Advances in Condensed Matter Physics

Volume 2012 (2012), Article ID 129139, 5 pages

doi:10.1155/2012/129139

Research Article

Fabrication and Optoelectrical Properties of IZO/ Heterostructure Solar Cells by Thermal Oxidation

Cheng-Chiang Chen, Lung-Chien Chen, and Yi-Hsuan Lee

Department of Electro-optical Engineering, National Taipei University of Technology, 1, Section 3, Chung-Hsiao E. Road, Taipei 106, Taiwan

Received 14 March 2012; Accepted 30 March 2012

Academic Editor: Nigel Wilding

Copyright © 2012 Cheng-Chiang Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Indium zinc oxide (IZO)/cupper oxide (Cu₂O) is a nontoxic nature and an attractive all-oxide candidate for low-cost photovoltaic (PV) applications. The present paper reports on the fabrication of IZO/Cu₂O heterostructure solar cells which the Cu₂O layers were prepared by oxidation of Cu thin films deposited on glass substrate. The measured parameters of cells were the short-circuit current (), the open-circuit voltage (), the maximum output power (), the fill factor (FF), and the efficiency (η), which had values of 0.11 mA, 0.136 V, 5.05 μW, 0.338, and 0.56%, respectively, under AM 1.5 illumination.

1. Introduction

Cu₂O has long been considered an attractive to silicon, and there are other semiconductors being favored for the fabrication of cheaper solar cells for terrestrial applications [1–4]. The Cu₂O is intrinsically P-type, has a band gap (Eg) of 2.17 eV [5], and is expected to have a maximum theoretical efficiency of 20% [6, 7]. Its advantages include: its nontoxic nature, abundance of the starting material (copper), and cheap production processing. Cuprous oxide is of particular interest in the field of solar energy, because of their low cost, abundance of the starting material (copper), and its nontoxic nature and the light-to-electricity conversion efficiency [8]. IZO/Cu₂O is an attractive all-oxide candidate for low-cost photovoltaic (PV) applications. The indium zinc oxide (IZO) films have been attracting a lot of attention because of good conductivity [9], high optical transparency [10], low deposition temperature, and very smooth surface roughness [11] in comparison with ITO films.

Therefore, Cu₂O thin films can be prepared by various techniques, such as reactive sputtering [12], MOCVD [13], electrochemical deposition [14–17], solution chemical [18–20], and direct oxidation of Cu sheets [21], of which the magnetron sputtering presents many advantages as low-cost approach, large scale, and easy control. Some groups reported ZnO/Cu₂O solar cells prepared [21, 22]. The efficiency was to be 1-2%. In this work, the effect of the conditions of deposition of the P-type Cu₂O thin films by the radiofrequency magnetron sputtering method is studied; in particular, the effect of changing the temperature and time of annealing to obtain the different performances of P-type Cu₂O thin layers on crystal quality was investigated. Finally, the fabrication and optoelectronic performance of an IZO/Cu₂O solar cell is considered.

2. Experimental

In the study, devices that had cupper (Cu) layers of one micron thick were deposited with a purity of 99.995% on a glass substrate by magnetron reactive sputtering from high-purity Cu targets in argon (Ar) gas at a flow rate of 15 sccm and a stable pressure of 3 × 10⁻³ Torr. The sputtering system consisted of a vacuum chamber with a target and a substrate copper holder, a turbo molecular pump parallel to a rotary pump that provides a high vacuum, a radio frequency (RF) power supply at 13.56 MHz, and mass flow controllers that maintain a steady gas flow rate. The target was cleaned by presputtering with Ar plasma for 5 min prior to each deposition process. In all our Cu deposition experiments the RF power and the gas pressure were kept constant at 50 W and 3.1 × 10⁻³ Torr, respectively. The Cu₂O layers were obtained by oxidation of Cu layers annealing at various temperatures for 10 min in air. For easy and convenient characterization of the same material, Cu₂O films were prepared on glass to measure the electronic characteristics by Hall measurements and their absorbance from transmittance spectra. The films’ crystallinity was studied by X-ray diffraction (XRD) and exhibits a polycrystalline structure. Indium-oxide-doped zinc oxide (IZO, In : Zn = 1 : 9) was deposited onto the Cu₂O layer by sputtering, and indium electrodes were formed by the evaporation onto both the surface of the IZO layer and the Cu₂O layer, respectively, to complete IZO/Cu₂O heterostructure solar cells. Figure 1 shows the crosssection of the completed structure. Additionally, the current density-voltage (J-V) characteristics were determined using a Keithley 2420 programmable source meter under irradiation by a 100 W xenon lamp. Finally, the irradiation power density on the surface of the sample was calibrated as 100 W/m².

