Bismuth vanadate (BiVO₄) with a band gap of ∼2.4 eV has emerged as one of the visible photocatalysts that can absorb light below 520 nm. The electron/hole pairs that are generated following BiVO₄ band gap excitation are effective for water splitting, especially when BiVO₄ is combined with other metal oxides such as WO₃. We report a solution processed method for designing transparent WO₃/BiVO₄ heterojunction electrodes and observe a synergistic effect on the photoelectrochemical activity of WO₃/BiVO₄, with the combined system performing dramatically better than either individual component. Using ultrafast transient absorption spectroscopy, we elucidated the electronic interaction between WO₃ and excited BiVO₄. Moreover, the photocatalytic reduction of thionine by WO₃/BiVO₄ as well as by each individual oxide component is used to track electron injection processes and determine the energetics of the studied systems. In the composite WO₃/BiVO₄ film a shifted quasi-Fermi level results, due to electronic equilibration between the two materials. The better performance of WO₃/BiVO₄ heterojunction electrodes is thus a consequence of the electron injection from BiVO₄ into WO₃, followed by back electron transfer from WO₃ to the holes in BiVO₄.

The purpose of this Viewpoint is to dispel a commonly held misconception when comparing diffusion lengths and discuss how variation in the measuring techniques can bring about differences in the measured values. The diffusion length, Ld, of electrons or holes in a semiconductor is defined by the average distance the relevant charge moves in the semiconductor. It is influenced by the average distance the relevant charge moves in the semiconductor (for example, in photovoltaic cells, which is the topic of interest in this Viewpoint) and recombination/extraction from the semiconductor.

The optoelectronic properties of hybrid perovskites are a strong function of their physical structure, and understanding the fundamental steps involved in the formation of these films can aid in the optimization and rational design of devices with tailored properties. Here we investigate the structural and optical characteristics of CH₃NH₃PbI₃ films prepared from solutions composed of stoichiometric and nonstoichiometric quantities of lead iodide and methylammonium iodide precursors. In the presence of excess organohalide salt, a precursor phase composed of various iodoplumbate complexes is stabilized. The complexes dominate the optical properties of as-deposited films. Upon thermal treatment, the iodoplumbate precursor phase gradually evolves into the final tetragonal perovskite structure. Employing transient absorption spectroscopy, we have succeeded in tracking this transformation and gain insight into the interplay between the solid-state precursor and perovskite phases at various stages of formation. Correlation between time-resolved spectroscopic data and structural character can aid in better defining the structure–property relationship of hybrid perovskite thin films.

CdSe/CdS core/shell semiconductor nanorods (NR) with rod-in-rod morphology offer new strategies for designing highly emissive nanostructures. The interplay between energetically matched semiconductors results in enhanced emission from the CdSe core. In order to further evaluate the cooperative role of these two semiconductors in a core/shell geometry, we have probed the photoinduced charge transfer between CdSe/CdS core/shell semiconductor NR and methyl viologen (MV²⁺). The quenching of the emission by the electron acceptor, MV²⁺, as well as the production of electron transfer product MV^•+ depends on the aspect ratio (l/w) of the NR thus pointing out the role of CdS shell in determining the overall photocatalytic efficiency. Transient absorption measurements show that the presence of MV²⁺ influences only the bleaching recovery of the CdS shell and not of the CdSe core recovery. Thus, optimization of shell aspect ratio plays a crucial role in maximizing the efficiency of this photocatalytic system.

Charge carrier diffusion coefficient and length are important physical parameters for semiconducting materials. Long-range carrier diffusion in perovskite thin films has led to remarkable solar cell efficiencies; however, spatial and temporal mechanisms of charge transport remain unclear. Here we present a direct measurement of carrier transport in space and in time by mapping carrier density with simultaneous ultrafast time resolution and ~50-nm spatial precision in perovskite thin films using transient absorption microscopy. These results directly visualize long-range carrier transport of ~220 nm in 2 ns for solution-processed polycrystalline CH₃NH₃PbI₃, thin films. Variations of the carrier diffusion coefficient at the μm length scale have been observed with values ranging between 0.05 and 0.08 cm² s⁻¹. The spatially and temporally resolved measurements reported here underscore the importance of the local morphology and establish an important first step towards discerning the underlying transport properties of perovskite materials.

A need to understand the excited-state behavior of organic–inorganic hybrid perovskites, such as CH₃NH₃PbI₃, has arisen due to the rapid development of perovskite solar cells. The photoinduced processes leading to the efficient charge separation observed in these materials remain somewhat elusive. This Perspective presents an overview of the initial attempts to characterize the excited-state and charge recombination dynamics in the prototypical material CH₃NH₃PbI₃. While much has been accomplished in designing high-efficiency solar cells, the multifaceted nature of the CH₃NH₃PbI₃ excited state offers ample challenges for the photovoltaic community to better comprehend. Building on this foundation may enable us to tackle the stability concerns that have shadowed the rise of perovskite solar cells. Furthermore, a better understanding of the excited-state properties can provide insight into the specific properties that have thrust this material to the forefront of photovoltaic research.

When molecules or clusters are within the proximity of metal particles, their electronic transitions can be drastically enhanced. We have now probed the off-resonance excitation of molecule-like, glutathione-capped gold clusters (Au-GSH) in the close proximity of larger (plasmonic) Au and Ag nanoparticles. The excited state absorption spectrum of Au-GSH* is obtained with monophotonic excitation. The characteristic absorption of Au-GSH* allows us to probe the influence of excited plasmonic nanoparticles coupled with the clusters. Although infrared (775 nm) lasers pulses do not produce Au-GSH*, the excited states of these clusters are formed when coupled with metal (Au, Ag) nanoparticles. Interestingly, the coupled excitation of Au-GSH/AgNP with 775 nm laser pulses also results in an enhanced field effect, as seen from increased plasmon response of the metal nanoparticles. Transient absorption measurements confirm the synergy between these two inherently different nanomaterials, causing them to display greater excitation features. Better understanding of metal cluster–metal nanoparticle interactions will have important implications in designing light harvesting systems, and optoelectronic devices.

Perovskite solar cells employing hybrid organic–inorganic halide perovskites (e.g., CH₃NH₃PbI₃) have taken the photovoltaic community by storm. In the short time since being deemed its own class of emerging photovoltaic technologies by the National Renewable Energy Laboratory (October, 2013), the certified record efficiency of perovskite solar cells has increased nearly 50%, from 14.1 to 20.1% (http://www.nrel.gov/ncpv/). In addition, several groups have reported reproducible efficiencies in excess of 16%. These devices show great promise for commercial applications as they combine low-cost fabrication techniques with earth-abundant materials yet still deliver efficiencies rivaling traditional photovoltaic technologies. The possibility of using them in building facades or as a top cell in a tandem perovskite–Si architecture only increases their desirability. However, there is currently a dire need in the field for increased care on the part of authors in reporting their photovoltaic performance and on the side of reviewers and the scientific community at large in discriminating and evaluating reported results. Our hope is that this Viewpoint brings to light some of the issues pertaining to perovskite solar cells and provides the field with best practices for measuring and reporting perovskite solar cell performance.

