Abstract
1. Introduction
Since p-type cuprous oxide (p-Cu2O) has a high theoretical conversion efficiency of about 20%, it has long attracted research attention as a solar cell material[
In this paper, we describe the improvement of the photovoltaic properties of Cu2O-based heterojunction solar cells using AZO thin films prepared by the sputtering apparatus with our newly developed multi-chamber system.
2. Experimental
Cu2O sheets were prepared by oxidizing copper sheets (0.2-mm thick with 99.96% purity) using heat treatment in a furnace with a controlled ambient atmosphere, described in detail elsewhere[
To incorporate Na into the oxidized Cu2O sheets, the sheets impregnated with NaCl powder (purity: 99.9%, KANTO KAGAKU Co. Ltd.) were heat-treated at 700 °C in an Ar gas atmosphere for 1 h[
Figure 1.(Color online) Cross-sectional structure of AZO/p-Cu2O solar cell.
3. Results and discussion
3.1. AZO target-Cu2O sheet distance and sputtering voltage dependence of photovoltaic properties for AZO/p-Cu2O solar cell
When an AZO layer is formed on a p-Cu2O sheet by the magnetron sputtering method, the following two causes are considered the cause of the deterioration of the photovoltaic properties: (1) the physical damage on the Cu2O sheet surface due to bombardment by the sputtered particles; (2) excessive oxidation of the Cu2O sheet surface by oxygen ions. Fig. 2 shows the typical current–voltage (J–V) characteristics of the AZO/p-Cu2O heterojunction solar cells prepared with different target-Cu2O sheet distances. It should be noted that the J–V characteristics of the AZO thin film/p-Cu2O heterojunction solar cells were dependent on the target-Cu2O sheet distance. Fig. 3 shows the conversion efficiency (η), the fill factor (FF), VOC, and JSC as functions of the target-Cu2O sheet distance for AZO/p-Cu2O heterojunction solar cells fabricated with a target-Cu2O sheet distance from 30 to 55 mm. The photovoltaic properties did not substantially change until the distance between the target Cu2O sheets became about 35 mm. However, as the distance further increased, the photovoltaic properties gradually decreased. By increasing the distance between the substrate targets, we expect to reduce the physical damage on the surface of the Cu2O sheet due to the bombardment. Unfortunately, the photovoltaic properties deteriorated. This result suggests that the deterioration of the photovoltaic property accompanying the increase in the distance between the target and the substrate is not primarily caused by the physical damage on the Cu2O sheet’s surface due to the bombardment. In addition, Fig. 4 shows the typical J–V characteristics for AZO/p-Cu2O heterojunction solar cells prepared with different sputtering voltages: 380 and 700 V. It should be noted that the J–V characteristics of the AZO thin film/p-Cu2O heterojunction solar cells were strongly dependent on the sputtering voltage. As the sputtering voltage increases, the bombardment effect is expected to increase, but the photovoltaic characteristics will slightly improve. Fig. 5 shows the J–V characteristics measured under dark conditions obtained in an AZO/p-Cu2O heterojunction solar cell (Fig. 4), and when reverse bias voltage was applied, the leakage current decreased in the AZO/p-Cu2O solar cell fabricated by forming AZO film with higher sputtering voltage. This suggests that an improvement of the p-n junction, as seen in the AZO/Cu2O heterojunction, was achieved by a higher sputtering voltage. These above results suggest that the cause of the deterioration of the photovoltaic property of the AZO/p-Cu2O solar cell, fabricated by forming AZO thin film on the Cu2O sheet by the sputtering method, is not the physical bombardment of the sputtering particles. The cause of the deterioration of the photoelectric conversion characteristics of the AZO/p-Cu2O solar cell, where AZO thin film is formed on the Cu2O sheet by the sputtering method, is mainly attributable to excessive oxidation of the Cu2O sheet’s surface by chemically active oxygen ions to the p-n junction interface.
