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.

全钙钛矿串联太阳能电池比单结钙钛矿太阳能电池具有更高的功率转换效率（PCE），同时保持较低的制造成本^1-3。然而，它们的性能仍然在很大程度上受到混合 Pb-Sn 窄带隙钙钛矿子电池性能不佳的限制，这主要是由于钙钛矿薄膜表面上的高陷阱密度^4-6。尽管混合 2D/3D 钙钛矿的异质结可以减少表面复合，但这种常见策略会导致传输损失，从而限制器件填充因子^7-9。在这里，我们在 Pb-Sn 钙钛矿/电子传输层界面开发了一种具有 II 型能带结构的不混溶 3D/3D 双层钙钛矿异质结，以抑制界面非辐射复合并促进电荷提取。双层钙钛矿异质结是通过混合蒸发/溶液处理方法在混合 Pb-Sn 窄带隙钙钛矿顶部沉积一层铅卤化物宽带隙钙钛矿形成的。这种异质结构使我们能够将具有 1.2 µm 厚吸收层的 Pb-Sn 钙钛矿太阳能电池的 PCE 提高到 23.8%，同时具有 0.873 V 的高开路电压 (V oc ) 和 82.6% 的高_填充因子。因此，我们证明全钙钛矿串联太阳能电池的 PCE 达到创纪录的 28.5%（认证为 28.0%）。在模拟单太阳光照下连续运行 600 小时后，封装的串联器件仍能保持 90% 以上的初始性能。