Title: THz to UV spectroscopic ellipsometry probing photovoltaic devices
Speaker: Prof. Nikolas Podraza
Abstract: Spectroscopic ellipsometry is sensitive to the optical response and thicknesses of different component materials in multilayer structures like solar cells. The available spectral range for table-top ellipsometers has expanded throughout the millimeter (THz), infrared, visible, and ultraviolet wavelengths with different information contained within the optical properties of each spectral range. For semiconducting materials for solar cells, the bandgap and above gap electronic transitions are obtained over the near infrared to ultraviolet, phonon modes and chemical bonding are accessible in the infrared, and free carrier absorption is detected in the infrared to THz. The sensitivity of spectroscopic ellipsometry enables potential extraction of these material characteristics for each component in a multilayer stack. Strategies for analysis of different wavelength range ellipsometric spectra, including simultaneous analysis of multiple data sets, will be discussed for crystalline and polycrystalline semiconductors, transparent conducting oxides, and glass and dielectric materials. THz range spectroscopic ellipsometry and optical Hall effect measurements based on THz ellipsometric spectra collected under different applied magnetic fields are used to extract electronic properties of semiconductors when using the Drude model for these same materials as well as complete solar cell devices. Structural, optical, and electronic property information gained from ellipsometry is used to simulate photovoltaic device performance properties including external quantum efficiency and current-voltage characteristics of complete solar cells based on silicon, cadmium telluride, and hybrid organic-inorganic lead halide based perovskite absorbers to assess sources of optical and electronic losses.
References: 1. M. K. Mainali, P. Uprety, Z. Song, C. Wang, I. Subedi, B. Subedi, K. Ghimire, M. M. Junda, Y. Yan, N. J. Podraza, Materials Science in Semiconductor Processing, *170*, 107936 (2024).