DEVELOPMENT OF LARGE AREA, TEXTURED, HIGH QUALITY ZINC OXIDE TCO FILMS AND THEIR APPLICATION TO THIN SILICON DEVICES

摘要

High quality doped ZnO as a window material could play an important role in amorphous and nanocrystalline solar cell applications. In this work, we report on textured ZnO thin films deposited by reactive-environment hollow cathode sputtering (RE-HCS), which have superior optical and electrical properties. The films are intended for use as a glass/TCO superstrate for a-Si or hybrid a-Si/nc-Si thin-film solar modules. The overall process avoids a wet-etching step and doesn't need an expensive compound or alloy target, which makes it possible to manufacture solar cells in a clean and economic way. The doped, textured ZnO thin-films are deposited by sputtering from a 50cm long hollow cathode fitted with two zinc targets (with provision of oxygen as a reactive gas) onto a soda lime glass substrate which is at a temperature in the range of 180°C to 330°C. We have recently achieved the high mobility of 49.5cm~2/Vs at a carrier concentration of 4.42 × 10_(20)/cm~3 as determined by Hall effect measurements. This corresponds to a low film resistivity of 2.86 × 10~(-4) Ω.cm. High carrier mobility is needed to avoid free carrier absorption in the near infrared range while retaining a low resistivity. The morphology of the ZnO films can be controlled by deposition conditions such as pressure, reactive species, substrate temperature, and bias. The dynamic deposition rate is 23nm.m/min at 3kW power and the film uniformity is about ±10% over a width of 43cm. The efficiency of the single-junction a-Si solar cells made using these films as TCO on glass increased about 8% comparing to the efficiency of the cells made on commercial SnO_2 From this study, we demonstrate that large area, high quality, textured, and cost-effective ZnO films can be achieved by the RE-HCS process, and the films can be successfully used in device fabrication.