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Abstract: Electrical and photovoltaic properties of donor–acceptor composite system comprised of poly (3-phenyl azo methine thiophene) (PPAT) and 1, 1′–diallyl substituted 4, 4′-dipyridine (DADP) were investigated. A significant enhancement of photocurrent was observed when PPAT was blended with DADP. The increase in photocurrent has been explained in terms of efficient charge separation that resulted from the transfer of photo-excited electrons from PPAT to DADP. The strong quenching of fluorescence of PPAT was caused by the presence of DADP that indicates the photo-induced charge transfer from PPAT to DADP. The open circuit voltage (V oc) generated in the device is independent of the variation of work function of negative metal electrode that has been explained in terms of Fermi level pinning between DADP and metal via surface charges. The electrical characteristics of ITO/PPAT: DADP/Al photovoltaic device were determined by analyzing the dependence of short circuit photocurrent density (J sc) and V oc under illumination at different temperatures. The V oc decreases almost linearly with increasing temperature, while short-circuit photocurrent increases logarithmically with temperature and saturates at higher temperature above 330K. This dependence of J sc and V oc on temperature has been discussed in terms of possible mechanism that involves the photovoltage generation and charge carrier transport in the device under thermally activated state. The photovoltaic device made from PPAT: DADP blend has shown three times higher photosensitivity than that of made from pure PPAT. [Copyright &y& Elsevier]

The electrical and photoelectrical properties of an organic Schottky barrier device have been studied. The Schottky barrier device is based on spin-coated thin film of allyl viologen-doped polypyrrole (AV-PPy) sensitized with pyronine (PYR) dye, which is sandwiched between indium tin oxide (ITO)-coated glass and aluminum electrodes. The effect of dye sensitization on the electrical and photoelectrical parameters and photogeneration process has been described in this paper. It is found that the dye sensitization decreases the depletion layer width and diode quality factor of the device, whereas the mobility and carrier concentration increases on dye sensitization in AV-PPy thin film. The absorption spectrum of PYR complements that of AV-PPy in the visible region of the solar spectrum. It is also observed that the absorption spectra of PYR-sensitized AV-PPy thin film are broader and the number of photons absorbed in the visible region is higher than in the AV-PPy thin film. It has also been confirmed from the fluorescence spectra of PYR-sensitized AV-PPy thin film that the incident energy absorbed by the PYR dye does not give rise to fluorescence, and absorption of the incident light is generating the photocurrent. The PYR dye molecules enable the separation of charges photogenerated by the polymer before radiative recombination. [Copyright &y& Elsevier]

Abstract: Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly (3-phenyl hydrazone thiophene) (PPHT) as electron donor and titanium dioxide (TiO2) particles as an electron acceptor (n-type wide band gap semiconductor) have been studied. The blend films were spin coated from a common solvent mixture. The absorption peak and shape of the absorption spectra of PPHT:TiO2 (40vol%) indicate that it is a superposition of the absorption of PPHT and TiO2. From the cyclic voltammetry measurements, we have estimated the values of levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for PPHT. A strong fluorescence quenching indicates that the exciton dissociation and charge separation occurs successfully at PPHT:TiO2 interfaces formed in the bulk. This also enhances the possibility that the separated charges will reach the electrodes before recombining. Experimentally observed current–voltage characteristics of this device has been explained employing the metal–insulator–metal (MIM) model, where the intimate mixture of electron accepting and hole accepting materials is treated as a homogeneous intrinsic semiconductor. In this device, holes are solely transported along the pure donor phase and electrons transported along the paths of acceptor phase. Effect of TiO2 concentrations in the blend and thermal annealing of device has been described is detail. For low concentration of TiO2 below 20% the device performance is poor than that for pure PPHT, while for higher concentration of TiO2 significant improvement was obtained. The thermal annealing of the device also improves the photovoltaic response of the device, which may be due to the reduction in the recombination process. [Copyright &y& Elsevier]

Abstract: Photovoltaic properties and charge transfer process within the blend of p-type organic semi conducting copper phthalocyanine (CuPc) and inorganic n-type semiconductor zinc oxide (ZnO) nanoparticle have been determined. The photo-induced electron transfer mechanism in the composite thin film structure is predicted on the basis of its optical absorption behavior, photo-action spectra and photoluminescence (PL) characteristics, supported by current–voltage characteristics in dark and under illumination. An efficient electron transfer between donor (CuPc) and acceptor (ZnO) nanoparticle is explained by analyzing the photo-action spectra of the device and respective quenching of photoluminescence (PL) in composite thin film structure. The increase in photocurrent in composite blend based device is justified in terms of formation of bulk heterojunction between CuPc and ZnO nanoparticles. Also, the subsequent increase in photovoltaic response is ascribed to the formation of interconnecting network in between CuPc and ZnO nanoparticles. Our experimental results supports that photo-generation of charge carriers is induced by the production of excitons and their subsequent dissociation of into free carriers at ZnO–CuPc interface which is spread over the bulk region sandwiched between Al and ITO. Additionally, the optical and electrical properties of the composite blend based device have been discussed in details. The capacitance–voltage characteristics of the device support the formation of bulk heterojunction between CuPc and ZnO nanoparticles. [Copyright &y& Elsevier]