Ethylenediammonium-Based Hollow Pb/Sn Perovskites with Ideal Band Gap Yield Solar Cells with Higher Efficiency and Stability

The power conversion efficiency (PCE) of halide perovskite solar cells is now comparable to that of commercial solar cells. These solar cells are generally based on multication mixed-halide perovskite absorbers with nonideal band gaps of 1.5-1.6 eV. The PCE should be able to rise further if the solar cells could use narrower-band gap absorbers (1.2-1.4 eV). Reducing the Pb content of the semiconductors without sacrificing performance is also a significant driver in the perovskite solar cell research. Here, we demonstrate that mixed Pb/Sn-based perovskites containing the oversized ethylenediammonium (en) dication, {en}-FA(0.5)MA(0.5)Sn(0.5)Pb(0.5)I(3) (FA = formamidinium, MA = methylammonium), can exhibit ideal band gaps of 1.27-1.38 eV, suitable for the assembly of single-junction solar cells with higher efficiencies. The use of en dication creates a three-dimensional (3D) hollow inorganic perovskite structure, which was verified through crystal density measurements and single-crystal X-ray diffraction structural analysis as well as nuclear magnetic resonance measurements. The {en}FA(0.5)MA(0.5)Sn(0.5)Pb(0.5)I(3) structure has massive Pb/Sn vacancies and much higher chemical stability than the same structure without en and vacancies. This new property reduces the dark current and carrier trap density and increases the carrier lifetime of the Pb/Sn-based perovskite films. Therefore, solar cells using {en}-FA(0.5)MA(0.5)Sn(0.5)Pb(0.5)I(3) light absorbers have substantially enhanced air stability and around 20% improvement in efficiency. After overlaying a thin MABr top layer, we found that the {5% en}FA(0.5)MA(0.5)Sn(0.5)Pb(0.5)I(3) material gives an optimized PCE of 17.04%. The results highlight the strong promise of 3D hollow mixed Pb/Sn perovskites in achieving ideal band gap materials with higher chemical stability and lower Pb content for high-performance single-junction solar cells or multijunction solar cells serving as bottom cells.