Emitter Epitaxy for Crystalline Silicon Wafers and Thin Films: Solar Cells and Economical Aspects

Abstract

Epitaxial emitters grown by high temperature atmospheric pressure CVD can increase the spectral response of the solar cells at short wavelengths with optimised doping profiles. This paper presents the economical aspects, simulation and cell results of wafer and crystalline silicon thin-film solar cells with n-type epitaxial emitters. The economical potential of the emitter epitaxy is confirmed by cost calculations on system level and shows that using thin-film solar cells with epitaxial emitter could be up to 5% cheaper per watt peak than using corresponding cells with POCl3 diffusion. Simulation results based on experimental doping profiles with a moderate bulk and a higher surface doping level show that efficiencies up to 15.3% are possible with a 20 µm thick base. An optimum emitter thickness between 1 and 2 µm is found for an emitter doping level of 5x10 18 cm -3 . The experimental results tally nicely with the simulated values. To increase the current, a front texture is combined with the epitaxial emitter. For thin-films a texture is usually made by Plasma etching, which, however, induces shunts in our epitaxial emitter. Therefore, a subsequent short Phosphorus diffusion seems necessary to prevent these shunts. Solar cell results of thin-films on highly-doped Cz substrates reach efficiencies up to 15.2%.