# Fabrication and Characterization of  Copper System Compound Semiconductor  Solar Cells

Authors: Ryosuke Motoyoshi; Takeo Oku; Atsushi Suzuki; Kenji Kikuchi; Shiomi Kikuchi; Balachandran Jeyadevan; Jhon Cuya
Journal: Advances in Materials Science and Engineering
 (2010)

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## Abstract

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.


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## Body

## 1. 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, CuInS2 (CIS) is a well-known 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 CIS-based solar cells have been prepared by using CdS as buffer layers.Cd-free CIS-based solar cells with TiO2 and In2O3 have been fabricated by an evaporation method [2–4] and reported to show efficiencies of 3~9.5% [3, 4]. Titanium dioxide (TiO2) as an n-type semiconductor has also been used as electrode materials for dye-sensitized solar cells [5]. TiO2 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 [6–8]. 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/C60) fabricated using organic materials [11–13]. 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 TiO2/CIS structure by penetration of CIS nanocrystals into the porous TiO2 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 andCu2O) are well-known p-type oxide semiconductors. CuO and Cu2O 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 Cu2O solar cells by using a high-temperature annealing method and an expensive vacuum evaporation technique [14]. Heterojunction solar cells with Cu2O and ZnO have been fabricated by electrodeposition and photochemical deposition methods [15–17], 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 n-type semiconductors, which are called bulk heterojunction solar cells [6–8]. 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(C60) 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 spin-coating method, and to investigate the microstructures and photovoltaic properties forTiO2/CIS and CuO/C60 and Cu2O:C60 structures. The solar cells were investigated by microstructure analysis, optical absorption, and electronic property measurements.


## 2. Experimental Procedures

### 2.1. Fabrication of Dioxide/Copper Indium Disulfide Solar Cells

Two types of solar cells withTiO2/CIS and CIS/TiO2 structures were fabricated as shown in Figures 1(a) and 1(b), respectively. TiO2 and CIS layers were coated on precleaned F-doped SnO2 (FTO) glass plates (Asahi Glass, ~9.3 Ω/□) by using a spin-coating method. TiO2 layers with a thickness of ~10 μm were prepared by squee-gee coating using a TiO2 paste (0.6 g/mL) of fine TiO2 powders (P25, Sigma Aldrich) in an acetylacetone/polyethylene glycol solution and were annealed at 450°C for 30 minutes in air [22].
Copper acetate monohydrate, Cu(CH3COO)2·H2O (0.5 mol/L, Sigma Aldrich, 99.99%) and indium acetate, In(CH3COO)3 (0.5 mol/L, Sigma Aldrich, 99.99%) were dissolved into 2-propanol/monoethanolamine with a ratio of 4 : 1 and 1-propanol/diethanolamine with a ratio of 4 : 1, respectively, and the two solutions were then mixed. The molar ratio of Cu:In was 1 : 1. Cu-In layers of CIS precursor substance were prepared by spin coating at 1000 rpm using a mixed Cu-In solution. The Cu-In films were annealed at 200°C under an N2 atmosphere for 30 minutes. The films were sulfurated by dipping in sulfur solution (1.0 mol/L) in o-dichlorobenzene. After annealing at 500°C for 30 minutes. in a N2(95%)/H2 (5%) mixture gas, CIS layers with a thickness of ~100 nm were formed from the Cu-In films.Structures of (a) FTO/TiO2/CIS/Al, (b) FTO/CIS/TiO2/Al, (c) FTO/CuO/C60/Al heterojunction solar cells, and (d) ITO/PEDOT:PSS/Cu2O:C60/Al bulk heterojunction solar cells.
(a)(b)(c)(d)


