n‐Type emitter epitaxy for crystalline silicon thin‐film solar cells

Abstract

Crystalline silicon thin-film (cSiTF) solar cells are an attractive alternative to bulk silicon solar cells. At Fraunhofer ISE we follow the concept of the epitaxial wafer equivalent (EpiWE), where 20 mm 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 PH 3 /H 2 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 POCl 3 diffusion. Efficiencies up to 14Á9 and 13Á6% for large area cSiTF solar cells on highly doped Cz and mc, respectively, are presented.

Dopant mapping in highly p-doped silicon by micro-Raman spectroscopy at various injection levels

“…Within the layers, the dopant concentration is quite uniform; however, close to the surface, the concentration suddenly drops. Such a decrease has already been observed for the growth of highly phosphorous doped epitaxial layers 20 and has been attributed to out-diffusion of dopant after the end of growth during cooling down. Therefore, in order to get high dopant uniformity within the Raman probed area, 1 lm of the surface was etched off from the Raman samples by applying a polishing etchant.…”

Efficiency (>15%) for thin‐film epitaxial silicon solar cells on 70 cm2 area offspec silicon substrate using porous silicon segmented mirrors

“…Most recent discussion on economical aspects of EpiWE, by Schmich et al [4]. predict the total cell costs reduction advantage of EpiWE with in situ emitters of 0Á24% s/W p over the industrial wafer cell technology.…”

3D epitaxial growth through holes for the fabrication of thin-film solar cells

“…The target frontside thicknesses of these layers are 2 mm for the BSF, 20 mm for the base and 1 mm for the emitter, just as for the standard WaferEquivalent cell. The specific parameters of the deposition equipment and process have been described elsewhere [8].…”