Fabrication and Characterization of Copper System Compound Semiconductor Solar Cells

Copper system compound semiconductor solar cells were produced by a spin-coating method, and their cell performance and structures were investigated. Copper indium disulfide(CIS-) based solar cells with titanium dioxide (TiO2) were produced on F-doped SnO2 (FTO). A device based on an FTO/CIS/TiO2 structure provided better cell performance compared to that based on FTO/TiO2/CIS structure. Cupric oxide(CuO-) and cuprous oxide(Cu2O-) based solar cells with fullerene (C60) were also fabricated on FTO and indium tin oxide (ITO). The microstructure and cell performance of the CuO/C60 heterojunction and the Cu2O:C60 bulk heterojunction structure were investigated. The photovoltaic devices based on FTO/CuO/C60 and ITO/Cu2O:C60 structures provided short-circuit current density of 0.015 mAcm−2 and 0.11 mAcm−2, and open-circuit voltage of 0.045 V and 0.17 V under an Air Mass 1.5 illumination, respectively. The microstructures of the active layers were examined by X-ray diffraction and transmission electron microscopy.


Introduction
Chalcopyrite semiconductors are expected as one of the alternative materials to silicon solar cells.The features of chalcopyrite semiconductors include high optical absorption efficiency, low light degradation, and high radiation resistance.Copper indium disulfide, CuInS 2 (CIS) is a wellknown p-type chalcopyrite semiconductor.CIS is a suitable material for high-efficiency solar cells because its bandgap (1.5 eV) is close to the ideal energy gap, which is well matched with the solar spectrum.In addition, the band structure shows direct transition, which provides a high efficiency.The highest efficiency of 12% as the absorber layer has been obtained for the CIS solar cells by using the expensive vacuum evaporation technique [1].Most of the efficient CISbased solar cells have been prepared by using CdS as buffer layers.
Cd-free CIS-based solar cells with TiO 2 and In 2 O 3 have been fabricated by an evaporation method [2][3][4] and reported to show efficiencies of 3∼9.5% [3,4].Titanium dioxide (TiO 2 ) as an n-type semiconductor has also been used as electrode materials for dye-sensitized solar cells [5].TiO 2 is also a safe and low-cost material for environmentally harmonized solar cells.A low-cost method is essential for the mass production of CIS-based solar cells, and spin coating is a more economical method compared with vacuum evaporation [6][7][8].CIS thin films deposited by sol-gel spin-coating methods have also been investigated and reported [9,10].The resulting structure closely resembles that of polymeric bulk heterojunction cells (polymer/C 60 ) fabricated using organic materials [11][12][13].The bulk heterojunction structures contribute to increase the p-n heterojunction interface for photoelectric conversion region in the devices.The aim of the present work was to investigate whether the TiO 2 /CIS structure by penetration of CIS nanocrystals into the porous TiO 2 layer could function as a bulk heterojunction structure.
Oxide semiconductors are one of the alternatives to silicon solar cells.Features of oxide semiconductors are high optical absorption and low cost of raw materials.
Copper oxides (CuO and Cu 2 O) are well-known p-type oxide semiconductors.CuO and Cu 2 O are suitable materials for high-efficiency solar cells because those direct bandgaps (∼1.5 eV and ∼2.0 eV) are close to the ideal energy gap for solar cells, and are well matched with the solar spectrum.The highest efficiency of ∼2% has been obtained for Cu 2 O solar cells by using a high-temperature annealing method and an expensive vacuum evaporation technique [14].Heterojunction solar cells with Cu 2 O and ZnO have been fabricated by electrodeposition and photochemical deposition methods [15][16][17], and an efficiency of ∼0.001% was reported [17].However, p-n heterojunction interface area for photoelectric conversion is small.One of improvements is to increase the p-n heterojunction interface by blending of p-type and ntype semiconductors, which are called bulk heterojunction solar cells [6][7][8].Although CuO is used as a hole transfer layer and a barrier layer for dye-sensitized solar cells [18,19], few solar cells with CuO used as a p-type semiconductor active layer have been investigated.The advantage of CuO is a simple production method.
Fullerene (C 60 ) is a good acceptor material for solar cells, and has been used as n-type semiconductor active layer for organic thin film solar cells [12,20].A low-cost method is essential for mass production of CuO-based solar cells, and spin coating is a more economical method compared to vacuum evaporation [6,7].CuO thin films deposited by sol-gel spin-coating methods have also been investigated and reported [21].
The purpose of the present work was to prepare copper-based semiconductor solar cells by using a spincoating method, and to investigate the microstructures and photovoltaic properties for TiO 2 /CIS and CuO/C 60 and Cu 2 O:C 60 structures.The solar cells were investigated by microstructure analysis, optical absorption, and electronic property measurements.Cu 2 O nanoparticles were synthesized by reducing copper-amine complex with 1-heptanol in the presence of tetramethyl ammonium hydroxide.The details of the synthesis scheme were as follows: Copper acetate and oleylamine were introduced into 1-heptanol and heated under a nitrogen atmosphere at 120 • C for one hour to remove the moisture in the system.Then, tetramethyl ammonium hydroxide (TMAOH) dissolved in 1-hepatanol (∼150 • C) was introduced and the suspension was heated at 150 • C for 2 hours.Next, the suspension was cooled to room temperature and Cu 2 O nanoparticles were recovered by centrifuging.Finally, the Cu 2 O nanoparticles were washed with methanol to remove excess oleylamine, and the Cu 2 O nanoparticles (∼100 mg) were dispersed in toluene (10 mL).

