Showing papers by "Prashant V. Kamat published in 2013"

Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics.

Abstract: The recent surge in the utilization of semiconductor nanostructures for solar energy conversion has led to the development of high-efficiency solar cells. Some of these recent advances are in the areas of synthesis of new semiconductor materials and the ability to tune the electronic properties through size, shape, and composition and to assemble quantum dots as hybrid assemblies. In addition, processes such as hot electron injection, multiple exciton generation (MEG), plasmonic effects, and energy-transfer-coupled electron transfer are gaining momentum to overcome the efficiency limitations of energy capture and conversion. The recent advances as well as future prospects of quantum dot solar cells discussed in this perspective provide the basis for consideration as “The Next Big Thing” in photovoltaics.

Metal-Cluster-Sensitized Solar Cells. A New Class of Thiolated Gold Sensitizers Delivering Efficiency Greater Than 2%

Abstract: A new class of metal-cluster sensitizers has been explored for designing high-efficiency solar cells. Thiol-protected gold clusters which exhibit molecular-like properties have been found to inject electrons into TiO2 nanostructures under visible excitation. Mesoscopic TiO2 films modified with gold clusters deliver stable photocurrent of 3.96 mA/cm2 with power conversion efficiencies of 2.3% under AM 1.5 illumination. The overall absorption features and cell performance of metal-cluster-sensitized solar cells (MCSCs) are comparable to those of CdS quantum-dot-based solar cells (QDSCs). The relatively high open-circuit voltage of 832 mV and fill factor of 0.7 for MCSCs as compared to QDSCs show the viability of these new sensitizers as alternatives to semiconductor QDs and sensitizing dyes in the next generation of solar cells. The superior performance of MCSCs discussed in this maiden study lays the foundation to explore other metal clusters with broader visible absorption.

Graphitic design: prospects of graphene-based nanocomposites for solar energy conversion, storage, and sensing.

Abstract: Graphene not only possesses interesting electrochemical behavior but also has a remarkable surface area and mechanical strength and is naturally abundant, all advantageous properties for the design of tailored composite materials. Graphene–semiconductor or −metal nanoparticle composites have the potential to function as efficient, multifunctional materials for energy conversion and storage. These next-generation composite systems could possess the capability to integrate conversion and storage of solar energy, detection, and selective destruction of trace environmental contaminants or achieve single-substrate, multistep heterogeneous catalysis. These advanced materials may soon become a reality, based on encouraging results in the key areas of energy conversion and sensing using graphene oxide as a support structure. Through recent advances, chemists can now integrate such processes on a single substrate while using synthetic designs that combine simplicity with a high degree of structural and composition...

Tandem-layered quantum dot solar cells: tuning the photovoltaic response with luminescent ternary cadmium chalcogenides.

Abstract: Photon management in solar cells is an important criterion as it enables the capture of incident visible and infrared photons in an efficient way. Highly luminescent CdSeS quantum dots (QDs) with a diameter of 4.5 nm were prepared with a gradient structure that allows tuning of absorption and emission bands over the entire visible region without varying the particle size. These crystalline ternary cadmium chalcogenides were deposited within a mesoscopic TiO(2) film by electrophoretic deposition with a sequentially-layered architecture. This approach enabled us to design tandem layers of CdSeS QDs of varying band gap within the photoactive anode of a QD solar cell (QDSC). An increase in power conversion efficiency of 1.97-2.81% with decreasing band gap was observed for single-layer CdSeS, thus indicating varying degrees of photon harvesting. In two- and three-layered tandem QDSCs, we observed maximum power conversion efficiencies of 3.2 and 3.0%, respectively. These efficiencies are greater than the values obtained for the three individually layered photoanodes. The synergy of using tandem layers of the ternary semiconductor CdSeS in QDSCs was systematically evaluated using transient spectroscopy and photoelectrochemistry.

CuInS2-Sensitized Quantum Dot Solar Cell. Electrophoretic Deposition, Excited-State Dynamics, and Photovoltaic Performance

Abstract: Ternary metal chalcogenides such as CuInS2 offer new opportunities to design quantum dot solar cells (QDSC). Chemically synthesized CuInS2 quantum dots (particle diameter, 2.6 nm) have been successfully deposited within the mesoscopic TiO2 film using electrophoretic deposition (150 V cm–1 dc field). The primary photoinduced process of electron injection from excited CuInS2 into TiO2 occurs with a rate constant of 5.75 × 1011 s–1. The TiO2/CuInS2 films are photoactive and produce anodic photocurrent with a power conversion efficiency of 1.14%. Capping the TiO2/CuInS2 film with a CdS layer decreases the interfacial charge recombination and thus offers further improvement in the power conversion efficiency (3.91%). The synergy of using CdS as a passivation layer in the composite film is also evident from the increased external quantum efficiency of the electrode in the red region where only CuInS2 absorbs the incident light.