Humidity has been an important factor, in both negative and positive ways, in the development of perovskite solar cells, and will prove critical in the push to commercialize this exciting new photovoltaic technology. The interaction between CH₃NH₃PbI₃ and H₂O vapor is investigated by characterizing the ground state and excited state optical absorption properties, and probing morphology and crystal structure. These systematic undertakings elucidate the complex interaction inherent in this system, demonstrating that H₂O exposure does not simply only CH₃NH₃PbI₃ to revert to PbI₂. It is shown that, in the dark, H₂O is able to complex with the perovskite, forming a hydrate product similar to (CH₃NH₃)₄PbI₆•2H₂O. This causes a decrease in absorption across the visible region of the spectrum and a distinct change in the crystal structure of the material. Femtosecond transient absorption spectroscopic measurements show the effect that humidity has on the ultrafast excited state dynamics of CH₃NH₃PbI₃. More importantly, the deleterious effects of humidity on complete solar cells, specifically on photovoltaic efficiency and stability, are explored in light of these spectroscopic understandings.

The quest for economic, large scale hydrogen production has motivated the search for new materials and device designs capable of splitting water using only energy from the sun. Here we introduce an all solution-processed tandem water splitting assembly composed of a BiVO₄ photoanode and a single-junction CH₃NH₃PbI₃ hybrid perovskite solar cell. This unique configuration allows efficient solar photon management, with the metal oxide photoanode selectively harvesting high energy visible photons and the underlying perovskite solar cell capturing lower energy visible-near IR wavelengths in a single-pass excitation. Operating without external bias under standard AM 1.5G illumination, the photoanode-photovoltaic architecture, in conjunction with an earth-abundant cobalt phosphate catalyst, exhibits a solar-to-hydrogen conversion efficiency of 2.5% at neutral pH. The design of low-cost tandem water splitting assemblies employing single-junction hybrid perovskite materials establishes a potentially promising new frontier for solar water splitting research.

Current interest in quantum dot solar cells (QDSCs) motivates an understanding of the electron transfer dynamics at the quantum dot (QD) – metal oxide (MO) interface. Employing transient absorption spectroscopy, we have monitored the electron transfer rate (k_et) at this interface as a function of the bridge molecules that link QDs to TiO₂. Using mercaptoacetic acid (MAA), 3-mercaptopropionic acid (3-MPA), 8-mercaptooctanoic acid (8-MOA) and 16-mercaptohexadecanoic acid (16-MHA) we observe an exponential attenuation of ket with increasing linker length, which has been attributed to the tunneling of the electron through the insulating linker molecule. We model the electron transfer reaction using both rectangular and trapezoidal barrier models that have been discussed in the literature. The one electron reduction potential (equivalent to the lowest unoccupied molecular orbital or LUMO) of each molecule as determined by cyclic voltammetry (CV) was used to estimate the effective barrier height presented by MAA, 3-MPA, 8-MOA and16-MHA at the CdSe-TiO₂ interface. The electron transfer rate (k_et) calculated for each CdSe-TiO₂ interface using both models showed the results in agreement with the experimentally determined trend. This demonstrates that electron transfer between CdSe and TiO₂ can be viewed as electron tunneling through a layer of organic linking molecules and provides a useful method for predicting electron transfer rate constants.

Glutathione-capped gold nanoclusters (Au_x-GSH NCs) are anchored along with a sensitizing squaraine dye on a TiO₂ surface to evaluate the cosensitizing role of Au_x-GSH NCs in dye-sensitized solar cells (DSSCs). Photoelectrochemical measurements show an increase in the photoconversion efficiency of DSSCs when both sensitizers are present. The observed photoelectrochemical improvements in cosensitized DSSCs are more than additive effects as evident from the increase in photovoltage (ΔV as high as 0.24 V) when Au_x-GSH NCs are present. Electron equilibration and accumulation within gold nanoclusters increase the quasi-Fermi level of TiO₂ closer to the conduction band and thus decrease the photovoltage penalty. A similar beneficial role of gold nanoclusters toward boosting the V_oc and enhancing the efficiency of Ru(II) polypyridyl complex-sensitized solar cells is also discussed.

TiO₂/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open-circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 ºC). Annealing in N₂ produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N₂-annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo-induced charge trapping, in particular at absorber grain boundaries of the air versus N₂-annealed systems, correlated with the formation of Cd–O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface-oxidation, which both impedes electron injection from the CdSe to the TiO₂, but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers.

The rapid increase in efficiency of methylammonium lead halide perovskite solar cells necessitates further investigation into the nature of perovskite absorption features and optical properties. Films obtained from the deposition of solutions containing lead halides and the CH₃NH₃⁺ organic cation is known to yield the CH₃NH₃PbI₃ perovskite structure upon annealing. In examining the precursor solution used in the processing of CH₃NH₃PbI₃ solar cells, we find that Pb₂⁺ readily forms plumbate complexes in the presence of excess iodide ions and exhibits characteristic absorption bands at 370 (PbI₃^-) and 425 nm (PbI₄^2-). Through comparative spectral analysis of the absorption features of charge transfer complexes in the solution phase and the final solid-state perovskite films, we are able to fully classify the absorption features in the excited state of CH₃NH₃PbI₃ across the transient absorption spectrum recorded following laser pulse excitation. In particular, we attribute the broad photoinduced absorption to a charge-transfer excited state, and show correlation between the photoinduced absorption and 480 nm bleach signals. These observations lead us to propose a band structure composed of two distinct transitions that is consistent with the various spectral features and kinetic behavior of the CH₃NH₃PbI₃ excited state. Characterization of this unique dual excited state nature provides further insight into the optoelectronic behavior of hybrid lead halide perovskite films and thus aids in elucidating their exceptional photovoltaic properties.

The optical and electronic properties of quantum dots (QDs) which are drastically affected by their size have major impact on their performance in devices like solar cells. We now report the size dependent solar cell performance for CuInS₂ QDs capped with 1-dodecanethiol. Pyramidal shaped CuInS₂ QDs with diameter between 2.9 nm and 5.3 nm have been synthesized and assembled on mesoscopic TiO₂ films by electrophoretic deposition. Time resolved emission and transient absorption spectroscopy measurements have ascertained the role of internal and surface defects in determining the solar cell performance. An increase in power conversion efficiency (PCE) was observed with increasing size of QDs, with maximum values of 2.14 and 2.51% for 3.9 and 4.3 nm size particles, respectively. The drop in PCE observed for larger QDs (5.3 nm) is attributed to decreased charge separation following bandgap excitation. Since the origin of photocurrent generation in CuInS₂ QDSC arises from the defect dominated charge carriers it offers the opportunity to further improve the efficiency by controlling these defect concentrations.

Electrocatalytic activity of mesoscopic TiO₂ films towards the reduction of CO₂ is probed by depositing a nanostructured film on a glassy carbon electrode. The one-electron reduction of CO₂ in acetonitrile seen at an onset potential of -1.1 V (vs. NHE) is ~0.5 V lower than the one observed with a glassy carbon electrode. The electrocatalytic role of TiO₂ is elucidated through spectroelectrochemistry and product analysis. Ti³⁺ species formed when TiO₂ film is subjected to negative potentials have been identified as active reduction sites. Binding of CO₂ to catalytically active Ti³⁺ followed by the electron transfer facilitates the initial one-electron reduction process. Methanol was the primary product when the reduction was carried out in wet acetonitrile.

