Surface characterisation of wafers for silicon-heterojunction solar cells

Abstract

Silicon-heterojunction solar cells have been gaining interest and market share in the last years. In a very short time, this technology has been able to reach 21% lab-scale efficiency [1] and 6% of the whole world photovoltaic production [2] by combining the good properties of crystalline or multicrystalline silicon with the advantages of thin-film silicon technology.

In contrast to the conventional cells based on thermal diffusion, silicon-heterojunction cells are basically surface- (or interface-) dominated. Junctions are not formed by doping part of a solid, but growing a thin film to a surface. This makes surface properties absolutely critical for the proper performance of the resulting device. The key role of surface treatments previous to thin-film deposition is widely reported in literature [3], [4]. Studies show that making a good silicon-heterojunction cell from a textured wafer is much more difficult than preparing it from a polished one. Finding the reasons for this difference and understanding the influence of the chemical composition and microstructure of the silicon surface on cell properties is the aim of the research described in the present paper.

The work has been focused on three kinds of 〈1 0 0〉 p-type monocrystalline-silicon wafers, respectively, float-zone polished, Czochralski rough (not polished, not textured), and Czochralski textured. Only one type of treatment has been covered: HF at different times, applied for different concentrations. Chemical composition has been analysed by AR-XPS (angle resolved X-ray photoelectron spectroscopy) measurements and possible microstructural changes have been monitored by SEM (scanning electron microscopy).