Experimental Procedures
A thin layer of polyethylenedioxythiophene doped with polystyrene-sulfonic acid (PEDOT:PSS) (Sigma Aldrich) was spin-coated at 2000 rpm on precleaned indium tin oxide (ITO) glass plates (Geomatec Co., Ltd., ∼10 Ω/ ).After annealing at 100 • C for 10 minutes in N 2 atmosphere, semiconductor layers were prepared on a PEDOT layer by spin coating using a mixed solution of a toluene with Cu 2 O nanoparticle and a C 60 solution (16 mg/mL) with C 60 powders (Material Technologies Research, 99.98%) in odichlorobenzene.The thickness of the bulk heterojunction structure was approximately ∼150 nm.The Cu 2 O:C 60 layers were annealed at 100 • C for 30 minutes in N 2 atmosphere.
For all the present copper system compound semiconductor solar cells, aluminum (Al) metal contacts with a thickness of ∼100 nm were deposited as top electrodes, and annealed at 140 • C for 20 minutes in N 2 atmosphere.Schematic illustrations of the present solar cells are shown in Figure 1.

Measurements of Solar Cells.
The current density-voltage (J-V ) characteristics (Hokuto Denko Corp., HSV-100) of the solar cells were measured both in the dark and under illumination at 100 mW/cm 2 by using an AM 1.5 solar simulator (San-ei Electric, XES-301S) in N 2 atmosphere.The solar cells were illuminated through the side of the FTO and ITO substrates, and the illuminated area was 0.16 cm 2 .Optical absorption of the solar cells was investigated by means of UVvisible spectroscopy (Hitachi, Ltd., U-4100).The microstructures of the CIS, copper oxides, and C 60 thin films were investigated by X-ray diffractometer (XRD, PHILIPS X'Pert-MPD System) with CuKα radiation operating at 40 kV and 40 mA, scanning electron microscopy (SEM, Hitachi, Ltd., S-3200N), energy dispersive X-ray analysis (EDX, EMAX-5770W), and transmission electron microscopy (TEM) operating at 200 kV (Hitachi, Ltd., H-8100).