Quantum Dot Surface Chemistry: Ligand Effects and Electron Transfer Reactions

Abstract: With the increased interest in quantum dot sensitized solar cells (QDSCs) there comes a need to better understand how surface modification of quantum dots (QDs) can affect the excited state dynamics of QDs, electron transfer at the QD–metal oxide (MO) interface, and overall photoconversion efficiency of QDSCs. We have monitored the surface modification of solution based QDs via the steady state absorption and emission characteristics of colloidal CdSe passivated with β-alanine (β-Ala). The trap-remediating nature of the β-Ala molecule, arising from the Lewis basicity of the amine group, is realized from the hypsochromic shifts seen in excitonic absorption and emission bands as well as an increase in fluorescence quantum yield. Transient absorption measurements of CdSe–TiO2 films prepared with and without β-Ala as a linker molecule further reveal the role of the surface modifier in influencing excited state electron transfer processes. Electron transfer at this interface was found to be dependent on the me...

Making Graphene Holey. Gold-Nanoparticle-Mediated Hydroxyl Radical Attack on Reduced Graphene Oxide

Abstract: Graphene oxide (GO) and reduced graphene oxide (RGO) have important applications in the development of new electrode and photocatalyst architectures. Gold nanoparticles (AuNPs) have now been employed as catalyst to generate OH• and oxidize RGO via hydroxyl radical attack. The oxidation of RGO is marked by pores and wrinkles within the 2-D network. Nanosecond laser flash photolysis was used in conjunction with competition kinetics to elucidate the oxidative mechanism and calculate rate constants for the AuNP-catalyzed and direct reaction between RGO and OH•. The results highlight the use of the AuNP-mediated oxidation reaction to tune the properties of RGO through the degree of oxidation and/or functional group selectivity in addition to the nanoporous and wrinkle facets. The ability of AuNPs to catalyze the photolytic decomposition of H2O2 as well as the hydroxyl radical-induced oxidation of RGO raises new issues concerning graphene stability in energy conversion and storage (photocatalysis, fuel cells, L...

Trap and Transfer. Two-Step Hole Injection Across the Sb2S3/CuSCN Interface in Solid-State Solar Cells

Abstract: In solid-state semiconductor-sensitized solar cells, commonly known as extremely thin absorber (ETA) or solid-state quantum-dot-sensitized solar cells (QDSCs), transfer of photogenerated holes from the absorber species to the p-type hole conductor plays a critical role in the charge separation process. Using Sb2S3 (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. The hole transfer from excited Sb2S3 into CuSCN, which limits the overall power conversion efficiency of these solar cells, is now independently studied using transient absorption spectroscopy. In the Sb2S3 absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S–•) species. This trapped hole is transferred from the Sb2S3 absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. This process was monitored through the spectroscopic signal seen for the S–• species in Sb2S3, pr...

Reduced Graphene Oxide–Silver Nanoparticle Composite as an Active SERS Material

Abstract: Selectivity and enhanced sensitivity for SERS measurements are highly desirable for environmental and analytical applications. Interaction of a target molecule with SERS substrate plays a pivotal role in determining the magnitude of enhancement and spectral profile of the SERS signal. A reduced graphene oxide–Ag nanoparticle (RGO-Ag NP) composite has been designed to boost SERRS sensitivity of a porphyrin derivative. Complexation between 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP) porphyrin and the RGO-Ag NP composite is evidenced by a red-shifted porphyrin absorption band. Results indicate complexation is influential in improved surface-enhanced resonance Raman (SERRS) signal for TMPyP and thus offers an advantage for target molecule detection at low concentration levels. The combined effects of RGO and Ag NPs in the enhancement of SERS signal of TMPyP are discussed.

Direct observation of spatially heterogeneous single-layer graphene oxide reduction kinetics.

Abstract: Graphene oxide (GO) is an important precursor in the production of chemically derived graphene. During reduction, GO’s electrical conductivity and band gap change gradually. Doping and chemical functionalization are also possible, illustrating GO’s immense potential in creating functional devices through control of its local hybridization. Here we show that laser-induced photolysis controllably reduces individual single-layer GO sheets. The reaction can be followed in real time through sizable decreases in GO’s photoluminescence efficiency along with spectral blueshifts. As-produced reduced graphene oxide (rGO) sheets undergo additional photolysis, characterized by dramatic emission enhancements and spectral redshifts. Both GO’s reduction and subsequent conversion to photobrightened rGO are captured through movies of their photoluminescence kinetics. Rate maps illustrate sizable spatial and temporal heterogeneities in sp2 domain growth and reveal how reduction “flows” across GO and rGO sheets. The observe...

