Cu2O Nanocolumns on ZnO-Coated Glass Substrate for Solar Cell Applications

Cuprous oxide (Cu 2 O) 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 Cu 2 O thin films with various elements, compositions, and surface structures. The p-Cu 2 O thin films thus formed were characterized by FESEM and XRD. After annealing at 500C, the bilayer structure which consisted of Cu nanoclusters on the surface of a film of Cu 2 O nanocolumns was observed. The Cu 2 O solar cell with the bilayered structure exhibited poor power conversion efficiency.

According to the literature, the performance of Cu 2 Obased solar cells is significantly affected by the crystallinity of Cu 2 O, because Cu metal can easily be formed at the surface of the Cu 2 O films or at the interface between the TCO and the Cu 2 O films when thin films of TCO are deposited on the Cu 2 O films. Therefore, this study investigates the deposition of the Cu 2 O 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 Cu 2 O thin films.

Experimental
In this study, Cu 2 O 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 O 2 (flow rate of 1-3 sccm) gas at a stable pressure of 3 × 10 −3 Torr, and the temperature and time of annealing varied to yield Cu 2 O thin films with various properties. The flow rates of both argon and oxygen gases were individually monitored using mass flow controllers.     Figure 1 shows the top-view and cross-sectional FESEM images of the microstructures of the Cu 2 O films that had been annealed at 300, 400, and 500 ∘ C. Two-dimensional grain boundaries of the Cu 2 O 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 Cu (200) Cu (111) Intensity (a.u.) 2 (deg) . 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.

Results and Discussion
To elucidate the annealing mechanism, the phase was identified. Figure 2 Figure 2(b) presents the mechanism of formation of the Cu-Cu 2 O bilayer. 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 Cu 2 O films linearly fell while the mobility declined to ∼2-4 cm 2 /Vs. The reduction of resistivity of the Cu 2 O films after annealing may be attributed to the segregation of the Cu-rich nanoclusters from the Cu 2 O film, which is shown in Figure 1(e). Also, unlike other studies, a film with relatively low mobility was obtained by postthermal annealing [22][23][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 cm 2 /Vs owing to the increase in the size of the grains in the Cu 2 O nanocolumns and the Cu-rich nanoclusters. Figure 4(a) presents the absorption measurements for the Cu 2 O 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 Cu 2 O that can be found in the literature [25][26][27]. The extrapolation of the linear region of the curves to the horizontal axis gives the band-gap energies of Cu 2 O 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 Cu 2 O film with higher oxygen content has a smaller lattice constant, a larger band gap, and a higher resistivity. Figure 5(a) displays the cross-section of the completed structure and Figure 5  shunt resistance. The series resistance is caused mainly by the Cu 2 O structure that is formed from the nanocolumns, which degrades carrier transport. The shunt resistance is produced by charge leakage from the edges of the Cu 2 O nanocolumns.

Conclusions
Cuprous oxide (Cu 2 O) 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 Cu 2 O nanocolumns film following annealing at 500 ∘ C. The measured parameters of the cells were the short-circuit current density ( sc ), the opencircuit voltage ( oc ), and the efficiency ( ), with values of 0.0325 mA/cm 2 , 0.1 V, and 0.092%, respectively. The Cu 2 O solar cell with the bilayer structure had a poor power conversion efficiency because of the nanocolumn structure.