Low-Cost CuIn 1 − x Ga x Se 2 Ultra-Thin Hole-Transporting Material Layer for Perovskite/CIGSe Heterojunction Solar Cells

: This paper presents a new type of solar cellwith enhanced optical-current characteristics using an ultra-thin CuIn 1 − x Ga x Se 2 hole-transporting material (HTM) layer (<400 nm). The HTM layer was between a bi-layer Mo metal-electrode and a CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite active absorbing material. It promoted carrier transportand led to an improved device with good ohmic-contacts. The solar cell was prepared as a bi-layer Mo/CuIn 1 − x Ga x Se 2 /perovskite/C 60 /Ag multilayer of nano-structures on an FTO (ﬂuorine-doped tin oxide) glass substrate. The ultra-thin CuIn 1 − x Ga x Se 2 HTM layers were annealed at various temperatures of 400, 500, and 600 ◦ C. Scanning electron microscopy studies revealed that the nano-crystal grain size of CuIn 1 − x Ga x Se 2 increased with the annealing temperature. The solar cell results show an improved optical power conversion efﬁciency at ~14.2%. The application of the CuIn 1 − x Ga x Se 2 layer with the perovskite absorbing material could be used for designing solar cells with a reduced HTM thickness. The CuIn 1 − x Ga x Se 2 HTM has been evidenced to maintain a properopen circuit voltage, short-circuit current density and photovoltaic stability.


Introduction
Hybrid organic-inorganic perovskite materials exhibit the advantages of easy fabrication, a large absorption coefficient, an adjustable energy band gap, high carrier mobility, and a long charge carrier diffusion length [1][2][3][4]. Metal halide-based perovskite solar cells (PSCs) have aimed for a better power conversion efficiency (PCE) for the next generationof photovoltaics [5][6][7].It should be pointed out that even if a lot of effort has been spent on the development of novel alternative solar cells like quantum dot solar cells [8] and single-material organic solar cells [9], perovskite-like solar cells are the most promising type of photovoltaic devices. To comprehend the green renewable energy demand, not only the cost-effectiveness of photovoltaic systems but also the expansion of its installation locations should be carefully established. Since the utility market facility has to regulate its growth rate, building-integrated photovoltaics is a potential candidate to solve such green renewable energy expectation. toa serial resistance. The Schottky barrier has been reported with a height of approximately 0.8 eV for the p-doped CuInSe 2 (011)/Mo metal-contact film [41]. The molybdenum-diselenide (MoSe 2 ) film has significant effects on the behavior of the Mo/CuIn 1−x Ga x Se 2 layer interface [42,43]. Mo-O and Mo-O-Se compounds were discovered while being selenized with a Mo film deposited on the substrate with a CuInS film [44]. For a substrate temperature higher than 550 • C, the MoSe 2 film thickness increased witha higher annealing temperature [45]. The nano-crystal film orientation and growth rate of the MoSe 2 film was dependent upon the selenization temperature. It has been indicated that the construction of a MoSe 2 film at the Mo/CuIn 1−x Ga x Se 2 layer interface during a three-stage process using the annealing procedure [46]. However, the MoSe 2 film did not form under copper-rich conditions, and the characteristics of the MoSe 2 film relied on the CuIn 1−x Ga x Se 2 deposition technique and the annealing conditions [47,48].
In the past years, researchers have studied the contribution of nano-materials to the efficiency of ultra-thin CuIn 1−x Ga x Se 2 solar cells; that would lead to even thinner and more cost-effective devices [49,50]. In addition, a highly defective CuIn 1−x Ga x Se 2 /TCO interface could affect the connection to the depletion region in a thinned CuIn 1−x Ga x Se 2 solar cell, resulting in a high recombination rate between the absorber layer and the back contact layer [51,52]. This recombination and reformation phenomenon is more pronounced in ultra-thin CuIn 1−x Ga x Se 2 solar cells. The ultra-thin CuIn 1−x Ga x Se 2 HTM structures could be fabricated for the advancement of innovative concepts to increase the CIGSe-based device's performance. It could alsoreducesolar cell device cost, andmaintain perovskite absorber layer's open circuit voltage (V oc ), short-circuit current density (J 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 HTMs witha 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−x Ga 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−x Ga x S 2 (CIGS), CuIn 1−x Ga 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/CuInGaSSe 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−x Ga x Se 2 HTM to be a unique Cu-based inorganic HTM for solar cells. The perovskite solar cells with thinned CuIn 1−x Ga 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.

