Effects of Annealing on Characteristics of Cu2ZnSnSe4/CH3NH3PbI3/ZnS/IZO Nanostructures for Enhanced Photovoltaic Solar Cells

This paper presents new photovoltaic solar cells with Cu2ZnSnSe4/CH3NH3PbI3(MAPbI3)/ZnS/IZO/Ag nanostructures on bi-layer Mo/FTO (fluorine-doped tin oxide) glasssubstrates. The hole-transporting layer, active absorber layer, electron-transporting layer, transparent-conductive oxide layer, and top electrode-metal contact layer, were made of Cu2ZnSnSe4, MAPbI3 perovskite, zincsulfide, indium-doped zinc oxide, and silver, respectively. The active absorber MAPbI3 perovskite film was deposited on Cu2ZnSnSe4 hole-transporting layer that has been annealed at different temperatures. TheseCu2ZnSnSe4 filmsexhibitedthe morphology with increased crystal grain sizesand reduced pinholes, following the increased annealing temperature. When the perovskitefilm thickness was designed at 700 nm, the Cu2ZnSnSe4 hole-transporting layer was 160 nm, and the IZO (indium-zinc oxide) at 100 nm, and annealed at 650 °C, the experimental results showed significant improvements in the solar cell characteristics. The open-circuit voltage was increased to 1.1 V, the short-circuit current was improved to 20.8 mA/cm2, and the device fill factor was elevated to 76.3%. In addition, the device power-conversion efficiency has been improved to 17.4%. The output power Pmax was as good as 1.74 mW and the device series-resistance was 17.1 Ω.


Introduction
Photovoltaic (PV) devices provide electrical energy directly from sunlight, and have been one of the promising technologies in the renewable energy industry. The further improvement in the efficiency and reliability and also reduction in cost are in great demand, for the healthy development of global economy. The organic metal halide materials introduced a new generation ofthin-filmPVs, due to the excellent characteristics for light harvesting in solar cells. The optical-to-electrical power-conversion efficiency (PCE) of the lead halide perovskite-(CH 3 NH 3 PbI 3 , or MAPbI 3 ) based PV device has been increased substantially in recent years [1][2][3][4][5]. The MAPbI 3 substance favors efficient carrier generation and transport to electrodes. It can absorb sunlight radiation from ultra-violet to infrared region. The electron-hole diffusion length could exceed 1 µm in a tri-halide perovskite absorber [6]. Kim et al. developed the first solid-state lead halide perovskite PV with fluorinated tin oxide (FTO)/TiO 2 /MAPbI 3 /2,2 ,7,7 -tetrakis(N,N-di-p-methoxyphenylamino)-,9,9 spiro-bifluorene (SpiroOMeTAD)/Au nanostructures [7]. Later, Jeng et al. reported the first inverted planar structure

Materials and Methods
In this study, bi-layer Mo film was sputtered on FTO glass substrate as a back metal electrode contact layer. The Mo film was prepared by RF magnetron sputtering system using commercial Mo target (Ultimate Materials Technology Co., Miaoli, Taiwan). In this fabrication process, the bottom layer was deposited at a higher Ar flow working pressure, using high-power parameters, and the Ar flow rate and RF power were maintained at 70 sccm and 110 W. On the other hand, the top layer was deposited at a lower Ar flow working pressure, using lower-power parameters, and the Ar flow rate and RF power were maintained at 35 sccm and 55 W for preserving better adhesion. The bi-layer Mo films exhibited both low resistivity and good adhesion. It has been measured that the Mo bi-layer had a film resistivity of 4.37 × 10 −4 Ω-cm, which was much lower than that of single-layer at 2.2 × 10 −2 Ω-cm. Each layer had a thickness of~100 nm. The Mo film also acted as a reflective layer on these multi-layered solar cells. This Mo film that was deposited under high argon pressure would be under tensile stress and adhere successfully with the substrate, but with low conductivity. The deposition by low argon pressure would render compressive stress that had low resistivity but adhered poorly to interface on the substrate.
Moreover, Cu 2 ZnSnSe 4 film was deposited by RF magnetron sputtering using a Cu 2 ZnSnSe 4 target (Ultimate Materials Technology Co., Miaoli, Taiwan) on bi-layer Mo. The argon flow rate and RF power were maintained at 40 sccm and 70 W, respectively. It was adopted as the HTM layer. It should promote the carrier transporting and result in a beneficial device by providing a conductive ohmic-contact. The ultra-thin Cu 2 ZnSnSe 4 HTM (<200 nm) surface roughness corresponded to the optical absorber thickness transition, and could affect interface recombination of electrons and electron-holes. The root-mean-square surface roughness was low at~20 nm, measured by atomic force microscopy. The Cu 2 ZnSnSe 4 film thickness has been prepared at 40-160 nm approximately. It was then further thermally treated by the annealing temperature at 350, 450, 550, or 650 • C in a tube furnace for about 60 min in order to get magnificent crystallization.
