Structural order of thin film silicon made at 100 °C

Abstract

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 .

Decoupling high surface recombination velocity and epitaxial growth for silicon passivation layers on crystalline silicon

“…The disorder parameter Br of the a-Si:H is about 2.15 eV for samples I and III. The A and Br of sample I are similar to the values reported by Rath et al [28] for device quality a-Si:H deposited with VHF PECVD at 100 °C with S F = 5%.…”

Thin film silicon n–i–p solar cells deposited by VHF PECVD at 100°C substrate temperature

“…The photoresponse increased to 180 after annealing at 100 1C for 2 h. A microcrystalline p-type layer was chosen as a window layer for the cells, due to its high optical bandgap and good electrical properties. It is known that the nucleation of crystallites in the boron-doped silicon layer depends on the type of the substrate [12]. For this reason and for characterization purposes, the p-layer was grown on top of a stack consisting of a 20 nm a-Si:H layer (deposited on glass) and a buffer layer of about 2 nm thickness.…”