Efficiencies of Electron Injection from Excited Sensitizer Dyes to Nanocrystalline ZnO Films as Studied by Near-IR Optical Absorption of Injected Electrons

Characterizing Interfacial Structures of Dye-Sensitized Solar Cell Working Electrodes

In this feature article, we discuss the fundamental use of materials-characterization methods that directly determine structural information on the dye···TiO2 interface in dye-sensitized solar cells (DSCs). This interface is usually buried within the DSC and submerged in solvent and electrolyte, which renders such metrological work nontrivial. We will show how ex-situ X-ray reflectometry (XRR), atomic-force microscopy (AFM), grazing-incidence X-ray scattering (GIXS), pair-distribution-function analysis of X-ray diffraction data (gaPDF), and in-situ neutron reflectometry (NR) can be used to deliver specific structural information on the dye···TiO2 interface regarding dye anchoring, dye aggregation, molecular dye orientation, intermolecular spacing between dye molecules, interactions between the dye molecules and the TiO2 surface, and interactions between the dye molecules and the electrolyte components and precursors. Some of these materials-characterization techniques have been developed specifically for this purpose. We will demonstrate how the direct acquisition of such information from materials-characterization experiments is crucial for assembling a holistic structural picture of this interface, which in turn can be used to develop DSC design guidelines. Moreover, we will show how these methodologies can be used in the experimental-validation process of "design-to-device" pipelines for big-data- and machine-learning-based materials discovery. We conclude with an outlook on further developments of this design-to-device approach as well as the materials characterization of more dye···TiO2 interfacial structures that involve known DSC dyes using the methods described herein. In addition, we propose to combine these formally disparate metrologies so that their complementary merits can be exploited simultaneously. New metrologies of this kind could serve as a "one-stop-shop" for the materials characterization of surfaces, interfaces, and bulk structures in DSCs and other devices with layered architectures.

Interfacial Charge-Transfer Transitions for Direct Charge-Separation Photovoltaics

Photoinduced charge separation (PCS) plays an essential role in various solar energy conversions such as photovoltaic conversion in solar cells. Usually, PCS in solar cells occurs stepwise via solar energy absorption by light absorbers (dyes, inorganic semiconductors, etc.) and the subsequent charge transfer at heterogeneous interfaces. Unfortunately, this two-step PCS occurs with a relatively large amount of the energy loss (at least ca. 0.3 eV). Hence, the exploration of a new PCS mechanism to minimize the energy loss is a high-priority subject to realize efficient solar energy conversion. Interfacial charge-transfer transitions (ICTTs) enable direct PCS at heterogeneous interfaces without energy loss, in principle. Recently, several progresses have been reported for ICTT at organic-inorganic semiconductor interfaces by our group. First of all, new organic-metal oxide complexes have been developed with various organic and metal-oxide semiconductors for ICTT. Through the vigorous material development and fundamental research of ICTT, we successfully demonstrated efficient photovoltaic conversion due to ICTT for the first time. In addition, we revealed that the efficient photoelectric conversion results from the suppression of charge recombination, providing a theoretical guiding principle to control the charge recombination rate in the ICTT system. These results open up a way to the development of ICTT-based photovoltaic cells. Moreover, we showed the important role of ICTT in the reported efficient dye-sensitized solar cells (DSSCs) with carboxy-anchor dyes, particularly, in the solar energy absorption in the near IR region. This result indicates that the combination of dye sensitization and ICTT would lead to the further enhancement of the power conversion efficiency of DSSC. In this feature article, we review the recent progresses of ICTT and its application in solar cells.

Effects of the terminal donor unit in dyes with D-D-π-A architecture on the regeneration mechanism in DSSCs: a computational study

This theoretical study on dye-sensitized solar cells (DSSCs) includes design strategies for dye donor units to improve the efficiency of DSSCs, and further illuminates the organic dye regeneration mechanism. We have designed a series of new organic sensitizers based on a D-D-π-A architecture to exhibit easy electron transfer and to have remarkable light harvesting properties in the visible region by density functional theory (DFT) and time-dependent (TD)-DFT calculations. Furthermore, the interaction of the organic sensitizers with the conventional redox electrolyte using the triiodide/iodide couple (I3-/I-) is investigated. Our calculations indicate that incorporation of strong electron-donating groups remarkably improves the charge transfer characteristics, optoelectronic properties and rapid dye regeneration as compared to less electron donating substituents. In addition, our study demonstrates the possibility of second electron injection from the oxidized dye complex to the semiconductor surface, which further confirms our recently proposed dye regeneration mechanism.