The MAPbI 3 film was deposited on grown Cu 2 ZnSnSe 4 HTM layer and by one-step spin-coating process for the inverted structures of perovskite solar cells. The photovoltaic characteristics would Nanomaterials 2020, 10, 521 4 of 16 be investigated to reveal the relationships between the properties and structures. The single-step deposition involved the dissolution of PbI 2 and MAI (CH 3 NH 3 I) in a co-solvent, consisted of equal volumes of dimethyl sulfoxide and gamma-butyrolactone. This perovskite precursor solution was spin-coated using parameters of 1000 and 5000 rpm for 10 and 18 s, respectively, in a nitrogen-filled glove box. The wet film was then quenched by dropping 50 µL of anhydrous toluene at 15 s. Afterwards, the perovskite film was further annealed at 100 • C for 10 min. The MAPbI 3 thin-film had a thickness of 700 nm approximately.
Zinc sulfide film is an n-type semiconductor material and was adopted as an ETL in this multi-layered nanostructure PVs. It was prepared using a commercial target (Ultimate Materials Technology Co., Miaoli, Taiwan) by RF sputtering system. The Ar flow rate and RF power were controlled at 30 sccm and 50 W, respectively. The ZnS film was about 50 nm in thickness. The film was attributed to not only thinner ETL layer deposition, but also to extract electrons from the MAPbI 3 active absorber layer. It also required quenching at 100 • C in a tube furnace for about 10 min to achieve the ideal p-n junction, crystallization, and better ohmic-contact. The ZnS film could improve the multi-layer structures by alloying, plastic distortion and thermal annealing. It would bond with upper IZO transparent conductive oxide film, while conducting the necessary optical-current passing through, thus enhancing optical-to-electrical conversion performance.
Indium-doped zinc oxide is an n-type semiconductor material and was adopted as a transparent conductive oxide film, deposited by RF magnetron sputtering using a commercially available IZO target (Ultimate Materials Technology Co., Miaoli, Taiwan). The deposition parameters were argon flow rate at 30 sccm and RF power at 50 W. The IZO film had a thickness of 100 nm approximately. The low-temperature and high-mobility amorphous nature rendered excellent characteristics for the ZnS (ETL)/MAPbI 3 /CZTSe (HTM) heterojunction solar cells.
At last, the top Ag metal electrode film was deposited over the IZO n-type semiconductor TCO. The Ag metal ingot was prepared on a tungsten metal-boat in the vacuum chamber of an evaporation system. The tungsten boat was connected to an external power supply and provided with a maximum current of 90 A. The Ag metal film had a thickness of~100 nm. A shadow mask has been adopted to define an active area of 0.5 × 0.2 cm 2 during the Ag deposition. The nanostructured PV was investigated by X-ray diffraction (XRD) using PANalytical X'Pert Pro DY2840 system (Malvern Panalytical, Almelo, Netherlands) with Cu Kα (λ= 0.1541 nm) radiation. The crystalline surface morphology was studied by scanning electron microscopy (Zeiss Gemini SEM, Jena, Germany). A micro-Raman spectroscopy analysis was employed using Jon-YvonLabRAM system (Horiba-HR800, Kyoto, Japan). The photoluminescence (PL) results were scanned by a fluorescence spectrophotometer (Hitachi, F-7000, Tokyo, Japan). The electron spectroscopy chemical analysis (ESCA) spectra were examined using PHI-5000 system (ULVAC, Versaprobe-II, Kanagawa, Japan). The solar cell PV characteristics were studied by a Keithley 2420 programmable source instrument under 1000 W xenon illumination with a forward scan rate of 0.1 V/s. Figure 1 shows the complete scheme of the nanostructured MAPbI 3 perovskite solar cell device with the Cu 2 ZnSnSe 4 HTM layer. The corresponding energy levels of the planar architecture device of Ag/IZO/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO are illustrated in Figure 2. The ultra-thin Cu 2 ZnSnSe 4 HTM has been deposited between the bi-layer Mo metal-electrode and the MAPbI 3 active absorber layer to improve carrier transporting. The energy level diagram indicated that it is a heterojunction planar photovoltaic solar cell.

