A. K. Chauhan

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.

Electrochemical grafting of octyltrichlorosilane monolayer on Si

The octyltrichlorosilane (OTS) monolayer on hydrated Si (111) surfaces has been grafted by cyclic voltammetry (CV) using tetrabutylammonium perchlorate in dry methanol as supporting electrolyte. The percentage of OTS coverage, calculated from the current value at −1V, after 30 CV scans was found to be >97%, which is independently confirmed by atomic force microscopy. A mechanism of electrochemical grafting of OTS on Si via formation of Si–Si bonds is proposed. Current-voltage characteristics and impedance spectroscopic measurements on Al/OTS/Si structures reveal realization of a true OTS/Si interface.

Electro-optical properties of copper phthalocyanines (CuPc) vacuum deposited thin films

Copper phthalocyanine (CuPc) thin films grown on glass substrates by the vacuum deposition method were investigated using photo absorption–transmission as well as photoreflectance spectroscopy with respect to different thickness. The surface morphology was investigated using scanning electron microscopy (SEM). Structural properties were studied by grazing angle XRD diffraction. The vibrational studies were carried out using Fourier transform infrared spectroscopy (FT-IR). The wettability was studied using a contact angle meter.

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.

Enhanced Thermoelectric Properties of Selenium-Deficient Layered TiSe2–x: A Charge-Density-Wave Material

In the present work, we report on the investigation of low-temperature (300-5 K) thermoelectric properties of hot-pressed TiSe2, a charge-density-wave (CDW) material. We demonstrate that, with increasing hot-pressing temperature, the density of TiSe2 increases and becomes nonstoichiometric owing to the loss of selenium. X-ray diffraction, scanning electron microscopy, and transimission electron microscopy results show that the material consists of a layered microstructure with several defects. Increasing the hot-press temperature in nonstoichiometric TiSe2 leads to a reduction of the resistivity and enhancement of the Seebeck coefficient in concomitent with suppression of CDW. Samples hot-pressed at 850 °C exhibited a minimum thermal conductivity (κ) of 1.5 W/m·K at 300 K that, in turn, resulted in a figure-of-merit (ZT) value of 0.14. This value is higher by 6 orders of magnitude compared to 1.49 × 10(-7) obtained for cold-pressed samples annealed at 850 °C. The enhancement of ZT in hot-pressed samples is attributed to (i) a reduced thermal conductivity owing to enhanced phonon scattering and (ii) improved power factor (α(2)σ).

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.

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.