Midgap states in a-Si:H and a-SiGe:H p-i-n solar cells and Schottky junctions by capacitance techniques

Abstract

The midgap density of states (MGDOS) in a‐SiGe:H alloys is investigated by capacitance measurements on p‐i‐n solar cells. Past work on thick a‐Si:H Schottky barriers is extended to thin a‐SiGe:H p‐i‐n cells. Four methods of determining the MGDOS from the measured capacitance are described, and each is applied to two p‐i‐n devices having 0% and 62% Ge in the i layers, respectively. The first method involves fitting an equivalent circuit model to the measured admittance. Close agreement is found 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 admittance analysis. A second method determines g₀ from the limiting capacitance at high temperature. The third and fourth methods extract g₀ from the dependence of capacitance on voltage bias. One of these is novel, presented here for the first time. Thus, a unique feature of this study is the application of several different capacitance methods to standard p‐i‐n solar cell devices. Agreement within ±25% is found among the values of the MGDOS from the four methods. The MGDOS increases exponentially from (1–2)×10¹⁶ to (3–4)×10¹⁷/cm³ eV as the Ge increases from 0% to 62%, in general agreement with results of others.