### 2.2. Fabrication of Copper Oxide/Fullerene Solar Cells

CuO andC60 layers were coated on precleaned FTO glass plates (Asahi Glass, ~9.3 Ω/□) by using a spin-coating method. Copper acetate monohydrate, Cu(CH3COO)2·H2O (0.5 mol/L, Sigma Aldrich, 99.99%) was dissolved into 2-propanol/monoethanolamine. Cu layers of CuO precursor substance were prepared by spin coating at 1000 rpm using the mixture Cu solution. After annealing at 300°C under air atmosphere for 3 hours, CuO layers with a thickness of ~100 nm were formed from the Cu(II) films. C60 layers with a thickness of ~100 nm were prepared on a CuO layer by spin coating using a C60 solution (16 mg/mL) with C60 powders (Material Technologies Research, 99.98%) in o-dichlorobenzene, and were annealed at 100°C for 30 minutes in N2 atmosphere.Cu2O 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 Cu2O nanoparticles were recovered by centrifuging. Finally, the Cu2O nanoparticles were washed with methanol to remove excess oleylamine, and the Cu2O nanoparticles (~100 mg) were dispersed in toluene (10 mL). 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 N2 atmosphere, semiconductor layers were prepared on a PEDOT layer by spin coating using a mixed solution of a toluene with Cu2O nanoparticle and a C60 solution (16 mg/mL) with C60 powders (Material Technologies Research, 99.98%) in o-dichlorobenzene. The thickness of the bulk heterojunction structure was approximately ~150 nm. The Cu2O:C60 layers were annealed at 100°C for 30 minutes in N2 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 N2 atmosphere. Schematic illustrations of the present solar cells are shown in Figure 1.


### 2.3. 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/cm2 by using an AM 1.5 solar simulator (San-ei Electric, XES-301S) in N2 atmosphere. The solar cells were illuminated through the side of the FTO and ITO substrates, and the illuminated area was 0.16 cm2. Optical absorption of the solar cells was investigated by means of UV-visible spectroscopy (Hitachi, Ltd., U-4100). The microstructures of the CIS, copper oxides, and C60 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).


## 2.1. Fabrication of Dioxide/Copper Indium Disulfide Solar Cells

Two types of solar cells withTiO2/CIS and CIS/TiO2 structures were fabricated as shown in Figures 1(a) and 1(b), respectively. TiO2 and CIS layers were coated on precleaned F-doped SnO2 (FTO) glass plates (Asahi Glass, ~9.3 Ω/□) by using a spin-coating method. TiO2 layers with a thickness of ~10 μm were prepared by squee-gee coating using a TiO2 paste (0.6 g/mL) of fine TiO2 powders (P25, Sigma Aldrich) in an acetylacetone/polyethylene glycol solution and were annealed at 450°C for 30 minutes in air [22].
Copper acetate monohydrate, Cu(CH3COO)2·H2O (0.5 mol/L, Sigma Aldrich, 99.99%) and indium acetate, In(CH3COO)3 (0.5 mol/L, Sigma Aldrich, 99.99%) were dissolved into 2-propanol/monoethanolamine with a ratio of 4 : 1 and 1-propanol/diethanolamine with a ratio of 4 : 1, respectively, and the two solutions were then mixed. The molar ratio of Cu:In was 1 : 1. Cu-In layers of CIS precursor substance were prepared by spin coating at 1000 rpm using a mixed Cu-In solution. The Cu-In films were annealed at 200°C under an N2 atmosphere for 30 minutes. The films were sulfurated by dipping in sulfur solution (1.0 mol/L) in o-dichlorobenzene. After annealing at 500°C for 30 minutes. in a N2(95%)/H2 (5%) mixture gas, CIS layers with a thickness of ~100 nm were formed from the Cu-In films.Structures of (a) FTO/TiO2/CIS/Al, (b) FTO/CIS/TiO2/Al, (c) FTO/CuO/C60/Al heterojunction solar cells, and (d) ITO/PEDOT:PSS/Cu2O:C60/Al bulk heterojunction solar cells.
(a)(b)(c)(d)