Figure 1: Schematic crosssection of the completed structure.

3. Results and Discussion

In order to obtain a better understanding of the thermal oxidation mechanism, phase identification was performed. Figure 2 shows the XRD spectrum of the measured Cu₂O films at various temperatures of 300, 400, and 500°C. The XRD diffraction shows that single phase of Cu₂O films is obtained for growth at different thermal oxidation temperature with diffraction peaks at 29.58° and 43.35° corresponding to the (110) and (111) planes of the cubic-structured Cu₂O, respectively. As oxidation temperature increases, the CuO peak decreases and eventually vanished at 500°C. The strong Cu peaks at temperatures below 300°C [23], the sudden stop of mass gain in the using thermogravimetric analysis results, together with the SEM images, imply that a self passivation mechanism may involve in the formation of a compact Cu₂O layer. We employed the X-ray diffraction technique to get main crystalline phases and the possible orientation of crystalline in the films prepared at optimum conditions.

Figure 2: X-ray diffraction patterns of the samples with thermal oxidation at various temperatures.

Figure 3 shows the FESEM micrographs of the Cu₂O layers with thermal treatment at various temperatures. The micrographs indicate that the surface of the film consisted of small particles. The average particle sizes were approximately (a) 70, (b) 100, and (c) 200 nm for 300, 400, and 500°C annealing, respectively. The particle sizes of layer are increases with oxidation temperature increase. That shows thin oxides formed, particularly developed in the grain boundary areas, implying a product of fast-diffusion processes, which might be responsible for the small value of activation energy at oxidation of 500°C [24].

(a)

(b)

(c)

Figure 3: SEM images of the samples with thermal oxidation at various temperatures.

Figure 4 plots both the carrier concentration and the mobility as a function of thermal oxidation annealing temperatures. As the thermal oxidation annealing temperatures are elevated to 500°C, the carrier concentration is likely to decrease, and at the same time, the mobility increases to 5 cm²/Vs. That is, a film with relatively high mobility is obtained by thermal oxidation annealing of 400°C samples, although the temperature required to obtain the same mobility by thermal oxidation annealing appears to be slightly higher than in the case that the film is directly formed by oxidation at elevated temperatures such as 400 to 500°C. The increase of mobility shown in Figure 4 may be attributed to this increase of grain size.

Figure 4: The carrier concentration and the mobility as a function of thermal oxidation annealing temperatures.

Figure 5(a) shows the results of the absorption measurements for the Cu₂O layers with treatment at various annealing temperatures for 10 min. The layers have a very high absorption in visible region resulting in good materials for the solar energy devices. According to this figure, the absorption increased continuously as the annealing temperature increased from 300 to 500°C owing to that the Cu content decreased. Figure 5(b) shows the results of the absorption measurements for the IZO layers. The IZO layer yields an absorption edge at ~3.25 eV. Therefore, the transparent semiconductor IZO layer is suitable as the upper layer for the IZO/Cu₂O solar cells.

(a)

(b)

Figure 5: Absorbance spectra for (a) Cu₂O films with thermal oxidation at various temperatures and (b) the relationship between absorption squared versus photon energy.

