Adam F. Halverson

General Strategy for Fabricating Transparent TiO2 Nanotube Arrays for Dye-Sensitized Photoelectrodes: Illumination Geometry and Transport Properties

We report on the preparation of transparent oriented titania nanotube (NT) photoelectrodes and the effect of illumination direction on light harvesting, electron transport, and recombination in dye-sensitized solar cells (DSSCs) incorporating these electrodes. High solar conversion efficiency requires that the incident light enters the cell from the photoelectrode side. However, it has been synthetically challenging to prepare transparent TiO(2) NT electrodes by directly anodizing Ti metal films on transparent conducting oxide (TCO) substrates because of the difficulties of controlling the synthetic conditions. We describe a general synthetic strategy for fabricating transparent TiO(2) NT films on TCO substrates. With the aid of a conducting Nb-doped TiO(2) (NTO) layer between the Ti film and TCO substrate, the Ti film was anodized completely without degrading the TCO. The NTO layer was found to protect the TCO from degradation through a self-terminating mechanism by arresting the electric field-assisted dissolution process at the NT-NTO interface. The illumination direction and wavelength of the light incident on the DSSCs were shown to strongly influence the incident photon-to-current conversion efficiency, light-harvesting, and charge-collection properties, which, in turn, affect the photocurrent density, photovoltage, and solar energy conversion efficiency. Effects of NT film thickness on the properties and performance of DSSCs were also examined. Illuminating the cell from the photoelectrode substantially increased the conversion efficiency compared with illuminating it from the counter-electrode side.

Trap-Free Transport in Ordered and Disordered TiO2 Nanostructures

Understanding the influence of different film structures on electron diffusion in nanoporous metal oxide films has been challenging. Because of the rate-limiting role that traps play in controlling the transport properties, the structural effects of different film architectures are largely obscured or reduced. We describe a general approach to probe the impact of structural order and disorder on the charge-carrier dynamics without the interference of transport-limiting traps. As an illustration of this approach, we explore the consequences of trap-free diffusion in vertically aligned nanotube structures and random nanoparticle networks in sensitized titanium dioxide solar cells. Values of the electron diffusion coefficients in the nanotubes approached those observed for the single crystal and were up to 2 orders of magnitude greater than those measured for nanoparticle films with various average crystallites sizes. Transport measurements together with modeling show that electron scattering at grain boundaries in particle networks limits trap-free diffusion. In presence of traps, transport was 10(3)-10(5) times slower in nanoparticle films than in the single crystal. Understanding the link between structure and carrier dynamics is important for systematically altering and eventually controlling the electronic properties of nanoscaled materials.

Perturbation of the electron transport mechanism by proton intercalation in nanoporous TiO2 films.

This study addresses a long-standing controversy about the electron-transport mechanism in porous metal oxide semiconductor films that are commonly used in dye-sensitized solar cells and related systems. We investigated, by temperature-dependent time-of-flight measurements, the influence of proton intercalation on the electron-transport properties of nanoporous TiO(2) films exposed to an ethanol electrolyte containing different percentages of water (0-10%). These measurements revealed that increasing the water content in the electrolyte led to increased proton intercalation into the TiO(2) films, slower transport, and a dramatic change in the dependence of the thermal activation energy (E(a)) of the electron diffusion coefficient on the photogenerated electron density in the films. Random walk simulations based on a microscopic model incorporating exponential conduction band tail (CBT) trap states combined with a proton-induced shallow trap level with a long residence time accounted for the observed effects of proton intercalation on E(a). Application of this model to the experimental results explains the conditions under which E(a) dependence on the photoelectron density is consistent with multiple trapping in exponential CBT states and under which it appears at variance with this model.

Study of the Electronic Properties of Matched Na-Containing and Reduced-Na CuInGaSe2 Samples Using Junction Capacitance Methods

Junction capacitance methods were used to examine a matched pair of CuInGaSe 2 (CIGS) thin film solar cells, one with Na incorporated into the absorber and the other with a diffusion barrier to inhibit the Na incorporation from the soda-lime glass. Typical cells showed a 50% increase in efficiency with the addition of Na. Forward biased admittance spectroscopy revealed a large defect density located near the CdS/CIGS heterojunction in the reduced Na samples not present in the higher Na samples. This may be responsible for the lower V oc , contributing to the loss in efficiency when Na is not added. Drive-level capacitance profiles revealed free carrier densities of 3×10 14 cm -3 and 1.1×10 14 cm -3 for the higher and reduced Na samples, respectively. Transient photocapacitance spectra indicated a slight improvement in absorber properties with the addition of Na, but not enough to account for the large loss in efficiency.

Electron paramagnetic resonance as a probe of technologically relevant processing effects on CdTe solar cell materials

The performance of polycrystalline thin film CdTe solar cells is limited by as yet poorly understood deep level defects which serve as recombination centers. Electron paramagnetic resonance (EPR) has unrivaled analytical power and sensitivity in the identification of deep level defects in semiconductors; however, very little EPR literature exists with regard to technologically relevant processing of present day polycrystalline CdTe solar cells. In this study we explore the effects of several features important in these present day processing techniques: (1) effects of CdCl2 etching and (2) effects of Cu on the CdTe. Cu is widely used with CdTe solar cells and is thought to play a significant role in CdTe solar cells performance. Although the EPR spectra in all the cases we explored are dominated by a substitutional Mn site signal, our results are clearly sensitive to multiple changes caused by the aforementioned processing parameters.

The effect of a metallic Ni core on charge dynamics in CdS-sensitized p-type NiO nanowire mesh photocathodes

We report on the synthesis and photoelectrochemical characterization of photocathodes based on CdS-sensitized Ni–NiO core-shell nanowire mesh inverse opals (IOs). Compared to the NiO–CdS nanowire mesh IO electrode, the hole diffusion coefficient of the CdS-sensitized Ni–NiO core-shell nanowire mesh IO photocathode was one order of magnitude larger, indicating that the Ni core facilitated hole transfer in nanostructured p-type electrodes. As a result, the charge-collection efficiency of the Ni–NiO core-shell nanowire mesh IO electrode was shown to be essentially 100%.