Light Energy Conversion Devices Research Articles

(Invited) Semiconductor Nanostructures for Light Energy Conversion

This talk will review some of the key developments of semiconductor quantum dots based light energy conversion. Semiconductor nanostructures are finding new ways to design light energy conversion devices (e.g., thin film solar cells and light emitting devices). The decreased consumption of energy during the manufacture and the lessened use of semiconductor materials lowers the overall carbon footprint with energy payback time less than a year for such devices. The early studies which focused on the synthesis of various semiconductor nanostructures and exploration of their size dependent optical and electronic properties have led to their their integration in high efficiency thin film solar cells.. Unlike solar cell devices, photocatalytic processes are yet to deliver conversion efficiencies and stability that can make them compatible for practical applications. Understanding the limitations imposed by interfacial charge transfer processes remains a key to further advance photocatalytic systems for chemical energy storage. Recent developments in utilizing semiconductor nanostructures for light energy conversion and storage will be discussed.

Size Dependence of Ultrafast Electron Transfer from Didodecyl Dimethylammonium Bromide-Modified CsPbBr3 Nanocrystals to Electron Acceptors.

Charge transfer efficiencies in all-inorganic lead halide perovskite nanocrystals (NCs) are crucial for applications in photovoltaics and photocatalysis. Herein, CsPbBr3 NCs with different sizes are synthesized by varying the ligand contents of didodecyl dimethylammonium bromide at room temperature. Adding benzoquinone (BQ) molecules leads to a decrease in the PL intensities and PL decay times in NCs. The electron transfer (ET) efficiency (ηET) increases with NC size in complexes of CsPbBr3 NCs and BQ molecules (NC-BQ complexes), when the same concentration of BQ is maintained, as investigated by transient photobleaching and photoluminescence spectroscopies. Controlling the same number of attached BQ acceptor molecules per NC induces the same ηET in NC-BQ complexes even though with different NC sizes. Our findings provide new insights into ultrafast charge transfer behaviors in perovskite NCs, which is important for designing efficient light energy conversion devices.

Symmetry-Breaking Charge Separation in Molecular Constructs for Efficient Light Energy Conversion

The generation of electron–hole radical pair at the active layer of organic photovoltaics through symmetry-breaking charge separation (SB-CS) has a crucial role in enhancing open-circuit voltage (Voc) and thereby increasing power conversion efficiency. Since the SB-CS materials achieve intramolecular charge separation with a negligible energetic driving force and decelerated charge recombination (CR) rate, SB-CS has been subjected to extensive experimental and theoretical studies. This Focus Review assesses the fundamentals of photosynthetic reaction centers, especially the “special pair”, and discusses how covalent control over the geometric arrangement, surrounding dielectric medium, and substitutions on multichromophoric perylenediimide architecture affects the energy landscape of SB-CS and CR. We systematically summarize the kinetically favored undesirable radiative and non-radiative deactivation channels of SB-CS and CR processes on diverse chromophoric arrangements. Here, we suggest new rational design principles to fine-tune the electron-transfer dynamics at the molecular level to improve the performance of light–energy conversion devices.

Interplay between Light and Functionalized Silk Fibroin and Applications.

Silkworm silk has been considered to be a luxurious textile for more than five thousand years. Native silk fibroin (SF) films have excellent (ca. 90%) optical transparency and exhibit fluorescence under UV light. The silk dyeing process is very important and difficult, and methods such as pigmentary coloration and structural coloration have been tested for coloring silk fabrics. To functionalize silk that exhibits fluorescence, the in vivo and in vitro assembly of functional compounds with SF and the resulting amplification of fluorescence emission are examined. Finally, we discuss the applications of SF materials in basic optical elements, light energy conversion devices, photochemical reactions, sensing, and imaging. This review is expected to provide insight into the interaction between light and silk and to inspire researchers to develop silk materials with a consideration of history, material properties, and future prospects.

