Quantum efficiency measured at different voltage varies in a wide range. Compensation of the window layer, quality of the main junction and the value of back contact barrier can be derived from the voltage dependent quantum efficiency at different wavelength region. Depletion width and diffusion length of the minority carrier can be calculated from the apparent quantum efficiency. Relationship of the depletion width, diffusion length of the minority carrier and apparent quantum efficiency is presented in the article. A new method to calculate the depletion width and diffusion length of the minority carrier is proposed. Furthermore, we discussed the feasibility of studying thin film solar cells via its voltage dependent quantum efficiency.