## 1 Introduction
This recombination and reformation phenomenon is more pronounced in ultra-thin CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) solar cells. The ultra-thin CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM structures could be fabricated for the advancement of innovative concepts to increase the CIGSe-based device's performance. It could alsoreducosolar cell device cost, andmaintain perovskite absorber layer's open circuit voltage (V\({}_{\mathrm{oc}}\)), short-circuit current density (J\({}_{\mathrm{SC}}\)), fill factor (FF), and stability [53]. Furthermore, severalHTM materials require lithium salt doping to enhance hole mobility and performance. However, the hygroscopic nature of lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI) could cause a perovskite device todecompose, resulting in problems in stability. Therefore, a few investigators have developed other HTMMs with low cost and high stability for photovoltaic devices [54; 55; 56]. In comparison with other organic HTMs, p-type inorganic HTMs have already been studied due to their long-term stability and lower cost [57; 58; 59; 60]. It has been encouraging to investigate Cu-based inorganic HTMs with good characteristics, and thus ultra-thin CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) can act as a new hole-transporting material in solar cells. The Cu-based chalcopyrite semiconductors, such as CuInS\({}_{2}\), CuInSe\({}_{2}\), CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)S\({}_{2}\) (CIGS), CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) (CIGSe) and CIGSSe, are prospective light-absorbing materials. Recently, CuInS\({}_{2}\) and CuInSe\({}_{2}\) quantum dots were used as inorganic HTMs to replace the organic hole-transporting spiro-OMeTAD [61]. The relatively low PCE of Cu-based chalcopyrite semiconductors still has room for improvement for the future generation solar cells. The energy band gaps of CuInGaS/CuInGaSe/CuInGaSe can be tuned from 0.98 to 2.40 eV, providing a wider optical-response for photovoltaic applications. In addition, fullerene C\({}_{60}\) has been shown to improve metal electrodes by alloying and high-temperature annealing [62; 63; 64]. The nonlinear optical characteristics can be derived from the unique three-dimensional microstructure, moving away from the local restriction or electron coupling in the fullerene. At the same time, fullerene material is quite cost-effective and abundant on the earth. The fullerene film can be deposited by an evaporating process. In this work, the function of a multilayer structurehas been studied by photoluminescence spectroscopy, microscopy, and impedance spectroscopy. This experiments would implement a process for the fabrication of ultra-thin CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM to be a unique Cu-based inorganic HTM for solar cells. The perovskite solar cells with thinned CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM were fabricated using a CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) active absorbing layer for renewable energy applications, suitable for the next generation thin-film solar cellsand also beneficial to environmental protection.