Materials and Methods
The FTO/glasssubstrate (Hartford Glass, TECA7-RL, IN, USA) was cleaned using acetone (Sigma-Aldrich, MO, USA), ethanol (Sigma-Aldrich, MO, USA), and de-ionizedwater, each for 10 min, and then dried in an oven. Mo film has good conductive properties and was deposited onthe substrate Appl. Sci. 2019, 9, 719 4 of 15 by a radio-frequency (RF) magnetron sputtering system. It was extremely salient to ascertain optimal sputtering conditions for Mo film because good adherence and low resistivity were necessary to ensure a high efficiency for solar cells. In order to ameliorate conventional architecture to develop a novel solar cell device structure, a thinner Mo material was the second back metal-electrode contact layer. The FTO material was the first layer of the back contactelectrode. Thus, the FTO also worked as an adhesion layer in the multilayer-structure of solar cells. Using a transparent conductive oxide film, such as FTO, leads to recombination to substantially lower the absorption at the Mo back metal-electrode contact layer, enhancing the efficiency for multi-junction solar cells. In this study, an RF sputtering system with a different power in conjunction with a different working Ar flow pressure was explored to prepare bi-layer Mo films. The bi-layer Mo films obtained at a low sputtering pressure experienced compressive stress, which displayed low electrical resistivity but adhered poorly to the substrate. On the other hand, the bi-layer Mo films using high-pressure deposition could experience tensile stress and would adhere well to the substrate but displayed high electrical resistivity. The deposition was thus modified to include a porous bottom layer and a dense top layer. The combination of a higher working pressure and a lower working pressure has been attempted. In this experiment, the bottom layer was deposited at a higher Ar flow pressure and the Ar flow rate and RF power were maintained at 80 sccm and 120 W. Additionally, the top layer was deposited at a lower Ar flow pressure condition and the Ar flow rate and RF power were maintained at 40 sccm and 60 W for better adhesion and to ensure a low resistivity. Each Mo film with an approximate thickness of 100 nm was deposited. The Mo appeared as an excellent choice for the back metal-contact material as a result of its relative stability at the annealing temperature, better ohmic-contact for alloying with copper and indium, and its low contact resistance to the CuIn 1−x Ga x Se 2 solar cells. The Mo film also acted as a reflective layer in the CuIn 1−x Ga x Se 2 solar cells. The CuIn 1−x Ga x Se 2 HTM films adjacent to the back-contact layer can also enhance the role of the back metal-electrode contact material as an optical reflector.
The CuIn 1−x Ga x Se 2 film was further deposited by an RF magnetron system using a CuIn 1−x Ga x Se 2 target on the bi-layer Mo/FTO glass substrates. The argon flow rate and RF power were maintained at 60 sccm and 80 W, respectively. The CuIn 1−x Ga x Se 2 film was sputtered on bi-layer Mo metal-electrode films as a hole-transport material layer. For ultra-thin CuIn 1−x Ga x Se 2 HTM (<400 nm), a surface roughness effect transforms the optical absorbing thickness, which provides shunt paths between the back-electrode and the front-electrode through the optical-conductive buffer layer. At the CIGSe-Mo interface, the diffusion of Se could escalate the structural quality of the CuIn 1−x Ga x Se 2 solar cells, the adherence of CuIn 1−x Ga x Se 2 /Mo metal-contact and the electrical contact at CuIn 1−x Ga x Se 2 /Mo metal-contact interface.
