Justin B. Sambur

Multiple Exciton Collection in a Sensitized Photovoltaic System

Two for One Solar cells often contain materials that absorb a broad spectrum of light above a certain frequency threshold, or band gap. Unfortunately, much of the energy contained in this light is wasted, because any balance exceeding the band gap tends to be dissipated as heat, rather than harnessed into electric current. Recent spectroscopic studies have shown that incident photons with energy several multiples of the band gap can transiently generate more than one current carrier, but the excess carriers tend to collapse before they can be diverted into the circuit. Sambur et al. (p. 63) now show that, when light-absorbing lead sulfide nanoparticles are carefully coupled to smoothly polished titanium dioxide crystalline electrodes, such excess carriers can be transferred into the circuit before collapsing. Generating more than one current carrier per absorbed photon raises prospects for improved solar-cell efficiency. Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO2 single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO2 facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.

Photoelectrochemical Characterization of Nanocrystalline Thin-Film Cu2ZnSnS4 Photocathodes

Cu₂ZnSnS₄ (CZTS) nanocrystals, synthesized by a hot injection solution method, have been fabricated into thin films by dip-casting onto fluorine doped tin oxide (FTO) substrates. The photoresponse of the CZTS nanocrystal films was evaluated using absorbance measurements along with photoelectrochemical methods in aqueous electrolytes. Photoelectrochemical characterization revealed a p-type photoresponse when the films were illuminated in an aqueous Eu(3+) redox electrolyte. The effects of CZTS stoichiometry, film thickness, and low-temperature annealing on the photocurrents from front and back illumination suggest that the minority carrier diffusion and recombination at the back contact (via reaction of photogenerated holes with Eu(2+) produced from photoreduction by minority carriers) are the main loss mechanisms in the cell. Low-temperature annealing resulted in significant increases in the photocurrents for films made from both Zn-rich and stoichiometric CZTS nanocrystals.

Sub-particle reaction and photocurrent mapping to optimize catalyst-modified photoanodes.

The splitting of water photoelectrochemically into hydrogen and oxygen represents a promising technology for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled—yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalysing water oxidation also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, especially for nanostructured photoanodes benefiting from small charge-carrier transport distances. Here we use super-resolution imaging, operated in a charge-carrier-selective manner and with a spatiotemporal resolution of approximately 30 nanometres and 15 milliseconds, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resolution (about 390 nanometres), the photocurrent associated with water oxidation, and find that the most active sites for water oxidation are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod—even though more improvement in photocurrent efficiency correlates with less reduction in onset potential (and vice versa) at the sub-particle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential reduction the optimal sites are the sites with more positive onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.

Approaches to Single-Nanoparticle Catalysis

Nanoparticles are among the most important industrial catalysts, with applications ranging from chemical manufacturing to energy conversion and storage. Heterogeneity is a general feature among these nanoparticles, with their individual differences in size, shape, and surface sites leading to variable, particle-specific catalytic activity. Assessing the activity of individual nanoparticles, preferably with subparticle resolution, is thus desired and vital to the development of efficient catalysts. It is challenging to measure the activity of single-nanoparticle catalysts, however. Several experimental approaches have been developed to monitor catalysis on single nanoparticles, including electrochemical methods, single-molecule fluorescence microscopy, surface plasmon resonance spectroscopy, X-ray microscopy, and surface-enhanced Raman spectroscopy. This review focuses on these experimental approaches, the associated methods and strategies, and selected applications in studying single-nanoparticle catalysis with chemical selectivity, sensitivity, or subparticle spatial resolution.

Photooxidation of Chloride By Oxide Minerals: Implications for Perchlorate On Mars

We show that highly oxidizing valence band holes, produced by ultraviolet (UV) illumination of naturally occurring semiconducting minerals, are capable of oxidizing chloride ion to perchlorate in aqueous solutions at higher rates than other known natural perchlorate production processes. Our results support an alternative to atmospheric reactions leading to the formation of high concentrations of perchlorate on Mars.