Results and Discussion
a heterojunction is much more heavily doped than the other side, the junction is nearly a one-sided heterojunction. To form a good quality heterojunction, the difference between the neighboring semiconductors' lattice constants should be small in order to minimize the density of interface states. The difference in electron hole affinity between two different materials should be small to minimize band discontinuity, and thermal expansion coefficients should be close as well. MAPbI3 film and Cu2ZnSnSe4 HTM film were likely annealed and sintered altogether.   a heterojunction is much more heavily doped than the other side, the junction is nearly a one-sided heterojunction. To form a good quality heterojunction, the difference between the neighboring semiconductors' lattice constants should be small in order to minimize the density of interface states. The difference in electron hole affinity between two different materials should be small to minimize band discontinuity, and thermal expansion coefficients should be close as well. MAPbI3 film and Cu2ZnSnSe4 HTM film were likely annealed and sintered altogether.     [3,21,31]. The MAPbI 3 film's nano-crystal was illustrated by one main crystal plane (110) corresponding to the 2θ diffraction peak at~14.3 • . The 2θ full-width at half-maximum (FWHM) was reduced from 0.39 • to 0.28 • when the annealing temperature was increased from 350 • C to 650 • C. Other MAPbI 3 crystal planes involved (220) at~29.2 • and (310) at~32.4 • . When the annealing temperature of the Cu 2 ZnSnSe 4 HTM film under the MAPbI 3 film was increased, the FWHM of the Cu 2 ZnSnSe 4 HTM film's nano-crystal was also improved. Its nano-crystal was illustrated by the main crystal plane (112) corresponding to the 2θ diffraction peak at~27.1 • . Its FWHM was reduced from 0.64 • to 0.51 • when the annealing temperature was increased from 350 to 650 • C. Other Cu 2 ZnSnSe 4 crystal planes could be illustrated by (204) at 2θ diffraction peak of~45.1 • , (312) at~53.8 • , and (008) at 65.8 • . The Mo film's nano-crystal was illustrated by the main crystal plane (110) at the 2θ diffraction peak at~40.5 • . Its FWHM was reduced from 0.98 • to 0.83 • when the annealing temperature was increased from 350 to 650 • C. Other Mo film's crystal planes included (200) at~58.6 • , and (211) at~73.5 • . The smaller FWHM demonstrated MoSe 2 nano-crystal and it could be illustrated by the main crystal plane (002) at the 2θ diffraction peak at~13.5 • . The FWHM was reduced from 0.13 • to 0.10 • when the annealing temperature was increased from 350 to 650 • C. Other MoSe 2 crystal planes included (004) at~28.2 • , (006) at~42.7 • , and (008) at~57.5 • . The Cu 2 ZnSnSe 4 HTM film has been annealed at 350, 450, 550, and 650 • C, respectively. As a result, the film's crystal quality was improved following the increased annealing temperature. Interestingly, the crystallization of MAPbI 3 active absorber layer was also preceded by the same annealing temperature. It has been noted that when one side of a heterojunction is much more heavily doped than the other side, the junction is nearly a one-sided heterojunction. To form a good quality heterojunction, the difference between the neighboring semiconductors' lattice constants should be small in order to minimize the density of interface states. The difference in electron hole affinity between two different materials should be small to minimize band discontinuity, and thermal expansion coefficients should be close as well. MAPbI 3 film and Cu 2 ZnSnSe 4 HTM film were likely annealed and sintered altogether. Furthermore, their energy band gaps were similar for trapping light simultaneously. It would be beneficial to constitute pairs of excited electrons and associated electron holes. Eventually, the carriers could increase the optical-electronic power-conversion efficiency and optical-current associated in the photovoltaic cell's multi-layer nanostructures. Figure 4 shows the top-view SEM micrographs of the surface morphology ofCu2ZnSnSe4 HTM layers after the various annealing temperatures. The magnetron sputtered film provided full surface coverage and was composed of small crystal grains ranging from tens of nm to one μm in size. After its deposition, the bi-layer Mo film became indiscernible. However, theCu2ZnSnSe4nano-crystal grainsize was significantly enlarged with the increased annealing temperature. For the 350°C -annealed sample, it exhibited relatively small crystal grains from tens to two hundred nm. It contained some pinhole-type of defects. Figure 4(b) showedCu2ZnSnSe4 film surface appearance with tens to four hundred nm in grain sizefor 450°C . Figure 4(c) showed film surface with tens to six hundred nm in the grain sizefor 550°C . It also contained less pinholes at the same magnification. Figure 