Daniel R. Gamelin

Efficient CdSe quantum dot-sensitized solar cells prepared by an improved successive ionic layer adsorption and reaction process

In pursuit of efficient quantum dot (QD)-sensitized solar cells based on mesoporous TiO(2) photoanodes, a new procedure for preparing selenide (Se(2-)) was developed and used for depositing CdSe QDs in situ over TiO(2) mesopores by the successive ionic layer adsorption and reaction (SILAR) process in ethanol. The sizes and density of CdSe QDs over TiO(2) were controlled by the number of SILAR cycles applied. After some optimization of these QD-sensitized TiO(2) films in regenerative photoelectrochemical cells using a cobalt redox couple [Co(o-phen)(3)(2+/3+)], including addition of a final layer of CdTe, over 4% overall efficiencies were achieved at 100 W/m(2) with about 50% IPCE at its maximum. Light-harvesting properties and transient voltage decay/impedance measurements confirmed that CdTe-terminated CdSe QD cells gave better charge-collection efficiencies and kinetic parameters than corresponding CdSe QD cells. In a preliminary study, a CdSe(Te) QD-sensitized TiO(2) film was combined with an organic hole conductor, spiro-OMeTAD, and shown to exhibit a promising efficiency of 1.6% at 100 W/m(2) in inorganic/organic hybrid all-solid-state cells.

Near-Complete Suppression of Surface Recombination in Solar Photoelectrolysis by “Co-Pi” Catalyst-Modified W:BiVO4

The influence of an earth-abundant water oxidation electrocatalyst (Co-Pi) on solar water oxidation by W:BiVO(4) has been studied using photoelectrochemical (PEC) techniques. Modification of W:BiVO(4) photoanode surfaces with Co-Pi has yielded a very large (∼440 mV) cathodic shift in the onset potential for sustained PEC water oxidation at pH 8. PEC experiments with H(2)O(2) as a surrogate substrate have revealed that interfacing Co-Pi with these W:BiVO(4) photoanodes almost completely eliminates losses due to surface electron-hole recombination. The results obtained for W:BiVO(4) are compared with those reported recently for Co-Pi/α-Fe(2)O(3) photoanodes. The low absolute onset potential of ∼310 mV vs RHE achieved with the Co-Pi/W:BiVO(4) combination is promising for overall solar water splitting in low-cost tandem PEC cells, and is encouraging for application of this surface modification strategy to other candidate photoanodes.

Solar Water Oxidation by Composite Catalyst/α-Fe2O3 Photoanodes

Electrodeposition of an amorphous cobalt catalyst layer over a high-surface-area alpha-Fe(2)O(3) photoanode causes a more than 350 mV cathodic shift in the onset potential for photoelectrochemical water oxidation using this anode and simulated solar irradiation. The catalyst layer is shown to deposit conformally onto the mesostructured alpha-Fe(2)O(3), leading to a large contact area at the interface between the two halves of the composite photoanode. Photoelectrochemical measurements show that the photocurrent generated from this composite photoanode still derives from alpha-Fe(2)O(3) excitation but is now accessible at an external bias several hundred millivolts below what is typically required for alpha-Fe(2)O(3) photoanodes alone, indicating a reduced external bias would be needed to drive overall water splitting. These results demonstrate modification of this prototypical photoanode material with a conformal layer of a competent electrocatalyst to separate the tasks of photon absorption and redox catalysis, a strategy that may have important and general ramifications for solar photoelectrochemical hydrogen generation.

Photo-assisted electrodeposition of cobalt–phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation

A photo-assisted electrodeposition approach was used to deposit a cobalt–phosphate water oxidation catalyst (“Co–Pi”) onto recently improved dendritic mesostructures of α-Fe2O3. A comparison between this approach, electrodeposition of Co–Pi, and Co2+ wet impregnation showed that photo-assisted electrodeposition of Co–Pi yields superior α-Fe2O3 photoanodes for photoelectrochemical water oxidation. Stable photocurrent densities of 1.0 mA cm−2 at 1.0 V and 2.8 mA cm−2 at 1.23 V vs. RHE measured under standard illumination and basic conditions were achieved. By allowing deposition only where visible light generates oxidizing equivalents, photo-assisted electrodeposition provides a more uniform distribution of Co–Pi onto α-Fe2O3 than obtained by electrodeposition. This approach of fabricating catalyst-modified metal-oxide photoelectrodes may be attractive for optimization in conjunction with tandem or hybrid photoelectrochemical cells.

Ferromagnetism in oxide semiconductors

Over the past five years, considerable work has been carried out in the exploration of candidate diluted oxide magnetic semiconductors with high Curie temperatures. Fueled by early experimental results and theoretical predictions, claims of ferromagnetism at and above room temperature in doped oxides have abounded. In general, neither the true nature of these materials nor the physical causes of the magnetism have been adequately determined. It is now apparent that these dilute magnetic systems are deceptively complex. We consider two well-characterizedn-type magnetically doped oxide semiconductors and explore the relationship between donor electrons and ferromagnetism.

