Characterization of protocrystalline silicon multilayer and its application to light-absorbing layers for solar cells

Abstract

The protocrystalline silicon (pc-Si:H) multilayer solar cell is very promising owing to its fast stabilization with low degradation against light irradiation. However, the pc-Si:H multilayer have not extensively been investigated in detail on its material characteristics yet. We present the material characteristics of pc-Si:H multilayer using a transmission electron microscopy (TEM), Raman spectroscopy, Fourier transform infra (FTIR) spectroscopy, phase-modulated spectroscopic ellipsometry (SE) and quantum efficiency (QE) measurement. A cross-sectional TEM (XTEM) micrograph shows that a pc-Si:H multilayer has a repeatedly layered structure consisting of low hydrogen-diluted $(S_L)$ and highly hydrogen-diluted $(S_H)$ sublayers. The crystalline-like objects (CLOs) embedded in a-Si:H matrix is also shown in a planar TEM image. The analysis of the Raman spectra presents that the medium-range-order (MRO) and short-range-order (SRO) of a pc-Si:H multilayer are improved. Thus, a pc-Si:H multilayer is more specific material than amorphous silicon (a-Si:H) because of its phase and structure. The peculiar material characteristics of a pc-Si:H multilayer is (1) a repeatedly layered structure, (2) an intermediated phase material, (3) the distribution of the CLO embedded in a-Si:H matrix of SH sublayer and (4) the improvement of MRO and SRO. We can also pay attention to the indirect evidence of CLO from the stretching mode at 2100 cm-1 of FTIR spectra, which is ascribed to clustered Si-H bonds at the boundary region between CLO and a-Si:H matrix. Therefore, we suggest a microstructural factor $(R^*)$ as the figure of merit (FoM) for evaluating of the pc-Si:H multilayer. To verify the suggested FoM, we performed the QE measurement of pc-Si:H multilayers with different SH sublayer deposition time $(t_{SH})$ during light illumination. We have observed the strong correlation between the light-induced degradation and the FoM. We prepared pc-Si:H multilayer solar cells in o...