Characterization of a-Si:H and a-SiGe:H p-i-n and Schottky junctions by admittance circuit modeling

The midgap density of states (MGDOS) and junction properties of a-Si:H and a-SiGe:H alloys are investigated by admittance measurements on p-i-n solar cells. An expression for the capacitance is derived from the frequency, temperature, and bias dependence of the deep state response. The measured admittance is analyzed with an equivalent circuit model. The density of states g₀, attempt-to-escape frequency, and the Fermi energy are adjusted to fit the measured admittance. Close agreement is found with the measured capacitance and conductance over a wide range of temperature and frequency. The single junction model is shown to apply equally well to p-i-n and Schottky diodes, justifying the neglect of the n-i junction and thin doped layers in the p-i-n device analysis. Fitted values of g₀ are confirmed by values of g₀ obtained from the limiting capacitance at high temperature