A strategy to minimize the energy offset in carrier injection from excited dyes to inorganic semiconductors for efficient dye-sensitized solar energy conversion

Photoinduced carrier injection from dyes to inorganic semiconductors is a crucial process in various dye-sensitized solar energy conversions such as photovoltaics and photocatalysis. It has been reported that an energy offset larger than 0.2-0.3 eV (threshold value) is required for efficient electron injection from excited dyes to metal-oxide semiconductors such as titanium dioxide (TiO2). Because the energy offset directly causes loss in the potential of injected electrons, it is a crucial issue to minimize the energy offset for efficient solar energy conversions. However, a fundamental understanding of the energy offset, especially the threshold value, has not been obtained yet. In this paper, we report the origin of the threshold value of the energy offset, solving the long-standing questions of why such a large energy offset is necessary for the electron injection and which factors govern the threshold value, and suggest a strategy to minimize the threshold value. The threshold value is determined by the sum of two reorganization energies in one-electron reduction of semiconductors and typically-used donor-acceptor (D-A) dyes. In fact, the estimated values (0.21-0.31 eV) for several D-A dyes are in good agreement with the threshold value, supporting our conclusion. In addition, our results reveal that the threshold value is possible to be reduced by enlarging the π-conjugated system of the acceptor moiety in dyes and enhancing its structural rigidity. Furthermore, we extend the analysis to hole injection from excited dyes to semiconductors. In this case, the threshold value is given by the sum of two reorganization energies in one-electron oxidation of semiconductors and D-A dyes.

Electron transport properties in dye-sensitized solar cells with {001} facet-dominant TiO2 nanoparticles

Dye-sensitized solar cells (DSSCs) with reactive {001} facet-dominant TiO₂ have attracted a great deal of attention owing to their high solar cell performance, despite the origin and the variation of the results being controversial. Here, we report the characteristic charge transport properties of DSSCs composed of {001} and {101} facet-dominant TiO₂ nanoparticles in order to explain the origin of solar cell performance. Based on transient photocurrent and photovoltage measurements and transient absorption spectroscopy, the energetics of TiO₂ semiconductors and dye sensitizers are utilized to understand the electron diffusion, recombination, and injection kinetics to determine solar cell performance. Novel strategies to improve DSSC performance by utilizing the characteristics of {001} facet-dominant TiO₂ nanoparticles are proposed, which are (1) enhancement of electron injection and (2) reduction of carrier recombination for J_SC and V_OC improvement, despite the drawback of slower electron diffusion in the mesoporous network of {001} facet-dominant TiO₂.

Probing single-molecule electron-hole transfer dynamics at a molecule-NiO semiconductor nanocrystalline interface

Interfacial charge transfer dynamics in dye-sensitized NiO nanoparticles are being investigated for photocathodes in p-type dye-sensitized solar cells. In the photoreaction, after fast electron transfer from NiO to a molecule, the recombination of the hole in the nanoparticles with the electron in a reduced molecule plays an important role in the charge separation process and solar energy harvesting. Nevertheless, knowledge of the interfacial charge recombination (CR) rate and its mechanism is still limited due to the complex photoinduced electron and hole dynamics and lack of characterization of the inhomogeneity of the dynamics. Here, we report our work on probing interfacial charge recombination dynamics in Zn(ii)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP) dye-sensitized NiO nanoparticles by correlating single-molecule fluorescence blinking dynamics with charge transfer dynamics using single-molecule photon-stamping spectroscopy. The correlated analyses of single-molecule fluorescence intensity, lifetime, and blinking reveal the intrinsic distribution and temporal fluctuation of interfacial charge transfer reactivity, which are closely related to site-specific molecular interactions and dynamics.

