| In this study, magnetron sputtered ZnO:Al thin films for application in silicon thin film solar cells have been systematically investigated. ZnO:Al thin films were prepared with non-reactive sputtering from dual rotatable ceramic targets (ZnO:Al2O3=99.5:0.5 wt%) or with reactive sputtering from dual rotatable metallic targets (Zn:Al=99.5:0.5 wt%). The size of glass substrates can be up to 30 cm x 30 cm. Both sputtering techniques can greatly enhance the utilization of target materials due to the rotating of targets during sputtering. As a promising transparent and conductive oxide (TCO) material, the optical and electrical properties of ZnO:Al films were characterized, examined and analyzed. Meanwhile, one of the main aims of the work is to develop a TCO deposition process with a high growth rate. In addition, for the practical application in silicon thin film solar cells, the surface structures of as-grown and after-etched ZnO:Al films were also characterized and examined in details.Magnetron sputtering of ZnO:Al thin films from dual rotatable ceramic targets were carried out under mid-frequency (MF) excitation. The influences of different deposition conditions such as substrate temperature, working pressure, discharge power, argon gas flow, oxygen gas flow and magnetron direction on various optical and electrical properties of ZnO:Al films were systematically investigated. In addition, etching behaviors and resulted morphologies were also studied. Low growth rate ZnO:Al films (at 2 kW) with low resistivity of 3.6x 10-4Ω·cm and high carrier mobility up to 50 cm2/Vs as well as optimum surface structures after etching, were achieved. High growth rate ZnO:Al films (at 14 kW) with a high rate close to 110 nm/min and nice optical properties were obtained. Optimized ZnO:Al films deposited from ceramic targets were applied inμc-Si:H andα-Si:H/μc-Si:H solar cells as front contacts. High efficiencies of 8.5% and 11.3% for both kinds of cells were achieved, respectively.ZnO:Al films were deposited from dual rotatable metallic Zn:Al targets by reactively sputtering depositions, which were control by a plasma emission monitoring (PEM) system and carried out under different deposition conditions. Reactively sputtered high rate ZnO:Al films (close to110 nm/min) with low resistivity close to 3.3×10-4Ω·cm and high mobility up to 47 cm2/Vs as well as high transmittance were achieved. It was found that as-grown and after-etched ZnO:Al films deposited at different deposition conditions displayed various morphologies.A novel chemical wet etching method i.e. the etching is first in diluted HF solution, and then in diluted HCl solution, was successfully applied to etch high rate ZnO:Al films, which were deposited using either high rate non-reactive or reactive sputtered technologies and greatly improved their surface structures. Good surface structures on both types of high rate ZnO:Al films were achieved. Optimized high rate ZnO:Al films treated with the novel etching method were applied inμc-Si:H and a-Si:H/μc-Si:H tandem solar cells and high efficiencies of up to 8.2% and 11.4% were achieved, respectively. Therefore, it is demonstrated that our developed technology based on the high rate ZnO:Al film deposition processes by dual rotatable ceramic or metallic targets together with the applied double etching method has a great potential for high-efficiency silicon thin film solar cells. |
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