Silicon CVD deposition for low cost applications in photovoltaics

This paper presents a low cost application of high-temperature silicon CVD depositions for photovoltaics. Crystalline silicon thin-film (cSiTF) solar cells could be an attractive alternative to bulk silicon solar cells. At Fraunhofer ISE we follow the concept of an epitaxial wafer equivalent (EpiWE), where 20 μm of silicon are deposited epitaxially by atmospheric pressure CVD on a cheap silicon substrate. The EpiWE can then be processed in a 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 emitter formation made by epitaxy could provide an alternative to the conventional POCl3 emitter-diffusion for cSiTF solar cells. In this paper the concept of the EpiWE is introduced and the RTCVD deposition reactor is described in detail. The lab-type horizontal CVD reactor was constructed at Fraunhofer ISE to obtain a cheap deposition tool with a sufficient layer quality for solar cells. The deposition process as well as the resulting emitter profiles is discussed with regard to their effect on the solar cells. Solar cell parameters of emitters on n-type Fz and on p-type cSiTF are presented with efficiencies up to 14.2% and 14.8%, respectively. The high open circuit voltages of more than 645 mV show the good performance of the epitaxial emitters.