Zinc oxide nanocrystal quenching of emission from electron-rich ruthenium-bipyridine complexes

A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods. Each compound incorporated one electron withdrawing 4,4'-dicarboxylic acid-2,2'-bipyridine and two bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4' positions. The electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the amine substituent increased. Density functional calculations established that the HOMO was delocalized over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine. The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data, the excited state reduction potentials were estimated to range from -0.93 to -1.03 V vs. NHE. Monodispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs on the small end of the particle size distribution had electron transfer rates that were uncompetitive with radiative and nonradiative decay mechanisms.

First application of the HETPHEN concept to new heteroleptic bis(diimine) copper(I) complexes as sensitizers in dye sensitized solar cells

We report here the synthesis and full chemical and physical characterizations of the first stable heteroleptic copper(I)-bis(diimine) complexes designed for implementation in dye sensitized solar cells (DSC). Thanks to the HETPHEN concept, pure and stable heteroleptic copper(I) complexes were isolated. Anchorage of the sensitizers was provided by 2,2'-biquinoline-4,4'-dicarboxylic acid (dcbqH2), while sterically challenged ligands 2,9-dimesityl-1,10-phenanthroline (L0) and N-hexyl-2,9-dimesityl-1,10-phenanthroline-[a:b]imidazo-(4'-dianisylaminophenyl) (L1) were used to complete the copper(I) coordination sphere. The resulting heteroleptic complexes C1 and C2 exhibit a broad MLCT transition spreading over a wide wavelength domain, especially when adsorbed onto nanoparticulate TiO2 photoanodes, providing a rather comprehensive visible light collection. The corresponding DSC were evaluated under AM 1.5 simulated solar light and rather weak performances were obtained owing to small J(sc) and V(oc). This is due to a combination of low extinction coefficient and poor driving forces for the various interfacial processes. However, significant improvements in the performances were monitored upon ageing in the dark, likely due to beneficial reorganization of the dye monolayers. The possibility to isolate stable asymmetric systems paves the way for structurally assisted photo-induced charge injection from the chemisorbed copper(i) based sensitizers into the conduction band of TiO2, through charge vectorialization.

Effect of metal oxide morphology on electron injection from CdSe quantum dots to ZnO

Performance of quantum dot sensitized solar cells relies on a rapid electron injection from quantum dot to metal oxide. We studied the injection process in CdSe-ZnO system by ultrafast time-resolved absorption spectroscopy for two types of acceptor morphologies—nanowires and nanoparticles' films. Based on comparison between experimental data and Marcus theory, we demonstrate that the acceptor morphology has a significant impact on electron injection due to (i) change in material permittivity and (ii) different density of the band-edge states. The results open a reference to improve injection efficiency in quantum dot-metal oxide system by selection of the acceptor morphology.

Quantitative evaluation of electron injection efficiency in dye-sensitized TiO(2) films

The efficiency of electron injection (Φ(inj)) in dye-sensitized nanocrystalline films was studied through transient absorption (TA) and time-resolved microwave conductivity (TRMC) measurements. Here, I show the absolute value of Φ(inj) for several dye-sensitized nanocrystalline films and discuss the relationship between Φ(inj) and the free energy change (-ΔG (inj)) for the injection process. Some systems exhibited lower Φ(inj) values even when -ΔG (inj) was sufficiently large to promote electron injection. Recent experimental findings are used to propose possible explanations for this phenomenon. Quantitative evaluation of Φ(inj) using TA and TRMC will give us new insights for developing high-performance solar cell devices.

Determining the orientation and molecular packing of organic dyes on a TiO2 surface using X-ray reflectometry

The determination of the orientation and molecular density for several porphyrin dyes adsorbed on planar TiO(2) surfaces using X-ray reflectometry (XRR) is reported. Adsorption of nanoscale water layers occurred rapidly upon exposure of freshly prepared TiO(2) surfaces to ambient conditions; however, this was successfully eliminated, resulting in clearly discernible adsorbed dye layers for sensitized surfaces. Adsorbed dye orientations, determined from computations constrained by the measured dye layer thickness, were calculated to have a binding tilt angle of 35°-40°. Combining the XXR data with the orientation models indicates that the porphyrins form densely packed surfaces with an intermolecular spacing of 3-4 Å, consistent with π-π stacking interactions. Changes in the molecular size of probe dyes were reflected in corresponding changes in the measured dye layer thickness, confirming the ability of this technique to resolve small variations in dye layer thickness and consequently adsorption orientation. Application of these results to understanding the behavior of dye-sensitized devices is discussed.