Results and Discussion
3.1.CIS/TiO 2 Solar Cells.The J-V characteristics of the TiO 2 /CIS and CIS/TiO 2 structures in the dark and under illumination were measured.The photocurrent was observed under illumination and both structures showed characteristic curves with open-circuit voltage and short-circuit current.The measured parameters of these solar cells are summarized in Table 1.A solar cell with the CIS/TiO 2 structure provided power conversion efficiency (η) of 1.5 × 10 −5 %, fill factor (FF) of 0.25, short-circuit current density (J SC ) of 6.0 × 10 −3 mA/cm 2 , and open-circuit voltage (V OC ) of 1.0×10 −2 V, which is better than those of the TiO 2 /CIS device.Figure 2 shows the measured optical absorption of the solar cells.The TiO 2 /CIS structure shows high absorption at 444, 510, 606, and 738 nm, which correspond to 2.8, 2.4, 2.0, and 1.7 eV, respectively.The CIS/TiO 2 structure also shows high optical absorption at 420, 488, 590, and 710 nm, which correspond to 2.6, 2.5, 2.1, and 1.7 eV, respectively.The absorption peaks at 444 nm and 420 nm for the TiO 2 /CIS and CIS/TiO 2 structures were due to TiO 2 , and the other peaks were due to CIS.As shown in Figure 2, the optical absorption between 400 nm and 800 nm for the TiO 2 /CIS solar cell is higher than that of CIS/TiO 2 structure.The increase in film thickness of the CIS layer in the TiO 2 /CIS structure through the penetration of the CIS nanocrystals into the porous TiO 2 layer results in the increase in the optical absorption of solar cells in the TiO 2 /CIS structure.
Microstructures of the CIS thin film were investigated by XRD, as shown in Figure 3(a).Diffraction peaks corresponding to CIS and indium oxide (In 2 O 3 ) were observed for the CIS thin film.The crystallite size was estimated using the Scherer equation: D = 0.9 λ/B cosθ, where λ, B, and θ represent the wavelength of the X-ray source, the full width at half maximum (FWHM), and the Bragg angle, respectively [23].From the FWHM of the 112 peak, the crystallite size of CIS was determined to be 13.1 nm.Elemental analysis of the CIS thin film was investigated by SEM and EDX.The atomic concentrations of the CIS thin film were copper (28.0%), indium (26.0%) and sulfur (46.0%).
Preparation conditions of the TiO 2 /CIS structure were different from those of the CIS/TiO 2 structure, and influence of different annealing temperatures of the TiO 2 layer was investigated.Figures 3(b indicate an anatase phase with a tetragonal system, and there was a slight amount of the rutile phase.The particle size of the anatase phase was estimated to be 26.1 nm from the Scherrer's equation.No differences between Figures 3(b) and 3(c) were observed on the crystalline structures of the TiO 2 layer.5(a), the porous TiO 2 layer has a composite structure with CIS nanocrystals.Therefore, the p-n junction interface area, which is the photoelectron conversion region in the TiO 2 /CIS solar cells, becomes larger compared to the ordinary heterojunction structure, leading to an increase in efficiency.As shown in Figure 2, the optical absorption between 400 nm and 800 nm for the TiO 2 /CIS solar cell is higher than that of CIS/TiO 2 structure.The increase of the CIS layer thickness in the TiO 2 /CIS structure through the penetration of the CIS nanocrystals into the porous TiO 2 layer would result in the increase of the optical absorption of TiO 2 /CIS solar cells.Charge separation for the TiO 2 /CIS solar cell would be higher from high optical absorption.However, the carrier recombination of electrons and holes would occur at the FTO/TiO 2 interface for the TiO 2 /CIS structure, and the energy conversion efficiency would not be improved by an increase of the p-n heterojunction interface in the present work.
Energy level diagrams of TiO 2 /CIS and CIS/TiO 2 solar cells are summarized as shown in Figures 5(c) and 5(d), respectively.Previously reported values were used for the energy levels [24,25].It has been reported that V OC is nearly proportional to the bandgap of the semiconductors [26].
Compared to a Si semiconductor with an indirect transition band structure, the CIS with a direct transition band structure is the more suitable for the optical absorption property.In addition, the ultrathin film of the CIS layers could provide an efficient charge injection because of the high optical absorption.Although Cu(In X , Ga 1−X )Se 2 (CIGS) has been mainly used as a p-type inorganic semiconductor for solar cells [27], CIS was applied instead of CIGS in the present work.Advantages of the present CIS are the lowcost reagent and the simple fabrication process.TiO cells is due to presence of impure compounds in the active layer.The defects produced by inadequate crystals would cause carrier recombination.Optimization of the CIS microstructures can increase the efficiency of solar cells.
There could be carrier recombination of electrons and holes at the rear surface of the n-type electrode (FTO).Solar cells with a buffer layer fabricated by addition of ultrathin TiO 2 or ZnO n-type semiconductor layers between FTO and TiO 2 have been reported [28,29].Energy conversion efficiency could be improved by addition of the buffer layers to inhibit carrier recombination at the contact surfaces.

