Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

/pdf/dye-sensitized-solar-cells-50b0w2bhsr.pdf

Dye-Sensitized Solar Cells

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

We demonstrate that semiconductor nanorods can be used to fabricate readily processed and efficient hybrid solar cells together with polymers. By controlling nanorod length, we can change the distance on which electrons are transported directly through the thin film device. Tuning the band gap by altering the nanorod radius enabled us to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum. A photovoltaic device consisting of 7-nanometer by 60-nanometer CdSe nanorods and the conjugated polymer poly-3(hexylthiophene) was assembled from solution with an external quantum efficiency of over 54% and a monochromatic power conversion efficiency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers. Under Air Mass (A.M.) 1.5 Global solar conditions, we obtained a power conversion efficiency of 1.7%.

http://science.sciencemag.org/content/sci/295/5564/2425.full.pdf

Hybrid Nanorod-Polymer Solar Cells

The charge injection dynamics of dye sensitization from a surface-bound dye (coumarin 343 (C343)) to the conduction band (CB) of the TiO2 is reported here for the first time. Ultrafast fluorescence dynamics demonstrate that the charge injection from the C343 dye to the CB of the TiO2 occurs on a time scale of ca. 200 fs. The charge injection efficiency is attributed to strong electronic coupling between the dye and TiO2 energy levels. The results yield a rate of injection of 5 × 1012 s-1.

Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye−Semiconductor (TiO2) Interface

Studies of interface formation on conventional semiconductor materials are typically carried out under relatively pristine conditions. However, for devices based on the use of electronic polymers there is also compelling interest in exploring the variations in contact behavior that might result under realistic manufacturing conditions like multilayer device assembly based on solution coating technology. Small molecule doped polymers (MDPs) developed principally as large area coatings for electrophotographic use are now finding wider device applications. These polymers are insulators capable of transporting excess injected charge with a unipolar drift mobility which can be tuned over a wide range by varying the concentration of transport active species. Most significant in the present context, MDPs can be rendered trap free by molecular design. These unique characteristics of MDPs make it possible to analyze the relative injection efficiencies of their interfaces with various contacts simply by a direct comparison of current–voltage characteristics with time of flight drift mobility measurements carried out on the same film coatings. In this way, and apart from their intrinsic interest and practical value, MDPs and closely related polymeric media provide the ideal venue for the study of contact phenomena on molecular solids. Almost all the present measurements were carried out by measuring dark hole injection from various preformed metal substrates into the MDP film TPD/polycarbonate. Under these circumstances it was found that while injection efficiency nominally scaled with the estimated interfacial energy step there was significant variance that in some cases could be clearly associated with the specific details of interfacial chemistry. For one exceptional case where Au was evaporated on the free surface of an already cast film a time and temperature dependent contact forming process could be delineated in which the interface systematically evolved from emission limited to ohmic.

Direct evaluation of contact injection efficiency into small molecule based transport layers: Influence of extrinsic factors

Conduction and Valence Band Edges of Porous Silicon Determined by Electron Transfer

The effects of liquid methanol on the photoluminescence intensity and FTIR spectra of red and blue emitting porous silicon were investigated. Hydrogen passivated red emitting samples exhibit quenching and recovery of photoluminescence intensity and broadening of the Si‐Hx stretch bands upon exposure to liquid methanol. Oxygen passivated red emitting samples exhibit no photoluminescence quenching. The sensitivity of the red emitting sample is due to the microstructure of porous silicon at the surface and the ability of methanol to penetrate the pores. The blue photoluminescence of thermally oxidized samples is quenched upon exposure to methanol. This is attributed to the solvent’s ability to change the surface passivation which modifies existing traps and introduces competitive recombination channels for electrons.

Julie M. Rehm

Papers

Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye−Semiconductor (TiO2) Interface

Direct evaluation of contact injection efficiency into small molecule based transport layers: Influence of extrinsic factors

Conduction and Valence Band Edges of Porous Silicon Determined by Electron Transfer

How methanol affects the surface of blue and red emitting porous silicon

Size Dependence of Radiative Rates in the Indirect Band Gap Material AgBr