High-efficiency, solid-state, dye-sensitized solar cells using hierarchically structured TiO₂ nanofibers

High-performance, room-temperature (RT), solid-state dye-sensitized solar cells (DSSCs) were fabricated using hierarchically structured TiO₂ nanofiber (HS-NF) electrodes and plastic crystal (PC)-based solid-state electrolytes. The electrospun HS-NF photoelectrodes possessed a unique morphology in which submicrometer-scale core fibers are interconnected and the nanorods are dendrited onto the fibers. This nanorod-in-nanofiber morphology yielded porosity at both the mesopore and macropore level. The macropores, steming from the interfiber space, afforded high pore volumes to facilitate the infiltration of the PC electrolytes, whereas the mesoporous nanorod dendrites offered high surface area for enhanced dye loading. The solid-state DSSCs using HS-NFs (DSSC-NF) demonstrated improved power conversion efficiency (PCE) compared to conventional TiO₂ nanoparticle (NP) based DSSCs (DSSC-NP). The improved performance (>2-fold) of the DSSC-NFs was due to the reduced internal series resistance (R(s)) and the enhanced charge recombination lifetime (τ(r)) determined by electrochemical impedance spectroscopy and intensity modulated photocurrent/photovoltage spectroscopy. The easy penetration of the PC electrolytes into HS-NF layers via the macropores reduces R(s) significantly, improving the fill factor (FF) of the resulting DSSC-NFs. The τ(r) difference between the DSSC-NF and DSSC-NP in the PC electrolytes was extraordinary (~14 times) compared to reported results in conventional organic liquid electrolytes. The optimized PCE of DSSC-NF using the PC electrolytes was 6.54, 7.69, and 7.93% at the light intensity of 100, 50, and 30 mW cm⁻², respectively, with increased charge collection efficiency (>40%). This is the best performing RT solid-state DSSC using a PC electrolyte. Considering the fact that most reported quasi-solid state or nonvolatile electrolytes require higher iodine contents for efficient ion transport, our HS-NFs are a promising morphology for such electrolytes that have limited ion mass transport.

Employing a Photosynthetic Antenna Complex to Interfacial Electron Transfer on ZnO Quantum Dot

Photosynthetic antenna complexes exhibit unidirectional energy-transport phenomena, which make them potential photosensitizers in interfacial electron-transfer processes. In the present study, we show the antenna function of phycocyanin-allophycocyanin (PC-APC) complex using transient emission and absorption spectroscopy. Interfacial electron-transfer dynamics in the PC-APC complex sensitized ZnO semiconductor quantum dot material is compared in native and denatured conditions. The downhill sequential energy transfer from a peripheral phycocyanin disk to a core allophycocyanin disk opens a new electron injection pathway from the allophycocyanin disk in addition to primary electron injection from directly photoexcited phycocyanin disk. Further, the large association of phycocayanobilin chromophores in PC-APC conjugates stabilizes the positive charge within the sensitizer, which leads to slower charge recombination in comparison to that in denatured condition. This study displays the antenna function of energy-efficient biomolecules in favor of better charge separation across the semiconductor interface.

Growth of Dumbbell-like ZnO Microcrystals under Mild Conditions and their Photoluminescence Properties

Large-scale uniform dumbbell-like ZnO microcrystals were successfully synthesized via a facile solution method under mild conditions. The as-prepared dumbbells, with lengths of 3.5-5.4 microm and diameters of 1.3-1.8 microm, possess a single-crystal hexagonal structure and grow along the [0001] direction. The influence of the reactant concentration on the size and shapes of the ZnO samples had been studied, and the results revealed that the reactant concentration plays a crucial role in determining final morphologies of the samples. Moreover, the evolution process of the dumbbell-like ZnO microcrystals was viewed by field-emission scanning electron microscopy (FE-SEM) characterization, and a possible formation mechanism was proposed. In addition, optical properties of the ZnO samples prepared at different reaction times were also investigated by photoluminescence (PL) spectroscopy. The room-temperature PL spectrum of the dumbbell-like ZnO microcrystals shows a strong UV emission peak. The UV emission is further identified to originate from the radiative free-exciton recombination by the temperature-dependent PL.

