James T. McLeskey

Water-soluble polythiophene∕nanocrystalline TiO2 solar cells

We report the characteristics of polymer∕nanocrystalline solar cells fabricated using an environmentally friendly water-soluble polythiophene and TiO2 in a bilayer configuration. The cells were made by dropping the polymer onto a TiO2 nanocrystalline film and then repeatedly sweeping a clean glass rod across the polymer as it dried. The devices showed an open circuit voltage of 0.81 V, a short circuit current density of 0.35mA∕cm2, a fill factor of 0.4, and an energy conversion efficiency of 0.13%. The water-soluble polythiophene showed significant photovoltaic behavior and the potential for use in solar cells.

Femtosecond pump–probe nondestructive examination of materials (invited)

Ultrashort-pulsed lasers have been demonstrated as effective tools for the nondestructive examination (NDE) of energy transport properties in thin films. After the instantaneous heating of the surface of a 100 nm metal film, it will take ∼100 ps for the influence of the substrate to affect the surface temperature profile. Therefore, direct measurement of energy transport in a thin film sample requires a technique with picosecond temporal resolution. The pump–probe experimental technique is able to monitor the change in reflectance or transmittance of the sample surface as a function of time on a subpicosecond time scale. Changes in reflectance and transmittance can then be used to determine properties of the film. In the case of metals, the change in reflectance is related to changes in temperature and strain. The transient temperature profile at the surface is then used to determine the rate of coupling between the electron and phonon systems as well as the thermal conductivity of the material. In the case of semiconductors, the change in reflectance and transmittance is related to changes in the local electronic states and temperature. Transient thermotransmission experiments have been used extensively to observe electron-hole recombination phenomena and thermalization of hot electrons. Application of the transient thermoreflectance (TTR) and transient thermotransmittance (TTT) technique to the study of picosecond phenomena in metals and semiconductors will be discussed. The pump–probe experimental setup will be described, along with the details of the experimental apparatus in use at the University of Virginia. The thermal model applicable to ultrashort-pulsed laser heating of metals will be presented along with a discussion of the limitations of this model. Details of the data acquisition and interpretation of the experimental results will be given, including a discussion of the reflectance models used to relate the measured changes in reflectance to calculated changes in temperature. Finally, experimental results will be presented that demonstrate the use of the TTR technique for measuring the electron–phonon coupling factor and the thermal conductivity of thin metallic films. The use of the TTT technique to distinguish between different levels of doping and alloying in thin film samples of hydrogenated amorphous silicon will also be discussed briefly.

Characteristics of water-soluble polythiophene: TiO2 composite and its application in photovoltaics

We have studied the characteristics of composites of an environmentally friendly water-soluble polythiophene sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (PTEBS) and TiO2. We observed that the ultraviolet-visible absorption spectrum of low molecular weight PTEBS is redshifted possibly due to the formation of aggregates. Cyclic voltammetry reveals the values of highest occupied molecular orbitals and lowest unoccupied molecular orbitals for PTEBS. A factor of 7 in photoluminescence quenching indicates that the exciton dissociation and charge separation occur successfully at the PTEBS:TiO2 (1:1 by weight) interface. This enhances the possibility that the separated charges will reach the electrodes before recombining. Scanning electron micrograph images show how the PTEBS and TiO2 are interconnected and form paths to the electrodes to improve charge transport. Photovoltaic devices with TiO2:PTEBS composite achieved an energy conversion efficiency of η=0.015%, a short circuit current of JSC=0.22mA∕cm2, ...

