An facile approach for improving device efficiency of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) bulk heterojunction solar cells is presented. This method is used by simply precasting a tiny thin P3HT layer with high crystallinity between PEDOT:PSS and photoactive P3HT:PC61BM layers. The high crystalline thin P3HT layers are casted from three different solvents such as dichloromethane (DCM), dichlorobenzene (o-DCB), and tetrahydrofuran (THF). It is demonstrated that THF used for thin P3HT layer preparation is a suitable solvent for yielding a high crystalline film, which is unreadily washed away during the solution processing of the active layer. The results indicate that the morphology of P3HT:PC61BM active layers strongly depend on the formation of P3HT buffer layer. A great morphology difference of P3HT:PC61BM is caused from crystallinity of P3HT buffer layers prepared by different solvents. The thin P3HT layer with high crystallinity can improve the crystalline degree of P3HT in the active layer, subsequently inducing the whole active layer to form a well self-assembled pathway for efficient charge transfer and transportation to their respective electrodes. Therefore, a dramatically enhanced short-circuit current density of the device is resulted. After optimization of thickness of the P3HT buffer layer, an improvement of the power conversion efficiency is obtained from 2.98% to 5.14%.