4(d)showed theCu2ZnSnSe4 film surface morphology for 650°C. It demonstrated better crystallization, with crystal grains as large as one μm in size. Much less pinholes could be found. Thus, the higher annealing temperature achieved high-quality surface HTM films. Additionally, the Cu2ZnSnSe4 HTM film's hole mobility was increased from 15.1 cm 2 /(V s) to 29 cm 2 /(V s), while the annealing temperature was increased from 350 to 650°C . The higher carrier mobility would help to reduce the device series-resistance, and improve performance for the nanostructured photovoltaic cells. Figure 5 shows the compositional dependence of Raman spectra of the Cu2ZnSnSe4 HTM nano-films after the various annealing temperature treatments. In this fabrication, Cu2ZnSnSe4 HTM film exhibited dominant spectra with intense Raman scattering main peak at 194cm −1 correlating with the optical phonon mode. Its intensity increased slightly with the increased thermal annealing temperature. Other Raman scattering peak intensities were found at 233 and 253 cm −1 . The Cu2ZnSnSe4 HTM crystal orientation could be found from polarization of Raman-scattered light with respect to the laser light, if the crystal structure's point group could be known. Furthermore, their energy band gaps were similar for trapping light simultaneously. It would be beneficial to constitute pairs of excited electrons and associated electron holes. Eventually, the carriers could increase the optical-electronic power-conversion efficiency and optical-current associated in the photovoltaic cell's multi-layer nanostructures. Figure 4 shows the top-view SEM micrographs of the surface morphology of Cu 2 ZnSnSe 4 HTM layers after the various annealing temperatures. The magnetron sputtered film provided full surface coverage and was composed of small crystal grains ranging from tens of nm to one µm in size. After its deposition, the bi-layer Mo film became indiscernible. However, the Cu 2 ZnSnSe 4 nano-crystal grainsize was significantly enlarged with the increased annealing temperature. For the 350 • C-annealed sample, it exhibited relatively small crystal grains from tens to two hundred nm. It contained some pinhole-type of defects. Figure 4b showed Cu 2 ZnSnSe 4 film surface appearance with tens to four hundred nm in grain sizefor 450 • C. Figure 4c showed film surface with tens to six hundred nm in the grain sizefor 550 • C. It also contained less pinholes at the same magnification. Figure 4d showed the Cu 2 ZnSnSe 4 film surface morphology for 650 • C. It demonstrated better crystallization, with crystal grains as large as one µm in size. Much less pinholes could be found. Thus, the higher annealing temperature achieved high-quality surface HTM films. Additionally, the Cu 2 ZnSnSe 4 HTM film's hole mobility was increased from 15.1 cm 2 /(V s) to 29 cm 2 /(V s), while the annealing temperature was increased from 350 to 650 • C. The higher carrier mobility would help to reduce the device series-resistance, and improve performance for the nanostructured photovoltaic cells.   Figure 6 shows the measurement results of the absorbance spectra of Cu2ZnSnSe4 HTM nano-films after the various annealing temperatures. The optical absorption properties of Cu2ZnSnSe4 in the visible region and near-infrared region are associated with electronic transitions and are also useful in comprehending electronic band conformations of semiconducting films. The optical spectra were recorded using a UV (ultraviolet) spectrophotometer at the wavelength range from 300 to 1200 nm. It was observed that the absorbance intensity increased with the increased annealing temperature. This was presumably caused by free-carrier absorption corresponding to conductivity. These absorption spectra illustrated that all Cu2ZnSnSe4 nano-films absorb over the entire visible region of electromagnetic waves. The absorption spectra data were analyzed following a classical equation for near edge optical absorption of semiconductors: αhν = A(hν −Eg) n , where α is absorption coefficient, hν is photon energy, Eg is energy band gap, A is constant, n can have values of 1/2, 2, 3/2 and 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect  Figure 5 shows the compositional dependence of Raman spectra of the Cu 2 ZnSnSe 4 HTM nano-films after the various annealing temperature treatments. In this fabrication, Cu 2 ZnSnSe 4 HTM film exhibited dominant spectra with intense Raman scattering main peak at 194 cm −1 correlating with the optical phonon mode. Its intensity increased slightly with the increased thermal annealing temperature. Other Raman scattering peak intensities were found at 233 and 253 cm −1 . The Cu 2 ZnSnSe 4 HTM crystal orientation could be found from polarization of Raman-scattered light with respect to the laser light, if the crystal structure's point group could be known.    