The photovoltaic and interfacial properties of heteorojunction polymer device consisting of p-type poly(3-octyl thiophene) and n-type poly(benzimidazobenzophenanthroline) are investigated. The photocurrent efficiency is enhanced at least by a factor of 100 in these bilayers compared to their respective single layer structures. The interfacial properties are studied using current-voltage characteristics, transient photocurrent measurement and modulated photocurrent spectroscopy. The open circuit photovoltage corresponds to the estimated fermi level difference of the polymers. The dispersive transport and interfacial charging in these bilayers are studied using transient and modulated photocurrent measurements.