double-perovskite stacks as a promising route toward high-efficiency, environmentally friendly photovoltaics and provide specific guidance for experimental realization via thickness engineering and interface control.

Source: Materials & Design

DOI: 10.1016/j.matdes.2026.117088