Thin film silicon solar cells that could be used as power supplies for devices on plastic, need to be processed at a low temperature of ≤ 100 • C (which is compatible for most of the plastics such as PEN, PES and many types of PET). However, deposition of amorphous silicon (a-Si:H) by a CVD process at temperatures lower than 200 • C usually leads to increased structural disorder and defects in the material, owing to a lowering of the diffusion length of precursors on the growing surface. By optimizing hydrogen to silane flow ratio for the deposition of the i-layer, we have been able to make solar cells at 100 • C on Asahi TCO substrates in the p-i-n configuration with efficiencies of 7.8 % by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) and 3.4 % by hot-wire chemical vapour deposition (HWCVD). The purpose of this paper is to study the structural order of optimal materials at low deposition temperature for solar cell applications.The dielectric functions obtained by spectroscopic ellipsometry (SE) of the thin film silicon materials on glass have been simulated by using the Tauc-Lorentz (TL) dispersion law [1]. The imaginary part of the dielectric function in a TL model is described by, ε im (E) =, where E 0 is the peak transition energy, E g is the gap energy, C is a broadening parameter, which is related to the disorder in the material and A is related to the film denseness [1]. We have fitted the dielectric function of three types of amorphous silicon samples, made at 100 • C by (i) HWCVD (ii) VHF-PECVD and (iii) standard PECVD (13.56 MHz), and two a-Si:H samples made at 200 • C as reference. The SE data are analysed taking into account the substrate, the thin film bulk and its surface roughness, which is considered as an overlayer formed by a mixture consisting of 50 % bulk material and 50 % voids The Table (given below) shows the fitting parameters of the ellipsometry data. As a reference, the C and A parameters of our device quality PECVD sample deposited at 200 • C are well comparable to literature values [1] and significantly smaller than that of 100 • C PECVD samples, confirming the effect of substrate temperature on the structural order. However, the VHF sample made at 100 • C using an optimum H 2 dilution shows denseness (A value) and structural order (C value) that are comparable to the sample made at 200 • C. The HWCVD materials in general have a less dense structure and higher roughness compared to the plasma deposited samples. This can be attributed to the absence of ion impact on the growing film. However, the A parameter of HWCVD samples is continuously improved with increasing dilution. Among all samples considered, it is observed that the C parameter reaches the lowest value (C = 1.67) for the HWCVD sample made at an optimum R value of 20, even though it was made only at 100 • C. The Raman spectra of these samples also confirm this high structural order [2]. This material also shows the best device quality with a photoresponse of > 10 5 .

The dielectric functions of thin film silicon materials on glass were measured by spectroscopic ellipsometry (SE) and simulated by using the Tauc-Lorentz (TL) dispersion law, which provided information on disorder (C) and density (A). A VHF plasma-deposited sample made at 100°C with an optimum hydrogen dilution shows density (relative packing density of Si-Si bonds as estimated by SE) and structural disorder that are comparable to samples made at 200°C. HWCVD materials made at 100°C at lower hydrogen dilution conditions have a less dense structure and higher roughness compared to the plasma-deposited samples. This can be attributed to the absence of ion bombardment on the growing film. Out of all samples investigated, the HWCVD sample made at a hydrogen to silane flow ratio value of 20 showed a remarkably low structural disorder (C = 1.67) even though the deposition temperature was only 100°C. A small bond angle variation of $6.4°as determined from its Raman spectrum, the presence of small (1-1.5 nm) dispersed crystalline-like islands in the silicon matrix, and sharp rings in the selective area diffraction pattern point towards a special ordered structure. The photoresponse of this material is >10 5 .