Figure 6 shows the I-V characteristics of the IZO/Cu₂O heterostructure with and without any illumination, respectively. The cell performance was measured under AM 1.5 illumination with a solar intensity of 10 mW/cm² at 25°C. The cell has an active area of 0.3 × 0.3 cm² and no antireflective coating. The Cu₂O layers were prepared by oxidation of Cu thin films at 500°C for 10 min. IZO/Cu₂O solar cells exhibited the following static parameters: of 0.11 mA ( of 1.22 mA/cm²) and of 0.136 V. As is well known, the fill factor (FF) can be described by [25]

where is the maximum output current, and is the maximum output voltage. Therefore, using the values of and deduced from Figure 6, the value of FF results is equal to 0.338. Similarly, the conversion efficiency (η) defined by [25]

with is the incident power, results are to be 0.56%. Low FF value and poor conversion efficiency are owing to the high series resistance (~760 Ω, in this study). The series resistance is mainly caused by the bulk resistance of materials, contacts, and interconnections. However, in this work, the high value of the series resistance may be caused by the Cu₂O layers and the contact resistance between the electrodes and the IZO layer and the Cu₂O layer.

Figure 6: I-V curves evaluated for IZO/Cu₂O heterostructure solar cells in the dark and under an illumination.

4. Conclusions

The present paper reports on the fabrication of IZO/Cu₂O heterostructure solar cells in which the Cu₂O layers were prepared by oxidation of Cu thin films deposited on glass substrate. The Cu₂O layers have a very high absorption in visible region resulting in good materials for the solar energy devices. The measured parameters of cells were the short-circuit current (), the open-circuit voltage (), the maximum output power (), the fill factor (FF), and the efficiency (η), which had values of 0.11 mA, 0.136 V, 5.05 μW, 0.338, and 0.56%, respectively, under AM 1.5 illumination. Therefore, IZO/Cu₂O is a nontoxic nature and an attractive all-oxide candidate for low-cost photovoltaic (PV) applications in the future.

Acknowledgment

The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract no. NSC 100-2215-E-027-057.

References

C. Noguet, M. Tapiero, C. Schwab, et al., “Cuprous oxide as a photovoltaic converter,” in Proceedings of the Photovoltaic Solar Energy Conference, p. 1170, Luxemburg, 1977.

A. A. Berezin and F. L. Weichman, “Photovoltaic effect in cuprous oxide-copper junctions in relation to the optical absorption spectrum of cuprous oxide,” Solid State Communications, vol. 37, no. 2, pp. 157–160, 1981.

M. Tapiero, C. Noguet, J. P. Zielinger, C. Schwab, and D. Pierrat, “Conversion photovoltaïgque dans Cu₂O,” Applied Physics Reviews, vol. 14, p. 231, 1979.

L. C. Olsen, F. W. Addis, and R. C. Bohara, “Investigation of Cu₂O solar cells,” in Proceedings of the 14th IEEE Photovoltaic Specification Conference, pp. 462–467, January 1980.

P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, and J. M. Tarascon, “Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries,” Nature, vol. 407, no. 6803, pp. 496–499, 2000.

S. S. Jeong, A. Mittiga, E. Salza, A. Masci, and S. Passerini, “Electrodeposited ZnO/Cu₂O heterojunction solar cells,” Electrochimica Acta, vol. 53, no. 5, pp. 2226–2231, 2008.

A. Mittiga, E. Salza, F. Sarto, M. Tucci, and R. Vasanthi, “Heterojunction solar cell with 2% efficiency based on a Cu₂O substrate,” Applied Physics Letters, vol. 88, no. 16, Article ID 163502, 2006.

E. Y. Wang, D. Trivich, H. Sawaiha, and G. Thomas, “Cuprous oxide Schottky photovoltaic cells as potential solar energy converters,” in Proceedings of the Complex International Conference, p. 643, Dharan, Saudi Arabia, 1975.

