Analysis and optimization of thin film photovoltaic materials and device fabrication by real time spectroscopic ellipsometry

Abstract

Methods of spectroscopic ellipsometry (SE) have been applied to investigate the growth and properties of the material components used in the three major thin film photovoltaics technologies: (1) hydrogenated silicon (Si:H); (2) cadmium telluride (CdTe); and (3) copper indium-gallium diselenide (CuIn_1-xGa_xSe² or CIGS). In Si:H technology, real time SE (RTSE) has been applied to establish deposition phase diagrams that describe very high frequency (vhf) plasmaenhanced chemical vapor deposition (PECVD) processes for hydrogenated silicon (Si:H) and silicon-germanium alloy (Si_1-xGe_x:H) thin films. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si_1-xGe_x:H component solar cells of multijunction devices are obtained when the i-layers are prepared under maximal H₂ dilution conditions. In CdTe technology, the magnetron sputter deposition of polycrystalline CdTe, CdS, and CdTe_1-xS_x thin films as well as the formation of CdS/CdTe and CdTe/CdS heterojunctions has been studied. The nucleation and growth behaviors of CdTe and CdS show strong variations with deposition temperature, and this influences the ultimate grain size. The dielectric functions ε of the CdTe_1-xS_x alloys have been deduced in order to set up a database for real time investigation of inter-diffusion at the CdS/CdTe and CdTe/CdS interfaces. In CIGS technology, strong variations in ε of the Mo back contact during sputter deposition have been observed, and these results have been understood applying a Drude relaxation time that varies with the Mo film thickness. Ex-situ SE measurements of a novel In₂S₃ window layer have shown critical point structures at 2.77±0.08 eV, 4.92±0.005 eV, and 5.64±0.005 eV, as well as an absorption tail with an onset near 1.9 eV. Simulations of solar cell performance comparing In₂S₃ and the conventional CdS have revealed similar quantum efficiencies, suggesting the possibility of a Cd-free window layer in CIGS technology.