The charged state density at the a-Si/c-Si interface is an important parameter in a heterojunction a-Si/c-Si solar cell. The extraction of the charged state density at the interface from measurements of lateral conductance is demonstrated by simulations. In a-Si/c-Si heterojunction an inversion layer is formed at the interface between a-Si and c-Si (heterointerface). The lateral conductance of the inversion layer is much higher than the doped or intrinsic a-Si layer conductance and the current primarily flows through this path. The increase of the charged state density at the heterointerface weakens the inversion and hence lowers the lateral conductance of these devices. This effect is studied in this work by applying a theoretical model developed in the commercial simulator Sentaurus. The simulation results based on this model have shown that in an optimized device structure the sensitivity of the measurement technique in determining the charged state density can be on the order of 1 × 1010/cm2.