Characterization of Nanoaerosol Size Change During Enhanced Condensational Growth

Increasing the size of nanoaerosols may be beneficial in a number of applications, including filtration, particle size selection, and targeted respiratory drug delivery. A potential method to increase particle or droplet size is enhanced condensational growth (ECG), which involves combining the aerosol with saturated or supersaturated air. In this study, we characterize the ECG process in a model tubular geometry as a function of initial aerosol size (mean diameters–150, 560, and 900 nm) and relative humidity conditions using both in vitro experiments and numerical modeling. Relative humidities (99.8–104%) and temperatures (25–39°C) were evaluated that can safely be applied to either targeted respiratory drug delivery or personal aerosol filtration systems. For inlet saturated air temperatures above ambient conditions (30 and 39°C), the initial nanoaerosols grew to a size range of 1000–3000 nm (1–3 m) over a time period of 0.2 s. The numerical model results were generally consistent with the experimental findings and predicted final to initial diameter ratios of up to 8 after 0.2 s of humidity exposure and 14 at 1 s. Based on these observations, a respiratory drug delivery approach is suggested in which nanoaerosols in the size range of 500 nm are delivered in conjunction with a saturated or supersaturated air stream. The initial nanoaerosol size will ensure minimal deposition and loss in the mouth-throat region while condensational growth in the respiratory tract can be used to ensure maximal lung retention and to potentially target the site of deposition.

Comparison of genetic algorithm to particle swarm for constrained simulation-based optimization of a geothermal power plant

The performance of a genetic algorithm is compared with that of particle swarm optimization for the constrained, non-linear, simulation-based optimization of a double flash geothermal power plant. Particle swarm optimization converges to better (higher) objective function values. The genetic algorithm is shown to converge more quickly and more tightly, resulting in a loss of solution diversity. Particle swarm optimization obtains solutions within 0.1% and 0.5% of the best known optimum in significantly fewer objective function evaluations than the genetic algorithm.

Nanostructured solid-state hybrid photovoltaic cells fabricated by electrostatic layer-by-layer deposition

We report on the fabrication of hybrid organic/inorganic photovoltaic cells utilizing layer-by-layer deposition of water-soluble polyions and nanocrystals. A bulk heterojunction structure was created consisting of alternating layers of the p-conductive polythiophene derivative poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] and n-conductive TiO2 nanoparticles. We fabricated working devices with the heterostructure sandwiched between suitable charge carrier blocking layers and conducting oxide and metal electrodes, respectively. We analyzed the influence of the thickness and nanostructure of the active layer on the cell performance and characterized the devices in terms of static and transient current response with respect to illumination and voltage conditions. We observed reproducible and stable photovoltaic behavior with photovoltages of up to 0.9 V.

A passive house with seasonal solar energy store: in situ data and numerical modelling

This paper presents parametric analysis of solar collector area and solar energy storage volume for a passive house in Galway, Ireland. Using the simulation tool Transient System Simulation Tool (TRNSYS), a model was developed to represent a 215 m2 home built to Passivhaus standards and incorporating a 10.6 m2 solar thermal collector and a 23 m3 solar thermal storage tank. This model was validated through comparison with data collected in situ from the operation of the home over the period of 1 year. Once validated, the model was used to investigate the effect of varying solar collector area and solar energy storage volume on the fraction of heat demand met by solar energy. Results indicate that increasing collector area from 10.6 to 20 m2 could increase total solar fraction from 0.47 to 0.63, decreasing fossil-fuel-derived energy demand at the home under study by a further 30%.

Electrokinetic separation of co-solutes into bimodal fibers by electrospinning

Composite and chemically/physically distinct fibers of sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (PTEBS) and polyethylene oxide (PEO) were formed by electrospinning from a homogeneous aqueous solution containing PTEBS and PEO co-solutes. Composite nanofibers of diameter of ∼60nm were electrospun from an aqueous solution. The addition of ammonium hydroxide (NH4OH) to the water solution resulted in “bimodal” electrospun fibers consisting of distinct large diameter white PEO fiber segments and small diameter black PTEBS fiber segments. The optical absorption spectrum of the composite PTEBS/PEO nanofibers did not exhibit the characteristic peak around 460nm, which is present in the bulk spectrum.