P. Veerender

Synthesis of vertically aligned polyaniline (PANI) nanofibers, nanotubes on APTMS monolayer and their field emission characteristics

Growth of aligned polyaniline (PANI) nanostructures on silicon modified with amino-silane self-assembled (3-aminopropyl)trimethoxysilane (APTMS) monolayer has been carried out by chemical polymerization in the presence of strong (HCl) and weak (CH3COOH) acids as dopant. Self assembly of aniline monomer leading to formation of aligned nanostructures on the APTMS monolayer is due to the amino (–NH2) moieties acting as growth centers. Interestingly, the nature of dopant acid (strong/weak) is found to determine the resultant form of the PANI nanostructures, either as nanofibers or nanotubes. The formation of PANI nanofibers and nanotubes has been revealed by scanning and transmission electron microscopy, whereas confirmation of the conducting phase of PANI is obtained from UV-visible and FTIR analysis. A plausible explanation illustrating the growth mechanism has been presented. Field emission studies of the PANI films have been carried out in a planar diode configuration at base pressure of ∼1 × 10−8 mbar. The turn-on field required to draw an emission current of 1 nA was found to 1.6 and 2.4 V μm−1 for aligned nanofibers and nanotubes, respectively. Interestingly, both the emitters exhibit good stability of the emission current over a duration of 4 h. The ease of the synthesis route and interesting field emission characteristics indicate the aligned PANI nanofibers and nanotubes as promising materials for field emission based applications.

Efficiency enhancement in dye sensitized solar cells through co-sensitization of TiO2 nanocrystalline electrodes

We have demonstrated that co-sensitization of TiO2 electrode with an inexpensive rhodamine 19 perchlorate laser dye along with N3 dye not only enhances the incident-photon-to-current conversion efficiency but also reduces dark current. Consequently, the devices yield an average power efficiency of 4.7% as against 2.3% and 0.6% obtained for N3 and rhodamine 19 perchlorate dye based devices, respectively. The improvement in efficiency is attributed to the enhanced dye absorption on TiO2 electrode as well as reduced dye aggregation that resulted from the usage of two dyes on different anchoring sites of single TiO2 electrode.

Investigation on the effects of thermal annealing on PCDTBT:PCBM bulk-heterojunction polymer solar cells

Bulk heterojunction solar cells of 3×3 array have been fabricated using a blend of PCDTBT:PCBM as active layer. Among nine devices fabricated, the best device exhibited efficiency 5% with standard deviation ∼ 20% in efficiency. Thermal annealing of the devices resulted in decrease of photovoltaic properties of all devices which was attributable to the change at the active layer/cathode interface of devices as supported by electrical and spectroscopic studies.

Graphene composite for improvement in the conversion efficiency of flexible poly 3-hexyl-thiophene:[6,6]-phenyl C71 butyric acid methyl ester polymer solar cells

The solution of thin graphene-sheets obtained from a simple ultrasonic exfoliation process was found to chemically interact with [6,6]-phenyl C71 butyric acid methyl ester (PCBM) molecules. The thinner graphene-sheets have significantly altered the positions of highest occupied molecular orbital and lowest unoccupied molecular orbital of PCBM, which is beneficial for the enhancement of the open circuit voltage of the solar cells. Flexible bulk heterojunction solar cells fabricated using poly 3-hexylthiophene (P3HT):PCBM-graphene exhibited a power conversion efficiency of 2.51%, which is a ∼2-fold increase as compared to those fabricated using P3HT:PCBM. Inclusion of graphene-sheets not only improved the open-circuit voltage but also enhanced the short-circuit current density owing to an improved electron transport.

Enhancement of the carrier mobility of conducting polymers by formation of their graphene composites

Conducting polymers (CP) with high charge carrier mobility are crucial for flexible organic electronic devices. However, the inherent carrier mobility of these polymers is very low. Therefore, methodologies need to be explored to improve the carrier transport in these polymers so that they can be efficiently used in organic electronic devices. Graphene, due to its exceptional electrical and mechanical properties, is a promising material to be examined for its possible incorporation in CP matrix to achieve the objective. We have prepared graphene composites of the conducting polymers following an optimized procedure and these were investigated for their charge transport properties. The mobility values were measured using electric field induced second harmonic generation (EFISHG) and field effect transistor (FET) transfer characteristics. Both the transient and average mobilities were found to increase significantly with the inclusion of graphene. This enhancement in mobility has been attributed to an ordered packing of the thinner and smaller graphene sheets with polymer chain and interfacial π–π interaction. To substantiate its usefulness in device applications, the effect of graphene inclusion was also investigated for polymer solar cells and it was observed that despite of reduction in open circuit voltage, device fabricated using graphene composites yielded about 20% higher efficiencies as compared to pristine conducting polymer devices.

One pot, single step, room temperature dielectrophoretic deposition of gold nanoparticles clusters on polyethylene terephthalate substrate.

The major challenge of plastic electronics is the deposition of gold nanoparticles (AuNPs) on flexible substrates at room temperature. Here, we show fast, single step, room temperature deposition of AuNPs on polyethylene terephthalate (PET) and biaxially oriented PET (BoPET) substrate by employing dielectrophoresis. The deposition has been carried out using two‐electrode system, with BoPET (or PET) and metallic (Pt or stain steel) mesh, under an AC signal of 20 kHz and 20 V peak‐to‐peak (Vpp) (signal for PET is 6 Vpp and 6 kHz). In this method, we show how to deposit AuNPs on PET‐like insulator by exploiting its polarization capability under an AC signal. The polarization of PET has been confirmed by change in the Raman spectra of the PET film under in situ AC signals. Furthermore, we show that using this dielectrophoretic deposition method, the PET films can be patterned by AuNPs at room temperature without any pre‐ and posttreatment.

Effect of Co-sensitization and acid treatment on TiO2 photoanodes in dye-sensitized solar cells

The performance of dye-sensitized solar cells is improved by simple formic acid treatment of TiO2 photoanodes. The treatment increases number of hydroxyl groups at TiO2 surface and thereby result in enhanced dye-loading and device efficiency. The best device showed an open circuit voltage of 0.62V, short-circuit current density 6.34mA/cm2, fill factor of 60%, and a power conversion efficiency of ∼12 % under 20mW/cm2 white light illumination.

Poly(2,7-carbazole) derivative based air stable and flexible organic field effect transistor

Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) which is known to be air stable, has been used as active layer to fabricate flexible organic field effect transistor. Poly(methyl methacrylate) has been used as gate dielectric. The mobility and on-off ratio for these devices were found to be 7×10−4cm2V−1s−1 and 1.5×102, respectively.

Metal-free organic dye for dye sensitized solar cells

We demonstrate a dye sensitized solar cell (DSSC) using a new organic dye (i.e. rhodamine 19 perchlorate) sensitized TiO2 electrode. The power conversion efficiency of the cell was found to be 1.1% under white light illumination. The binding modes of the dye to TiO2 electrode as well as the mechanism of the cell are discussed.

Electrochemical Polymerization of tetra-(4-hydroxyphenyl) porphyrin for Organic Solar Cells

Electrochemical oxidation of tetrakis-5,10,15,20-(4-hydroxyphenyl)porphyrin (THPP) on ITO electrodes was carried out. It leads to a uniform polymeric film (poly-THPP) with a nanostructured morphology. Poly-THPP electrodes as a bilayer organic solar cell and bulk heterojunction solar cells are investigated. The device with Phenyl-C61-butyric acid methyl ester (PCBM) integrated bilayer shows a modest photoactivity.