A geometrical model based on the stochastic nature of diffusion transport is proposed to formulate local collection efficiency in the nano-crystalline solar cells. It is shown that this parameter is given by eta(local)(x) approximate to 1 - x/L. Using this formula, we derived the generalized form of the well-known equation of total collection efficiency (eta(coll) = 1 - tau(IMPS)/tau(IMVS)), which can be used for all solar cells independent of their specific performance. The generalized equation is in good agreement with the simulation and empirical results reported by other groups. For more justification of the model validity, the results are compared with the non-linear diffusion equation. In addition, we propose an approach for direct estimation of the electron diffusion length based on the IMPS and IMVS responses. (C) 2016 Published by Elsevier Ltd.