Figure 6 shows the measurement results of the absorbance spectra of Cu2ZnSnSe4 HTM nano-films after the various annealing temperatures. The optical absorption properties of Cu2ZnSnSe4 in the visible region and near-infrared region are associated with electronic transitions and are also useful in comprehending electronic band conformations of semiconducting films. The optical spectra were recorded using a UV (ultraviolet) spectrophotometer at the wavelength range from 300 to 1200 nm. It was observed that the absorbance intensity increased with the increased annealing temperature. This was presumably caused by free-carrier absorption corresponding to  visible region and near-infrared region are associated with electronic transitions and are also useful in comprehending electronic band conformations of semiconducting films. The optical spectra were recorded using a UV (ultraviolet) spectrophotometer at the wavelength range from 300 to 1200 nm. It was observed that the absorbance intensity increased with the increased annealing temperature. This was presumably caused by free-carrier absorption corresponding to conductivity. These absorption spectra illustrated that all Cu 2 ZnSnSe 4 nano-films absorb over the entire visible region of electromagnetic waves. The absorption spectra data were analyzed following a classical equation for near edge optical absorption of semiconductors: αhν = A(hν − E g ) n , where α is absorption coefficient, hν is photon energy, E g is energy band gap, A is constant, n can have values of 1/2, 2, 3/2 and 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, separately. The Cu 2 ZnSnSe 4 energy band gap was determined by plotting a graph of hv versus (αhv) 2 , for the direct band gap. The energy band gap was designated by extrapolating the straight line portion to the energy axis, whose intercept to the x-axis should give the optical energy band gap [38]. An example graph for Cu 2 ZnSnSe 4 annealed at 650 • C is provided in Figure 7, and the calculation result of energy band gap has been 1.07-1.1 eV for the ultra-thin Cu 2 ZnSnSe 4 HTM which was deposited on bi-layer Mo/FTO glass substrate. It should promote a valence electron bound to an atom to a conduction electron. Such electrons then move freely within the HTM and become carriers that can conduct current. transitions, separately. The Cu2ZnSnSe4 energy band gap was determined by plotting a graph of hv versus (αhv) 2 , for the direct band gap. The energy band gap was designated by extrapolating the straight line portion to the energy axis, whose intercept to the x-axis should give the optical energy band gap [38]. An example graph for Cu2ZnSnSe4annealed at 650°C is provided in Figure 7, and the calculation result of energy band gap has been 1.07-1.1 eV for the ultra-thin Cu2ZnSnSe4 HTM which was deposited on bi-layer Mo/FTO glass substrate. It should promote a valence electron bound to an atom to a conduction electron. Such electrons then move freely within the HTM and become carriers that can conduct current.    Table 1 summarizes the PV characteristic parameters of these nanostructured solar cells, without MAPbI3 perovskite, under 100 mW/cm 2 illumination (air mass, AM1.5G). It has been evidenced that the transitions, separately. The Cu2ZnSnSe4 energy band gap was determined by plotting a graph of hv versus (αhv) 2 , for the direct band gap. The energy band gap was designated by extrapolating the straight line portion to the energy axis, whose intercept to the x-axis should give the optical energy band gap [38]. An example graph for Cu2ZnSnSe4annealed at 650°C is provided in Figure 7, and the calculation result of energy band gap has been 1.07-1.1 eV for the ultra-thin Cu2ZnSnSe4 HTM which was deposited on bi-layer Mo/FTO glass substrate. It should promote a valence electron bound to an atom to a conduction electron. Such electrons then move freely within the HTM and become carriers that can conduct current.    Table 1 summarizes the PV characteristic parameters of these nanostructured solar cells, without MAPbI3 perovskite, under 100 mW/cm 2 illumination (air mass, AM1.5G). It has been evidenced that the   Table 1 summarizes the PV characteristic parameters of these nanostructured solar cells, without MAPbI 3 perovskite, under 100 mW/cm 2 illumination (air mass, AM1.5G). It has been evidenced that the open-circuit voltage increased from 0.36 to 0.39 V, following the increased HTM layer thickness from 40 to 160 nm. The device short-circuit current was also enlarged from 6.47 to 9.46 mA/cm 2 . The device fill factor value would be amplified from 39.5% to 46.3%. The PV device power-conversion efficiency value was slightly increased from 0.92% to 1.71%, and the output power P max value was enhanced from 0.09 to 0.17 mW. Additionally, the Cu 2 ZnSnSe 4 film alone could not absorb enough photons, so that the PV cells exhibited poor PCE performance in the illustration.   