All-perovskite tandem solar cells with 3D/3D bilayer perovskite heterojunction.

All-perovskite tandem solar cells promise higher power-conversion efficiency (PCE) than single-junction perovskite solar cells while maintaining a low fabrication cost 1-3 . However, their performance is still largely constrained by the subpar performance of mixed Pb-Sn narrow-bandgap perovskite subcells, mainly due to a high trap density on the perovskite film surface 4-6 . Although heterojunctions with intermixed 2D/3D perovskites could reduce surface recombination, this common strategy induces transport losses and thereby limits device fill factors 7-9 . Here we develop an immiscible 3D/3D bilayer perovskite heterojunction with type-II band structure at the Pb-Sn perovskite/electron-transport layer interface to suppress the interfacial non-radiative recombination and facilitate charge extraction. The bilayer perovskite heterojunction is formed by depositing a layer of lead-halide wide-bandgap perovskite on top of the mixed Pb-Sn narrow-bandgap perovskite via a hybrid evaporation/solution processing method. This heterostructure allows us to increase the PCE of Pb-Sn perovskite solar cells having a 1.2-µm-thick absorber to 23.8%, together with a high open-circuit voltage (V oc ) of 0.873 V and a high fill factor of 82.6%. We thereby demonstrate a record-high PCE of 28.5% (certified 28.0%) in all-perovskite tandem solar cell. The encapsulated tandem devices retain over 90% of their initial performance after 600 hours of continuous operation under simulated one-sun illumination.