Date of Award
Doctor of Philosophy
Electrical and Computer Engineering
High quality muc-SiGe:H is a very promising alternative for high efficiency photovoltaic device. The preparation of this material requires very high hydrogen dilution ratio in PECVD system. The deposition rate was greatly limited by this high hydrogen ratio. In this work, ECR PECVD technique is used to deposit muc-SiGe:H material. The growth rate can be greatly enhanced by taking advantage of the high plasma density and low ion energy features of ECR and extremely high hydrogen dilution ratios are no longer necessary for muc-SiGe:H growth. Films with good crystallinity were prepared at hydrogen dilution ratio as low as 1:15. An intensive study has been completed for the muc-SiGe:H with 0 to over 75% Ge incorporated. The optical bandgap shrinks with the incorporation of Ge into the material. Raman spectra and the increase of activation energy and photosensitivity indicates the deterioration of crystallinity by adding Ge to the Si structure. Solar cell devices using muc-SiGe:H as the active layer were deposited on stainless steel substrates. Fill factors over 55% were achieved for muc-SiGe devices with less than 35% Ge. An muc-Si buffer layer between n+ and muc-SiGe:H n layer was used in the device design and this buffer layer revealed to be very beneficial to the device performance and the growth rate of muc-SiGe:H active layer. C-V measurements showed that the accidental oxygen leakage can raise the doping level to the order of 1E17cm-3 ppm level TMB can be mixed in the source gas to very effectively reduce the N-type doping brought by oxygen. Short circuit current was increased by the TMB counter doping. The minority carrier diffusion length was estimated from reversed bias QE and C-V measurements. In the muc-SiGe:H devices fabricated by ECR PECVD, the hole diffusion length is several tenth micrometers. The accidental doping in the muc-SiGe:H deteriorates the device performance by decreasing the minority carrier diffusion length. Compensating doping of TMB can increase minority carrier diffusion length Lp and improve short circuit current, and hence improve the conversion efficiency of solar cell device.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Zhu, Jianhua, "Hydrogenated microcrystalline silicon germanium material and photovoltaic devices prepared using ECR PECVD " (2003). Retrospective Theses and Dissertations. 760.