FABRICATION AND CHARACTERIZATION OF ELECTRODEPOSITED Cu2O P-N HOMOJUNCTION SOLAR CELLS

Kunhee Han,1 Xiaofei Han2 and Meng Tao1

1 Department of Electrical Engineering and 2 Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, Texas 76019, USA

ABSTRACT

Two device structures for electrodeposited Cu2O p-n homojunction solar cells, substrate and superstrate, have been fabricated and characterized. The Cu2O p-n homojunctions in these cells were prepared by a two-step sequential electrodeposition process. It was found that the open-circuit voltage of the cells was affected by solution pH during the deposition of the n-type Cu2O layer. Solution pH controls oxygen incorporation and thus the native point defects in Cu2O, which in turn controls the flat-band voltage of Cu2O. A thermal stability study indicates that the performance of the Cu2O solar cells is unstable at temperatures above 150 oC. For Cu2O superstrate solar cells, it was found that the thickness of the p-type Cu2O layer is critical to the performance of the cells. The open-circuit voltage, and thus the efficiency of the cells, is proportional to the thickness of the p-layer, and a 3 µm p-Cu2O layer is necessary for good performance. The highest efficiency of 0.15 % is achieved in a Cu2O substrate solar cell with an area of 0.01 cm2.

INTRODUCTION

Cuprous oxide (Cu2O) has the potential as the light absorber material in low-cost solar cells [1]. The source materials of Cu2O are naturally abundant. It can be deposited with inexpensive solution-based method such as electrodeposition. It has a direct band gap around 2.0 eV and is non-toxic [2]. Cu2O was among the first semiconductors to be investigated in the 1920’s for solid-state electronics [3]. It is known as a defect semiconductor with p-type conductivity due to Cu vacancies [4]. With the lack of n-type Cu2O, previous solar cells based on Cu2O as the absorber layer utilized metal/Cu2O Schottky junctions [5] or ZnO/Cu2O p-n heterojunctions [6,7]. Recently, we reported n-type behavior in electrodeposited Cu2O. This was accomplished by controlling the pH of the deposition solution [8]. The n-type Cu2O makes it feasible to fabricate a p-n homojunction Cu2O solar cell, considered to be a critical approach to increase the efficiency of Cu2O-based solar cells [4]. We have succeeded in demonstrating a p-n homojunction Cu2O solar cell [9]. In this paper, we report two different device structures for Cu2O solar cells, substrate and superstrate, which are fabricated by an inexpensive two-step electrodeposition process. The focus of this paper is the investigation of several key factors affecting device performance and thermal stability of these devices.

EXPERIMENTAL

The deposition of p- and n-type Cu2O was carried out potentiostatically without stirring of the deposition solution. A three-electrode electrochemical cell was used, where the counter electrode was a Pt foil and the reference electrode was Ag/AgCl/sat. NaCl. The working electrode for substrate-structured Cu2O cells was a 100 nm Au/75 nm Cr/glass slide, while that for superstrate-structured Cu2O cells was a 300 nm ITO/glass slide. The Cr layer was used as the adhesion layer for Au. Prior to deposition, the Au/Cr/glass and ITO/ glass slides were ultrasonicated in acetone for 15 minutes and then rinsed in deionized water.

In case of substrate-structured solar cells, p- and n-type Cu2O films were sequentially deposited in two different electrolyte solutions to make a p-n homojunction. A 2 µm p-type Cu2O film was first electrodeposited on an Au/Cr-coated glass substrate in an aqueous solution containing 0.4 M copper sulfate and 3 M sodium lactate at solution pH 13.0. The deposition potential was -0.3 V and deposition temperature 70 °C. A 0.5 µm n-type Cu2O film was directly deposited on p-type Cu2O in an aqueous solution containing 0.01 M copper acetate and 0.1 M sodium acetate with a varying solution pH from 5.2 to 6.4. The deposition potential was -0.1 V and deposition temperature 60 °C. For window layer deposition, 100 nm ITO was deposited on the n-type Cu2O layer by reactive RF sputtering. The device fabrication was finished by DC sputtering of Al for top contact. The finished devices were 1×1 mm2 and 4×10 mm2 in size. The top view of a finished device with size of 4×10 mm2 is shown in Figure 1.

Figure 1. Top view of a Cu2O p-n homojunction substrate solar cell with area of 0.4 cm2: four individual cells are shown.

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In case of superstrate-structured solar cells, a 0.8 µm n-type Cu2O film was first electrodeposited on a ITO-coated glass substrate in an aqueous solution containing 0.01 M copper acetate and 0.1 M sodium acetate at solution pH 5.2. The deposition potential was -0.1 V and deposition temperature 60 °C. A p-type Cu2O film with different thickness was then directly deposited on n-type Cu2O in an aqueous solution containing 0.4 M copper sulfate and 3 M sodium lactate at solution pH 13.0. The deposition potential was -0.5 V and deposition temperature 60 °C. For the back contact, which acts as the back reflector as well, 100 nm Au was deposited on the p-type Cu2O layer by thermal evaporation. The back Au electrode was round-shaped with an area 0.282 cm2, which defines the cell size. A Cu wire was then bonded to the Au electrode as the electrical lead. The top view of a finished device is shown in Figure 2. The performance of the cells was investigated with a Newport solar simulator calibrated at AM 1.5. The I-V data was obtained with a Keithley 2601 SourceMeter.

