In organic bulk heterojunction solar cells, the internal quantum efficiency can be considered as the product of the absorption efficiency, the exciton diffusion efficiency, the charge transfer efficiency, and the charge collection efficiency. In previous studies, it has been reported that charge transfer and charge collection is very high in these solar cells, approaching 100 %. With a Monte Carlo simulation approach that considers the hopping of charges within the two materials of the heterojunctions, it is highlighted that an estimate of the charge collection efficiency can be evaluated considering that a certain number of charges do not reach the electrodes because of the random arrangement of the two materials in the heterojunction. The case of P3HT:PCBM (1:1 ratio) has been analyzed in this study.