CdS Nanowire Solar Cells: Dual Role of Squaraine Dye as a Sensitizer and a Hole Transporter

Abstract: The squaraine dye (SQ) anchored onto CdS nanowires serves as a photosensitizing dye and a hole acceptor. This dual role of the squaraine dye has been successfully exploited in a nanowire solar cell to improve the photoconversion efficiency. Electrophoretic deposition of CdS NWs and CdS NWs+SQ composite onto conducting glass electrodes was performed to obtain robust photoanodes and evaluate the photovoltaic performance of nanowire solar cells (NWSCs). Whereas the sensitization property of the SQ extends the response of CdS NWSCs into the near-IR (NIR) region, its redox property facilitates shuttling of holes to the electrolyte and suppressing the charge recombination process. Transient absorption measurements confirm the formation of cation radical of the dye arising from these two processes. The dual role of the squaraine dye has enabled us to improve the power conversion efficiency of NWSCs by a factor of ∼20. Photoelectrochemical, spectroelectrochemical, and spectroscopic measurements provide insight in...

Galvanic Exchange on Reduced Graphene Oxide: Designing a Multifunctional Two-Dimensional Catalyst Assembly

Abstract: The two-dimensional network of reduced graphene oxide (RGO) is decorated with silver and gold nanoparticles. The silver nanoparticles deposited on RGO by photocatalytic reduction are subjected to galvanic exchange with Au3+ ions to transform them into gold nanoparticles. This compositional change on the RGO surface demonstrates RGO’s versatile ability to anchor a wide array of nanoparticles and facilitate chemical transformations. Coupled with RGO’s unique ability to capture and transport electrons, galvanic exchange is used to contrive a two-dimensional nanocatalyst mat. Raman studies show that metal nanoparticles anchored on reduced graphene oxide facilitate enhancement of Raman bands. Using methyl viologen as a probe we elucidate the photocatalytic activity of the semiconductor–RGO–metal nanoassembly and highlight the mediation of RGO in charge transfer processes.

CdSe nanowire solar cells using carbazole as a surface modifier

Abstract: Carbazole molecules containing thiol functional groups, when attached to CdSe nanowires (NWs), facilitate hole transport across semiconductor interfaces. The improved hole transfer rate is evidenced by increased electron lifetimes and better photovoltaic performance. Nanowire solar cells (NWSCs) with carbazole treatment delivered a power conversion efficiency of 0.46%, which is an order of magnitude improvement over untreated films. The illumination of the sample during the electrophoretic deposition of nanowires also had a profound effect in obtaining stable and higher photocurrents.

Photoactive porous silicon nanopowder.

Abstract: Bulk processing of porous silicon nanoparticles (nSi) of 50–300 nm size and surface area of 25–230 m2/g has been developed using a combustion synthesis method. nSi exhibits consistent photoresponse to AM 1.5 simulated solar excitation. In confirmation of photoactivity, the films of nSi exhibit prompt bleaching following femtosecond laser pulse excitation resulting from the photoinduced charge separation. Photocurrent generation observed upon AM 1.5 excitation of these films in a photoelectrochemical cell shows strong dependence on the thickness of the intrinsic silica shell that encompasses the nanoparticles and hinders interparticle electron transfer.

Nickel-doped MnO2 nanowires anchored onto reduced graphene oxide for rapid cycling cathode in lithium ion batteries

Abstract: Nickel-doped MnO2 nanowires were synthesized directly onto reduced graphene oxide (RGO) to generate a composite cathode material with improved high-rate cycling characteristics. The presence of RGO improves the electrochemical characteristics of the cathode in Li-ion half-cell architecture. Cyclic voltammetry, electrochemical impedance spectroscopy, and electrode cycling are confirm that RGO plays a major role in enhancing the ability of the NixMn(1-x)O2 to reversibly intercalate lithium ions at 1C rate. The chronocoulometric response of the RGO-based electrode shows the improvements originate from faster reaction kinetics and transport of Li+ coupled with increased specific capacitance and Li+ adsorption. (C) 2013 The Electrochemical Society. All rights reserved.

(Invited) Semiconductor-Metal Nanoparticles Anchored On Graphene Oxide: Photocatalysis and SERS Detection

Abstract: Our recent efforts have focused on utilizing graphene based assemblies for light energy conversi on, energy storage and sensing applications [1-4]. Electrophoretic deposition method has been employed to anchor single-layered GO onto to TiO 2 NP films. Under UV irradiation photogenerated electrons are transfe rred across graphene sheet. These electrons are capable of reducing Ag+ ions from solution and deposit Ag nanoparticles on the GO surface. By controlling the irradiation time we can tune the size and loading o f metal nanoparticles. The semiconductor-graphene-metal nanoparticle (SGM) films can be used as a photocatalyst or in se nsing applications. SGM films tested for SERRS enhancemen t of the porphyrin target molecule, TAPP showed over three orders of magnitude enhancement. Immersing th e SGM films in a solution containing TAPP results in interaction with RGO and this gets concentrated nea r the preexisting Ag nanoparticles. Increased concentrati on and SERS enhancement makes the detection of nanomolar concentration of molecules that exist in solution. In addition, these composites can also be used to develop new photocatalyst devices for water splitti ng reaction. The role of Graphene based assemblies in promoting charge separation will be discussed.