A systematic study of the photoluminescence quenching efficiencies in P3HT: PCBM blends showed a non-linear dependence on the PCBM concentration. We find a faster decrease in PL emission initially which later flattens out around 1:1 composition which tallies well with sample compositions known to give the best power conversion efficiencies. This implies that the exciton dissociation rates dominate the photocurrent generation in these films. We obtained a maximum of 91% photoluminescence quenching for films with a 1:1 blend ratio. A mean field based phenomenological model is presented, which very well describes our experimental results. The generation of free carriers due to various proposed mechanisms like dissociation and delocalization are collectively considered in the model. The model helps us understand the underlying physics and dependence of the quenching efficiency on parameters like excitation intensities. The proposed model will be useful in predicting the behaviour of exciton dissociation in new organic blends. © 2019 IOP Publishing Ltd.