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 ( V oc ) has recently been proposed to characterize interface states in a-Si : H/c-Si heterojunctions, and the interface defect density, D it , is estimated from the simulations of capacitance. In this paper, the theoretical diffusion capacitance, C D , 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 C D is developed as a function of D it , the excess minority carrier density out of c-Si depletion and diffusion regions, Δ n , and the carrier diffusion lengths in c-Si. C D decreases with increasing D it since the interface states act as recombination centres to decrease the excess carrier density in c-Si. The measurement is sensitive to D it down to 10 10 cm −2 eV −1 . Accordingly, in the measurement of Δ n and the carrier diffusion lengths in c-Si, the interface states can be characterized directly and accurately by the theoretical diffusion capacitance.