## 2 Materials and Methods
A thinner Mo film between FTO and CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) HTM could improve the recombination ofpairsof electron-holes at the interface. This phenomenon of recombination provided a means of gaining access to ultra-thin CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) HTM comparablewith the traditional bulk carrier of CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) solar cells. CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) HTM film with a thickness of 100-300 nm was deposited. The films were further treated by thermal annealing at 400, 500, or 600 \({}^{\circ}\)C in a tube furnace for 60 min in order to get exceptional crystallization. The solution-processed MAPbI\({}_{3}\) material was coated on a CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) film and fabricated as an inverted perovskite solar cell. PbI\({}_{2}\) and MAI were dissolved in 1 mL of a co-solvent, involving dimethyl sulfoxide and \(\gamma\)-butyrolactone (volume ratio = 1:1), to form a perovskite precursor solution. The precursor solution was spin-coated on the CuIn\({}_{1-\mathrm{X}}\)Ga\({}_{\mathrm{X}}\)Se\({}_{2}\) at 1000 rpm and 5000 rpm for 10 s and 20 s, respectively, in a glove chamber filled withhigh purity nitrogen (>99.99%). The wet spin-coating of the MAPbI\({}_{3}\) thin-film was processed by dropping 50 \(\upmu\)L of anhydrous toluene at 17 s. It was then annealed at 100 \({}^{\circ}\)C for 10 min. The perovskite film had a thickness of approximately 600 nm. Additionally, C\({}_{60}\) is an n-type semiconductor material and the function is an electronic transport buffer layer in this photovoltaic multilayer nano-structure. This fullerene powder was prepared on a molybdenum metal-boat using a vacuum evaporation system. The C\({}_{60}\) film with a thickness of 50 nm was deposited on the MAPbI\({}_{3}\) film. The fullerene was designated as an electron-transporting buffer layer. It could bond with the upper silver metal-electrode contact layer while conducting the necessary photo-current through the neighboring layers. Finally, the top silver metal-electrode contact film layer was deposited with thickness of 100 nm by athermal vacuum evaporator system (PSE-1.5 KVA). This solar cell device was shadowed with a finger mask to define an active area of 0.5 \(\times\) 0.2 cm\({}^{2}\) during the front silver metal-electrode material deposition. Figure 1 shows the complete device schematic diagram of the MAPbJ\({}_{3}\) perovskite solar cells with an ultra-thin CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) HTM layer. The crystallization results of the nano-structures of the thin-film solar cells were analyzed by a PANalyticalX'Pert Pro DY2840 X-ray diffraction (XRD) system with Cu K\(\alpha\) radiation (\(\lambda\) = 0.154 nm). A field-emission scanning electron microscope system (Zeiss Gemini SEM) was applied to take regard of the nano-crystal surface pattern of the thin-film devices. The photoluminescence (PL) was estimated using a fluorescence spectrophotometer (Hitachi F-7000). The solar cell materials were inocharacterized by micro-Raman spectroscopy (Horiba/Jon YvonLabRAM HR800). The current-density voltage (J-V) characteristics of the devices were measured using a Keithley 2420 programmable source meter system under irradiation from a 1000 W xenon lamp. The measurement forward scan rate was 0.1 V/s. The irradiation power density on the surface of the sample was calibrated to 1000 W/m\({}^{2}\).