Short circuit current density (J_sc) and open circuit voltage (V_oc) are two important parameters to evaluate the performance of organic solar cells. How to increase these two parameters without using novel material still remains a challenge. Two small molecules, zinc phthalocyanine (ZnPc) and 3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI), are used to fabricate ITO/ZnPc/ZnPc:PTCBI/PTCBI/ZnPc/Al photovoltaic device. We find that J_sc and V_oc are enhanced by using oblique deposition technique and ZnPc cathode buffer layer, respectively. Analysis of the active layer reveals phase segregation in obliquely deposited ZnPc:PTCBI bulk heterojunction layer. Field emission measurement is used to probe the band bending and internal field in ZnPc/PTCBI planar heterojunction. The effects of phase segregation and internal field are discussed. This work shows that careful assembly of donor and acceptor material is beneficial to small molecule photovoltaic device.