CuO/C
60 Solar Cells.J-V characteristics of the CuO/C 60 structures in the dark and under illumination were investigated.The photocurrent was observed under illumination, and the CuO/C 60 structure showed characteristic curves with short-circuit current and open-circuit voltage.Measured parameters of these solar cells are summarized in Table 1.A solar cell with a CuO/C 60 structure provided η of 1.7×10 −4 %, FF of 0.25, J SC of 0.015 mAcm −2 , and V OC of 0.045 V. Figure 6 shows the measured optical absorption of the CuO/C 60 solar cell.The absorption peaks around 350 nm in the CuO/C 60 structure were due to CuO, and absorption peaks around 338 nm, 440 nm, and 606 nm correspond to C 60 .
Figure 7 shows an X-ray diffraction pattern of the CuO thin films on glass substrates annealed at 300 • C for 3 hours in air.Diffraction peaks corresponding to CuO were observed for the CuO thin film, which consisted of a tenorite phase with monoclinic system (space group of C2/c and lattice parameters of a = 0.4653 nm, b = 0.3410 nm, c = 0.5108 nm, and β = 99.48• ).The crystallite size was estimated using Scherrer's equation.From the FWHM of the −111 peak, the crystallite size of CuO was determined Energy level diagram of CuO/C 60 solar cells is summarized as shown in Figure 9. Previously reported values were used for the energy levels [20,30,31].Compared to the Si semiconductor with an indirect transition band structure, CuO with a direct transition bandgap is more suitable for the optical absorption property.Although Cu 2 O has been mainly used as a p-type oxide semiconductor for solar cells [14][15][16][17], CuO was applied instead of Cu 2 O in the present work.The advantages of the present CuO are low-cost reagent and the simple fabrication process.
CuO-based solar cells prepared by a spin-coating method without vacuum evaporation were investigated in the present work.The low conversion efficiency of the present solar cells would be due to carrier recombination by the defects produced by inadequate crystalline, and presence of CuO compounds with heterogeneous grain size in the active layer.Formation of the high-quality CuO thin films could increase the efficiency of solar cells.1.A solar cell with a Cu 2 O:C 60 structure provided η of 4.3 × 10 −3 %, FF of 0.23, J SC of 0.11 mAcm −2 and V OC of 0.17 V.   nanocrystal structures of Cu 2 O. C 60 peaks [32] were not observed in this pattern, which would be due to dispersion of C 60 crystals.Energy level diagram of the Cu 2 O:C 60 solar cell is summarized as shown in Figure 13.Previously reported values were used for the energy levels [11,31,33].Cu 2 O with a direct transition bandgap is more suitable for the optical absorption property compared to Si.Although ZnO has been mainly used as an n-type oxide semiconductor for solar cells  Compared to previously reported Cu 2 O-based heterojunction solar cells [14], copper oxide-based bulk heterojunction and heterojunction solar cells prepared by a spincoating method without a vacuum system were investigated in the present work, as listed in Table 2.The present copper oxide-based bulk heterojunction and heterojunction solar cells have simple fabrication process and good cost performance.

Cu
The low conversion efficiency of the present solar cells would be due to aggregation of Cu 2 O nanoparticle in the active layer, which would cause carrier recombination.Formation of the Cu 2 O:C 60 thin films with homogeneous distribution of Cu 2 O nanoparticles could increase the efficiency of solar cells.

