n-type emitter epitaxy for crystalline silicon thin-film solar cells
Crystalline silicon thin-film (cSiTF) solar cells are an attractive alternative to bulk silicon solar cells. At Fraunhofer ISE the concept of the epitaxial wafer equivalent (EpiWE)is followed, where 20 micron of silicon are deposited epitaxially by high temperature atmospheric pressure CVD (APCVD) on a cheap silicon substrate. The EpiWE can then be processed using standard industrial cell production. Furthermore, it is possible to simplify the solar cell process when depositing the emitter by epitaxy in-situ after the growth of the base. The epitaxial emitter could be an alternative method of n-type emitter processing with adjustable emitter profile and short deposition time. This paper presents results of cSiTF solar cells with epitaxial emitters and photolithographic contacts, which prove the good performance of the emitter and the applicability of the EpiWE to conventional solar cell processes. Included in the discussion is a description of the CVD deposition principle developed at Fraunhofer ISE and the applied clean room solar cell process. The emitter doping profiles and concentrations, as well as the structural quality, are discussed in detail using SIMS and SEM data. The phosphorus out-diffusion during cooling is prevented by cooling the samples in a PH3/H2 atmosphere. Moreover, a double-layer emitter is formed having blue sensitive properties. The internal quantum efficiencies (IQEs) show that the emitter can be passivated as well as a 'standard' emitter formed by POCl3 diffusion. Efficiencies up to 14.9 % and 13.6 % for large area cSiTF solar cells on highly doped Cz and mc, respectively, are presented.