Vibha Saxena

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.

Effect of the crystallinity of silver nanoparticles on surface plasmon resonance induced enhancement of effective absorption cross-section of dyes

The effective absorption cross-section of dye, and therefore, the efficiency of dye-sensitized solar cell can be increased by surface plasmon resonance (SPR) of metal nanoparticles with enhanced dephasing time. Further, the dephasing time is proportional to the enhancement factor of electric field in the vicinity of nanoparticle surface, and is governed by size, shape, and dielectric constant of surrounding medium. In this paper, we demonstrate that crystallinity of silver nanoparticles plays an important role in enhancing the dephasing time of SPR. Our theoretical formulation indicates that the dephasing time is higher for single crystalline silver nanoparticles as compared to that of polycrystalline nanoparticles, which is attributed to the presence of scattering centers in the latter. This suggests that single crystalline silver nanoparticles are interesting candidates for the enhancement of effective absorption cross-section of dyes. In order to validate our theoretical formulation, we have synthesized single crystalline and polycrystalline silver nanoparticles and studied their effect on absorption cross-section of N719 dye. We observed that dye incorporated with single crystalline silver nanoparticles showed a significant enhancement as compared to polycrystalline silver nanoparticles (24.42% in solution, 21.01% in thin film form in single crystalline silver nanoparticles while 8.52% in solution, 7.97% in thin film form in polycrystalline silver nanoparticles, respectively).

Nanostructured PdO Thin Film from Langmuir–Blodgett Precursor for Room-Temperature H2 Gas Sensing

Nanoparticulate thin films of PdO were prepared using the Langmuir-Blodgett (LB) technique by thermal decomposition of a multilayer film of octadecylamine (ODA)-chloropalladate complex. The stable complex formation of ODA with chloropalladate ions (present in subphase) at the air-water interface was confirmed by the surface pressure-area isotherm and Brewster angle microscopy. The formation of nanocrystalline PdO thin film after thermal decomposition of as-deposited LB film was confirmed by X-ray diffraction and Raman spectroscopy. Nanocrystalline PdO thin films were further characterized by using UV-vis and X-ray photoelectron spectroscopic (XPS) measurements. The XPS study revealed the presence of prominent Pd(2+) with a small quantity (18%) of reduced PdO (Pd(0)) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, the band gap energy of PdO was estimated to be 2 eV, which was very close to that obtained from specular reflectance measurements. Surface morphology studies of these films using atomic force microscopy and field-emission scanning electron microscopy indicated formation of nanoparticles of size 20-30 nm. These PdO film when employed as a chemiresistive sensor showed H2 sensitivity in the range of 30-4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light.

Broadband enhancement in absorption cross-section of N719 dye using different anisotropic shaped single crystalline silver nanoparticles

We have synthesized spherical as well as different anisotropic shapes of Ag nanoparticles using chemical reduction of silver nitrate with different sodium borohydride and cetyltrimethylammonium bromide (CTAB) concentrations. The X-ray diffraction of all synthesized Ag nanoparticles revealed a fcc structure. Further capping of CTAB on the surface of Ag nanoparticles was confirmed by Fourier transform infrared spectroscopy. Morphological analysis using transmission electron microscopy showed the formation of different anisotropic shapes such as oval, rod, cubic, pentagonal and prismatic when the CTAB concentration was maintained at 4 mM. The single crystalline nature of all different shaped Ag nanoparticles was confirmed by Nano beam diffraction patterns. The presence of multiple Surface Plasmon Resonance (SPR) peaks for different anisotropic shapes of Ag nanoparticles was revealed by UV-vis spectroscopy and was further utilized in commonly used N719 dye in dye sensitized solar cells. It was observed that the incorporation of different anisotropic Ag nanoparticle shapes into the dye resulted in a broadband enhancement of its absorption (∼65%), which is much greater as compared to that incorporating spherical Ag nanoparticle shape (∼21%). This enhancement was attributed to the improvement in the effective cross-section of the dye upon incorporation of different anisotropic Ag nanoparticle shapes.

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.

Conducting Polymer Sensors, Actuators and Field-Effect Transistors

Conducting polymers have been widely used in various device applications. This is because their physical properties (optical, electrical, electrochemical, mechanical, etc.) can be easily tailored by simple redox reactions. Moreover, device fabrication is simple and cost effective. This chapter deals mainly with our work on various aspects of conducting polymers and their applications. First we describe synthesis of conducting polymers in various forms, such as nanofibres and crystalline and thin films. Different concepts, such as self-assembled templates, chemical, electrochemical and interfacial polymerization, have been used for synthesis. We then discuss applications of conducting polymers in three major areas: toxic gas sensors, actuators and field-effect transistors. In gas sensors, we describe sensing characteristics of polycarbazole Langmuir–Blodgett films, polyaniline nanofibres and composite films of poly(3-hexylthiophene):ZnO-nanowire as well as polypyrrole:ZnO-nanowire. We discuss fabrication of electrochemical actuators using PPy-DBS/Au free-standing films. Finally, characteristics of top-contact field-effect transistors fabricated using poly(3-hexylthiophene) as an active layer are described.

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.