The unique and promising properties of semiconducting organometal halide perovskites have brought these materials to the forefront of solar energy research. Here, we present new insights into the excited-state properties of CH_3>NH₃PbI₃ thin films through femtosecond transient absorption spectroscopy measurements. The photoinduced bleach recovery at 760 nm reveals that band-edge recombination follows second-order kinetics, indicating that the dominant relaxation pathway is via recombination of free electrons and holes. Additionally, charge accumulation in the perovskite films leads to an increase in the intrinsic bandgap that follows the Burstein–Moss band filling model. Both the recombination mechanism and the band-edge shift are studied as a function of the photogenerated carrier density and serve to elucidate the behaviour of charge carriers in hybrid perovskites. These results offer insights into the intrinsic photophysics of semiconducting organometal halide perovskites with direct implications for photovoltaic and optoelectronic applications.

The recent thrust in utilizing reduced graphene oxide (RGO) as a support for nanostructured catalyst particles has led to the claims of improved efficiency in solar cells, fuel cells, and photocatalytic degradation of pollutants. Specifically, the robust TiO₂ system is often coupled with RGO to improve charge separation and facilitate redox reactions. Here we probe the stability of RGO in the presence of UV-excited TiO₂ in aqueous media and establish its reactivity towards OH^• radicals, a primary oxidant generated at the TiO₂ surface. By probing changes in absorption, morphology and total organic carbon content (TOC) we conclusively demonstrate the vulnerability of RGO towards OH^• attack and raise the concern of its use in many applications where OH^• are likely to be formed. On the other hand, the OH^• radical-mediated mineralization could also enable new approaches in tackling environmental remediation of nanocarbons such as RGO, carbon nanotubes, and fullerenes.

Glutathione protected gold clusters exhibit size dependent excited state and electron transfer properties. Larger size clusters (e.g., Au₂₅GSH₁₈) with core-metal atoms display rapid (<1 ps) as well as slower relaxation (~200 ns) while homoleptic clusters (e.g., Au_10-12GSH_10-12) exhibit only slower relaxation. These decay components have been identified as metal-metal transition and ligand-to-metal charge transfer respectively. The short lifetime relaxation component becomes less dominant as the size of the gold cluster decreases. The long-lived excited state and ability to participate in electron transfer are integral for these clusters to serve as light harvesting antennae. A strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity was evidenced from the electron transfer to methyl viologen. The photoactivity of these metal clusters show increasing photocatalytic reduction yield (0.05 - 0.14) with decreasing cluster size, Au₂₅ < Au₁₈ < Au₁₅ < Au_10-12. Gold clusters, Au₁₈GSH₁₄, were found to have the highest potential as a photosensitizer based on the quantum yield of electron transfer and good visible light absorption properties.

Energy transfer coupled with electron transfer is a convenient approach to mimic photosynthesis in light energy conversion. Better understanding of mechanistic details of energy transfer processes is important to enhance the performance of dye or quantum dot sensitized solar cells. Energy transfer through both long range dipole based Förster Resonance Energy Transfer (FRET), and short range Dexter Energy Transfer (DET) mechanisms have been identified to occur between CdSe quantum dots (QDs) linked to a red-infrared absorbing squaraine dye through a short thiol functional group (SQSH). Solutions of SQSH linked to CdSe were investigated through steady-state and time resolved spectroscopy experiments to explore both mechanisms. Photoluminescence studies revealed that smaller QDs had higher energy transfer efficiencies than predicted by FRET, and femtosecond transient absorption experiments revealed faster energy transfer rates in smaller donor QD sizes. These findings supported A DET process dominating at small donor sizes. The presence of both processes illustrates multiple strategies for utilizing energy transfer in light harvesting assemblies and the required considerations in device design to maximize energy transfer gains through either mechanism.

Smart material nanoassemblies that can simultaneously sense and shoot low level contaminants from air and water are important for overcoming the threat of hazardous chemicals. Graphene oxide (GO) sheets deposited on mesoscopic TiO₂ films that underpin the deposition of Ag nanoparticles with UV-irradiation provide the foundation for the design of a smart material. The Ag particle size is readily controlled through precursor concentration and UV irradiation time. These semiconductor – graphene oxide – metal (SGM) films are SERS active and hence capable of sensing aromatic contaminants such as nitrobenzenethiol (NBT) in nanomolar range. Increased local concentration of organic molecule achieved through interaction with 2D carbon support (GO) facilitates low-level detection of contaminants. Upon UV irradiation of the NBT loaded SGM film, one can induce photocatalytic transformations. Thus, each component of the SGM film plays a pivotal role in aiding the detection and degradation of a contaminant dispersed in aqueous solutions. The advantage of using SGM films as multipurpose "detect and destroy" systems for nitroaromatic molecule is discussed.

The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]) offers new ways to modulate the electrochemical reduction of carbon dioxide. [emim][Tf₂N], when present as the supporting electrolyte in acetonitrile, decreases the reduction overpotential at a Pb electrode by 0.18 V as compared to tetraethylammonium perchlorate as the supporting electrolyte. More interestingly, the ionic liquid shifts the reaction course during the electrochemical reduction of carbon dioxide by promoting the formation of carbon monoxide instead of oxalate anion. With increasing concentration of [emim][Tf₂N], a carboxylate species with reduced CO₂ covalently bonded to the imidazolium ring is formed along with carbon monoxide. The results highlight the catalytic effects of the medium in modulating the CO₂ reduction products.

Successive Ionic Layer Adsorption and Reaction (SILAR) is a popular method of depositing the metal chalcogenide semiconductor layer on the mesoscopic metal oxide films for designing quantum dot sensitized solar cell (QDSSC) or Extremely Thin Absorber (ETA) solar cells. While this deposition method exhibits higher loading of light absorbing semiconductor layer than direct adsorption of pre-synthesized colloidal quantum dots, the chemical identity of these nanostructures and evolution of interfacial structure are poorly understood. We have now analyzed step-by-step SILAR deposition of CdSe films on mesoscopic TiO₂ nanoparticle films using x-ray absorption near-edge structure analysis and probed the interfacial structure of these films. The film characteristics interestingly show dependence on the order in which the Cd and Se are deposited, and the CdSe-TiO₂ interface is affected only during the first few cycles of deposition. Development of SeO₂ passivation layer in the SILAR-prepared films to form TiO₂/SeO₂/CdSe junction facilitates an increase in photocurrents and power conversion efficiencies of quantum dot solar cells when these films were integrated as photoanodes in a photoelectrochemical solar cell.

