a, Cross-sectional SEM image of an all-perovskite tandem solar cell. b, c, J–V (b) and EQE (c) curves of tandem cells with back subcell thicknesses of 750, 900 and 1,200 nm. d, e, J–V (d) and EQE and total absorptance (1-R) (e) curves of the best tandem device (with aperture area of 0.049 cm2) with approximately 1.2-μm-thick Pb–Sn perovskite absorber layer. R is the primary optical reflectance measured from the front glass side. f, J–V curve of a large-area tandem device (aperture area = 1.05 cm2). g, Continuous MPP tracking of the encapsulated CF3-PA and control tandem solar cells (with aperture area of 0.049 cm2) under simulated 1 Sun illumination (equivalent to AM1.5G, 100 mW cm−2, multicolor LED simulator) in ambient air with a humidity of 30–50%. The CF3-PA and control tandem devices had initial PCEs of 25.6% and 24.3% measured under a two-lamp simulator, respectively. The device temperature was approximately 35 oC during operation, which was a result of self-heating under solar illumination. There was no active cooling during device operation and the environmental temperature was kept at approximately 25 oC. T90 represents the lifetime at which the PCE declines to 90% of its initial value.