Abstract
Cuprous oxide (Cu2O) films were prepared on an indium tin oxide glass substrate by radiofrequency magnetron sputtering using a high-purity Cu target. The temperature of annealing was varied to obtain Cu2O thin films with various elements, compositions, and surface structures. The p-Cu2O thin films thus formed were characterized by FESEM and XRD. After annealing at 500∘C, the bilayer structure which consisted of Cu nanoclusters on the surface of a film of Cu2O nanocolumns was observed. The Cu2O solar cell with the bilayered structure exhibited poor power conversion efficiency.
1. Introduction
Cuprous oxide (Cu2O) is a potential oxide semiconductor with a direct band gap of 2.17 eV because the theoretical energy conversion efficiency of a Cu2O solar cell is of the order of 20% [1–3]. Undoped Cu2O, prepared by various techniques, is usually a p-type semiconducting material because of its native defects that are formed by Cu vacancies [4, 5]. Cu2O is of a nontoxic nature and is an attractive all-oxide candidate for low-cost photovoltaic (PV) applications. Cu2O has long been considered to be an attractive transparent conductor oxide (TCO) semiconductor, which is favorable for the fabrication of low-cost solar cells for terrestrial applications. Several groups have reportedly developed p-n junction solar cells with a TCO/Cu2O heterostructure, such as zinc oxide (ZnO)/Cu2O, indium-doped ZnO (IZO)/Cu2O, aluminum-doped ZnO (AZO)/Cu2O, and gallium-doped ZnO (GZO)/Cu2O heterojunction [6–11]. Cu2O thin films have been prepared by various methods, including reactive sputtering [12], metal-organic chemical vapor deposition (MOCVD) [13], electrochemical deposition [14–17], chemical dissolution [18–20], and the direct oxidation of Cu sheets [21].
According to the literature, the performance of Cu2O-based solar cells is significantly affected by the crystallinity of Cu2O, because Cu metal can easily be formed at the surface of the Cu2O films or at the interface between the TCO and the Cu2O films when thin films of TCO are deposited on the Cu2O films. Therefore, this study investigates the deposition of the Cu2O thin films by the magnetron sputtering method; in particular, it examines the effect of changing the temperature of annealing to vary the crystal quality of the Cu2O thin films.
2. Experimental
In this study, Cu2O films were prepared on ITO glass using a Cu target with 99.995% purity and a radiofrequency magnetron sputtering system. The Cu targets were in Ar (flow rate of 40 sccm) and O2 (flow rate of 1–3 sccm) gas at a stable pressure of Torr, and the temperature and time of annealing varied to yield Cu2O thin films with various properties. The flow rates of both argon and oxygen gases were individually monitored using mass flow controllers. Table 1 presents in detail the parameters of reactive magnetron sputtering for the preparation of Cu2O films. The elements, composition, and surface structure of Cu2O thin films were characterized by field emission scanning electron microscopy (FESEM) and XRD measurements. The mobility and carrier concentrations in the Cu2O thin films, and the resistances of the the films, were determined by making Hall measurements.
After the Cu2O films had been studied, ZnO/Cu2O heterostructure solar cells were fabricated. A ZnO film and then a Cu2O film were deposited on an ITO-coated glass substrate. Next, copper electrodes were formed by sputtering onto the surfaces of both of the ZnO film and the Cu2O film to complete the fabrication of the ZnO/Cu2O heterostructure solar cells. The current density-voltage (-) characteristics were measured 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 at 100 W/m2.
3. Results and Discussion
Figure 1 shows the top-view and cross-sectional FESEM images of the microstructures of the Cu2O films that had been annealed at 300, 400, and 500°C. Two-dimensional grain boundaries of the Cu2O films that were annealed at 400 and 500°C are clearly observed. The oxide scales on the sample that was annealed at 300°C exhibit compact clusters of fine grains, as shown in Figures 1(a) and 1(b). When the annealing temperature exceeded 400°C, porous, thin oxides formed, particularly developed in the grain boundary regions, implying that they were produced by fast diffusion processes, as shown in Figures 1(c) to 1(e), which may be responsible for the low value of the activation energy. Nanosize grains with sizes from 30 to 50 nm were obtained by varying the annealing temperature and flow rate of oxygen gas. After annealing at 500°C, as shown in Figure 1(e), the bilayer structure consisted of Cu nanoclusters on the surface of a film of Cu nanocolumns was observed.
