A solar cell is a photovoltaic device designed to convert sunlight into electrical power. We can categorize solar cells into organic and inorganic. The advantages of organic solar cell over inorganic ones are inexpensive to fabricate,light weight, flexible shape and have little potential for negative environmental impact.But, the disadvantages of organic solar cell is low power conversion efficiency and suffer from huge degradation.So,there are different approaches for improving the efficiency of organic solar cells. Some of them are selection of solar harvesting material which have low band gap and broad absorption, active layer structure design (such as using bulk heterojunction),and etc.This work is aimed at enhancing the performance of a bulk heterojunction solar cell made from APFO Green-6:PCBM (1:4 ratio).This can be achieved through creating new device architecture design(bulk heterojunction) and careful control on the morphology of the active layer using thermal annealing in order to get a higher power conversion efficiency. Thermal annealing increases the degree of crystallization of the cell,charge transport and then the efficiency of the organic cell.