## 3 Results and Discussion  
The morphology of the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film was investigated by SEM. Figure 2 showsthe top-view SEM morphology of the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM films following thermal annealing at various temperatures. The as-formed CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM films exhibited full surface coverage and were composed of crystal grains ranging from tens of nm to one \(\upmu\)m in size. It has also been evidenced that the nano-crystal grain size increased following the higher annealing temperature. A densely crystallized CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film could be acquired after sintering at the annealing temperature. However, the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film surface appearance at 400 \({}^{\circ}\)C, in Figure 2a, exhibited relatively smaller crystal grains from tens to 300 nm, also involving more pinholes. This defect phenomenon could facilitate incomplete bonding between the multilayer films. Figure 2b showsthe CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film surface appearance from 500 \({}^{\circ}\)C annealing with crystallization from tens to 600 nm in grain size, involving fewer pinholes at the same resolution. Figure 2c shows the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film surface annealed at 600 \({}^{\circ}\)C. It exhibited crystals from tens nm to 1 \(\upmu\)m in grain size, with scarce pinholes at the same resolution. The lower annealing temperature, on this occasion, led to a film surface involved in more defects or pinholes. Figure 1: A schematic diagram of the perovskite solar cell with ultra-thin CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) hole-transporting material layer. Figure 3 displays the X-ray diffraction spectra of the multi-layer structures. The MAPbI\({}_{3}\) film's nano-crystals exhibit one main crystal plane (110) corresponding to the diffraction peak at \(\sim\)14.3\({}^{\circ}\), indicating a strong preferential orientation in the growth direction. The other MAPbI\({}_{3}\) film's crystal plane (220) corresponds to the diffraction peak at \(\sim\)29.2\({}^{\circ}\), and crystal plane (310) corresponds to the diffraction peak at \(\sim\)32.4\({}^{\circ}\). On the other hand, the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film nano-crystals exhibit one main crystal plane (112) diffraction peak at \(\sim\)26.8\({}^{\circ}\), indicating a strong preferential orientation in the growth direction. It also shows crystal planes (204) (220) diffraction peak at \(\sim\)44.9\({}^{\circ}\), and crystal planes (312) (316) diffraction peak at \(\sim\)53.0\({}^{\circ}\). The bi-layer Mo film's nano-crystals exhibit one main crystal plane (110) diffraction peak at \(\sim\)40.5\({}^{\circ}\). Other bi-layer Mo film nano-crystals include a crystal plane (200) diffraction peak at \(\sim\)58.6\({}^{\circ}\) and a crystal plane (211) diffraction peak at \(\sim\)73.5\({}^{\circ}\). Additionally, the MoSe\({}_{2}\) film nano-crystals exhibit one main crystal plane (011) diffraction peak at \(\sim\)13.5\({}^{\circ}\), and crystal planes (224) (314) diffraction peak at \(\sim\)36.5\({}^{\circ}\), and crystal plane (002) diffraction peak at \(\sim\)57\({}^{\circ}\). The crystallization of MAPbI\({}_{3}\) active absorber layer was preceded by the ascension annealing temperature of the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film. The function of the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film was a hole-transporting layer and the function of MAPbI\({}_{3}\)was as an active photovoltaic layer. In addition, the CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM film and the MAPbI\({}_{3}\) film could be sintered together. Furthermore, the MAPbI\({}_{3}\) and CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM films' energy band gapscan be complementary for trapping light simultaneously.