Characterisation and Optimisation of Indium Tin Oxide Films Deposited by Pulsed DC Magnetron Sputtering for Heterojunction Silicon Wafer Solar Cell Applications

Amorphous/crystalline heterojunction silicon wafer solar cells are attracting attention in recent years due to their potential to achieve high conversion efficiencies at low fabrication temperatures. However, to date, only SANYO (Japan) has brought this technology to commercial mass production, mainly due to the high sensitivity of the solar cell parameters to the growth conditions. One significant difference between a heterojunction and a standard screen- printed silicon wafer solar cell is the current collection scheme. As a heterojunction silicon wafer solar cell is limited by its low emitter lateral conductivity, a transparent conductive oxide (TCO) is employed to improve the carrier transport, whilst also acting as an antireflective coating (ARC) for the front side. From the variety of TCOs, indium tin oxide (ITO) is one of the most promising candidates due to its high electrical conductivity and excellent optical transmittance. For deposition of very thin ITO films (in the 70-90 nm range, in order to function as ARC), the restrictions and requirements for both high electrical and optical properties are rather challenging. In this study we focus on TCO deposition using an industrial pulsed direct current (DC) magnetron sputtering system (FHR, Germany). The effects of the applied DC power, the oxygen partial pressure, the deposition temperature, and the annealing conditions are investigated. Encouraging results demonstrate a resistivity in the range of (4.6-5)x10-4 Ωcm and a transmission of above 90% in the visible range.