## 2.2. Fabrication of Copper Oxide/Fullerene Solar Cells

CuO andC60 layers were coated on precleaned FTO glass plates (Asahi Glass, ~9.3 Ω/□) by using a spin-coating method. Copper acetate monohydrate, Cu(CH3COO)2·H2O (0.5 mol/L, Sigma Aldrich, 99.99%) was dissolved into 2-propanol/monoethanolamine. Cu layers of CuO precursor substance were prepared by spin coating at 1000 rpm using the mixture Cu solution. After annealing at 300°C under air atmosphere for 3 hours, CuO layers with a thickness of ~100 nm were formed from the Cu(II) films. C60 layers with a thickness of ~100 nm were prepared on a CuO layer by spin coating using a C60 solution (16 mg/mL) with C60 powders (Material Technologies Research, 99.98%) in o-dichlorobenzene, and were annealed at 100°C for 30 minutes in N2 atmosphere.Cu2O 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 Cu2O nanoparticles were recovered by centrifuging. Finally, the Cu2O nanoparticles were washed with methanol to remove excess oleylamine, and the Cu2O nanoparticles (~100 mg) were dispersed in toluene (10 mL). 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 N2 atmosphere, semiconductor layers were prepared on a PEDOT layer by spin coating using a mixed solution of a toluene with Cu2O nanoparticle and a C60 solution (16 mg/mL) with C60 powders (Material Technologies Research, 99.98%) in o-dichlorobenzene. The thickness of the bulk heterojunction structure was approximately ~150 nm. The Cu2O:C60 layers were annealed at 100°C for 30 minutes in N2 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 N2 atmosphere. Schematic illustrations of the present solar cells are shown in Figure 1.


## 2.3. 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/cm2 by using an AM 1.5 solar simulator (San-ei Electric, XES-301S) in N2 atmosphere. The solar cells were illuminated through the side of the FTO and ITO substrates, and the illuminated area was 0.16 cm2. Optical absorption of the solar cells was investigated by means of UV-visible spectroscopy (Hitachi, Ltd., U-4100). The microstructures of the CIS, copper oxides, and C60 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).


## 3. Results and Discussion

### 3.1. CIS/TiO2 Solar Cells

TheJ-V characteristics of the TiO2/CIS and CIS/TiO2 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/TiO2 structure provided power conversion efficiency (η) of 1.5×10-5%, fill factor (FF) of 0.25, short-circuit current density (JSC) of 6.0×10-3 mA/cm2, and open-circuit voltage (VOC) of 1.0×10-2 V, which is better than those of the TiO2/CIS device.Table 1
Measured parameters of solar cells.
Layered samplesFTO/TiO2/CIS/AlFTO/CIS/TiO2/AlFTO/CuO/C60/AlITO/PEDOT:PSS/Cu2O:C60/AlVOC (V)1.1×10-30.0100.0450.17JSC (mA/cm2)3.6×10-46.0×10-30.0150.11FF0.250.250.250.23η (%)1.0×10-71.5×10-51.7×10-44.3×10-3Figure2 shows the measured optical absorption of the solar cells. The TiO2/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/TiO2 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 TiO2/CIS and CIS/TiO2 structures were due to TiO2, and the other peaks were due to CIS. As shown in Figure 2, the optical absorption between 400 nm and 800 nm for the TiO2/CIS solar cell is higher than that of CIS/TiO2 structure. The increase in film thickness of the CIS layer in the TiO2/CIS structure through the penetration of the CIS nanocrystals into the porous TiO2 layer results in the increase in the optical absorption of solar cells in the TiO2/CIS structure.Figure 2
Optical absorption of CIS/TiO2 heterojunction solar cells.Microstructures of the CIS thin film were investigated by XRD, as shown in Figure3(a). Diffraction peaks corresponding to CIS and indium oxide (In2O3) 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%).XRD pattern of (a) CIS thin film and (b)TiO2 thin film annealed at 450°C for 30 minutes in air and 500°C for 30 minutes in N2/H2 atmosphere and (c) TiO2 thin film annealed at 450°C for 30 minutes in air.
(a)(b)(c)Preparation conditions of theTiO2/CIS structure were different from those of the CIS/TiO2 structure, and influence of different annealing temperatures of the TiO2 layer was investigated. Figures 3(b) and 3(c) show X-ray diffraction patterns of the TiO2 thin films on the glass substrates annealed at 450°C for 30 minutes in air, and 500°C for 30 minutes in N2/H2 atmosphere and annealed at 450°C for 30 minutes in air, respectively. The X-ray diffraction patterns 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 TiO2 layer.Figures4(a) and 4(b) show a TEM image and an electron diffraction pattern of the TiO2/CIS composite film, respectively. The TEM image indicated TiO2 nanocrystals in the CIS matrix. Debye-Scherrer rings, which indicate polycrystalline structures of TiO2 and 112 reflection of the CIS crystal can be seen in Figure 4(b). Enlarged high-resolution electron microscopy (HREM) images of the TiO2 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 TiO2 and the 112 of the CIS crystals.(a) TEM image and (b) electron diffraction pattern ofTiO2/CIS structure. (c) Enlarged HREM images of TiO2 in (a) and (d) CIS in (a).
(a)(b)(c)(d)Figures5(a) and 5(b) show the microstructures of FTO/TiO2/CIS/Al and FTO/CIS/TiO2/Al, respectively. As shown in Figure 5(a), the porous TiO2 layer has a composite structure with CIS nanocrystals. Therefore, the p-n junction interface area, which is the photoelectron conversion region in the TiO2/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 TiO2/CIS solar cell is higher than that of CIS/TiO2 structure. The increase of the CIS layer thickness in the TiO2/CIS structure through the penetration of the CIS nanocrystals into the porous TiO2 layer would result in the increase of the optical absorption of TiO2/CIS solar cells. Charge separation for the TiO2/CIS solar cell would be higher from high optical absorption. However, the carrier recombination of electrons and holes would occur at the FTO/TiO2 interface for the TiO2/CIS structure, and the energy conversion efficiency would not be improved by an increase of the p-n heterojunction interface in the present work.Schematic microstructures of (a) FTO/TiO2/CIS/Al and (b) FTO/CIS/TiO2/Al solar cells. Energy level diagrams of (a) TiO2/CIS and (b) CIS/TiO2 heterojunction solar cells.
(a)(b)(c)(d)Energy level diagrams ofTiO2/CIS and CIS/TiO2 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 VOC 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. AlthoughCu(InX,Ga1-X)Se2 (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 low-cost reagent and the simple fabrication process. TiO2 and CIS have stable properties against sunlight, and prevent photodeterioration for the solar cells. CIS-based solar cells are expected to provide a long life time.CIS-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 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 ultrathinTiO2 or ZnO n-type semiconductor layers between FTO and TiO2 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.