A thinner Mo film between FTO and CuIn 1−x Ga 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−x Ga x Se 2 HTM comparablewith the traditional bulk carrier of CuIn 1−x Ga x Se 2 solar cells. CuIn 1−x Ga 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 • 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−x Ga 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 γ-butyrolactone (volume ratio = 1:1), to form a perovskite precursor solution. The precursor solution was spin-coated on the CuIn 1−x Ga 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 µL of anhydrous toluene at 17 s. It was then annealed at 100 • 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 athickness of 100 nm by athermal vacuum evaporator system (PSE-1.5KVA). This solar cell device was shadowed with a finger mask to define an active area of 0.5 × 0.2 cm 2 during the front silver metal-electrode material deposition. Figure 1 shows the complete device schematic diagram of the MAPbI 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 Kα radiation (λ = 0.154nm). 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 alsocharacterized 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 . with a finger mask to define an active area of 0.5 × 0.2 cm 2 during the front silver metal-electrode material deposition. Figure 1 shows the complete device schematic diagram of the MAPbI3 perovskite solar cells with an ultra-thin CuIn1-xGaxSe2 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 Kα radiation (λ = 0.154nm). 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 alsocharacterized 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 .

Figure 1.
A schematic diagram of the perovskite solar cell with ultra-thin CuIn1-xGaxSe2 hole-transporting material layer.

Results and Discussion
The morphology of the CuIn1-xGaxSe2 HTM film was investigated by SEM. Figure 2 showsthe top-view SEM morphology ofthe CuIn1-xGaxSe2 HTM films following thermal annealing at various temperatures. The as-formed CuIn1-xGaxSe2 HTM films exhibited full surface coverage and were composed of crystal grains ranging from tens of nm to one μm in size. It has also been evidenced that the nano-crystal grain size increased following the higher annealing temperature. A densely crystallized CuIn1-xGaxSe2 HTM film could be acquired after sintering at the annealing temperature. However, the CuIn1-xGaxSe2 HTM film surface appearance at 400°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 CuIn1-xGaxSe2 HTM film surface appearance from 500°C annealing with crystallization from tens to 600 nm in grain size, involving fewer pinholes at the same resolution. Figure 2c shows the CuIn1-xGaxSe2 HTM film surface annealed at 600°C. It exhibited crystals from tens nm to 1 μm 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 3 displays the X-ray diffraction spectra of the multi-layer structures. The MAPbI3 film's nano-crystals exhibit one main crystal plane (110) corresponding to the diffraction peak at ∼14.3°, indicating a strong preferential orientation in the growth direction. The other MAPbI3 film's crystal plane (220) corresponds to the diffraction peak at ∼29.2°, and crystal plane (310) corresponds to the

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 ofthe 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 µm 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 • 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 • 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 • C. It exhibited crystals from tens nm to 1 µm 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. diffraction peak at ∼32.4°. On the other hand, the CuIn1-xGaxSe2 HTM film nano-crystals exhibit one main crystal plane (112) diffraction peak at ∼26.8°, indicating a strong preferential orientation in the growth direction. It also shows crystal planes (204) (220) diffraction peak at ∼44.9°, and crystal planes (312) (316) diffraction peak at ∼53.0°. The bi-layer Mo film's nano-crystals exhibit one main crystal plane (110) diffraction peak at ∼40.5°. Other bi-layer Mo film nano-crystals include a crystal plane (200) diffraction peak at ∼58.6° and a crystal plane (211) diffraction peak at ∼73.5°. Additionally, the MoSe2 film nano-crystals exhibit one main crystal plane (011) diffraction peak at ∼13.5°, and crystal planes (224) (314) diffraction peak at ∼36.5°, and crystal plane (002) diffraction peak at ∼57°. The crystallization of MAPbI3 active absorber layer was preceded by the ascension annealing temperature of the CuIn1-xGaxSe2 HTM film. The function of the CuIn1-xGaxSe2 HTM film was a hole-transporting layer and the function of MAPbI3was as an active photovoltaic layer. In addition, the CuIn1-xGaxSe2 HTM film and the MAPbI3 film could be sintered together. Furthermore, the MAPbI3 and CuIn1-xGaxSe2 HTM films' energy band gapscan be complementary for trapping light simultaneously. As a result, it is composed of pairs of an excited electron and an associated 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 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 ∼14.3 • , indicating a strong preferential orientation in the growth direction. The other MAPbI 3 film's crystal plane (220) corresponds to the diffraction peak at ∼29.2 • , and crystal plane (310) corresponds to the diffraction peak at ∼32.4 • . 