Morphology-Controlled Synthesis of Monodisperse ZnO Troughs at the Air−Water Interface under Mild Conditions

In this work, an interfacial synthetic strategy to produce ZnO troughs at the air-water interface under mild conditions was demonstrated. The as-prepared ZnO trough was monodisperse, single crystalline, and pure. By manipulation of the magnet stirring time, the fabricated ZnO can be manipulated as an independent or intertwined trough. Various preparative parameters, such as initial reactants and their concentrations, had been examined, and the results revealed that troughlike ZnO could be only obtained by adopting zinc acetate and hexamethylenetetramine as precursors at concentrations larger than 0.01 M. The as-obtained ZnO, within the dimension of several micrometers, showed a trough-related photoluminescence emission at 356 nm. Moreover, the evolution process of a ZnO trough after reaction for 5, 30, 60, and 90 min was viewed by X-ray diffraction and field emission scanning electron microscopy characterization, and a possible mechanism was also proposed and discussed, which might help us to further our understanding toward the reaction mechanism at the air-water interface.

Mechanisms of electron injection from retinoic acid and carotenoic acids to TiO2 nanoparticles and charge recombination via the T1 state as determined by subpicosecond to microsecond time-resolved absorption spectroscopy: dependence on the conjugation length

To examine the mechanisms of electron injection to TiO2 in retinoic acid (RA) and carotenoic acids (CAs), including RA5, CA6, CA7, CA8, CA9, and CA11 having the number of conjugated double bonds n = 5, 6, 7, 8, 9, and 11, respectively, their subpicosecond time-resolved absorption spectra were recorded free in solution and bound to TiO2 nanoparticles in suspension. The time-resolved spectra were analyzed by singular-value decomposition (SVD) followed by global fitting based on an energy diagram consisting of the 3A(g)(-), 1B(u)(-), 1B(u)(+), and 2A(g)(-) singlet excited states, whose energies had been determined as functions of 1/(2n + 1) by the use of carotenoids with n = 9-13. It was found that electron injection took place from both the 1B(u)(+) and 2A(g)(-) states in RA5, CA6, CA7, and CA8, whereas only from the 1B(u)(+) state in CA9 and CA11. The electron-injection efficiencies were determined, by the use of the relevant time constants determined by the SVD and global-fitting analyses, to be in the following order: RA5 approximately CA6 < CA7 > CA8 > CA9 > CA11. To determine the mechanism of charge recombination via the T(1) state, submicrosecond time-resolved absorption spectra of RA5, CA6, CA7, and CA8 bound to TiO2 nanoparticles in suspension were recorded. The SVD and global-fitting analyses lead us to a new scheme, which includes the formation of the D(0)(*+) - T(1) complex followed by transformation to both the D(0)(*+) and T(1) states. On the other hand, their one-electron oxidation potentials were determined, and their singlet and triplet levels were scaled to the conduction band edge (CBE) of TiO2. The T(1) level was lower than, but closest to, the CBE in RA5, and it became lower in the order RA5, CA6, CA7, and CA8. Consistent with the energy gap between the CBE and the T(1) levels, the generation of the T(1) state (or in other words, charge recombination) decreased in the order RA5 > CA6 > CA7 > CA8.

Electron Transfer Dynamics from Organic Adsorbate to a Semiconductor Surface: Zinc Phthalocyanine on TiO2(110)

The femtosecond time evolutions of excited states in zinc phthalocyanine (ZnPC) films and at the interface with TiO2(110) have been studied by using time-resolved two-photon photoelectron spectroscopy (TR-2PPE). The excited states are prepared in the first singlet excited state (S1) with excess vibrational energy. Two different films are examined: ultrathin (monolayer) and thick films of approximately 30 A in thickness. The decay behavior depends on the thickness of the film. In the case of the thick film, TR-2PPE spectra are dominated by the signals from ZnPC in the film. The excited states decay with tau = 118 fs mainly by intramolecular vibrational relaxation. After the excited states cascaded down to near the bottom of the S1 manifold, they decay slowly (tau = 56 ps) although the states are located at above the conduction band minimum of the bulk TiO2. The exciton migration in the thick film is the rate-determining step for the electron transfer from the film to the bulk TiO2. In the case of the ultrathin film, the contribution of electron transfer is more evident. The excited states decay faster than those in the thick film, because the electron transfer competes with the intramolecular relaxation processes. The electronic coupling with empty bands in the conduction band of TiO2 plays an important role in the electron transfer. The lower limit of the electron-transfer rate was estimated to be 1/296 fs(-1). After the excited states relax to the states whose energy is below the conduction band minimum of TiO2, they decay much more slowly because the electron-transfer channel is not available for these states.