### 3.2. CuO/C60 Solar Cells

J-V characteristics of the CuO/C60 structures in the dark and under illumination were investigated. The photocurrent was observed under illumination, and the CuO/C60 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/C60 structure provided η of 1.7×10-4%, FF of 0.25, JSC of 0.015 mAcm-2, and VOC of 0.045 V.Figure6 shows the measured optical absorption of the CuO/C60 solar cell. The absorption peaks around 350 nm in the CuO/C60 structure were due to CuO, and absorption peaks around 338 nm, 440 nm, and 606 nm correspond to C60.Figure 6
Optical absorption of CuO/C60 heterojunction solar cell.Figure7 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 to be 16.0 nm. Figures 8(a) and 8(b) show a TEM image and an electron diffraction pattern of the CuO thin film annealed at 300°C, respectively. The TEM image indicated CuO nanocrystals with sizes of 10~20 nm, which agree well with XRD results. Debye-Scherrer rings in Figure 8(b) indicate polycrystalline structures of CuO.Figure 7
XRD patterns of CuO thin films annealed at300°C for 3 hours in air.(a) TEM image and (b) electron diffraction pattern of CuO structure.
(a)(b)Energy level diagram of CuO/C60 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 Cu2O has been mainly used as a p-type oxide semiconductor for solar cells [14–17], CuO was applied instead of Cu2O in the present work. The advantages of the present CuO are low-cost reagent and the simple fabrication process.Figure 9
Energy level diagram of FTO/CuO/C60/Al heterojunction solar cells.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.