Plasmon-Assisted Polarity Switching of a Photoelectric Conversion Device by UV and Visible Light Irradiation

The plasmon-induced charge separation between metallic nanoparticles and a semiconductor following an electron transfer process has been extensively studied as one of the mechanisms in plasmonic light energy conversion devices. In this study, we propose that the switching of photocurrent polarity can be realized by changing the rectification properties of plasmonic photoelectric conversion devices and utilizing the difference in carrier mobility between electrons and holes. We fabricated plasmonic photoelectric conversion devices using gold nanoparticles (Au-NPs), nickel oxide (NiO), and mobility-limited TiO2 (ML-TiO2) to control the photocurrent polarity according to irradiation wavelengths of visible and UV light. A pulsed laser deposition technique was employed to deposit the ML-TiO2 and NiO layers. The photoelectric properties were measured, and in situ spectroelectrochemical measurements were performed to investigate the relationship between the rectification properties of the plasmonic photoelectric...

Three-Dimensional Hetero-Integration of Faceted GaN on Si Pillars for Efficient Light Energy Conversion Devices.

An important pathway for cost-effective light energy conversion devices, such as solar cells and light emitting diodes, is to integrate III-V (e.g., GaN) materials on Si substrates. Such integration first necessitates growth of high crystalline III-V materials on Si, which has been the focus of many studies. However, the integration also requires that the final III-V/Si structure has a high light energy conversion efficiency. To accomplish these twin goals, we use single-crystalline microsized Si pillars as a seed layer to first grow faceted Si structures, which are then used for the heteroepitaxial growth of faceted GaN films. These faceted GaN films on Si have high crystallinity, and their threading dislocation density is similar to that of GaN grown on sapphire. In addition, the final faceted GaN/Si structure has great light absorption and extraction characteristics, leading to improved performance for GaN-on-Si light energy conversion devices.

Carbon Nanotube-Quantum Dot Nanohybrids: Coupling with Single-Particle Control in Aqueous Solution.

A strategy is reported for the controlled assembly of organic-inorganic heterostructures consisting of individual single-walled carbon nanotubes (SWCNTs) selectively coupled to single semiconductor quantum dots (QDs). The assembly in aqueous solution was controlled towards the formation of monofunctionalized SWCNT-QD structures. Photoluminescence studies in solution, and on surfaces at the single nanohybrid level, showed evidence of electronic coupling between the two nanostructures. The ability to covalently couple heterostructures with single particle control is crucial for the design of novel QD-based optoelectronic and light-energy conversion devices.

α-Cyclodextrin Functionalized Carbon Dots: Pronounced Photoinduced Electron Transfer by Aggregated Nanostructures

In recent past assembly of α-cyclodextrin (α-CD) functionalized carbon dots (α-CD-CDots) has been used in molecular recognition. These assemblies are effectively used for optical sensing by the help of electron transfer mechanism. Photoinduced electron transfer (PET) between CDots and surface-encapsulated derivatives of methyl viologen (MV2+) is used in the present work to follow the effect of the aggregated nanostructures. Formation of the nanoaggregates was confirmed by fluorescence anisotropy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). These aggregated nanostructures are found to reinforce the electron transfer dynamics between CDots and MV2+. PET was confirmed from steady-state and time-resolved fluorescence along with ultrafast transient absorption measurements. The present work elaborates kinetic details of PET in the formed nanotubular aggregates. The reported concepts may be prospective toward development of light energy conversion devices.

Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad and Its Nanocomposite Form in Order to Develop Light Energy Conversion Devices.

UV-vis absorption, steady state and time resolved spectroscopic investigations in pico and nanosecond time domain were made in the different environments on a novel synthesized dyad, 3-(2-methoxynaphthalen-1-yl)-1-(4-methoxyphenyl)prop-2-en-1-one (MNTMA) in its pristine form and when combined with gold (Au) nanoparticles i.e., in its nanocomposite structure. Both steady state and time resolved measurements coupled with the DFT calculations performed by using Gaussian 03 suit of software operated in the linux operating system show that though the dyad exhibits mainly the folded conformation in the ground state but on photoexcitation the nanocomposite form of dyad prefers to be in elongated structure in the excited state indicating its photoswitchable nature. Due to the predominancy of elongated isomeric form of the dyad in the excited state in presence of Au Nps, it appears that the dyad MNTMA may behave as a good light energy converter specially in its nanocomposite form. As larger charge separation rate (kcs ~ 4 x 10(8) s-1) is found relative to the rate associated with the energy wasting charge recombination processes (kcR ~ 3 x 10(5) s-1) in the nanocomposite form of the dyad, it demonstrates the suitability of constructing the efficient light energy conversion devices with Au-dyad hybrid nanomaterials.

