Electrical and optical characterization of monolithically integrated multi-junction (MJ) solar cells is complicated by the fact that the internal subcells are not directly accessible. Two important characteristics of the MJ solar cell are the internal subcell I-V curves and the spectral photocurrent response. Techniques to obtain the subcell I-V curves as well as the spectral photocurrent response have been well demonstrated, however, the close connection between these two different measurements has not been exploited. Using small signal analysis, we establish the relationship between the subcell I-V response and the voltage dependence of the spectral photocurrent response. By an inverse analysis, we also show how to derive the subcell I-V curves from the voltage dependent spectral photocurrent response, providing a new and independent technique to extract the internal subcell I-V curves.