In this paper, the degradation of fill factor with time of organic bulk heterojunction solar cells has been investigated up to 312 h. The experimental data of P3HT: PCBM solar cells has been analyzed theoretically to determine the parameters which affect the FF. The existing empirical formula has been applied to degradation data, and it was found to deviate drastically as the cell degrades, indicating that the correct behavior of solar cell is not imitated using the current FF formula. In view of the discrepancy, the expression for fill factor has been modified taking into account the material and device properties, which directly influence the working of a solar cell. All the values can be determined experimentally. The results of the modified expression gives better theoretical fit of FF with time. The results highlight the parameters which should be targeted in order to overcome the short lifetime of organic solar cells. (C) 2017 Elsevier B.V. All rights reserved.