Lithium Ion Effect on Electron Injection from a Photoexcited Coumarin Derivative into a TiO2 Nanocrystalline Film Investigated by Visible-to-IR Ultrafast Spectroscopy

The dynamics of ultrafast electron injection from a coumarin derivative (NKX-2311), which is an efficient photosensitizer for dye-sensitized solar cells, into the conduction band of TiO(2) nanocrystalline films have been investigated by means of femtosecond transient absorption spectroscopy in a wide wavelength range from 600 nm to 10 mum. In the absence of Li(+) ions, electron injection into the TiO(2) conduction band occurred in about 300 fs. In the presence of Li(+) ions, however, electron injection occurred within approximately 100 fs, and the oxidized dye generated was found to interact with nearby Li(+) ions. Possible positions of Li(+) ion attachment to the dye molecule were examined by means of semiempirical molecular orbital calculations. The electron injection efficiency was found to increase by a factor of 1.37 in the presence of Li(+) ions. The effects of Li(+) ions on the energy of the TiO(2) conduction band and the electronic interaction between the dye molecule and Li(+) ions are discussed, and the major cause for the acceleration of electron injection was suggested to be a conduction-band shift of TiO(2).

ULTRAFAST ELECTRON TRANSFER AT THE MOLECULE-SEMICONDUCTOR NANOPARTICLE INTERFACE

Electron transfer across the molecule-semiconductor interface is a fundamental process that is relevant to many applications of nanoparticles, such as dye-sensitized solar cells and molecular electronics. This review summarizes recent progress in understanding electron transfer dynamics from molecular adsorbates to semiconductor nanoparticles. Photoexcitation of molecular adsorbates to their excited states is followed by electron injection into semiconductor nanoparticles. The products of electron injection (oxidized adsorbate and electrons in semiconductor) are monitored by their electronic and vibrational spectra, allowing direct measurement of injection rate. The dependence of injection rate on the properties of semiconductor nanoparticle, molecular adsorbate, intervening bridging and anchoring group, and interfacial environment are discussed and compared with Marcus theory of interfacial electron transfer.

Hybrid Zinc Oxide Conjugated Polymer Bulk Heterojunction Solar Cells

Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and crystalline ZnO nanoparticles (nc-ZnO) as electron acceptor have been studied. Composite nc-ZnO:MDMO-PPV films were cast from a common solvent mixture. Time-resolved pump-probe spectroscopy revealed that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived (milliseconds) charge-separated state. The photovoltaic effect in devices, made by sandwiching the active nc-ZnO:MDMO-PPV layer between charge-selective electrodes, has been studied as a function of the ZnO concentration and the thickness of the layer. We also investigated changing the degree and type of mixing of the two components through the use of a surfactant for ZnO and by altering the size and shape of the nc-ZnO particles. Optimized devices have an estimated AM1.5 performance of 1.6% with incident photon to current conversion efficiencies up to 50%. Photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy have been used to gain insight in the morphology of these blends.

Ultrafast Electron Transfer between Molecule Adsorbate and Antimony Doped Tin Oxide (ATO) Nanoparticles

Ultrafast transient IR spectroscopy has been used to examine the effect of doping on interfacial electron transfer (ET) dynamics in Re(dpbpy)(CO)(3)Cl (dpbpy = 4,4'-(CH(2)PO(OH)(2))2-2,2'-bipyridine) (ReC1PO(3)) sensitized ATO (Sb:SnO(2)) nanocrystalline thin films. In films consisting of particles with 0%, 2% and 10% Sb dopant, the rates of electron injection from the adsorbate excited state to ATO were independent of and the rates of the recombination increased with the doping level. The observed similar forward electron injection rates were attributed to negligible changes of available accepting states in the conduction band at the doping levels studied. The dependence of the recombination rate on conduction band electron density and a possible mechanism for the recombination process were discussed.