Since the second industrial revolution, social development driven by oil has led to a serious greenhouse effect [[1], [2], [3], [4]]. To alleviate the greenhouse effect and promote clean energy, perovskite solar cells (PSCs) have undergone rapid development [[5], [6], [7], [8]]. The first generation of PSCs were mainly organic-inorganic hybrid PSCs. For example, in 2009, Kojima et al. first used CH3NH3PbI3 as the perovskite light-absorbing material. Since then, more and more organic-inorganic PSCs have entered people's fields of vision. Although the preparation process is simple, their stability is poor, and they are easily affected by light and humidity. The service life is relatively short compared to traditional silicon solar cells. Moreover, the materials contain toxic elements such as lead, which pose potential risks to the environment and health. Therefore, it is necessary to develop more stable and non-toxic PSCs [9]. To overcome the drawback of the easy decomposition of organic-inorganic PSCs at room temperature, metal cations such as Rb+ are usually used as substitutes. Replacing MA+ with Rb+ can improve the photoelectric conversion efficiency of perovskite solar cells. This is because using Rb+ can increase the lattice constant and crystal symmetry of the perovskite, effectively reducing lattice defects and improving charge transfer efficiency. For the toxic heavy metal lead, Sn and Ge are generally used as substitutes. For Sn, although it can effectively reduce environmental pollution, its large ionic radius can cause lattice distortion and affect electron transport in the absorber layer. Moreover, Sn is prone to undergo redox reactions in PSCs, leading to reduced stability. Ge is not only an environmentally friendly material but also more widely distributed on Earth than Pb. In addition, Ge has higher chemical stability in PSCs, which can reduce degradation and damage during long-term use. Compared to Pb, Ge ions have a smaller radius, which can reduce lattice distortion and defects, thereby improving charge transfer efficiency. Therefore, using Ge as a substitute for the toxic heavy metal Pb is undoubtedly a good strategy [10]. In PSCs that are too thick, metal cations are prone to dissolve and migrate, and the thick PSCs may also cause electrolyte diffusion, thereby reducing the stability of the PSCs. Designing ultra-thin PSCs can not only reduce the use of materials and production costs but also reduce the stress generated inside the PSCs when the temperature and humidity change, thereby improving the stability of the PSCs. Therefore, it is necessary to design ultra-thin PSCs.

In this work, we designed an ultra-thin PSC with a thickness of only 700 mm. By substituting the unstable organic cations with the metal cation Rb+ and replacing the toxic Pb element and unstable Sn ions in PSCs with Ge, we've introduced notable enhancements. Additionally, through Density Functional Theory (DFT) analysis of RbGeI3, we discovered its excellent mechanical stability and resilience. This characteristic allows for reduced stress during fabrication and operation, promising prolonged stability in the long term.

To make the simulation closer to reality, we passivated the surface of the perovskite absorber layer, namely the interface defect layer (IDL), which played a role in the charge transport layer to some extent. We optimized the thickness, bandgap, defect density, electron affinity, and other parameters of each layer through the structure model of the cell established by SCAPS and calculated their optimal values. In addition, we also used DFT to calculate the RbGeI3, including its density of states (DOS), partial density of states (PDOS), charge difference density, and elastic properties. Our work greatly reduces carrier recombination and provides a reference for future research on Ge-based ultra-thin PSCs. In summary, our work doesn't merely focus on improving efficiency; rather, it strikes a balance between efficiency enhancement and device stability. This dual consideration provides a reference for the future design of PSCs with long-term stability.