Showing papers by "Prashant V. Kamat published in 2008"

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters

Abstract: The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...

TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide

Abstract: Graphene oxide suspended in ethanol undergoes reduction as it accepts electrons from UV-irradiated TiO2 suspensions. The reduction is accompanied by changes in the absorption of the graphene oxide, as the color of the suspension shifts from brown to black. The direct interaction between TiO2 particles and graphene sheets hinders the collapse of exfoliated sheets of graphene. Solid films cast on a borosilicate glass gap separated by gold-sputtered terminations show an order of magnitude decrease in lateral resistance following reduction with the TiO2 photocatalyst. The photocatalytic methodology not only provides an on-demand UV-assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.

Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe−TiO2 Architecture

Abstract: Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency

Decorating Graphene Sheets with Gold Nanoparticles

Abstract: Renewed interest in graphene architectures has opened up new avenues to utilize them in electronic and optoelectronic applications. The desire to design graphene−metal nanohybrid assemblies has led us to explore a solution-based approach of chemical reduction of AuCl4- ions in graphene suspensions. The gold particles anchored on octadecylamine functionalized graphene are readily suspendable in THF medium. The dependence of particle stability on the graphene concentration and SEM analysis indicate that the gold nanoparticles are well dispersed on graphene sheets. Transient absorption spectroscopy measurements suggest that the ultrafast disappearance of plasmon absorption and its recovery are unaffected by the presence of graphene.

Quantum dot solar cells. Electrophoretic deposition of CdSe-C60 composite films and capture of photogenerated electrons with nC60 cluster shell.

Abstract: Encapsulation with a nC60 cluster shell facilitates capture of photogenerated electrons from CdSe quantum dots following visible light excitation. Electrophoretic deposition of CdSe−C60 composite clusters on optically transparent electrodes (OTE/SnO2) produce photoactive films that exhibit photoelectrochemical activity. The observed photoconversion efficiency (IPCE) of ∼4% is significantly greater than those observed with CdSe or nC60 films.

Single-Walled Carbon Nanotube Scaffolds for Dye-Sensitized Solar Cells

Abstract: The influence of single-walled carbon nanotube (SWCNT) architectures for facilitating charge transport in mesoscopic semiconductor films has been probed using a TiO2/Ru(II) trisbipyridyl complex system. Both transient absorption and emission measurements indicate that the SWCNT network in the film has no noticeable influence on the charge injection process from the excited Ru(II) trisbipyridyl complex into TiO2 particles. However, it plays an important role in improving the charge separation, as the rate of back electron transfer between the oxidized sensitizer (Ru(III)) and the injected electrons becomes slower in the presence of the SWCNT scaffold. The beneficial aspect of charge collection by SWCNT has been further explored by carrying out photoelectrochemical measurements. The dye-sensitized solar cells constructed using this SWCNT scaffold show an improvement in the photocurrent generation. However, this improvement in photocurrent generation is neutralized by a lower photovoltage as the apparent Fer...

Solvent dependence of the charge-transfer properties of a quaterthiophene-anthraquinone dyad

Abstract: An electron donor–acceptor dyad (quaterthiophene–anthraquinone) mediates ultrafast intramolecular photoinduced charge separation and consequent charge recombination when in polar or moderately polar solvents. Alternatively, non-polar media completely impedes the initial photoinduced electron transfer by causing enough destabilization of the charge-transfer state and shifting its energy above the energy of the lowest locally excited singlet state. Furthermore, femtosecond transient-absorption spectroscopy reveals that for the solvents mediating the initial photoinduced electron-transfer process, the charge recombination rates were slower than the rates of charge separation. This behavior of donor–acceptor systems is essential for solar-energy-conversion applications. For the donor–acceptor dyad described in this study, the electron-transfer driving force and reorganization energy place the charge-recombination processes in the Marcus inverted region.

Harvesting Infrared Photons with Croconate Dyes

Abstract: Excited-state properties of the croconate dyes were investigated with an aim to utilize them as light harvesting assemblies in the infrared (IR) region (λmax ∼ 865 nm and M−1 c−1 = (1.4−4.2) × 105 M−1 s−1). The excited singlet of the monomeric dye quickly deactivates (lifetime 4−7 ps) without undergoing intersystem crossing to generate triplet. The triplet excited-state produced via triplet–triplet energy transfer method show relatively long life (lifetime 7.2 µs). The dye molecules when deposited as thin film on optically transparent electrodes or on nanostructured TiO2 film form H-aggregates with a blue-shifted absorption maximum around 660 nm. The excitons formed upon excitation of the dye aggregates undergo charge separation at the TiO2 and SnO2 interface. The H-aggregates in the film are photoactive and produce anodic current (IPCE of 1.2% at 650 nm) when employed in a photoelectrochemical cell. Spectroscopic and photoelectrochemical experiments that highlight the usefulness of croconate dyes in IR l...

Nanomaterial scaffolds for electron transport

Abstract: Embodiments of the present invention provide nanomaterial scaffolds for transporting electrons. There is provided a single wall carbon nanotube (SWCNT) architecture employed as a conducting scaffold in semiconductor based photoelectrochemical cells. SWCNT architecture provides a nanotube network to disperse nanoparticles and/or quantum dots, whether ordered or randomized. As a result, an increase in incident photon conversion to charge carrier conversion efficiency (IPCE) represents a beneficial role of SWCNT architecture as a conducting scaffold to facilitate charge collection and charge transport in nanostructured semiconductor films. Embodiments may be used for solar cells based on semiconductor quantum dots and nanostructures, solar hydrogen production, microcapacitors and storage batteries, solar-fuel cell hybrids, etc.

Excited-state and photoelectrochemical behavior of pyrene-linked phenyleneethynylene oligomer.

Abstract: An oligophenyleneethynylene (OPE), 1,4-bis(phenyleneethynyl)-2,5-bis(hexyloxy)benzene (2), is coupled with pyrene to extend the conjugation and allow its use as a light-harvesting molecule [Py−OPE ...

Fullerene-Based Supramolecular Nanoclusters with Poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] for Light Energy Conversion

Abstract: Organized composite molecular nanoassemblies of fullerene and poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) prepared in acetonitrile/toluene mixed solvent absorb light over the entire spectrum of visible light. The highly colored composite clusters can be assembled as a three-dimensional array onto nanostructured SnO2 films by electrophoretic deposition approach. The composite cluster films exhibit an incident photon-to-photocurrent efficiency (IPCE) as high as 18%, which is significantly higher than that of a molecular assembly composed of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-21H,23H-porphyrin (H2P) and C60 prepared in the same manner (4%). The maximum IPCE value increases to 25% at an applied bias potential of 0.2 V vs saturated calomel reference electrode (SCE). The power conversion efficiency of a MEH-PPV and C60 assembly-modified electrode is determined to be 0.24%. The photocurrent generation properties observed with MEH-PPV and C60 clusters demonstrate the synergy of these systems towards yielding efficient photoinduced charge separation within these composite nanoclusters.