Using a one-dimensional theoretical drift-diffusion model, we have proposed analytical solutions for the equations of excitons, electrons, and holes of a bulk heterojunctions organic solar cell. The main purpose is to predict and interpret the role of morphological effects on the performance of the device. We have highlighted the role of morphology parameters such as the donor domain size, the thickness of the active layer and the electron and hole motilities. We have chosen PCDTBT: PCBM in two different D/A weight rations (1:4 and 1:1) as a test organic blend (donor-acceptor) material. Some experimental works of other groups, has showed that despite the less absorption coefficient of device 1:4, it shows high efficiency in compare to device 1:1. We have attributed this behavior to two main mechanisms: i: the small phase domain size of device 1:4 and ii: percolated network of the blend which creates continuous pathways for carriers to move. In order to determine the reality of proposed model, we have studied the characteristic parameters of the device and also studied the dependence of characteristic parameters on device’s parameters such as the active layer thickness, donor domain size and carrier’s mobility. The Results shows a good agreement with experimental observations, which means that the model is satisfactory for discussing and predicting the bulk heterojunction organic solar cell’s performance.