### 3.3.Cu2O:C60 Solar Cells

The CuO/C60 solar cell was a heterojunction structure. To increase the interfacial area of p-n junction, Cu2O/C60 bulk heterojunction solar cells were prepared. The J-V characteristics of the Cu2O:C60 structure in the dark and under illumination were measured. The photocurrent was observed under illumination, and the Cu2O:C60 structure showed photovoltaic properties. The measured parameters of the solar cells are summarized in Table 1. A solar cell with a Cu2O:C60 structure provided η of 4.3×10-3%, FF of 0.23, JSC of 0.11 mAcm-2 and VOC of 0.17 V.Figure10 shows optical absorption of the Cu2O:C60 solar cells. An absorption peak around 324 nm for the Cu2O:C60 structure is due to Cu2O, and the absorption peak around 362 nm and 476 nm corresponds to C60.Figure 10
Optical absorption ofCu2O:C60 bulk heterojunction solar cells.Microstructures of theCu2O:C60 thin films were investigated by XRD, as shown in Figure 11. Diffraction peaks corresponding to Cu2O and C60 were observed for the Cu2O:C60 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 C60 phase with cubic system (space group of Fm3m and a lattice parameter of a = 1.4166 nm). The crystallite sizes of Cu2O and C60 were determined to be 7.2 nm and 25.7 nm, respectively.Figure 11
XRD pattern ofCu2O:C60 thin film.Figures12(a) and 12(b) show a TEM image and a selected area diffraction pattern of the Cu2O:C60 thin film annealed at 100°C, respectively. The TEM image indicated aggregated Cu2O nanocrystals with sizes of 10~20 nm. Line broadening of the Debye-Scherrer rings in Figure 12(b) indicates nanocrystal structures of Cu2O. C60 peaks [32] were not observed in this pattern, which would be due to dispersion of C60 crystals. Optimization of the nanocomposite structure with Cu2O and C60 would increase the efficiencies of the solar cells.(a) TEM image and (b) electron diffraction pattern ofCu2O:C60 structure.
(a)(b)Energy level diagram of theCu2O:C60 solar cell is summarized as shown in Figure 13. Previously reported values were used for the energy levels [11, 31, 33]. Cu2O 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 [14–17], C60 was applied instead of ZnO in the present work. Advantages of the present C60 are a good acceptor for solar cells and the simple fabrication process.Figure 13
Energy level diagram of ITO/PEDOT:PSS/Cu2O:C60/Al bulk heterojunction solar cells.Compared to previously reportedCu2O-based heterojunction solar cells [14], copper oxide-based bulk heterojunction and heterojunction solar cells prepared by a spin-coating 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.Table 2
Comparison of copper oxide-based solar cells.
Sampleη (%)Fabrication processAnnealing temperatures(°C)ReferencesHeterojunctionCu2O/ZnO2.01Evaporation and annealing1000[1]Heterojunction CuO/C601.7×10-4Spin coating300Present workBulk heterojunctionCu2O:C604.3×10-3Spin coating140Present workThe low conversion efficiency of the present solar cells would be due to aggregation ofCu2O nanoparticle in the active layer, which would cause carrier recombination. Formation of the Cu2O:C60 thin films with homogeneous distribution of Cu2O nanoparticles could increase the efficiency of solar cells.