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 ∼26.8 • , indicating a strong preferential orientation in the growth direction. It also shows crystal planes (204) (220) diffraction peak at ∼44.9 • , and crystal planes (312) (316) diffraction peak at ∼53.0 • . The bi-layer Mo film's nano-crystals exhibit one main crystal plane (110) diffraction peak at ∼40.5 • . Other bi-layer Mo film nano-crystals include a crystal plane (200) diffraction peak at ∼58.6 • and a crystal plane (211) diffraction peak at ∼73.5 • . Additionally, the MoSe 2 film nano-crystals exhibit one main crystal plane (011) diffraction peak at ∼13.5 • , and crystal planes (224) (314) diffraction peak at ∼36.5 • , and crystal plane (002) diffraction peak at ∼57 • . 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. As a result, it is composed of pairs of an excited electron and an associated 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 MAPbI3/CIGSe/bi-layer Mo/FTO glass following thermal annealing at the various temperatures. One main peak of the PL spectrum wavelength at ∼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°C, the PL intensity of the MAPbI3 film deposited on the CuIn1-xGaxSe2   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 ∼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 • C, the PL intensity of the MAPbI 3 film deposited on the CuIn 1−x Ga x Se 2 HTM film was enhanced. The MAPbI 3 /CuIn 1−x Ga x Se 2 HTM nano-crystals grown by annealing resulted in ohmic-contact construction and low contact resistance of the CuIn 1−x Ga x Se 2 HTM film with bi-layer Mo film. The CuIn 1−x Ga 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. 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°C, the PL intensity of the MAPbI3 film deposited on the CuIn1-xGaxSe2   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 • 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 g ) at 1.25 eV for the CuIn 1−x Ga x Se 2 HTM grown on a bi-layer Mo/FTO glass-substrate. The band-gap was obtained by extrapolating the linear portion of (αhν) 2 vs. hν graph, where α, h and ν 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. HTM film was enhanced. The MAPbI3/CuIn1-xGaxSe2 HTM nano-crystals grown by annealing resulted in ohmic-contact construction and low contact resistance of the CuIn1-xGaxSe2 HTM film with bi-layer Mo film. The CuIn1-xGaxSe2 HTM film was conceived to havegood contact with the active MAPbI3 absorbing layer in the solar cells to improve device stability. Figure 5 shows the SEM cross-sectional view of the MAPbI3 on CIGSe HTM/bi-layer Mo/FTO glass multi-layer structure, with annealing at 600°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/C60/MAPbI3/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 (Eg) . Figure 6. Tauc plot for measuring the energy band gap for CuIn1-xGaxSe2 HTM grown on a bi-layer Mo/FTO glass substrate. The red line was used to extrapolate the band-gap.    Furthermore, the fill factor has been defined as the ratio of the output power Pmax to the device open-circuit voltage and short-circuit current at the maximum electric power output. This is the maximum power of the rectangle in the current-voltage characteristic curve. In accordance with the current-density voltage curve and the PCE value, it has been suggested that the optimal thickness of the CuIn 1−x Ga x Se 2 hole-transporting film layer is at~200 nm. This sample exhibited a slightly higher open-circuit voltage along with the lowest device series-resistance. In this study, it has been desired to develop ultra-thin HTM layers for solar cell applications. The CIGS film quality has been improved and the HTM layer thickness was decreased. In fact, the 300 nm sample also exhibited a reasonably good PSC performance. On the other hand, when the CIGS layer was tried at a thickness below 100 nm (such as 30-50 nm, not shown in this report), not only was the PSC performance was degraded but also the deviation increased appreciably.