Figure 9 displays the graphic J-V curves of Ag/ZnS/MAPbI3/Cu2ZnSnSe4/Mo/FTO nanostructured solar cells, with the MAPbI3 perovskite nanostructures on bi-layer Mo. The Cu2ZnSnSe4 HTM layer thickness has been fixed at 160 nm, but was thermally treated at the various annealing temperature 350-650°C . The measurements were, again, under 100 mW/cm 2 illumination. Table 2 lists the derived PV characteristic parameters of the Ag/ZnS/MAPbI3/Cu2ZnSnSe4/Mo/FTO nanostructured solar cells. It has been clearly evidenced with significant improvements in all the parameters. The open-circuit voltage was increased from 0.89 to 0.98 V following the increased annealing temperature from 350 to 650°C . The short-circuit current was also expanded from 19.9 to 20.4 mA/cm 2 . The device fill factor value could be increased from 68.6% to 71.3%. The PV device power-conversion efficiency value was strengthened from 12.2% to 14.3%, and the output power Pmax value was enhanced from 1.22 to 1.43 mW.   Figure 9 displays the graphic J-V curves of Ag/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO nanostructured solar cells, with the MAPbI 3 perovskite nanostructures on bi-layer Mo. The Cu 2 ZnSnSe 4 HTM layer thickness has been fixed at 160 nm, but was thermally treated at the various annealing temperature 350-650 • C. The measurements were, again, under 100 mW/cm 2 illumination. Table 2 lists the derived PV characteristic parameters of the Ag/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO nanostructured solar cells. It has been clearly evidenced with significant improvements in all the parameters. The open-circuit voltage was increased from 0.89 to 0.98 V following the increased annealing temperature from 350 to 650 • C. The short-circuit current was also expanded from 19.9 to 20.4 mA/cm 2 . The device fill factor value could be increased from 68.6% to 71.3%. The PV device power-conversion efficiency value was strengthened from 12.2% to 14.3%, and the output power P max value was enhanced from 1.22 to 1.43 mW. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 16  In addition, Figure 10 displays the J-V curves of the Ag/IZO/ZnS/MAPbI3/Cu2ZnSnSe4/Mo/FTO nanostructured solar cells at the various HTM thermal annealing temperatures under 100 mW/cm 2 illumination. The TCO layer in thickness of 100 nm IZO film has been inserted between the ZnS ETL and top Ag electrode. Table 3 presents the derived PV characteristic parameters of the Ag/IZO/ZnS/MAPbI3/Cu2ZnSnSe4/Mo/FTO nanostructured solar cells. Furthermore, it has been clearly evidenced with enhancements in all the parameters. The open-circuit voltage was amplified from 0.97 to 1.10 V, following the annealing temperature that was increased from 350 to 650 °C. The short-circuit current was slightly increased from 20.5 to 20.8 mA/cm 2 . The device fill factor value was slightly diminished to 76.3%. The PV device power-conversion efficiency value was further increased from 15.5% to 17.4%. Additionally, the device series-resistance was decreased from 20.2 to 17.1 Ω., and the device output power Pmax value could be enhanced from 1.55 to 1.74 mW.  In addition, Figure 10 displays the J-V curves of the Ag/IZO/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO nanostructured solar cells at the various HTM thermal annealing temperatures under 100 mW/cm 2 illumination. The TCO layer in thickness of 100 nm IZO film has been inserted between the ZnS ETL and top Ag electrode. Table 3 presents the derived PV characteristic parameters of the Ag/IZO/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO nanostructured solar cells. Furthermore, it has been clearly evidenced with enhancements in all the parameters. The open-circuit voltage was amplified from 0.97 to 1.10 V, following the annealing temperature that was increased from 350 to 650 • C. The short-circuit current was slightly increased from 20.5 to 20.8 mA/cm 2 . The device fill factor value was slightly diminished to 76.3%. The PV device power-conversion efficiency value was further increased from 15.5% to 17.4%. Additionally, the device series-resistance was decreased from 20.2 to 17.1 Ω., and the device output power P max value could be enhanced from 1.55 to 1.74 mW. Figure 11 shows the PL spectral measurement results of MAPbI 3 perovskite nano-films onCu 2 ZnSnSe 4 /Mo/FTO following the various thermal annealing temperatures. The spectra were examined by fluorescence spectrophotometer. It has been clearly evidenced with one main peak at the wavelength of~768 nm. The intensity was also enhanced by the increased annealing temperature. The intensity of PL spectrum is relative to lifetime of the injected electrons and electron-holes combined to form excitons. An exciton indicates a mobile energy constitution by an excited electron and a relative electron-hole. Anincrease in the number of excitons could simultaneously increase the electron/electron-hole recombination, thus the PL intensity.   