Glutathione capped metal nanoclusters (Aux-GSH NCs) which exhibit molecular like properties are employed as a photosen-sitizer for hydrogen generation in a photoelectrochemical cell (PEC) and a photocatalytic slurry reactor. The reversible re-duction (E0= -0.63 V vs. RHE) and oxidation (E0= 0.97 V and 1.51 V vs. RHE) potentials of these metal nanoclusters make them suita-ble for driving the water splitting reaction. When a mesoscopic TiO₂ film sensitized by Au_x-GSH NCs was used as the pho-toanode with a Pt counter electrode in aqueous buffer solution (pH = 7), we observe significant photocurrent activity under visible light (400 - 500 nm) excitation. Additionally, sensitizing Pt/TiO₂ nanoparticles with Au_x-GSH NCs in an aqueous slurry system and irradiating with visible light produced H₂ at a rate of 0.3 mmole of hydrogen/hr/gram of Au_x-GSH NCs. The rate of H₂ evolution was significantly enhanced (~5 times) when a sacrificial donor, such as EDTA, was introduced into the sys-tem. Using metal nanoclusters as a photosensitizer for hydrogen generation lays the foundation for the future exploration of other metal nanoclusters with well controlled numbers of metal atoms and capping ligands.

Excited state interaction between CdSe quantum dots (QD) of different sizes (2.3, 3.2 and 4.2 nm diameter) and graphene oxide (GO) has been probed by depositing them as films on conducting glass electrodes. The emission of smaller CdSe QDs (2.3 nm) is quenched by GO three times faster than that of larger QDs (4.2 nm). Electrophoretic deposition method has allowed us to sequentially deposit single or multiple layers of different sized QDs and GO assemblies on conducting glass electrode and achieve the modulation of photoresponse in photoelectrochemical solar cells. Superior photoconversion efficiency through the incorporation of GO is attributed to the improved charge separation in the composite assembly.

Semiconductor nanostructures are attractive for designing low cost solar cells with tunable photoresponse. The recent advances in size and shape selective synthesis have enabled the design of quantum dot solar cells with photoconversion efficiencies greater than 5%. In order to make them competitive with other existing thin film or polycrystalline photovoltaic technologies, it is important to overcome kinetic barriers for charge transfer at semiconductor interfaces. This feature focuses on the limitations imposed by slow hole transfer in improving solar cell performance and its role in the stability of metal chalcogenide solar cells. Strategies to improve the rate of hole transfer through surface modified redox relays offer new opportunities to overcome the hole transfer limitation. The mechanistic and kinetic aspects of hole transfer in Quantum Dot Solar Cells (QDSCs), Nanowire Solar Cells (NWSCs) and Extremely Thin Absorber (ETA) solar cells are discussed.

CdS, CdSe and ternary CdSe_xS_(1-x) are some of the most widely studied II-VI semiconductors due to their wide range of applications and promising performance in numerous systems. One-dimensional semiconductor nanowires offer the ability to conduct charges efficiently along the length of the wire which has potential charge transport benefits compared to nanoparticles. Herein, we report a simple, inexpensive synthetic procedure for high quality CdSeS nanowires where the composition can be easily modulated from pure CdSe to pure CdS by simply adjusting the Se:S precursor ratio. This allows for tuning of the absorption and emission properties of the nanowires across the visible spectrum. The CdSeS nanowires have a wurtzite crystal structure and grow along the [001] direction. As measured by femtosecond transient absorption spectroscopy, the short component of the excited state lifetime remains relatively constant at ~10 ps with increasing Se; however the contribution of this short lifetime component increased dramatically from 8.4% to 57.7% with increasing Se content. These CdSeS nanowires offer facile synthesis and widely adjustable optical properties, characteristics which give them broad potential applications in the fields of optoelectronics, and photovoltaics.

Cycloparaphenylene molecules, commonly known as 'carbon nanohoops,' have the potential to serve as building blocks in constructing carbon nanotube architectures. The singlet and triplet excited state characteristics of [9]-cycloparaphenylene ([9]CPP) and [12]-cycloparaphenylene ([12]CPP) have now been elucidated using time resolved transient absorption and emission techniques. The fluorescence quantum yields (Φ) of [9]CPP and [12]CPP were determined to be 0.46 and 0.83 respectively. Rates of non-radiative recombination (k_nr), radiative recombination (k_r) and intersystem crossing (kisc) determined in this study indicate that radiative decay dominates in these nanohoop structures. The triplet quantum yields determined through energy transfer with excited biphenyl triplet were 0.18 and 0.13 for [9]CPP and [12]CPP respectively. The rate of triplet state quenching by oxygen was measured to be 1.7 × 10³ s^-1 ([9]CPP) and 1.9 × 10³ s^-1 ([12]CPP). The excited state dynamics established in this study enable us to understand the behavior of a carbon nanotube-like structure on a single subunit level.

Hole transfer to dissolved sulfide species in liquid junction CdSe quantum dot sensitized solar cells is relatively slow when compared to electron transfer from CdSe to TiO₂. Controlled exposure of cadmium chalcogenide surfaces to copper ions followed by immersion in sulfide solution promotes development of interfacial CuxS layer, which mediates hole transfer to polysulfide electrolyte by collection of photogenerated holes from CdSe. In addition, Cu_xS was also found to interact directly with defect states on the CdSe surface and quench emission characteristic of electron traps resulting from selenide vacancies. Together these effects were found to work in tandem to deliver 6.6% power conversion efficiency using Mn-doped CdS and CdSe co-sensitized quantum dot solar cell. Development of n-p interfacial junction at the photoanode-electrolyte interface in quantum dot solar cells unveils new means for designing high efficiency liquid junction solar cells.

Quantum dot (QD) surface chemistry is an emerging field in semiconductor nanocrystal related research. Along with size manipulation, the careful control of QD surface chemistry allows modulation of the optical properties of a QD suspension. Even a single molecule bound to the surface can introduce new functionalities. Herein, we summarize the recent advances in QD surface chemistry and the resulting effects on optical and electronic properties. Specifically, the feature article focuses addresses three main issues: (i) How surface chemistry affects the optical properties of QDs, (ii) How it influences the excited state dynamics and (iii) How one can manipulate surface chemistry to control the interactions between QDs and metal oxides, metal nanoparticles and in self-assembled QD monolayers.

This work reports on the use of an internal electrostatic field to facilitate charge separation at inorganic-organic interfaces, analogous to those in hybrid solar cells. Systematic charge transfer studies show that the donor-acceptor charge transfer rate is highly sensitive to the direction of the internal electric field.

Transfer of photogenerated holes from the absorber species to the p-type hole conductor is fundamental to the performance of solid-state sensitized solar cells. In this study, we comprehensively investigate hole diffusion in the Sb₂S₃ absorber and hole transfer across the Sb₂S₃–CuSCN interface in the TiO₂/Sb₂S₃/CuSCN system using femtosecond transient absorption spectroscopy, carrier diffusion modeling, and photovoltaic performance studies. Transfer of photogenerated holes from Sb₂S₃ to CuSCN is found to be dependent on Sb₂S₃ film thickness, a trend attributed to diffusion in the Sb₂S₃ absorber. However, modeling reveals that this process is not adequately described by diffusion limitations alone as has been assumed in similar systems. Therefore, both diffusion and transfer across the Sb₂S₃–CuSCN interface are taken into account to describe the hole transfer dynamics. Modeling of diffusion and interfacial hole transfer effects reveal that interfacial hole transfer, not diffusion, is the predominate factor dictating the magnitude of the hole transfer rate, especially in thin (< 20 nm) Sb₂S₃ films. Lastly, the implications of these results are further explored by photovoltaic measurements using planar TiO₂/Sb₂S₃/CuSCN solar cells to elucidate the role of hole transfer in photovoltaic performance.