Figure 2. Top view of a Cu2O p-n homojunction superstrate solar cell: two individual cells are shown.

RESULTS AND DISCUSSION Dark I-V characteristics of a Cu2O p-n homojunction

substrate solar cell with an area of 1×1 mm2 are shown in Figure 3. P-type and n-type Cu2O films were deposited in solutions with pH 13.0 and 5.2, respectively. The I-V characteristics show the rectification effect of a p-n junction, although the rectification ratio is low and there is also significant series resistance in the cell. The photovoltaic performance of a p-n homojunction Cu2O solar cell is shown in Figure 4. The open-circuit voltage, 0.32 V, is slightly higher than that for an n-ZnO/p-Cu2O heterojunction cell [10].

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Figure 3. Dark I-V characteristic of a Cu2O p-n homojunction substrate solar cell with area of 0.01cm2.

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FF: 35.33 %η: 0.15 %

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Figure 4. I-V characteristic under illumination of a Cu2O p-n homojunction substrate solar cell with area of 0.01 cm2.

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FF: 27.36%η: 0.02%

Area: 0.01cm2

AM: 1.0

Figure 5. I-V characteristic under illumination of a Cu2O p-n homojunction solar cell, in which the n-type Cu2O layer was deposited at solution pH 6.4.

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For comparison, a Cu2O p-n homojunction solar cell, in which the n-type Cu2O layer was deposited at solution pH 6.4, was prepared and its photovoltaic performance is shown in Figure 5. The open-circuit voltage for this cell is significantly lower than the cell in Fig. 4. It has been reported that the flat-band voltage (Vfb) of Cu2O is significantly affected by solution pH during deposition [11]. It is speculated that the decreased open-circuit voltage is caused by changes in the flat-band voltage due to increased solution pH, from 5.2 to 6.4.

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Figure 6. Performance change of Cu2O substrate solar cell with area of 0.4 cm2 before (a) and after (b) thermal treatment at 150 oC in air for 20 minutes.

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voltage (V) Figure 7. Performance of three superstrate Cu2O solar cells with different p-type Cu2O layer thickness. N-type Cu2O layer thickness is fixed to 0.8 µm. (a) 3.0 µm p-layer, (b) 1.5 µm p-layer, and (c) 0.8 µm p-layer.

A thermal stability study was performed on a Cu2O substrate solar cell with area of 0.4 cm2. The cell was annealed in air at 150 oC and then 300 oC sequentially, each for 20 minutes. The open-circuit voltage decreased from the 107 mV to 74 mV, and the short-circuit current also decreased from 0.44 mA to 0.36 mA after annealing,

as shown in Fig. 6. After 300 oC annealing, there was no photovoltaic effect for the cell. It is believed that different types of native point defects in each type of Cu2O, i.e. Cu vacancies in p-type Cu2O and O vacancies in n-type Cu2O, diffuse across the p-n junction at temperatures close to 300 oC, which annihilates the p-n junction.

Superstrate Cu2O solar cells with different thicknesses of the p-type Cu2O layer, 3.0, 1.5, and 0.8 µm, were prepared to investigate the effect of p-type layer thickness on cell performance. The thickness of the n-type Cu2O layer was fixed to 0.8 µm in all these cells. The open-circuit voltage of the superstrate cells increases linearly with the thickness of the p-type Cu2O layer, while the short-circuit current remains almost the same, as shown in Fig. 7. This is a typical phenomenon of the shunt effect. It was found that surface roughness of the superstrate cells was larger than the substrate cells, probably caused by the thicker n-type Cu2O layer in the superstrate cells (0.8 µm vs. 0.5 µm). It is speculated that on the rough surface of n-type Cu2O, more shunt paths, such as n-Cu2O regions not covered by p-Cu2O, are more likely to form as the thickness of the p-type Cu2O layer decreases. It was noticed that a ~3 µm p-Cu2O layer is necessary to form a good-quality p-n junction in the superstrate structure cells.

CONCLUSION Two different structures of Cu2O p-n homojunction

solar cells, substrate and superstrate, were successfully fabricated from a two-step electrodeposition process. The open-circuit voltage of the cells was affected by solution pH during the deposition of n-type Cu2O. A thermal stability study shows that the performance of the Cu2O solar cells is not stable at temperatures close to 300 oC. The efficiency of the superstrate cells was affected by the thickness of the p-type layer, and the superstrate cell with a 3 µm p-type Cu2O layer showed a minimal shunt effect. The low efficiency, along with the low short-circuit current, in both Cu2O substrate and superstrate solar cells is largely attributed to the high resistivities of both p-type and n-type Cu2O. Therefore, doping in both p-type and n-type Cu2O to reduce their resistivities is necessary for better efficiency.

ACKNOWLEDGEMENT

This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0002062.

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