Figure 11 shows the PL spectral measurement results of MAPbI3 perovskite nano-films onCu2ZnSnSe4/Mo/FTO following the various thermal annealing temperatures. The spectra were examined by fluorescence spectrophotometer. It has been clearly evidenced with one main peak at the wavelength of ~768 nm. The intensity was also enhanced by the increased annealing temperature. The intensity of PL spectrum is relative to lifetime of the injected electrons and electron-holes combined to form excitons. An exciton indicates a mobile energy constitution by an excited electron and a relative electron-hole. Anincrease in the number of excitons could simultaneously increase the electron/electron-hole recombination, thus the PL intensity.      Figure 11 shows the PL spectral measurement results of MAPbI3 perovskite nano-films onCu2ZnSnSe4/Mo/FTO following the various thermal annealing temperatures. The spectra were examined by fluorescence spectrophotometer. It has been clearly evidenced with one main peak at the wavelength of ~768 nm. The intensity was also enhanced by the increased annealing temperature. The intensity of PL spectrum is relative to lifetime of the injected electrons and electron-holes combined to form excitons. An exciton indicates a mobile energy constitution by an excited electron and a relative electron-hole. Anincrease in the number of excitons could simultaneously increase the electron/electron-hole recombination, thus the PL intensity.   Figure 12 displays the external quantum efficiency (EQE) spectrum measurement results based on Ag/IZO/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO nanostructured solar cells after the various annealing temperatures. The EQE curves for the planar solar cells have been well increased in the wavelength range of 300~800 nm from the 350 • C to 650 • C samples. Among the results of these measurements, the maximum EQE value reached nearly 85% at 550 nm for the 650 • C sample, as compared to 63% for the 350 • C sample. The higher EQE value could suggest a reduction of recombination centers. Planar solar cells involved translational electrical field distribution. It was determined entirely by 1-dimensional resonance.
temperatures. The EQE curves for the planar solar cells have been well increased in the wavelength range of 300~800 nm from the 350°C to 650°C samples. Among the results of these measurements, the maximum EQE value reached nearly 85% at 550 nm for the 650°C sample, as compared to 63% for the 350°C sample. The higher EQE value could suggest a reduction of recombination centers. Planar solar cells involved translational electrical field distribution. It was determined entirely by 1-dimensional resonance.  Figure 13 shows the cross-sectional SEM micrograph of the Ag/IZO/ZnS/MAPbI3/Cu2ZnSnSe4/ Mo/FTO nanostructures on glass substrate. This sample has been annealed at 650°C . The MAPbI3 perovskite solar cell was evidenced with a glossy cross-section, contained few pinholes and micro-crack type of defects. The planar solar cell had uniform layer distribution through the magnetron sputtering technique and the spin-coating process. The thickness of each constituent layer could be clearly identified. Consequently, the nano-crystals were of high quality from this fabrication process, and the PV cell characteristics should be warranted in this study. In addition, Figure 14 shows the SEM micrographs of MAPbI3 perovskite films on Cu2ZnSnSe4/Mo/FTO following the various thermal annealing temperatures. The crystal grains exhibited good surface coverage and were significantly increased in size with the increased annealing temperature.  Figure 13 shows the cross-sectional SEM micrograph of the Ag/IZO/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 / Mo/FTO nanostructures on glass substrate. This sample has been annealed at 650 • C. The MAPbI 3 perovskite solar cell was evidenced with a glossy cross-section, contained few pinholes and micro-crack type of defects. The planar solar cell had uniform layer distribution through the magnetron sputtering technique and the spin-coating process. The thickness of each constituent layer could be clearly identified. Consequently, the nano-crystals were of high quality from this fabrication process, and the PV cell characteristics should be warranted in this study. In addition, Figure 14 shows the SEM micrographs of MAPbI 3 perovskite films on Cu 2 ZnSnSe 4 /Mo/FTO following the various thermal annealing temperatures. The crystal grains exhibited good surface coverage and were significantly increased in size with the increased annealing temperature.