階層化高分子組織体を用いた電子機能材料

The most efficient electric devices in nature will be the light harvesting system in biological membrane. Photosensitive proteins complex, such as photosystem I (PS I), photosystem II (PS II), and electron transport proteins complex such as cytochrome were hierarchically assembled in the bilayer membrane based on their redox potentials. Electrons, which were produced by photoinduced electron transfer in PS II are unidirectionally transferred to cytochrome b6f then PS I following the potential gradient in the membrane. We can produce similar hierarchical molecular assemblies using the Langmuir-Blodgett (LB) technique. The unidirectional electron current flow has been achieved in hierarchical assembled LB films to produce efficient light-energy conversion devices. In this paper, we prepare hierarchical assembled polymer films using the LB technique. It has been reported that poly (N-dodecylacrylamide) forms a stable polymer monolayer “polymer nanosheets” at the air-water interface. Several electrofunctional polymer nanosheets were assembled in a tailor-made manner to achieve the unidirectional electron transfer. We applied the unidirectional electron transfer to produce other unique electrofunctional devices such as molecular memory and mutli-color electrochromism. The results indicate that we can produce unique electric devices by not just mimicking functions in nature but extracting the essence of their functions and applying them.

Decoration of CdS nanoparticles on MWCNT's by simple solution chemistry

Simple and inexpensive solution chemistry is applied for decoration/formation CdS nanoparticles on multiwall carbon nanotubes (MWCNT's). Simple ion-by-ion mechanism is utilized for the deposition of CdS nanoparticles on MWCNT's. The size of the nanoparticles is controlled by tuning the preparative parameters which is most favorably based on ion-by-ion mechanism. The use of alkaline medium is completely avoided. Complete decoration of CdS nanoparticles on MWCNT's with cubic structure is observed by structural and surface morphological studies. The photoluminescence spectroscopy shows the two intense luminescence bands, i.e. blue and green, evidencing the presence of small size and agglomerated big size particles of CdS over MWCNT's (size-effect). The modification of MWCNT's with CdS presented in this study opens up new avenues in the design of optoelectronic and light-energy conversion devices.

Control of Plasmonic Superradiance in Metallic Nanoparticle Assembly by Light-Induced Force and Fluctuations.

The possibility of simultaneous control of the configuration and optical functions of a metallic nanoparticle (NP) assembly by light-induced force (LIF) and thermal fluctuations has been demonstrated on the basis of self-consistent theory of LIF and nonequilibrium dynamics. It has been clarified that the NPs are arranged parallel to the polarization of the focused laser beam under the balance of LIF and the electrostatic repulsive force due to the ions on the surface of NPs. Particularly, in such a NP assembly consisting of high-density NPs, the light-scattering rate (radiative decay) of localized surface plasmon polaritons (LSPPs) can be drastically enhanced to be greater than 100 meV (10 times that of single NPs), and the spectral width is also greatly broadened due to the superradiance effect. The results will provide a foundation of the principles for designing a NP assembly with controllable light scattering for highly efficient broad-band light energy conversion devices.

The Rates of Charge Separation and Energy Destructive Charge Recombination Processes Within an Organic Dyad in Presence of Metal-Semiconductor Core Shell Nanocomposites