T. Minami, T. Kakumu, and S. Takata, “Preparation of transparent and conductive In₂O₃–ZnO films by radio frequency magnetron sputtering,” Journal of Vacuum Science & Technology A, vol. 14, no. 3, article 1704, 5 pages, 1996.

N. Naghavi, A. Rougier, C. Marcel, C. Guery, J. B. Leriche, and J. M. Tarascon, “Characterization of indium zinc oxide thin films prepared by pulsed laser deposition using a Zn₃In₂O₆ target,” Thin Solid Films, vol. 360, no. 1-2, pp. 233–240, 2000.

A. Kaijo, Display Imaging, vol. 4, p. 143, 1996.

S. Izhizuka, S. Kato, T. Maruyama, and K. Akimoto, “Nitrogen doping into Cu₂O thin films deposited by reactive radio-frequency magnetron sputtering,” Japanese Journal of Applied Physics, vol. 40, pp. 2765–2768, 2001.

S. Jeong and E. S. Aydil, “Heteroepitaxial growth of Cu₂O thin film on ZnO by metal organic chemical vapor deposition,” Journal of Crystal Growth, vol. 311, no. 17, pp. 4188–4192, 2009.

K. Nakaoka and K. Ogura, “Electrochemical preparation of p-type cupric and cuprous oxides on platinum and gold substrates from copper(II) solutions with various amino acids,” Journal of the Electrochemical Society, vol. 149, no. 11, pp. C579–C585, 2002.

A. K. Mukhopadhyay, A. K. Chakraborty, A. P. Chatterjee, and S. K. Lahiri, “Galvanostatic deposition and electrical characterization of cuprous oxide thin films,” Thin Solid Films, vol. 209, no. 1, pp. 92–96, 1992.

S. Joseph and P. V. Kamath, “Electrochemical deposition of Cu₂O on stainless steel substrates: promotion and suppression of oriented crystallization,” Solid State Sciences, vol. 10, no. 9, pp. 1215–1221, 2008.

T. Fujiwara, T. Nakaue, and M. Yoshimura, “Direct fabrication and patterning of Cu₂O film by local electrodeposition method,” Solid State Ionics, vol. 175, no. 1–4, pp. 541–544, 2004.

L. Zhang, H. Li, Y. H. Ni, J. Li, K. M. Liao, and G. C. Zhao, “Porous cuprous oxide microcubes for non-enzymatic amperometric hydrogen peroxide and glucose sensing,” Electrochemistry Communications, vol. 11, p. 812, 2009.

C. H. Kuo and M. H. Huang, Journal of the American Chemical Society, vol. 130, p. 12815, 2008.

G. Jimenez-Cadena, E. Comini, M. Ferroni, and G. Sberveglieri, “Synthesis of Cu₂O bi-pyramids by reduction of Cu(OH)₂ in solution,” Materials Letters, vol. 64, no. 3, pp. 469–471, 2010.

A. Mittiga, E. Salza, F. Sarto, M. Tucci, and R. Vasanthi, “Heterojunction solar cell with 2% efficiency based on a Cu₂O substrate,” Applied Physics Letters, vol. 88, no. 16, Article ID 163502, 2006.

T. Minami, T. Miyata, K. Ihara, Y. Minamino, and S. Tsukada, “Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu₂O heterojunction devices,” Thin Solid Films, vol. 494, no. 1-2, pp. 47–52, 2006.

Y. Zhu, K. Mimura, and M. Isshiki, “Oxidation Mechanism of Cu₂O to CuO at 600–1050°C,” Oxidation of Metals, vol. 62, no. 3/4, pp. 207–222, 2004.

W. Gao, H. Gong, J. He, A. Thomas, L. Chan, and S. Li, “Oxidation behaviour of Cu thin films on Si wafer at 175–400°C,” Materials Letters, vol. 51, p. 78, 2001.

Sze S. M., Physics of Semiconductor Devices, John Wiley & Sons, New York, NY, USA, 2nd edition, 1981.