The influence of metal work function on the barrier heights of metal/pentacene junctions

The electronic structure of Cu(111)/pentacene and Ag(111)/pentacene interfaces were investigated with photoelectron spectroscopy and the hole barrier heights were determined to be 0.74 and 0.90 eV, respectively. When combined with previous measurements of the Au(111)/pentacene interface, the slope of the plot of metal work function against barrier height for Schottky barrier formation was determined to be 0.36, in agreement with current-voltage (I−V) measurements in the literature. However, the absolute barrier heights from photoemission measurements are 0.16 eV higher. The offset between the I−V measurement and the x-ray and ultraviolet photoelectron spectroscopy measurements was attributed to differences in how the highest occupied molecular orbital position is determined. Photoemission data indicates that at low coverages the pentacene molecules lie flat on the metal substrates, whereas at higher coverages the molecular orientation changes to orient the long molecular axis normal to the surface. Thicke...

Morphologies, structures, and interfacial electronic structure of perylene on Au(111)

Various coverages of perylene thin films on Au(111) were investigated using scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. A Schottky junction formed between Au(111) and perylene consisted of a large 0.65 eV interface dipole and a hole barrier height of 0.85 eV. A wetting layer of approximately 4 A thickness was initially formed followed by island formation, consistent with Stranski–Krastanov growth. Room temperature STM investigations of nominal one monolayer perylene films revealed symmetry equivalent domains and two different stable commensurate lattice structures. Perylene film growth mode, film structure and the energy level diagram are discussed.

Size Selective Photoetching of CdSe Quantum Dot Sensitizers on Single-Crystal TiO2

Cadmium selenide quantum dots covalently attached to and photosensitizing single-crystal TiO2 surfaces are observed to corrode under illumination in aqueous electrolyte containing iodide as a regenerator. Comparison of photocurrent spectra before and after long-term monochromatic illumination indicated that the CdSe QD sensitizers photocorroded and decreased in size until their band gap energy exceeded the excitation energy. This wavelength-dependent photoelectrochemical etching mechanism can be used to tune the size distribution of surface adsorbed QDs and may account for the instability of QD sensitized solar cells that do not employ sulfide-based electrolytes.

Interfacial Morphology and Photoelectrochemistry of Conjugated Polyelectrolytes Adsorbed on Single Crystal TiO2

The nanoscale morphology and photoactivity of conjugated polyelectrolytes (CPEs) deposited from different solvents onto single crystal TiO(2) were investigated with atomic force microscopy (AFM) and photocurrent spectroscopy. CPE surface coverages on TiO(2) could be incremenentally increased by adsorbing the CPEs from static solutions. The solvents used for polymer adsorption influenced the surface morpohology of the CPEs on the TiO(2) surface. Photocurrent spectroscopy measurements in aqueous electrolytes, using iodide as a hole scavenger, revealed that the magnitude of the sensitized photocurrents was related to the surface coverages and the degree of aggregation of the CPEs as determined by AFM imaging. Absorbed photon-to-current efficiencies approaching 50% were measured for CPE layers as thick as 4 nm on TiO(2). These results suggest that precise control of CPE morphology at the TiO(2) interface can be achieved through optimization of the deposition conditions to improve the power conversion efficiencies of polymer-sensitized solar cells.

Patternable Solvent-Processed Thermoplastic Graphite Electrodes

Since their invention in the 1950s, composite carbon electrodes have been employed in a wide variety of applications, ranging from batteries and fuel cells to chemical sensors, because they are easy to make and pattern at millimeter scales. Despite their widespread use, traditional carbon composite electrodes have substandard electrochemistry relative to metallic and glassy carbon electrodes. As a result, there is a critical need for new composite carbon electrodes that are highly electrochemically active, have universal and easy fabrication into complex geometries, are highly conductive, and are low cost. Herein, a new solvent-based method is presented for making low-cost composite graphite electrodes containing a thermoplastic binder. The electrodes, which are termed thermoplastic electrodes (TPEs), are easy to fabricate and pattern, give excellent electrochemical performance, and have high conductivity (700 S m-1). The thermoplastic binder enables the electrodes to be hot embossed, molded, templated, and/or cut with a CO2 laser into a variety of intricate patterns. Crucially, these electrodes show a marked improvement in peak current, peak separation, and resistance to charge transfer over traditional carbon electrodes. The impact of electrode composition, surface treatment (sanding, polishing, plasma treatment), and graphite source were found to significantly impact fabrication, patterning, conductivity, and electrochemical performance. Under optimized conditions, electrodes generated responses similar to more expensive and difficult to fabricate graphene and highly oriented pyrolytic graphite electrodes. The TPE electrode system reported here provides a new approach for fabricating high performance carbon electrodes with utility in applications ranging from sensing to batteries.