An ultra-thin CuIn 1−x Ga x Se 2 HTM (<400 nm) has been deposited between the MAPbI 3 active absorber material and the bi-layer Mo metal-electrode to promote carrier transport, leading to an improved device witha goodohmic-contact. Additionally, innovative structures of bi-layer Mo films were deposited onto FTO/glass substrates. Eventually, Figure 8 shows the ultra-thin CuIn 1−x Ga x Se 2 hole-transporting material detailed investigation on the compositional dependence of the Raman spectra, grown on a bi-layer Mo/FTO glass substrate. In this work, the CuIn 1−x Ga x Se 2 HTM film exhibited dominant spectra with an intense Raman scattering peak at 174 cm −1 , corresponding to the optical phonon mode that was characteristic of the chalcopyrite nano-crystals and nano-structures. The Raman intensities at 174 cm −1 , 214 cm −1 and 240 cm −1 were slightly increased while increasing the annealing temperature. The CuIn 1−x Ga x Se 2 HTM crystal orientation could be found from the polarization of Raman-scattered light with respect to the laser light if the crystal structure's point group was known. . Figure 8. Compositional dependence of Raman spectra of CuIn1-xGaxSe2 hole-transporting material layer that was grown on a bi-layer Mo/FTO glass substrate.
reasonably good PSC performance. On the other hand, when the CIGS layer was tried at a thickness below 100 nm (such as 30-50 nm, not shown in this report), not only was the PSC performance was degraded but also the deviation increased appreciably. An ultra-thin CuIn1-xGaxSe2HTM (<400 nm) has been deposited between the MAPbI3 active absorber material and the bi-layer Mo metal-electrode to promote carrier transport, leading to an improved device witha goodohmic-contact. Additionally, innovative structures of bi-layer Mo films  Figure 9 shows the device measured external quantum efficiency (EQE) curve results due to optical modification with CuIn 1−x Ga x Se 2 hole-transporting material. The spectra included experimentally measured EQE curves with an entire major spectral feature, which reproduced in both wavelength and amplitude. Notably, the EQE results with ultra-thin CuIn 1−x Ga x Se 2 HTM exhibited the maximum EQE of~78% for the 600 • C-annealed sample, observed between 500 nm and 600 nm. It was attributed to the combined effects of Fabry-Perot resonances and efficient coupling into the waveguide modes of the absorber layer [67,68]. Planar CuIn 1−x Ga x Se 2 solar cells contained translational invariant and an electric field distribution by resonances.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In summary, sputtering-processed CuIn 1−x Ga x Se 2 nano-crystals have been successfully applied as a novel Cu-based inorganic HTM for solar cells. The PCE of the CuIn 1−x Ga x Se 2 HTM-based photovoltaic device has been reached at 14.2%, which is quite encouraging for conventional solar cells with Cu-based chalcopyrite semiconductor HTMs. In addition, the CuIn 1−x Ga x Se 2 HTM/MAPbI 3 perovskite layers that involved MoSe 2 exhibited a good ohmic-contact. This work provided a promising candidate of Cu-based inorganic HTM for stable perovskite solar cells, and dedicated investigation about characteristics of silver/C 60 /CH 3 NH 3 PbI 3 /CIGSe/bi-layer Mo/FTO multilayer of nano-structured devices. The SEM morphology observation on CuIn 1−x Ga x Se 2 HTM film following thermal annealing at 400, 500, and 600 • C revealed crystal grains ranging from tens nm to one µm in size. The pinhole and defect density wasalso decreased with the increasing annealing temperature. The experimental results of MAPbI 3 perovskite solar cells with the optimal 200 nm CuIn 1−x Ga x Se 2 HTM nano-structures that were annealed at 600 • C showed a better PCE of 14.2%. This device exhibited that the V OC , J SC , FF, PCE, Rs, and Pmax were 1.02 V, 20.9 mA/cm 2 , 66.6%, 14.2%, 15.8 Ω, and 1.42 mW, respectively.