## 3 Results and Discussion
As a result, it is composed of pairs of an excited electron and an associated  
Figure 2: Top-view scanning electron microscope images of the ultra-thin CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\) hole-transporting material layer following thermal annealing at the various temperatures (**a**) 400 \({}^{\circ}\)C, (**b**) 500 \({}^{\circ}\)C, and (**c**) 600 \({}^{\circ}\)C.  
electron-hole for recombination. Subsequently, it can escalate the photon-electron power-conversion efficiency and the associated optical-current for the multilayer structure cells. Figure 4 shows the photoluminescence (PL) spectra of the MAPbI\({}_{3}\)/CIGSe/bi-layer Mo/FTO glass following thermal annealing at the various temperatures. One main peak of the PL spectrum wavelength at \(\sim\)767 nm is observed. The PL spectrum is light emission from any form of matter after the absorption of photons or electromagnetic radiation. The intensity is associated with the lifetime of the injected electrons and holes that were combined to form excitons in the solar cells. The excitons revealed mobile concentration energy formation by an excited electron and an associated electron-hole. As the excitons intensified during the examination, the number of excitons was enhanced and the electron/electron-hole recombined with each other. As the annealing temperature was increased from 400 to 600 \({}^{\circ}\)C, the PL intensity of the MAPbI\({}_{3}\) film deposited on the CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM film was enhanced. The MAPbI\({}_{3}\)/CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM nano-crystals grown by annealing resulted in ohmic-contact construction and low contact resistance of the CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM film with bi-layer Mo film. The CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM film was conceived to havegood contact with the active MAPbI\({}_{3}\) absorbing layer in the solar cells to improve device stability. Figure 3: The X-ray diffraction pattern results of CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) (MAPbI\({}_{3}\)) perovskite solar cells with CuIn\({}_{1-\mathrm{x}}\)Ga\({}_{\mathrm{x}}\)Se\({}_{2}\) HTM (hole-transporting material) multi-layer structures. Figure 5 shows the SEM cross-sectional view of the MAPbI\({}_{3}\) on CIGSe HTM/bi-layer Mo/FTO glass multi-layer structure, with annealing at 600 \({}^{\circ}\)C. The solar cells have been composed of a relatively smooth CIGSe surface, involving few pinholes. Nevertheless, a complete solar cell multi-layer structure of Ag/C\({}_{60}\)/MAPbI\({}_{3}\)/CIGSe/Mo/FTO was clearly revealed with a densely packed columnar architecture. Hereafter, Figure 6 shows a Tauc plot for measuring the energy band gap (\(E_{\rm g}\)) at 1.25 eV for the CuIn\({}_{1-\rm x}\)Ga\({}_{\rm x}\)Se\({}_{2}\) HTM grown on a bi-layer Mo/FTO glass-substrate. The band-gap was obtained by extrapolating the linear portion of \((\it{ahv})^{2}\) vs. \(\it{hv}\) graph, where \(\alpha\), \(\it{h}\) and \(\
u\) represent the absorption, Planck constant and radiation frequency [65, 66]. The energy band gap is required to elevate a valence electron bound to an atom to become a conduction electron and a hole, which can be free to move as charge carriers to conduct an electrical current. Figure 4: PL (photoluminescence) spectra of MAPbI\({}_{3}\)/CIGSe HTM/bi-layerMo/FTO (fluorine-tin-oxide) glass following various thermal annealing temperatures at 400, 500, and 600 \({}^{\circ}\)C.  
Figure 5: The field-emission SEM cross-sectional view of CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) (MAPbI\({}_{3}\)) the perovskite solar cell witha CuIn\({}_{1-\rm x}\)Ga\({}_{\rm x}\)Se\({}_{2}\) HTM/bi-layer Mo/FTO glass substrate. The CuIn\({}_{1-\rm x}\)Ga\({}_{\rm x}\)Se\({}_{2}\)HTM layer was annealed at 600 \({}^{\circ}\)C.  
Figure 7 shows the plotted current-density voltage (J-V) curves of MAPbI\({}_{3}\) perovskite solar cells with the ultra-thin CuIn\({}_{1-x}\)Ga\({}_{x}\)Se\({}_{2}\)HTM nano-structures on bi-layer Mo/FTO glass substrates.