Steady state and time resolved spectroscopic measurements were made at the ambient temperature on an organic dyad, 1-(4-Chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone (MNCA), where the donor 1-methoxynaphthalene (1 MNT) is connected with the acceptor p-chloroacetophenone (PCA) by an unsaturated olefinic bond, in presence of Ag@TiO2 nanoparticles. Time resolved fluorescence and absorption measurements reveal that the rate parameters associated with charge separation, k(CS), within the dyad increases whereas charge recombination rate k(CR) reduces significantly when the surrounding medium is changed from only chloroform to mixture of chloroform and Ag@TiO2 (noble metal-semiconductor) nanocomposites. The observed results indicate that the dyad being combined with core-shell nanocomposites may form organic-inorganic nanocomposite system useful for developing light energy conversion devices. Use of metal-semiconductor nanoparticles may provide thus new ways to modulate charge recombination processes in light energy conversion devices. From comparison with the results obtained in our earlier investigations with only TiO2 nanoparticles, it is inferred that much improved version of light energy conversion device, where charge-separated species could be protected for longer period of time of the order of millisecond, could be designed by using metal-semiconductor core-shell nanocomposites rather than semiconductor nanoparticles only.

Photocatalytic Events of CdSe Quantum Dots in Confined Media. Electrodic Behavior of Coupled Platinum Nanoparticles

The electrodic behavior of platinum nanoparticles (2.8 nm diameter) and their role in influencing the photocatalytic behavior of CdSe quantum dots (3.4 nm diameter) has been evaluated by confining both nanoparticles together in heptane/dioctyl sulphosuccinate/water reverse micelles. The particles spontaneously couple together within the micelles via micellar exchange processes and thus facilitate experimental observation of electron transfer reactions inside the water pools. Electron transfer from CdSe to Pt is found to occur with a rate constant of 1.22 × 10(9) s(-1). With the use of methyl viologen (MV(2+)) as a probe molecule, the role of Pt in the photocatalytic process is established. Ultrafast oxidation of the photogenerated MV(+•) radicals indicates that Pt acts as an electron sink, scavenging electrons from MV(+•) with a rate constant of 3.1 × 10(9) s(-1). The electron transfer between MV(+•) and Pt, and a drastically lower yield of MV(+•) under steady state irradiation, confirms the ability of Pt nanoparticles to discharge electrons quickly. The kinetic details of photoinduced processes in CdSe-Pt assemblies and the electrodic behavior of Pt nanoparticles provide important information for the development of light energy conversion devices.

Nature of charge separation and recombination processes within an organic dyad having short spacer

With the help of electrochemical, steady state and time resolved fluorescence (fluorescence lifetimes by using time correlated single photon counting technique) and nanosecond laser flash photolysis methods, the nature of charge separation along with the energy wasting charge recombination processes within a short-chained organic dyad 1-(4-Bromo-phenyl)-3-(2-methoxy-naphthalen-1-yl)-propenone (MNBA) has been revealed. In MNBA , the donor 2-methoxynaphthalene (2MNT) is connected with the acceptor p-bromoacetophenone (PBA) by an unsaturated olefinic bond. Though in the ground state elongated type structure (E-form) is observable from NMR spectra but on photoexcitation, another conformers possibly of the nature of folded type isomeric species (termed as Z-isomer) were also apparent from time resolved fluorescence measurements. However, preponderance of elongated form in the excited singlet state has been established from this time resolved measurements. NMR study on photoirradiated sample and theoretical predictions from computations using CIS method with Lanl2DZ basis set also indicate in favor of the propositions made on the formations of the two possible conformers and the stability of elongated isomeric species in the electronic excited state from the experimental results. The energy wasting charge recombination rate, k CR , determined from the transient absorption measurements by nanosecond laser flash photolysis (LFP) technique was found to be ( k CR , ∼1.9×10 5 s −1) significantly lower than the charge-separation rate, k cs ∼9.4×10 7 s −1, measured from the time resolved fluorescence. This observation demonstrates that MNBA may serve as an efficient candidate to construct artificial light energy conversion devices or components of molecular photovoltaic cells.

Development of a Nanocomposite System by Combining an Organic Dyad 1-(4-chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-Propenone with Semiconductor TiO<SUB>2</SUB> Nanoparticles

Steady state and time resolved spectroscopic measurements on an organic dyad, 1-(4-chlorophenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone, where the donor 1-methoxynaphthalene is connected with the acceptor p-chloroacetophenone by an unsaturated olefinic bond, in presence of TiO2 nanoparticles were made at the ambient temperature. Time resolved fluorescence measurements reveal that the rate parameters associated with charge separation, kCS, within the dyad increases whereas charge recombination rate reduces when the surrounding medium is changed from chloroform to TiO2 nanoparticles. The observed results indicate that the dyad being combined with TiO2 nanoparticles may form organic-inorganic nanocomposite systems useful for developing light energy conversion devices.

