Organic cations in hybrid organic-inorganic perovskite solar cells are susceptible to decomposition under high temperatures and ultraviolet light, leading to a decline in power conversion efficiency (PCE). All-inorganic perovskite solar cells exhibit both high PCE and superior photothermal stability, making them promising candidates for single-junction and tandem photovoltaic applications. The mixed-halide perovskite CsPbI2Br has garnered significant attention as a top cell in semi-transparent and tandem solar cells owing to its excellent thermal stability and suitable bandgap (1.90 eV). Although the PCE of CsPbI2Br-based solar cells is nearing its theoretical limit, the non-radiative recombination-induced energy losses remaining a major barrier to further performance enhancement. This non-radiative recombination is primarily caused by inadequate band alignment between the absorption layer and the transport layer, resulting in open-circuit voltage (VOC) losses and reduced short-circuit current density (JSC). Two-dimensional perovskite passivation formed via solution processing could mitigate interfacial recombination, but it also could impede efficient charge transport. Constructing three-dimensional perovskite structures not only provides an effective solution to these limitations but also enhances sunlight absorption and facilitates carrier transport. In this study, we propose a Dual-absorption-layer Perovskite Heterojunction (DPHJ) strategy, which involves integrating a staggered type-II perovskite heterojunction (p-pCsPbI2Br-CsPbIBr2) into the absorption layer of the top cell in an all-perovskite tandem solar cell. The result of simulation indicates that stacking a 100 nm-thick CsPbIBr2 layer atop a 300 nm-thick CsPbI2Br layer significantly enhances the PCE of the single-junction device from 19.46% to 22.29%. This improvement is primarily attributed to band bending at the CsPbI2Br/CsPbIBr2 interface, which enhances the built-in electric field, facilitates carrier transport, and suppresses non-radiative recombination within the absorption layer. Compared with the tandem solar cell utilizing a single-absorption-layer CsPbI2Br top cell, the DPHJ-based tandem solar cell significantly increases the VOC (from 2.16 V to 2.25 V) and enhances the JSC (from 15.96 mA×cm-2 to 16.76 mA×cm-2). As a result, the DPHJ-based tandem solar cell achieves a high theoretical PCE of 32.47%. In addition, the DPHJ-based tandem solar cell exhibits a significantly enhanced external quantum efficiency in the 500-580 nm wavelength range, which could be attributed to the band-edge absorption of CsPbIBr2. This enhanced absorption generates more photogenerated carriers, thereby significantly improving the JSC. The results of this study surpass the experimentally reported VOC and PCE values of current CsPbI2Br single-junction and all-perovskite tandem solar cells. Further experimental results show that compared with the single-layer CsPbI2Br (E2= 101.9 meV, electron-phonon coupling strength γac=1.2×10-2，γLO=6.9×103), the double-absorption-layer film exhibits a higher exciton binding energy (E2= 110.7 meV) and reduced electron-phonon coupling strength (γac=1.1×10-2，γLO=6.3×103), which helps suppress phase segregation and enhances both optical and thermal stability—favorable for fabricating long-term stable all-perovskite tandem solar cells. By focusing on absorption layer design, this work provides new insights and theoretical guidance for enhancing the efficiency and stability of all-perovskite tandem solar cells. It presents a versatile design concept for optimizing absorption layers in all-perovskite multijunction cells and is expected to drive further advancements in this field.
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