The hydroxyl radical (•OH) is one of the main oxidative species in aqueous phase advanced oxidation processes, and its initial reactions with organic pollutants are important to understand the transformation and fate of organics in water environments. Insights into the kinetics and mechanism of •OH mediated degradation of the model environmental endocrine disruptor, dimethyl phthalate (DMP), have been obtained using radiolysis experiments and computational methods. The bimolecular rate constant for the •OH reaction with DMP was determined to be (3.2 ± 0.1) × 109 M^–1s^–1. The possible reaction mechanisms of radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) were considered. By comparing the experimental absorption spectra with the computational results, it was concluded that the RAF and HAT were the dominant reaction pathways, and OH-adducts (•DMPOH₁, •DMPOH₂) and methyl type radicals •DMP(-H)α were identified as dominated intermediates. Computational results confirmed the identification of transient species with maximum absorption around 260 nm as •DMPOH₁ and •DMP(-H)α, and these radical intermediates then converted to monohydroxylated dimethyl phthalates and monomethyl phthalates. Experimental and computational analyses which elucidated the mechanism of •OH-mediated degradation of DMP are discussed in detail.

The excited state behavior of luminescent gold clusters provide new insights in understanding their photocatalytic activity in the visible region. The excited state of glutathione protected gold nanoclusters (AuGSH) which is characterized by the long-lived excited state (τ = 780 ns) arises from the ligand-to-metal type transition. These AuGSH clusters are in a partially oxidized state, (Au(I))and are readily reduced by chemical or electrochemical methods. Interestingly a metal core transition with short lived lifetime (τ <3 ps) appears along with a longer lifetime in reduced AuGSH clusters. The role of oxidation state of gold clusters in dictating the photocatalytic reduction of methyl viologen is discussed.

Organo-lead halide perovskite solar cells have emerged as one of the most promising candidates for the next generation of solar cells. To date, these perovskite thin film solar cells have exclusively employed organic hole conducting polymers which are often expensive and have low hole mobility. In a quest to explore new inorganic hole conducting materials for these perovskite based thin film photovoltaics, we have identified copper iodide as a possible alternative. Using copper iodide, we have succeeded in achieving a promising power conversion efficiency of 6.0% with excellent photocurrent stability. The open-circuit voltage, compared to the best spiro-OMeTAD devices, remains low and is attributed to higher recombination in CuI devices as determined by impedance spectroscopy. However, impedance spectroscopy revealed that CuI exhibits two orders of magnitude higher electrical conductivity than spiro-OMeTAD which allows for significantly higher fill factors. Reducing the recombination in these devices could render CuI as a cost effective competitor to spiro-OMeTAD in perovskite solar cells.

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 observed heterogeneous reduction kinetics provides mechanistic insight into GO's conversion to chemically derived graphene and highlights opportunities for overcoming its dynamic, chemical disorder.

The power conversion efficiency of semiconductor nanowire (NW) based solar cells as compared to quantum dot solar cell (QDSC) has remained lower, and efforts to improve the photovoltaic performance of semiconductor NWs continue. We have now succeeded in using a layered architecture of CdS and CdSe NWs for improving the photovoltaic performance of nanowire solar cell (NWSC). The photoanode designed with sequentially deposited films of CdSe and CdS NWs delivered a power conversion efficiency of 1%. This efficiency of CdSe/CdS composite is an order of magnitude improvement over single nanowire system (CdS or CdSe) based solar cell. The improvement seen in the CdSe/CdS composite film is attributed to charge rectification and improvement of electron and hole separation and transport in the opposite direction. Impedance spectroscopy demonstrates the beneficial effect of type II structure in CdSe/CdS sequential deposition through lower transport resistance, which remains a dominating effect in dictating the overall performance of the NWSC.

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 into the multifaceted role of squaraine dye in improving the performance of NWSCs.

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 Ni_xMn_(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.

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, providing direct evidence for the hole transfer dynamics in ETA solar cells. Elucidation of the hole transfer mechanism from Sb₂S₃ to CuSCN represents a significant step toward understanding charge separation in Sb₂S₃ solar cells, and provides insight into the design of new architectures for higher efficiency devices.

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 dependent on the method of QD deposition: CdSe-TiO₂ (direct deposition, k_et = 1.5 × 10^10 s^-1), CdSe-linker-TiO₂ (attaching linker molecule first to TiO₂ so that β-Ala interaction is minimal, k_et = 2.4 × 10^9 s^-1) or linker-CdSe-linker-TiO₂ (linkage via full β-Ala encapsulation in solution prior to deposition, k_et = 6.4 × 10^8 s^-1). These results imply that the surface chemistry of colloidal CdSe plays an important role in mediating electron transfer reactions.

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.

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, Li-ion batteries, etc.). Understanding RGO oxidation by free radicals will aid in maintaining the long-term stability of RGO-based functional composites where intimate contact with radical species is inevitable.

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.

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.

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.

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.

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.

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 TiO₂ 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.

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 nano-particles and facilitate chemical transformations. Coupled with RGO's unique ability to capture and transport electrons, galvanic exchange is used to contrive a two-dimensional nano catalyst 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.

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 selectivity. This progress represents the beginning of a transformative movement leveraging the advancements of single-purpose chemistry toward the creation of composites designed to address whole-process applications.

The promising field of graphene nanocomposites for sensing and energy applications is based on fundamental studies that explain the electronic interactions between semiconductor or metal nanoparticles and graphene. In particular, reduced graphene oxide is a suitable composite substrate because of its two-dimensional structure, outstanding surface area, and electrical conductivity. In this Account, we describe common assembly methods for graphene composite materials and examine key studies that characterize its excited state interactions. We also discuss strategies to develop graphene composites and control electron capture and transport through the 2D carbon network. In addition, we provide a brief overview of advances in sensing, energy conversion, and storage applications that incorporate graphene-based composites. With these results in mind, we can envision a new class of semiconductor– or metal–graphene composites sensibly tailored to address the pressing need for advanced energy conversion and storage devices.

Graphene oxide (GO) serves as a two-dimensional carbon nano-mat to anchor catalyst nanoparticles. We have developed a photocatalyst assembly by anchoring ZnO and Ag nanoparticles on graphene oxide sheets suspended in ethanol. Upon photoirradiation, the electrons are transferred from ZnO to GO to produce reduced graphene oxide (RGO). The ZnO–RGO composites are further decorated with Ag nanoparticles by reducing Ag+ ions quantitatively with excess electrons stored in RGO. Under continuous UV-illumination we observe charging of ZnO nanoparticles as evidenced by the shift in absorption edge. However, no shift in the band edge is seen for ZnO–RGO or ZnO–RGO–Ag composites under UV irradiation indicating the quick discharge of electrons on RGO surface. Such charge–discharge phenomenon on the graphene oxide sheet was further probed by carrying out reduction of methyl viologen. Improved charge separation and selectivity in the reduction process was achieved in these graphene based photocatalytic assemblies.