Figure 2.(Color online) Typical
Figure 3.(Color online) Conversion efficiency (
Figure 4.(Color online) Typical
Figure 5.(Color online)
3.2. Photovoltaic properties for AZO/Cu2O solar cell prepared using multi-chamber MSD apparatus
As mentioned above, to improve the photovoltaic property of the AZO/p-Cu2O solar cell fabricated by forming AZO thin film by the sputtering method, the Cu2O sheet surface’s excessive oxidation must be reduced by oxygen ions. However, in the AZO thin film deposition process by the sputtering method, to remove oxygen and moisture from the target surface, we must process the generated plasma using pre-sputtering. In this pre-sputtering process, the Cu2O sheet’s surface is protected by a shutter, but since the sputtering gas pressure is as high as 0.6 Pa, preventing excessive oxidation of the Cu2O’s surface is difficult by a shutter due to plasma wraparound. Our newly developed multi-chamber sputtering apparatus has deposition and loading chambers, and we can prevent excessive oxidation by oxygen ions by retracting the Cu2O sheet into the loading chamber during the pre-sputtering process.
As one example, as a function of the pre-sputtering time, typical J–V characteristics are shown in Fig. 6 for AZO/p-Cu2O heterojunction solar cells measured under AM1.5G solar illumination. Pre-sputtering was carried out in a deposition chamber before introducing the p-Cu2O sheet from the loading chamber. Next a p-Cu2O sheet was introduced into the deposition chamber, and then the n+-AZO thin films were prepared at RT at a pure Ar pressure of 0.6 Pa. As seen in Fig. 6, we drastically improved the J–V characteristics by increasing the pre-sputtering time to 10 min. These results suggest that the p-Cu2O sheet’s surface was degraded by exposure to excessive oxygen plasma in the deposition chamber without the pre-sputtering process. On the other hand, when pre-sputtering was performed for more than 15 min, the amount of oxygen, supplied from the moisture adsorbed on the target surface, decreased. As a result, oxygen was deprived on the Cu2O surface, and the interface state deteriorated. We obtained the highest efficiency of 3.21% in an AZO/p-Cu2O heterojunction solar cell prepared with a 10 min pre-sputtering time. Fig. 6 also shows typical J–V characteristics for AZO/p-Cu2O heterojunction solar cells prepared using PLD. The J–V characteristics of the AZO/p-Cu2O heterojunction solar cells, prepared using the sputtering method with 10 min pre-sputtering, exhibited better properties than the PLD method (Fig. 6). The solar cell’s leakage current prepared by magnetron sputtering with 10 min pre-sputtering, measured under a reversed bias, was as low as the solar cell prepared by PLD. This suggests that greater improvement of the p–n junction, as seen in the AZO/Cu2O heterojunction, can be achieved by magnetron sputtering methods with 10 min pre-sputtering to decrease the recombination associated with defects at the interface between AZO and Cu2O.
Figure 6.(Color online) Typical
4. Conclusion
We demonstrated that using a newly developed multi-chamber sputtering apparatus for preparing AZO thin film not only as the n-type semiconductor layer but also as a transparent electrode greatly improves the performance of AZO/p-Cu2O heterojunction solar cells. We significantly improved the photovoltaic properties by AZO/p-Cu2O heterojunction solar cells fabricated on p-type Cu2O sheets that were prepared by the thermal oxidation of Cu sheets. The high efficiency obtained in the heterojunction solar cells may be attributable to a decrease of the defect levels at the interface between the AZO thin film and the Cu2O sheet. The highest efficiency (3.21%) was obtained in an AZO/p-Cu2O heterojunction solar cell. This value achieved the same or higher characteristics than the solar cell with a similar structure prepared by the PLD method. Therefore, the newly developed multi-chamber sputtering apparatus is promising as a practical n-type semiconductor thin film that forms technology for Cu2O-based solar cells.
Acknowledgment
The authors acknowledge the technical assistance of R. Takahashi and N. Ogawa in the experiments. This research was partly supported by Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No.15K04723).
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