To elucidate the annealing mechanism, the phase was identified. Figure 2(a) presents the XRD pattern of the Cu2O films that were treated at various annealing temperatures. XRD diffraction shows that single phase of Cu2O films was formed by growth at different annealing temperatures, yielding diffraction peaks at 36.45° and 42.33° that corresponded to the (111) and (200) planes of the cubic-structured Cu2O. At high annealing temperatures, the Cu2O(111) peak increased and Cu peaks appeared at 400 and 500°C. The Cu2O(111) peak shifted slightly from 36.45° to 36.93°. The outgassing of oxygen from the surface of the film into the Cu2O nanocolumns and the formation of Cu clusters on the surface of the Cu2O film may have contributed to the appearance of the Cu peaks and the shift of the Cu2O peaks. Figure 2(b) presents the mechanism of formation of the Cu-Cu2O bilayer.
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(b)
Figure 3 plots both the resistivity and the mobility as functions of thermal annealing temperature for annealing periods of 10 and 20 min. The sample that was annealed for 10 min had superior electrical characteristics than the sample that had been annealed for 20 min. As the thermal annealing temperature increased, the resistivity of Cu2O films linearly fell while the mobility declined to ~2–4 cm2/Vs. The reduction of resistivity of the Cu2O films after annealing may be attributed to the segregation of the Cu-rich nanoclusters from the Cu2O film, which is shown in Figure 1(e). Also, unlike other studies, a film with relatively low mobility was obtained by postthermal annealing [22–24]. The reduction of mobility in Figure 3 is attributable to the transportation of carriers from one nanocolumn to another nanocolumn. However, as the annealing temperature increased to 500°C, the mobility in the sample that had been annealed for 10 min increased by ~5 cm2/Vs owing to the increase in the size of the grains in the Cu2O nanocolumns and the Cu-rich nanoclusters.
Figure 4(a) presents the absorption measurements for the Cu2O layers following postannealing treatment at various temperatures for 10 min. The layers absorb very strongly in the visible region, and so they are favorable materials for use in solar energy devices. According to this figure, the absorption increased continuously with the annealing temperature from 300 to 500°C owing to a drop in Cu content. Figure 4(b) plots absorption squared as a function of photon energy as determined from the transmittance in Figure 4(a). The as-deposited sample and the sample that was postannealed at 300°C had an absorption edge at ~2.1 eV. This value is consistent with other results for the band-gap energy of Cu2O that can be found in the literature [25–27]. The extrapolation of the linear region of the curves to the horizontal axis gives the band-gap energies of Cu2O following postannealing at 400 and 500°C, which are ~2.45 and ~2.55 eV, respectively. This result may be attributed to the incorporation of a larger amount of oxygen in the film, making it nonstoichiometric following postannealing. A nonstoichiometric Cu2O film with higher oxygen content has a smaller lattice constant, a larger band gap, and a higher resistivity.
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(b)
Figure 5(a) displays the cross-section of the completed structure and Figure 5(b) shows the FESEM image of the structure. The Cu2O nanocolumns collapse in the image following the preparation of the sample for FESEM observation. Figure 6 plots the - characteristics of the ZnO/Cu2O heterostructure with and without illumination. The cell performance was measured under AM 1.5 illumination with a solar intensity of 100 mW/cm2 at 25°C. The cell had an active area of 0.3 × 0.3 cm2 and no antireflective coating. ZnO/Cu2O solar cells exhibited the following static parameters; of 0.0325 mA/cm2, of 0.1 V, FF of 0.283, and a conversion efficiency () of 0.092%. The low FF value and poor conversion efficiency are caused by the high series resistance and low shunt resistance. The series resistance is caused mainly by the Cu2O structure that is formed from the nanocolumns, which degrades carrier transport. The shunt resistance is produced by charge leakage from the edges of the Cu2O nanocolumns.
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4. Conclusions
Cuprous oxide (Cu2O) films were prepared on an indium tin oxide glass substrate by radiofrequency magnetron sputtering using a highly pure Cu target. The bilayer structure comprised Cu nanoclusters on the surface of Cu2O nanocolumns film following annealing at 500°C. The measured parameters of the cells were the short-circuit current density (), the open-circuit voltage (), and the efficiency (), with values of 0.0325 mA/cm2, 0.1 V, and 0.092%, respectively. The Cu2O solar cell with the bilayer structure had a poor power conversion efficiency because of the nanocolumn structure.
Acknowledgment
Financial support of this paper was provided by the National Science Council of the Republic of China under Contract no. NSC 101-2221-E-027-054.