temperatures. The EQE curves for the planar solar cells have been well increased in the wavelength range of 300~800 nm from the 350°C to 650°C samples. Among the results of these measurements, the maximum EQE value reached nearly 85% at 550 nm for the 650°C sample, as compared to 63% for the 350°C sample. The higher EQE value could suggest a reduction of recombination centers. Planar solar cells involved translational electrical field distribution. It was determined entirely by 1-dimensional resonance.  Figure 13 shows the cross-sectional SEM micrograph of the Ag/IZO/ZnS/MAPbI3/Cu2ZnSnSe4/ Mo/FTO nanostructures on glass substrate. This sample has been annealed at 650°C . The MAPbI3 perovskite solar cell was evidenced with a glossy cross-section, contained few pinholes and micro-crack type of defects. The planar solar cell had uniform layer distribution through the magnetron sputtering technique and the spin-coating process. The thickness of each constituent layer could be clearly identified. Consequently, the nano-crystals were of high quality from this fabrication process, and the PV cell characteristics should be warranted in this study. In addition, Figure 14 shows the SEM micrographs of MAPbI3 perovskite films on Cu2ZnSnSe4/Mo/FTO following the various thermal annealing temperatures. The crystal grains exhibited good surface coverage and were significantly increased in size with the increased annealing temperature.  Figure 15 demonstrates the secondary ion mass spectrometry (SIMS) depth profile of the Cu 2 ZnSnSe 4 HTM nano-film grown on bi-layer Mo/FTO glass substrate, using ESCA PHI-5000 system (Ulvac-PHI, Kanagawa, Japan). This sample has been annealed at 650 • C. The addition of Cu-based Cu 2 ZnSnSe 4 material, including selenide and MoSe 2 inter-film, had beneficial effect on the power conversion efficiency. The SIMS analysis provided quantitative depth profiling with good depth resolution. These features became essential to characterize the ultra-thin Cu 2 ZnSnSe 4 HTM solar cells, where possible variations on structure or composition could lead to significant changes. In this study, Cu 2 ZnSnSe 4 HTM film was prepared by magnetron sputtering with 160 nm in thickness, and bi-layer Mo back contact layer at about 200 nm on FTO glass substrate, all well evidenced in the depth profile. It has been noted that the use of a large collection area would provide a more representative sampling of the results. Nevertheless, the SIMS depth profile can be reconstructed after the deposition analysis to provide compositional changes at different locations.  Figure 15 demonstrates the secondary ion mass spectrometry (SIMS) depth profile of the Cu2ZnSnSe4 HTM nano-film grown on bi-layer Mo/FTO glass substrate, using ESCA PHI-5000 system (Ulvac-PHI, Kanagawa, Japan).This sample has been annealed at 650°C . The addition of Cu-based Cu2ZnSnSe4 material, including selenide and MoSe2 inter-film, had beneficial effect on the power conversion efficiency. The SIMS analysis provided quantitative depth profiling with good depth resolution. These features became essential to characterize the ultra-thin Cu2ZnSnSe4 HTM solar cells, where possible variations on structure or composition could lead to significant changes. In this study, Cu2ZnSnSe4HTM film was prepared by magnetron sputtering with 160 nm in thickness, and bi-layer Mo back contact layer at about 200 nm on FTO glass substrate, all well evidenced in the depth profile. It has been noted that the use of a large collection area would provide a more representative sampling of the results. Nevertheless, the SIMS depth profile can be reconstructed after the deposition analysis to provide compositional changes at different locations.

Conclusions
In summary, one-step magnetron sputtered Cu2ZnSnSe4 nano-films have been successfully applied as novel Cu-based inorganic HTM forMAPbI3 perovskite nanostructured photovoltaics. The

Conclusions
In summary, one-step magnetron sputtered Cu 2 ZnSnSe 4 nano-films have been successfully applied as novel Cu-based inorganic HTM for MAPbI 3 perovskite nanostructured photovoltaics. The adequate control in nano-film quality significantly improved the PV characteristic parameters of Ag/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO nanostructured solar cells. When the Cu 2 ZnSnSe 4 HTM thickness was designed at 160 nm, and the thermal annealing temperature wasat 650 • C, its open-circuit voltage was increased to 0.98 V, and short-circuit current was increased to 20.4 mA/cm 2 . The device fill factor value was increased to 71.3%, the power-conversion efficiency value was elevated to 14.3%, and the output power P max value was enhanced to 1.43 mW. Furthermore, the additional inclusion of transparent conductive IZO could further enhance the PV characteristic parameters of the Ag/IZO/ZnS/MAPbI 3 /Cu 2 ZnSnSe 4 /Mo/FTO nanostructured solar cells. The open-circuit voltage was enhanced to 1.10 V, and the short-circuit current was increased to 20.8 mA/cm 2 . The device fill factor value was improved to 76.3%, the power-conversion efficiency value was increased to 17.4%, the device series-resistance was decreased to 17.1 Ω., and the device output power P max value could be enhanced to 1.74 mW. Therefore, the Cu 2 ZnSnSe 4 HTM used in this study would help the development of perovskite PV technology.