In-situ plasma monitoring of PECVD a-Si:H(i)/a-Si:H (n) surface passivation for heterojunction solar cells application

摘要

Silicon-based solar cell manufacturing via plasma enhanced chemical vapor deposition (PECVD) of both active/passive layers is investigated. In addition, in-situ plasma diagnostics of the deposition process can be monitored in real-time. Two types of complementary diagnostics, namely optical emission spectroscopy (OES) and quadruple mass spectrometry (QMS) are applied to an PECVD reactor. Furthermore, the impact of chamber wall conditioning on the solar cell performance is experimentally investigated based on symmetrical stacks structure (a-Si:H(i) / CZ wafer (n)/ a-Si:H(i)) and the n-type hydrogenated amorphous silicon (a-Si:H) growth process conditions were optimized. Silicon heterojunction (SHJ) solar cell back surface field (BSF) layer was prepared by conventional radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) and the processing conditions in terms of the phosphine flow (1-20sccm), hydrogen dilution ratio (R=H2/SiH4) as 1-5 and symmetrical stacks structure using a (PH3/SiH4/H2/Ar) mixture were experimentally optimized. In addition, characterization of effective carrier lifetime (τ_eff), electrical and structure properties as well as correlation with the hydrogen dilution ratio were systematically discussed with the emphasis on the effectiveness of passivation layer. A high quality intrinsic/n-type a-Si:H layer stack BSF layer with measured effective carrier lifetime (τ_eff) of 1.8ms (which counts for implied Voc 0.707 V), can be consistently obtained and this improved passivation layer can be primarily attributed to the synergy of chemical and field effect to significantly reduce the surface recombination.