Organosulfide–Halide Perovskite Heterojunction Enables Electron Transport Layer-Free CsPbI₃ Perovskite Solar Cells

Inorganic cesium lead iodide (CsPbI3) perovskite with a bandgap of approximately 1.70 eV is ideal for top cells in 2-terminal tandem devices. Fullerene derivatives, widely used as electron transport materials in top cells, suffer from short-wavelength parasitic absorption and high cost. Here, we create a heterojunction by forming a CYS(PbBr2) perovskite layer over CsPbI3 using cysteamine (CYS: +NH3(CH2)2S–) and PbBr2, replacing fullerene derivatives and resulting in a shallower Fermi level. This heterojunction facilitates effective electron transfer and transport in the absence of fullerene derivatives and enables an enhancement of power conversion efficiency from 12.11 to 16.47% for inverted CsPbI3 perovskite solar cells. This intrinsic stable heterogeneous layer effectively stabilizes the CsPbI3 layer through a robust Pb–S bond, rendering PSCs retaining 63% of their initial efficiency after 600 h aging at 20% relative humidity. This work offers a promising fullerene-free strategy which could reduce the absorption loss in the tandem devices.