Dual-absorption-layer heterojunction strategy for enhancing photovoltaic performance of all-perovskite tandem solar cell

Organic cations in hybrid organic-inorganic perovskite solar cells are susceptible to decomposition under high temperatures and ultraviolet light, leading their power conversion efficiency (PCE) to decrease. 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 CsPbI<sub>2</sub>Br has received much attention as a top cell in semi-transparent and tandem solar cells due to its excellent thermal stability and suitable bandgap (1.90 eV). Although the PCE of CsPbI<sub>2</sub>Br-based solar cells is approaching its theoretical limit, the energy loss caused by non-radiative recombination remains a major barrier to further improving performance. This non-radiative recombination is mainly caused by inadequate band alignment between the absorption layer and the transport layer, resulting in the loss of open-circuit voltage (<i>V</i><sub>OC</sub>) and decrease of short-circuit current density (<i>J</i><sub>SC</sub>). Two-dimensional perovskite passivation formed through solution processing can mitigate interfacial recombination, but it can also 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-pCsPbI<sub>2</sub>Br-CsPbIBr<sub>2</sub>) into the absorption layer of the top cell in an all-perovskite tandem solar cell. The simulation result indicates that stacking a 100-nm-thick CsPbIBr<sub>2</sub> layer atop a 300-nm-thick CsPbI<sub>2</sub>Br layer greatly enhances the PCE of the single-junction device from 19.46% to 22.29%. This improvement is mainly attributed to band bending at the CsPbI<sub>2</sub>Br/CsPbIBr<sub>2</sub> 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 CsPbI<sub>2</sub>Br top cell, the DPHJ-based tandem solar cell significantly increases <i>V</i><sub>OC</sub> from 2.16 to 2.25 V and <i>J</i><sub>SC</sub> from 15.96 to 16.76 mA⋅cm<sup>–2</sup>. 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 a wavelength range of 500–580 nm, which can be attributed to the band-edge absorption of CsPbIBr<sub>2</sub>. This enhanced absorption generates more photogenerated carriers, thereby significantly improving the <i>J</i><sub>SC</sub>. The <i>V</i><sub>OC</sub> and PCE values in this study exceed those experimentally reported values of current CsPbI<sub>2</sub>Br single-junction and all-perovskite tandem solar cells. Compared with the single-layer CsPbI<sub>2</sub>Br (<i>E</i><sub>2</sub> = 101.9 meV, electron-phonon coupling strength <inline-formula><tex-math id="M3">\begin{document}$ {\gamma _{{\text{ac}}}} = 1.2 \times {10^{ - 2}},{\text{ }}{\gamma _{{\text{LO}}}} = 6.9 \times {10^3} $\end{document}</tex-math></inline-formula>), the double-absorption-layer film exhibits a high exciton binding energy (<i>E</i><sub>2</sub> = 110.7 meV) and reduced electron-phonon coupling strength (<inline-formula><tex-math id="M4">\begin{document}$ {\gamma _{{\text{ac}}}} = 1.1 \times {10^{ - 2}},{\text{ }}{\gamma _{{\text{LO}}}} = $\end{document}</tex-math></inline-formula><inline-formula><tex-math id="M4-1">\begin{document}$ 6.3 \times {10^3} $\end{document}</tex-math></inline-formula>), which helps suppress phase segregation and enhance both optical and thermal stability, which is favorable for fabricating long-term stable all-perovskite tandem solar cells. This work provides new ideas and theoretical guidance for improving the efficiency and stability of all-perovskite tandem solar cells. In addition, it also proposes a universal design concept for optimizing absorption layers in all-perovskite multijunction cells, which is expected to further advance the research in this field.