Silver nanoclusters complexed with dihydrolipoic acid (DHLA) exhibit molecular-like excited-state properties with well-defined absorption and emission features. The 1.8 nm diameter Ag nanoparticles capped with Ag₈ clusters exhibit fluorescence maximum at 660 nm with a quantum yield of 0.07%. Although the excited state is relatively short-lived (τ 130 ps), it exhibits significant photochemical reactivity. By introducing MV²⁺ as a probe, we have succeeded in elucidating the interfacial electron transfer dynamics of Ag nanoclusters. The formation of MV^+• as the electron-transfer product with a rate constant of 2.74 × 1010 s^–1 confirms the ability of these metal clusters to participate in the photocatalytic reduction process. Basic understanding of excited-state processes in fluorescent metal clusters paves the way toward the development of biological probes, sensors, and catalysts in energy conversion devices.

Few-layered reduced graphene oxide–Pt composites are prepared using a combination of two ultrasound frequencies at 20 kHz and 211 kHz. Such a unique dual frequency arrangement operating in tandem, yields large exfoliated graphene sheets with platinum nanoparticles dispersed on them. The extent of reduction achieved by the use of this dual frequency sonication arrangement is evaluated by XPS, IR, and Raman spectroscopies. Transmission electron and atomic force microscopies confirm the morphology of resulting assemblies to be bi- and single layered sheets. These composites show good electrocatalytic activity towards methanol oxidation.

With increased interest in semiconductor nanoparticles for use in quantum dot solar cells there comes a need to understand the long-term photostability of such materials. Colloidal CdSe quantum dots (QDs) were suspended in toluene and stored in combinations of light/dark and N₂/O₂ to simulate four possible benchtop storage environments. CdSe QDs stored in a dark, oxygen-free environment were observed to better retain their optical properties over the course of 90 days. The excited state lifetimes, determined through femtosecond transient absorption spectroscopy, of air-equilibrated samples exposed to light exhibit a decrease in average lifetime (0.81 ns) when compared to samples stored in a nitrogen/dark environment (8.3 ns). A photoetching technique commonly used for controlled reduction of QD size was found to induce energetic trap states to CdSe QDs and accelerate the rate of electron–hole recombination. X-ray absorption near edge structure (XANES) analysis confirms surface oxidation, the extent of which is shown to be dependent on the thickness of the ligand shell.

With increased interest in semiconductor nanoparticles for use in quantum dot solar cells there comes a need to understand the long-term photostability of such materials. Colloidal CdSe quantum dots (QDs) were suspended in toluene and stored in combinations of light/dark and N2/O2 to simulate four possible benchtop storage environments. CdSe QDs stored in a dark, oxygen-free environment were observed to better retain their optical properties over the course of 90 days. The excited state lifetimes, determined through femtosecond transient absorption spectroscopy, of air-equilibrated samples exposed to light exhibit a decrease in average lifetime (0.81 ns) when compared to samples stored in a nitrogen/dark environment (8.3 ns). A photoetching technique commonly used for controlled reduction of QD size was found to induce energetic trap states to CdSe QDs and accelerate the rate of electron–hole recombination. X-ray absorption near edge structure (XANES) analysis confirms surface oxidation, the extent of which is shown to be dependent on the thickness of the ligand shell.

Single- to few-layer graphene oxide (GO) sheets have been successfully anchored onto TiO2 films using electrophoretic deposition. Upon UV illumination of TiO2–GO films, photogenerated electrons from TiO2 are captured by GO. These electrons are initially used in GO's reduction, while additional electron transfer results in storage across its sp2 network. In the presence of silver ions, deposition of silver nanoparticles (NPs) is accomplished on the GO surface opposite the TiO2, thus confirming the ability of GO to transport electrons through its plane. Illumination-controlled reduction of silver ions allows for simple selection of particle size and loading, making these semiconductor–graphene–metal (SGM) films ideal for custom catalysis and sensor applications. Initial testing of SGM films as surface-enhanced resonance Raman (SERRS) sensors produced significant target molecule signal enhancements, enabling detection of nanomolar concentrations.

Reduced graphene oxide (RGO) has become a common substrate upon which active intercalation materials are anchored for electrochemical applications such as supercapacitors and lithium ion batteries. The unique attributes of RGO, including high conductivity and porous macrostructure, are often credited for enhanced cycling and capacity performance. Here we focus on probing the electrochemical response of α-MnO₂/RGO composite used as an electrode in a lithium ion battery cell and elucidating the mechanistic aspects of the RGO on the commonly observed improvements in cycling and capacity. We find that electron storage properties of RGO enables better electrode kinetics, more rapid diffusion of Li+ to intercalation sites, and a greater capacitance effect during discharge. Further investigation of the length of the one-dimensional nanowire morphology of the α-MnO₂ has allowed us to differentiate between the innate characteristics of the MnO2 and those of the RGO. RGO coupled with long nanowires (>5 μm) exhibited the best performance in all tests and retained 150 mAh/g capacity after 20 cycles at 0.4C rate.

Graphene based 2-D carbon nanostructures provide new opportunities to fortify semiconductor based light harvesting assemblies. Electron and energy transfer rates from photoexcited CdSe colloidal quantum dots (QDs) to graphene oxide (GO) and reduced graphene oxide (RGO) were isolated by analysis of excited state deactivation lifetimes as a function of degree of oxidation and charging in (R)GO. Apparent rate constants for energy and electron transfer determined for CdSe–GO composites were 5.5 × 10⁸ and 6.7 × 10⁸ s^–1, respectively. Additionally, incorporation of GO in colloidal CdSe QD films deposited on conducting glass electrodes was found to enhance the charge separation and electron conduction through the QD film, thus allowing three-dimensional sensitization. Photoanodes assembled from CdSe–graphene composites in quantum dot sensitized solar cells display improved photocurrent response (150%) over those prepared without GO.

Manipulation of energy and electron transfer processes in a light harvesting assembly is an important criterion to mimic natural photosynthesis. We have now succeeded in sequentially assembling CdSe quantum dot (QD) and squaraine dye (SQSH) on TiO₂ film and couple energy and electron transfer processes to generate photocurrent in a hybrid solar cell. When attached separately, both CdSe QDs and SQSH inject electrons into TiO₂ under visible–near-IR irradiation. However, CdSe QD if linked to TiO₂ with SQSH linker participates in an energy transfer process. The hybrid solar cells prepared with squaraine dye as a linker between CdSe QD and TiO₂ exhibited power conversion efficiency of 3.65% and good stability during illumination with global AM 1.5 solar condition. Transient absorption spectroscopy measurements provided further insight into the energy transfer between excited CdSe QD and SQSH (rate constant of 6.7 × 10¹⁰ s^–1) and interfacial electron transfer between excited SQSH and TiO2 (rate constant of 1.2 × 10¹¹ s^–1). The synergy of covalently linked semiconductor quantum dots and near-IR absorbing squaraine dye provides new opportunities to harvest photons from selective regions of the solar spectrum in an efficient manner.

The demand for clean energy will require the design of nanostructure-based light-harvesting assemblies for the conversion of solar energy into chemical energy (solar fuels) and electrical energy (solar cells). Semiconductor nanocrystals serve as the building blocks for designing next generation solar cells, and metal chalcogenides (e.g., CdS, CdSe, PbS, and PbSe) are particularly useful for harnessing size-dependent optical and electronic properties in these nanostructures.