Conclusions
Copper system compound semiconductor solar cells were produced and characterized.A device based on the CIS/TiO 2 structure provided η of 1.5 × 10 −5 %, FF of 0.25, J SC of 6.0 × 10 −3 mA/cm 2 , and V OC of 1.0 × 10 −2 V, which showed better performance compared to that of a device based on the TiO 2 /CIS structure.The devices based on the CuO/C 60 and Cu 2 O : C 60 structures were fabricated by a spin-coating method which provided η of 1.7 × 10 −4 % and 4.3 × 10 −3 %, FF of 0.25 and 0.23, J SC of 0.015 mAcm −2 and 0.11 mAcm −2 , and V OC of 0.045 V and 0.17 V, respectively.X-ray diffraction analysis indicated the formation of CIS, CuO, and Cu 2 O nanocrystal structures, respectively.
Energy level diagrams of CIS/TiO 2 , CuO/C 60 , and Cu 2 O:C 60 solar cells were proposed.Separated holes could transfer from the valence band of the copper system compound semiconductors to the transparent conducting oxide electrodes, and separated electrons could transfer from the conduction band of the copper system compound semiconductors to the Al electrodes, respectively.
Formation of the high-quality in active layers with inorganic nanoparticles would improve the efficiencies of the solar cells.The bulk heterojunction structure with dispersed inorganic nanoparticles in present work was effective for the improvement of current density.

Figure 3 :
Figure2shows the measured optical absorption of the solar cells.The TiO 2 /CIS structure shows high absorption at 444, 510, 606, and 738 nm, which correspond to 2.8, 2.4, 2.0, and 1.7 eV, respectively.The CIS/TiO 2 structure also shows high optical absorption at 420, 488, 590, and 710 nm, which correspond to 2.6, 2.5, 2.1, and 1.7 eV, respectively.The absorption peaks at 444 nm and 420 nm for the TiO 2 /CIS and CIS/TiO 2 structures were due to TiO 2 , and the other peaks were due to CIS.As shown in Figure2, the optical absorption between 400 nm and 800 nm for the TiO 2 /CIS solar cell is higher than that of CIS/TiO 2 structure.The increase in film thickness of the CIS layer in the TiO 2 /CIS structure through the penetration of the CIS nanocrystals into the porous TiO 2 layer results in the increase in the optical absorption of solar cells in the TiO 2 /CIS structure.Microstructures of the CIS thin film were investigated by XRD, as shown in Figure3(a).Diffraction peaks corresponding to CIS and indium oxide (In 2 O 3 ) were observed for the CIS thin film.The crystallite size was estimated using the Scherer equation: D = 0.9 λ/B cosθ, where λ, B, and θ represent the wavelength of the X-ray source, the full width at half maximum (FWHM), and the Bragg angle, respectively[23].From the FWHM of the 112 peak, the crystallite size of CIS was determined to be 13.1 nm.Elemental analysis of the CIS thin film was investigated by SEM and EDX.The atomic concentrations of the CIS thin film were copper (28.0%), indium (26.0%) and sulfur (46.0%).Preparation conditions of the TiO 2 /CIS structure were different from those of the CIS/TiO 2 structure, and influence of different annealing temperatures of the TiO 2 layer was investigated.Figures3(b) and 3(c) show X-ray diffraction patterns of the TiO 2 thin films on the glass substrates annealed at 450 • C for 30 minutes in air, and 500 • C for 30 minutes in N 2 /H 2 atmosphere and annealed at 450 • C for 30 minutes in air, respectively.The X-ray diffraction patterns

Figure 4 :
Figure 4: (a) TEM image and (b) electron diffraction pattern of TiO 2 /CIS structure.(c) Enlarged HREM images of TiO 2 in (a) and (d) CIS in (a).

