series of commercial organic materials was blended into MEHPPV (poly [2-methoxy, 5-(2-ethylhexoxy)1,4-phenylene vinylene]) and used as electron acceptors in the polymer heterojunction photovoltaic cells and photodiodes. Electrochemical cyclic voltammetry (CV) was used to determine the acceptors' lowest unoccupied molecule orbit (LUMO) levels. We demonstrated that the LUMO of the acceptor had remarkable effect on the open-circuit voltage for the donor and acceptor composite devices over similar to1 eV range at LUMO near 4.0 eV. The photoresponse can be further enhanced by the substitution of a long alkyl chain in the polar acceptor molecules. For device with the substituted acceptor doped into MEHPPV as an active layer, its photosensitivity at zero bias is increased by a factor of 4-0.00047 A/W in comparison with 0.00012 A/W for unsubstituted acceptor. At a voltage of -30 V, the photosensitivity reaches 0.14 A/W, the same level as for device from MEHPPV: PCBM at 0 V bias. A strong electric field and temperature dependence in the composite devices was also observed and discussed.