Regenerative PbS and CdS Quantum Dot Sensitized Solar Cells with a Cobalt Complex as Hole Mediator

Metal sulfide (PbS and CdS) quantum dots (QDs) were prepared over mesoporous TiO2 films by improved successive ionic layer adsorption and reaction (SILAR) processes. The as-prepared QD-sensitized electrodes were combined with a cobalt complex redox couple [Co(o-phen)3]2+/3+ to make a regenerative liquid-type photovoltaic cell. The optimized PbS QD-sensitized solar cells exhibited promising incident photon-to-current conversion efficiency (IPCE) of over 50% and an overall conversion efficiency of 2% at 0.1 sun in a regenerative mode. The overall photovoltaic performance of the PbS QD-sensitized cells was observed to be dependent on the final turn of the SILAR process, giving a better result when the final deposition was Pb2+, not S2-. However, in the case of CdS QD-sensitized cells, S2- termination was better than that of Cd2+. The cobalt complex herein used as a regenerative redox couple was found to be more efficient in generating photocurrents from PbS QD cells than the typical hole scavenger Na2S in a three-electrode configuration. The CdS-sensitized cell with this redox mediator also showed better defined current-voltage curves and an IPCE reaching 40%.

Light-Induced Spontaneous Magnetization in Doped Colloidal Quantum Dots

Saturated Magnetism in Photoexcited Nanocrystals Switching the magnetic state of semiconductors with either an electric field or by light absorption is a key requirement for spintronics, in which devices are based on electronic spin state rather than charge. In semiconductor nanoparticles doped with magnetic ions, excitons can form a spin state, a magnetic polaron, but often the effect is limited to low temperatures (below 30 kelvin) and does not saturate in the absence of an applied magnetic field. Beaulac et al. (p. 973) report the synthesis of Mn-doped CdSe nanocrystals in which the quantum confinement effects lead to long exciton lifetimes. Photoexcitation results in exchange fields that can exceed 30 Tesla at low temperatures and that persist even up to room temperature in the absence of an applied magnetic field. Long-lifetime excited states created by quantum confinement effects enable the light-induced magnetization of a quantum dot. An attractive approach to controlling spin effects in semiconductor nanostructures for applications in electronics is the use of light to generate, manipulate, or read out spins. Here, we demonstrate spontaneous photoinduced polarization of manganese(II) spins in doped colloidal cadmium selenide quantum dots. Photoexcitation generates large dopant-carrier exchange fields, enhanced by strong spatial confinement, that lead to giant Zeeman splittings of the semiconductor band structure in the absence of applied magnetic fields. These internal exchange fields allow spontaneous magnetic saturation of the manganese(II) spins to be achieved at zero external magnetic field up to ~50 kelvin. Photomagnetic effects are observed all the way up to room temperature.

Above-room-temperature ferromagnetic Ni2+-doped ZnO thin films prepared from colloidal diluted magnetic semiconductor quantum dots

We report the preparation of spin-coated nickel-doped zinc oxide nanocrystalline thin films using high-quality colloidal diluted magnetic semiconductor (DMS) quantum dots as solution precursors. These films show robust ferromagnetism with Curie temperatures above 350K and 300K saturation moments up to 0.1Bohr magnetons per nickel. These results demonstrate a step toward the use of colloidal zero-dimensional DMS nanocrystals as building blocks for the bottom-up construction of more complex ferromagnetic semiconductor nanostructures.

Tunable Dual Emission in Doped Semiconductor Nanocrystals

Colloidal manganese-doped semiconductor nanocrystals have been developed that show pronounced intrinsic high-temperature dual emission. Photoexcitation of these nanocrystals gives rise to strongly temperature dependent luminescence involving two distinct but interconnected emissive excited states of the same doped nanocrystals. The ratio of the two intensities is independent of nonradiative effects. The temperature window over which pronounced dual emission is observed can be tuned by changing the nanocrystal energy gap during growth. This unique combination of properties makes this new class of intrinsic dual emitters attractive for ratiometric optical thermometry applications.

Composite photoanodes for photoelectrochemical solar water splitting

Photoelectrochemical (PEC) water splitting is an attractive approach to capturing and storing the earths abundant solar energy influx. The challenging four-electron water-oxidation half-cell reaction has hindered this technology, giving rise to slow water oxidation kinetics at the photoanode surfaces relative to competitive loss processes. In this perspective, we review recent efforts to improve PEC efficiencies by modification of semiconductor photoanode surfaces with water-oxidation catalysts that can operate at low overpotentials. This approach allows separation of the tasks of photon absorption, charge separation, and surface catalysis, allowing each to be optimized independently. In particular, composite photoanodes marrying nanocrystalline and molecular/non-crystalline components provide flexibility in adjusting the properties of each component, but raise new challenges in interfacial chemistries.