This Account focuses on photoinduced electron transfer processes in quantum dot sensitized solar cells (QDSCs) and discusses strategies to overcome the limitations of various interfacial electron transfer processes. The heterojunction of two semiconductor nanocrystals with matched band energies (e.g., TiO₂ and CdSe) facilitates charge separation. The rate at which these separated charge carriers are driven toward opposing electrodes is a major factor that dictates the overall photocurrent generation efficiency. The hole transfer at the semiconductor remains a major bottleneck in QDSCs. For example, the rate constant for hole transfer is 2–3 orders of magnitude lower than the electron injection from excited CdSe into oxide (e.g., TiO₂) semiconductor. Disparity between the electron and hole scavenging rate leads to further accumulation of holes within the CdSe QD and increases the rate of electron–hole recombination. To overcome the losses due to charge recombination processes at the interface, researchers need to accelerate electron and hole transport.

The power conversion efficiency for liquid junction and solid state quantum dot solar cells, which is in the range of 5–6%, represents a significant advance toward effective utilization of nanomaterials for solar cells. The design of new semiconductor architectures could address many of the issues related to modulation of various charge transfer steps. With the resolution of those problems, the efficiencies of QDSCs could approach those of dye sensitized solar cells (DSSC) and organic photovoltaics.

The Perspective focuses on photoinduced electron transfer between semiconductor–metal and semiconductor–semiconductor nanostructures and factors that influence the rate of electron transfer at the interface. The storage and discharge properties of metal nanoparticles play an important role in dictating the photocatalytic performance of semiconductor–metal composite assemblies. Both electron and hole transfer across the interface with comparable rates are important in maintaining high photocatalytic efficiency and stability of the semiconductor assemblies. Coupled semiconductors of well-matched band energies are convenient to improve charge separation. Furthermore, semiconductor and metal nanoparticles assembled on reduced graphene oxide sheets offer new ways to design multifunctional catalyst mat. The fundamental understanding of charge-transfer processes is important in the future design of light-harvesting assemblies.

To make Quantum Dot Sensitized Solar Cells (QDSC) competitive, it is necessary to achieve power conversion efficiencies comparable to other emerging solar cell technologies. By employing Mn²⁺ doping of CdS, we have now succeeded in significantly improving QDSC performance. QDSC constructed with Mn-doped-CdS/CdSe deposited on mesoscopic TiO₂ film as photoanode, Cu2S/Graphene Oxide composite electrode, and sulfide/polysulfide electrolyte deliver power conversion efficiency of 5.4%.

A transformative approach is required to meet the demand of economically viable solar cell technology. By making use of recent advances in semiconductor nanocrystal research, we have now developed a one-coat solar paint for designing quantum dot solar cells. A binder-free paste consisting of CdS, CdSe, and TiO₂ semiconductor nanoparticles was prepared and applied to conducting glass surface and annealed at 473 K. The photoconversion behavior of these semiconductor film electrodes was evaluated in a photoelectrochemical cell consisting of graphene–Cu2S counter electrode and sulfide/polysulfide redox couple. Open-circuit voltage as high as 600 mV and short circuit current of 3.1 mA/cm² were obtained with CdS/TiO₂–CdSe/TiO₂ electrodes. A power conversion efficiency exceeding 1% has been obtained for solar cells constructed using the simple conventional paint brush approach under ambient conditions. Whereas further improvements are necessary to develop strategies for large area, all solid state devices, this initial effort to prepare solar paint offers the advantages of simple design and economically viable next generation solar cells.

The photoresponse of quantum dot solar cells (QDSCs) has been successfully extended to the near-IR (NIR) region by sensitizing nanostructured TiO₂–CdS films with a squaraine dye (JK-216). CdS nanoparticles anchored on mesoscopic TiO₂ films obtained by successive ionic layer adsorption and reaction (SILAR) exhibit limited absorption below 500 nm with a net power conversion efficiency of 1% when employed as a photoanode in QDSC. By depositing a thin barrier layer of Al₂O₃, the TiO₂–CdS films were further modified with a NIR absorbing squaraine dye. Quantum dot sensitized solar cells supersensitized with a squariand dye (JK-216) showed good stability during illumination with standard global AM 1.5 solar conditions, delivering a maximum overall power conversion efficiency (η) of 3.14%. Transient absorption and pulse radiolysis measurements provide further insight into the excited state interactions of squaraine dye with SiO₂, TiO₂, and TiO₂/CdS/Al₂O₃ films and interfacial electron transfer processes. The synergy of combining semiconductor quantum dots and NIR absorbing dye provides new opportunities to harvest photons from different regions of the solar spectrum.

Polysulfide electrolyte that is employed as a redox electrolyte in quantum dot sensitized solar cells provides stability to the cadmium chalcogenide photoanode but introduces significant redox limitations at the counter electrode through undesirable surface reactions. By designing reduced graphene oxide (RGO)-Cu₂S composite, we have now succeeded in shuttling electrons through the RGO sheets and polysulfide-active Cu2S more efficiently than Pt electrode, improving the fill factor by 75%. The composite material characterized and optimized at different compositions indicates a Cu/RGO mass ratio of 4 provides the best electrochemical performance. A sandwich CdSe quantum dot sensitized solar cell constructed using the optimized RGO-Cu₂S composite counter electrode exhibited an unsurpassed power conversion efficiency of 4.4%.

Iridium oxide, a water oxidation cocatalyst, plays an important role in mediating the hole transfer process of a UV-irradiated TiO₂ system. Spectroscopic identification of trapped holes has enabled their characterization in colloidal TiO₂ suspension and monitoring of the transfer of trapped holes to IrO₂. Titration of trapped holes with potassium iodide yields an estimate of three holes per particle during 7 min of UV irradiation of TiO₂ suspension in ethanol containing 5% acetic acid. The hole transfer to IrO2 occurs with a rate constant of 6 × 10⁵ s^–1. Interestingly, IrO₂ also catalyzes the recombination of trapped holes with reduced oxygen species. The results discussed here provide a mechanistic and kinetic insight into the catalytic role of IrO₂ in the photogenerated hole transfer process.

UV irradiation of TiO₂ nanoparticles in the presence of Ag+ ions results in the quantitative reduction and deposition of silver on its surface. Continued UV irradiation following the deposition of Ag on the TiO₂ surface causes a blue shift in the surface plasmon peak from 430 to 415 nm as these particles become charged with excess electrons. Under UV irradiation, both the charging and discharging of electrons occur at different rates, thus allowing the system to attain a steady state. Upon stopping the UV irradiation, a fraction of these electrons remain stored. The electron storage is dependent on the amount of Ag deposited on TiO₂ nanoparticles with maximum capacity seen at 8.6 μM of Ag in a suspension containing 5.8 mM of TiO₂. Such electron charging and discharging processes in semiconductor–metal composites need to be taken into account while evaluating the plasmon resonance induced effects in photocatalysis and photoelectrochemistry.