Figures 4 (
Figures 4(a) and 4(b) show a TEM image and an electron diffraction pattern of the TiO 2 /CIS composite film, respectively.The TEM image indicated TiO 2 nanocrystals in the CIS matrix.Debye-Scherrer rings, which indicate polycrystalline structures of TiO 2 and 112 reflection of the CIS crystal can be seen in Figure 4(b).Enlarged highresolution electron microscopy (HREM) images of the TiO 2 and CIS in Figure 4(a) are shown in Figures 4(c) and 4(d), respectively, revealing the lattice fringes of the 101 of the TiO 2 and the 112 of the CIS crystals.Figures 5(a) and 5(b) show the microstructures of FTO/TiO 2 /CIS/Al and FTO/CIS/TiO 2 /Al, respectively.As shown in Figure5(a), the porous TiO 2 layer has a composite structure with CIS nanocrystals.Therefore, the p-n junction interface area, which is the photoelectron conversion region in the TiO 2 /CIS solar cells, becomes larger compared to the ordinary heterojunction structure, leading to an increase in efficiency.As shown in Figure2, the optical absorption between 400 nm and 800 nm for the TiO 2 /CIS solar cell is higher than that of CIS/TiO 2 structure.The increase of the CIS layer thickness in the TiO 2 /CIS structure through the penetration of the CIS nanocrystals into the porous TiO 2 layer would result in the increase of the optical absorption of TiO 2 /CIS solar cells.Charge separation for the TiO 2 /CIS solar cell would be higher from high optical absorption.However, the carrier recombination of electrons and holes

Figure 7 :
Figure 7: XRD patterns of CuO thin films annealed at 300 • C for 3 hours in air.
2 O:C 60 Solar Cells.The CuO/C 60 solar cell was a heterojunction structure.To increase the interfacial area of p-n junction, Cu 2 O/C 60 bulk heterojunction solar cells were prepared.The J-V characteristics of the Cu 2 O:C 60 structure in the dark and under illumination were measured.The photocurrent was observed under illumination, and the Cu 2 O:C 60 structure showed photovoltaic properties.The measured parameters of the solar cells are summarized in Table

Figure 8 :
Figure 8: (a) TEM image and (b) electron diffraction pattern of CuO structure.

Figure 10 Figure 9 :Figure 10 :
Figure 10 shows optical absorption of the Cu 2 O:C 60 solar cells.An absorption peak around 324 nm for the Cu 2 O:C 60 structure is due to Cu 2 O, and the absorption peak around 362 nm and 476 nm corresponds to C 60 .Microstructures of the Cu 2 O:C 60 thin films were investigated by XRD, as shown in Figure 11.Diffraction peaks corresponding to Cu 2 O and C 60 were observed for the Cu 2 O:C 60 thin film, and they consisted of a cuprite phase with cubic system (space group of Pn3m and a lattice parameter of a = 0.4250 nm), and a C 60 phase with cubic system (space group of Fm3m and a lattice parameter of a = 1.4166 nm).The crystallite sizes of Cu 2 O and C 60 were determined to be 7.2 nm and 25.7 nm, respectively.Figures 12(a) and 12(b) show a TEM image and a selected area diffraction pattern of the Cu 2 O:C 60 thin film annealed at 100 • C, respectively.The TEM image indicated aggregated Cu 2 O nanocrystals with sizes of 10∼20 nm.Line broadening of the Debye-Scherrer rings in Figure 12(b) indicates

[ 14 -
17], C 60 was applied instead of ZnO in the present work.Advantages of the present C 60 are a good acceptor for solar cells and the simple fabrication process.

Table 1 :
Measured parameters of solar cells.
• C under air atmosphere for 3 hours, CuO layers with a thickness of ∼100 nm were formed from the Cu(II) films.C 60 layers with a thickness of ∼100 nm were prepared on a CuO layer by spin coating using a C 60 solution (16 mg/mL) with C 60 powders (Material Technologies Research, 99.98%) in odichlorobenzene, and were annealed at 100 • C for 30 minutes in N 2 atmosphere.

Table 2 :
Comparison of copper oxide-based solar cells.
Optimization of the nanocomposite structure with Cu 2 O and C 60 would increase the efficiencies of the solar cells.