Characterization of interface states in a-Si : H/c-Si heterojunctions by an expression of the theoretical diffusion capacitance

Capacitance spectroscopy under illumination and at a forward bias close to the open-circuit voltage (Voc) has recently been proposed to characterize interface states in a-Si : H/c-Si heterojunctions, and the interface defect density, Dit, is estimated from the simulations of capacitance. In this paper, the theoretical diffusion capacitance, CD, is presented for measurement to directly characterize the interface states. By solving the excess minority carrier density in c-Si when interface states are introduced, the expression of CD is developed as a function of Dit, the excess minority carrier density out of c-Si depletion and diffusion regions, Δn, and the carrier diffusion lengths in c-Si. CD decreases with increasing Dit since the interface states act as recombination centres to decrease the excess carrier density in c-Si. The measurement is sensitive to Dit down to 1010 cm-2 eV-1. Accordingly, in the measurement of Δn and the carrier diffusion lengths in c-Si, the interface states can be characterized directly and accurately by the theoretical diffusion capacitance.