## 3.1. CIS/TiO2 Solar Cells

TheJ-V characteristics of the TiO2/CIS and CIS/TiO2 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/TiO2 structure provided power conversion efficiency (η) of 1.5×10-5%, fill factor (FF) of 0.25, short-circuit current density (JSC) of 6.0×10-3 mA/cm2, and open-circuit voltage (VOC) of 1.0×10-2 V, which is better than those of the TiO2/CIS device.Table 1
Measured parameters of solar cells.
Layered samplesFTO/TiO2/CIS/AlFTO/CIS/TiO2/AlFTO/CuO/C60/AlITO/PEDOT:PSS/Cu2O:C60/AlVOC (V)1.1×10-30.0100.0450.17JSC (mA/cm2)3.6×10-46.0×10-30.0150.11FF0.250.250.250.23η (%)1.0×10-71.5×10-51.7×10-44.3×10-3Figure2 shows the measured optical absorption of the solar cells. The TiO2/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/TiO2 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 TiO2/CIS and CIS/TiO2 structures were due to TiO2, and the other peaks were due to CIS. As shown in Figure 2, the optical absorption between 400 nm and 800 nm for the TiO2/CIS solar cell is higher than that of CIS/TiO2 structure. The increase in film thickness of the CIS layer in the TiO2/CIS structure through the penetration of the CIS nanocrystals into the porous TiO2 layer results in the increase in the optical absorption of solar cells in the TiO2/CIS structure.Figure 2
Optical absorption of CIS/TiO2 heterojunction solar cells.Microstructures of the CIS thin film were investigated by XRD, as shown in Figure3(a). Diffraction peaks corresponding to CIS and indium oxide (In2O3) 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%).XRD pattern of (a) CIS thin film and (b)TiO2 thin film annealed at 450°C for 30 minutes in air and 500°C for 30 minutes in N2/H2 atmosphere and (c) TiO2 thin film annealed at 450°C for 30 minutes in air.
(a)(b)(c)Preparation conditions of theTiO2/CIS structure were different from those of the CIS/TiO2 structure, and influence of different annealing temperatures of the TiO2 layer was investigated. Figures 3(b) and 3(c) show X-ray diffraction patterns of the TiO2 thin films on the glass substrates annealed at 450°C for 30 minutes in air, and 500°C for 30 minutes in N2/H2 atmosphere and annealed at 450°C for 30 minutes in air, respectively. The X-ray diffraction patterns 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 TiO2 layer.Figures4(a) and 4(b) show a TEM image and an electron diffraction pattern of the TiO2/CIS composite film, respectively. The TEM image indicated TiO2 nanocrystals in the CIS matrix. Debye-Scherrer rings, which indicate polycrystalline structures of TiO2 and 112 reflection of the CIS crystal can be seen in Figure 4(b). Enlarged high-resolution electron microscopy (HREM) images of the TiO2 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 TiO2 and the 112 of the CIS crystals.(a) TEM image and (b) electron diffraction pattern ofTiO2/CIS structure. (c) Enlarged HREM images of TiO2 in (a) and (d) CIS in (a).
(a)(b)(c)(d)Figures5(a) and 5(b) show the microstructures of FTO/TiO2/CIS/Al and FTO/CIS/TiO2/Al, respectively. As shown in Figure 5(a), the porous TiO2 layer has a composite structure with CIS nanocrystals. Therefore, the p-n junction interface area, which is the photoelectron conversion region in the TiO2/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 TiO2/CIS solar cell is higher than that of CIS/TiO2 structure. The increase of the CIS layer thickness in the TiO2/CIS structure through the penetration of the CIS nanocrystals into the porous TiO2 layer would result in the increase of the optical absorption of TiO2/CIS solar cells. Charge separation for the TiO2/CIS solar cell would be higher from high optical absorption. However, the carrier recombination of electrons and holes would occur at the FTO/TiO2 interface for the TiO2/CIS structure, and the energy conversion efficiency would not be improved by an increase of the p-n heterojunction interface in the present work.Schematic microstructures of (a) FTO/TiO2/CIS/Al and (b) FTO/CIS/TiO2/Al solar cells. Energy level diagrams of (a) TiO2/CIS and (b) CIS/TiO2 heterojunction solar cells.
(a)(b)(c)(d)Energy level diagrams ofTiO2/CIS and CIS/TiO2 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 VOC 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. AlthoughCu(InX,Ga1-X)Se2 (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 low-cost reagent and the simple fabrication process. TiO2 and CIS have stable properties against sunlight, and prevent photodeterioration for the solar cells. CIS-based solar cells are expected to provide a long life time.CIS-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 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 ultrathinTiO2 or ZnO n-type semiconductor layers between FTO and TiO2 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.


## 3.2. CuO/C60 Solar Cells

J-V characteristics of the CuO/C60 structures in the dark and under illumination were investigated. The photocurrent was observed under illumination, and the CuO/C60 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/C60 structure provided η of 1.7×10-4%, FF of 0.25, JSC of 0.015 mAcm-2, and VOC of 0.045 V.Figure6 shows the measured optical absorption of the CuO/C60 solar cell. The absorption peaks around 350 nm in the CuO/C60 structure were due to CuO, and absorption peaks around 338 nm, 440 nm, and 606 nm correspond to C60.Figure 6
Optical absorption of CuO/C60 heterojunction solar cell.Figure7 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 to be 16.0 nm. Figures 8(a) and 8(b) show a TEM image and an electron diffraction pattern of the CuO thin film annealed at 300°C, respectively. The TEM image indicated CuO nanocrystals with sizes of 10~20 nm, which agree well with XRD results. Debye-Scherrer rings in Figure 8(b) indicate polycrystalline structures of CuO.Figure 7
XRD patterns of CuO thin films annealed at300°C for 3 hours in air.(a) TEM image and (b) electron diffraction pattern of CuO structure.
(a)(b)Energy level diagram of CuO/C60 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 Cu2O has been mainly used as a p-type oxide semiconductor for solar cells [14–17], CuO was applied instead of Cu2O in the present work. The advantages of the present CuO are low-cost reagent and the simple fabrication process.Figure 9
Energy level diagram of FTO/CuO/C60/Al heterojunction solar cells.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.