Electron transfer in pristine and functionalised single-walled carbon nanotubes

Since their very first days, electron transfer has always played a special role in carbon nanotubes' life. In view of their structural and electronic uniqueness, carbon nanotubes have been proposed either as bulk electrode materials for sensing and biosensing in advanced electrochemical devices, or as molecular-sized electrodes for very fast electrode kinetics investigations. Alternatively, electron transfer has been used to probe the electronic properties of carbon nanotubes by either direct voltammetric inspection or coupling with spectroscopic techniques, ultimately allowing, in the case of true solutions of individual uncut single-walled carbon nanotubes (SWNTs), to single-out their redox potentials as a function of diameter. For their redox properties, as emerged from these studies, SWNTs represent unique building blocks for the construction of photofunctional nanosystems to be used in efficient light energy conversion devices.

Nanostructured thin films of C60-aniline dyad clusters: electrodeposition, charge separation, and photoelectrochemistry.

Clusters of C60-aniline dyads are deposited as thin films on nanostructured SnO2 electrodes under the influence of an electric field. At low applied DC voltage (<5 V) the clusters in toluene/acetonitrile (1:3) mixed solvent grow in size (from 160 nm to approximately 200 nm in diameter) while at higher voltages (>50 V) they are deposited on the electrode surface as thin films. The C60- aniline dyad cluster films when cast on nanostructured SnO2 films are photoelectrochemically active and generate photocurrent under visible light excitation. These nanostructured fullerene films are capable of delivering relatively large photocurrents (up to approximately 0.2 mA cm(-2), photoconversion efficiency of 3-4%) when employed as photoanodes in photoelectrochemical cells. Both luminescence and transient absorption studies confirm the formation of charge transfer product (C60 anion) following UV/Vis excitation of these films. Photo-induced charge separation in these dyad clusters is followed by the electron injection from C60-anion moiety into the SnO2 nanocrystallites. The oxidized counterpart is reduced by the redox couple present in the electrolyte, thus regenerating the dyad clusters. The feasibility of casting high surface area thin fullerene films on electrode surfaces has opened up new avenues to utilize dyad molecules of sensitizer bridge donor type in light energy conversion devices, such as solar cells.

Nanometric surface design of size‐quantized semiconductor microcrystals

Semiconductor microcrystals (Q‐particles) with diameters less that 100 Å, modified with viologen have recently been produced. One potential application for such particles would be for light energy conversion devices such as photocatalysts and photosenstizers in wet solar cells. The viologen moieties have the ability to function as electron mediators between the semiconductor particles and acceptors in solution. The preparation and potential applications of these ineresting materials are discussed.

Photosensitized electron injection in colloidal semiconductors

Photosensitized electron injection in semiconductor particles was studied by using aqueous eosin (EO)/colloidal TiO/sub 2/ as the model system. Adsorption of EO onto the surface of TiO/sub 2/ occurs quantitatively at pH <6 and leads to red-shifted absorption and emission spectra. Electron injection in the conduction band of the TiO/sub 2/ particles takes place from the S/sub 1/ state of adsorbed eosin and occurs with a rate constant of 8.5 x 10/sup 8/ s/sup -1/ and a quantum yield of 38% (pH 3). The quantum yield decreases at high eosin occupancy of the particles due to concentration quenching. Back-electron transfer occurs via a rapid intraparticle reaction between EO/sup +/ e/sub CB//sup -/ pairs associated with the same TiO/sub 2/ host aggregate and via a slower process involving bulk diffusion. The rate constant for intraparticle recombination is 2 x 10/sup 5/ s/sup -1/, i.e., 4 x 10/sup 3/ times slower than that for electron injection. This enables slight-induced charge separation to be sustained on a colloidal TiO/sub 2/ particle for several microseconds, which is sufficient to trap the electron by a noble metal deposit. Implications for light energy conversion devices are discussed.