Negative differential resistance and photovoltaic phenomena observed in nanostructured organic heterojunction

Single layer organic photovoltaic devices based on copper phthalocyanine (CuPc) sandwiched between indium tin oxide (ITO) coated glass substrate and aluminum (Al) electrode have been fabricated. The interface dipoles formed at the metal/organic interface play a key role in determining the barrier for charge (electron or hole) injection between the metal electrode and significantly affects the efficiency of organic based electronic and optoelectronic devices. The origin of interface dipoles formed at the metal/organic (Al/CuPc) interface is assumed to be the charge transfer process between the organics (CuPc) and the metal (Al) electrode. Such a device shows forward rectifying property under dark condition when ITO kept at positive bias and Al kept at negative bias. Negative differential resistance (NDR) effect has been observed at high reverse bias voltage. Under white light excitation the device shows reverse rectifying property with a high open circuit voltage. It appears that the interface dipoles formed at the Al/CuPc junction act as exciton dissociation centers. NDR effect can be explained due to charge accumulation layer at the AI/CuPc interface and hence interface dipoles and/or band bending in CuPc. Asymmetry in dark current-voltage (I-V) characteristics has been explained due to strengthening or weakening the dipole field on the application of reverse bias or forward bias at the junction interface, respectively. Various photovoltaic parameters have been calculated from the I-V characteristics of the devices under illumination through ITO electrode.