## 3.3.Cu2O:C60 Solar Cells

The CuO/C60 solar cell was a heterojunction structure. To increase the interfacial area of p-n junction, Cu2O/C60 bulk heterojunction solar cells were prepared. The J-V characteristics of the Cu2O:C60 structure in the dark and under illumination were measured. The photocurrent was observed under illumination, and the Cu2O:C60 structure showed photovoltaic properties. The measured parameters of the solar cells are summarized in Table 1. A solar cell with a Cu2O:C60 structure provided η of 4.3×10-3%, FF of 0.23, JSC of 0.11 mAcm-2 and VOC of 0.17 V.Figure10 shows optical absorption of the Cu2O:C60 solar cells. An absorption peak around 324 nm for the Cu2O:C60 structure is due to Cu2O, and the absorption peak around 362 nm and 476 nm corresponds to C60.Figure 10
Optical absorption ofCu2O:C60 bulk heterojunction solar cells.Microstructures of theCu2O:C60 thin films were investigated by XRD, as shown in Figure 11. Diffraction peaks corresponding to Cu2O and C60 were observed for the Cu2O:C60 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 C60 phase with cubic system (space group of Fm3m and a lattice parameter of a = 1.4166 nm). The crystallite sizes of Cu2O and C60 were determined to be 7.2 nm and 25.7 nm, respectively.Figure 11
XRD pattern ofCu2O:C60 thin film.Figures12(a) and 12(b) show a TEM image and a selected area diffraction pattern of the Cu2O:C60 thin film annealed at 100°C, respectively. The TEM image indicated aggregated Cu2O nanocrystals with sizes of 10~20 nm. Line broadening of the Debye-Scherrer rings in Figure 12(b) indicates nanocrystal structures of Cu2O. C60 peaks [32] were not observed in this pattern, which would be due to dispersion of C60 crystals. Optimization of the nanocomposite structure with Cu2O and C60 would increase the efficiencies of the solar cells.(a) TEM image and (b) electron diffraction pattern ofCu2O:C60 structure.
(a)(b)Energy level diagram of theCu2O:C60 solar cell is summarized as shown in Figure 13. Previously reported values were used for the energy levels [11, 31, 33]. Cu2O 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 [14–17], C60 was applied instead of ZnO in the present work. Advantages of the present C60 are a good acceptor for solar cells and the simple fabrication process.Figure 13
Energy level diagram of ITO/PEDOT:PSS/Cu2O:C60/Al bulk heterojunction solar cells.Compared to previously reportedCu2O-based heterojunction solar cells [14], copper oxide-based bulk heterojunction and heterojunction solar cells prepared by a spin-coating 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.Table 2
Comparison of copper oxide-based solar cells.
Sampleη (%)Fabrication processAnnealing temperatures(°C)ReferencesHeterojunctionCu2O/ZnO2.01Evaporation and annealing1000[1]Heterojunction CuO/C601.7×10-4Spin coating300Present workBulk heterojunctionCu2O:C604.3×10-3Spin coating140Present workThe low conversion efficiency of the present solar cells would be due to aggregation ofCu2O nanoparticle in the active layer, which would cause carrier recombination. Formation of the Cu2O:C60 thin films with homogeneous distribution of Cu2O nanoparticles could increase the efficiency of solar cells.


## 4. Conclusions

Copper system compound semiconductor solar cells were produced and characterized. A device based on the CIS/TiO2 structure provided η of 1.5×10-5%, FF of 0.25, JSC of 6.0×10-3 mA/cm2, and VOC of 1.0×10-2 V, which showed better performance compared to that of a device based on the TiO2/CIS structure. The devices based on the CuO/C60 and Cu2O : C60 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, JSC of 0.015 mAcm-2 and 0.11 mAcm-2, and VOC of 0.045 V and 0.17 V, respectively. X-ray diffraction analysis indicated the formation of CIS, CuO, and Cu2O nanocrystal structures, respectively.Energy level diagrams of CIS/TiO2, CuO/C60, and Cu2O:C60 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.



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*Source: 562842-2010-09-20.xml*