Aerogel Templated ZnO Dye‐Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO 2 have exhibited solar energy-conversion efficiencies of over 10% and remain one of the most promising candidates for cost-effective solar energy conversion devices. [1,2] The most efficient DSSCs reported to date utilize a high surface area photoanode, which allows for good light harvesting with a moderate extinction dye, in contact with the I 3 À /I À couple which acts as a redox mediator.[3] The good performance of DSSCs is partially attributed to the slow dark reaction, i.e., electron transfer from TiO 2 to I 3 À via a potentially multielectron process. The slow dark reaction kinetics allows for excellent charge collection despite the relatively slow (millisecond) transport through the nanoparticle network.[4]The I 3 À /I À couple, however, has several disadvantages, including limitations on the open-circuit voltage related to the redox potential of the mediator species. In order to push device performance beyond its current limits, a faster redox shuttle, which requires a smaller overpotential to reduce the oxidized dye, is likely necessary, either to increase the photovoltage (provided that the dark current in not enhanced) or to allow alternate dyes with increased spectral coverage to be used. Replacing iodide/ triiodide with faster one-electron, outer-sphere redox reagents such as ferrocene, however, has resulted in significantly worse overall performance. [5,6] Although such fast redox species efficiently reduce the oxidized dye, rapid recombination between electrons in the TiO 2 and the redox shuttle results in poor electron collection. [5][6][7] Therefore, an alternate redox shuttle will concomitantly require faster charge transport through the metal oxide frameworks to allow for complete charge collection under cell operating conditions. Several interesting photoanode architectures have been fabricated with reduced dimensionality, a design feature that is expected to accelerate charge transport. Among these new architectures are hydrothermally grown ZnO nanorod arrays, ZnO nanotubes, and TiO 2 nanotubes. [8][9][10] The ZnO nanorod arrays have been shown to exhibit much faster transport than comparable ZnO nanoparticle networks. [11,12] While nanorod devices showed promising efficiencies of 1.5%, further improvement requires overcoming the technical challenge of increasing the relatively low surface area that currently limits light-harvesting.[8] Nominally one-dimensional ZnO nanotubes showed efficiencies of 1.6%, but with limited lightharvesting again a significant performance limiting factor.[9]Here we introduce a new core-shell material as a pseudo-one dimensional ZnO photoanode produced from coating templates of high aspect ratio substructures, exhibiting initial efficiencies up to 2.4% under AM 1.5 illumination when incorporated in a DSSC. Inert low density, high surface area silica aerogel films, featuring a large range of controllable thickness and porosity, are prepared as substructure templates. The aerogel templates are coated wi...

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

“…[33][34][35] Also, because ALD is not a line-of-sight technique, it can be used to make nanostructured 'thin films' which are prepared in an identical fashion to the model thin films, but have increased light absorption. [36][37][38] Thus lessons learned with our thin film model system can be directly applied to nanostructured architectures with precisely controlled dimensions fabricated via ALD.…”

Electron Transport in Dye-Sensitized Solar Cells Based on ZnO Nanotubes: Evidence for Highly Efficient Charge Collection and Exceptionally Rapid Dynamics

“…4,[10][11][12][13] Access to these architectures has been facilitated by several attractive fabrication strategies, among them (a) surfactant-controlled crystallization, 10,14 (b) controlled film corrosion, 15 and (c) templated atomic-layer deposition (ALD). 11,[16][17][18][19][20][21][22][23][24] Pseudo 1-D nanostructures, particularly with single or polycrystalline domains, are especially interesting since electrons traveling through the nanoparticle semiconductors are believed to move under diffusive control. 25 Here we quantify the characteristic conductivity and capacitance of the photoanodes, Figure 1, in assembled ZnO nanotube DSCs via electrochemical impedance spectroscopy (EIS).…”

A design for large-area fast photo-detectors with transmission-line readout and waveform sampling

“…When compared to conventional channel plates, aerogel-templated materials have larger open area ratios, larger surface areas, and are relatively easy to fabricate in large flat form factors. We have already demonstrated that aerogels can both be functionalized and work well as photo-anodes for solar cells [58][59][60][61][62][63][64][65][66][67][68][69].…”