Highly photoactive porphyrin is shown to form charge-transfer complex with silver nanoparticles. Complexation of tetra(4-aminophenyl) porphyrin (TAPP) with Ag nanoparticles is confirmed by ground-state absorption and Raman spectroscopy. Strong Raman enhancement indicates both electromagnetic and chemical enhancement. Evidence of chemical enhancement includes a selective enhancement of porphyrin Raman bands. Fast charge separation in the complex is indicated by ultrafast transient absorption and fluorescence upconversion measurements. The charge-separated state is shown to have a lifetime of 116 ± 6 ps. Porphyrin substituents are shown to play a role in the formation of charge-transfer complex.

A bottom-up strategy has been developed to construct a multiple electron transfer system composed of organic/inorganic ternary composites (porphyrin, zinc oxide nanoparticles, reduced graphene oxide) on a semiconducting electrode without impairing the respective donor–acceptor components. The hierarchical electron transfer cascade system exhibited remarkably high photocurrent generation with an incident-photon-to-current efficiency of up to ca. 70%.

The development of organic/inorganic hybrid nanocomposite systems that enable efficient solar energy conversion has been important for applications in solar cell research. Nanostructured carbon-based systems, in particular C60, offer attractive strategies to collect and transport electrons generated in a light harvesting assembly. We have assembled CdSe–C60 nanocomposites by chemically linking CdSe quantum dots (QDs) with thiol-functionalized C60. The photoinduced charge separation and collection of electrons in CdSe QD–C60 nanocomposites have been evaluated using transient absorption spectroscopy and photoelectrochemical measurements. The rate constant for electron transfer between excited CdSe QD and C60 increased with the decreasing size of the CdSe QD (7.9 × 10⁹ s^–1 (4.5 nm), 1.7 × 10¹⁰ s^–1 (3.2 nm), and 9.0 × 10¹⁰ s^–1 (2.6 nm)). Slower hole transfer and faster charge recombination and transport events were found to dominate over the forward electron injection process, thus limiting the deliverance of maximum power in CdSe QD–C60-based solar cells. The photoinduced charge separation between CdSe QDs and C60 opens up new design strategies for developing light harvesting assemblies.

The presence of sulfide/polysulfide redox couple is crucial in achieving stability of metal chalcogenide (e.g., CdS and CdSe)-based quantum dot-sensitized solar cells (QDSC). However, the interfacial charge transfer processes play a pivotal role in dictating the net photoconversion efficiency. We present here kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO₂ nanoparticles. The kinetic rate constant (10⁷–10⁹ s^–1) for the hole transfer obtained from the emission lifetime measurements suggests slow hole scavenging from CdSe by S^2– is one of the limiting factors in attaining high overall efficiency. The presence of the oxidized couple, by addition of S or Se to the electrolyte, increases the photocurrent, but it also enhances the rate of back electron transfer.

Understanding CdSe quantum dot (QD) adsorption phenomena on mesoscopic TiO₂ films is important for improving the performance of quantum dot sensitized solar cells (QDSSCs). A kinetic adsorption model has been developed to elucidate both Langmuir-like submonolayer adsorption and QD aggregation processes. Removal of surface-bound trioctylphosphine oxide as well as the use of 3-mercaptopropionic acid (MPA) as a molecular linker improved the adsorption of toluene-suspended QDs onto TiO₂ films. The adsorption constant Kad for submonolayer coverage was (6.7 ± 2.7) × 10³ M^–1 for direct adsorption and (4.2 ± 2.0) × 10⁴ M^–1 for MPA-linked assemblies. Prolonged exposure of a TiO₂ film to a CdSe QD suspension resulted in the assembly of aggregated particles regardless of the method of adsorption. A greater coverage of TiO₂ was achieved with smaller QDs due to reduced size constraints. Ultrafast transient absorption spectroscopy demonstrated faster electron injection into TiO₂ from directly adsorbed QDs (k_ET = 7.2 × 10⁹ s^–1) compared with MPA-linked QDs (k_ET = 2.3 × 10⁹ s^–1). The adsorption kinetic details presented in this study are useful for controlling CdSe QD adsorption on TiO₂ and designing efficient photoanodes for QDSSCs.

This article focuses on graphene-based electrodes for electrochemical energy conversion and storage devices. As elaborated in the other feature articles in this issue, graphene is a 2D “flat mat” consisting of a honeycomb-like structure of carbon atoms with sp² bonding character for each carbon. It exhibits excellent electrical conductivity and mechanical strength, and can be synthesized in a number of ways. Most syntheses involve first oxidizing graphite to graphene oxide (GO), a widely used method developed many years ago and referred to as Hummers Method. The graphitic carbon is dissolved in sulfuric acid along with other oxidants such as sodium nitrate during sonication, and KMnO₄ is slowly added to complete the oxidation process. Washing and rinsing allows for a fully exfoliated GO suspension in water, ethanol, and other polar solvents, that can be prepared as a stable colloid for months. The GO can be subsequently reduced in a number of ways by UV irradiation or thermal treatment, sonolysis, or chemical treatment with a strong reducing agent such as hydrazine and is referred to as reduced graphene oxide (RGO). The use of RGO in lithium ion batteries is still in its infancy and may provide a practical and inexpensive way to substantially improve the performance of these electrochemical energy storage devices, serving markets ranging from the long-sought electric vehicle as well as simple applications such as cell phones and laptop computers. GO is an extraordinarily useful material for the development of composite materials as a result of the oxygen moieties spread throughout the formerly-pristine carbon sheet. These highly electronegative species allow for the stability of the GO colloidal suspension as well as the binding of cations. Binding cationic or organic protonated anions (i.e., acetate) produces GO composites, and subsequently RGO composites that are formed from further chemical processing. The ability to selectively bind cationic materials has led to a swarm of activity surrounding the development of novel GO/RGOnanoparticle composite materials for a variety of applications. The development of electrodes for lithium ion batteries using the following scheme has produced a number of RGO-metal oxide composites already.

Photochemically generated methyl viologen radicals undergo electron transfer with graphene oxide (GO) in ethanol suspensions. This charge transfer interaction results in the reduction of GO as well as storage of electrons. The stored electrons can be utilized to reduce Ag⁺ ions and thus anchor silver nanoparticles on reduced graphene oxide (RGO). The spectroscopic experiments that elucidate the quantitative electron transfer and transmission electron microscopy that highlights the potential of designing metal–RGO assemblies are discussed.

Quantum dot-metal oxide junctions are an integral part of next-generation solar cells, light emitting diodes, and nanostructured electronic arrays. Here we present a comprehensive examination of electron transfer at these junctions, using a series of CdSe quantum dot donors (sizes 2.8, 3.3, 4.0, and 4.2 nm in diameter) and metal oxide nanoparticle acceptors (SnO₂, TiO₂, and ZnO). Apparent electron transfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy. The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. In contrast with dye-sensitized metal oxide electron transfer studies, our systems did not exhibit unthermalized hot-electron injection due to relatively large ratios of electron cooling rate to electron transfer rate. To investigate the implications of these findings in photovoltaic cells, quantum dot-metal oxide working electrodes were constructed in an identical fashion to the films used for the electron transfer portion of the study. Interestingly, the films which exhibited the fastest electron transfer rates (SnO₂) were not the same as those which showed the highest photocurrent (TiO₂). These findings suggest that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency.