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.

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%.

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.

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.

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.