We report on the microstructure and dynamics of electron transport and recombination in dye-sensitized solar cells (DSSCs) incorporating oriented TiO2 nanotube (NT) arrays. The morphology of the NT arrays, which were prepared from electrochemically anodized Ti foils, were characterized by scanning and transmission electron microscopies. The arrays were found to consist of closely packed NTs, several micrometers in length, with typical wall thicknesses and intertube spacings of 8-10 nm and pore diameters of about 30 nm. The calcined material was fully crystalline with individual NTs consisting of about 30 nm sized crystallites. The transport and recombination properties of the NT and nanoparticle (NP) films used in DSSCs were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. While both morphologies display comparable transport times, recombination was much slower in the NT films, indicating that the NT-based DSSCs have significantly higher charge-collection efficiencies than their NP-based counterparts. Dye molecules were shown to cover both the interior and exterior walls of the NTs. Analysis of photocurrent measurements indicates that the light-harvesting efficiencies of NT-based DSSCs were higher than those found for DSSCs incorporating NPs owing to stronger internal light-scattering effects.

We report on the influence of morphological disorder, arising from bundling of nanotubes (NTs) and microcracks in films of oriented TiO2 NT arrays, on charge transport and recombination in dye-sensitized solar cells (DSSCs). Capillary stress created during evaporation of liquids from the mesopores of dense TiO2 NT arrays was of sufficient magnitude to induce bundling and microcrack formation. The average lateral deflection of the NTs in the bundles increased with the surface tension of the liquids and with the film thicknesses. The supercritical CO2 drying technique was used to produce bundle-free and crack-free NT films. Charge transport and recombination properties of sensitized films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. Transport became significantly faster with decreased clustering of the NTs, indicating that bundling creates additional pathways via intertube contacts. Removing such contacts alters the transport mechanism from a combination of one and three dimensions to the expected one dimension and shortens the electron-transport pathway. Reducing intertube contacts also resulted in a lower density of surface recombination centers by minimizing distortion-induced surface defects in bundled NTs. A causal connection between transport and recombination is observed. The dye coverage was greater in the more aligned NT arrays, suggesting that reducing intertube contacts increases the internal surface area of the films accessible to dye molecules. The solar conversion efficiency and photocurrent density were highest for DSSCs incorporating films with more aligned NT arrays owing to an enhanced light-harvesting efficiency. Removing structural disorder from other materials and devices consisting of nominally one-dimensional architectures (e.g., nanowire arrays) should produce similar effects.

The objective of this research is to determine the operational characteristics key to efficient, low-cost, stable solar cells based on dye-sensitized mesoporous films (in collaboration with DOE's Office of Science Program). Toward this end, we have investigated the mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO 2 nanocrystals, improves the open-circuit photovoltage (V OC ) and short-circuit photocurrent density (J SC ). 1 We find that adding chenodeoxycholate not only shifts the TiO 2 conduction-band edge to negative potentials but also accelerates the rate of recombination. The net effect of these opposing phenomena is to produce a higher photovoltage. It is also found that chenodeoxycholate reduces the dye loading significantly but has only a modest effect on J SC . Implications of these results to developing more efficient cells are discussed. ObjectivesThe dye-sensitized solar cell (DSC) is a promising future generation concept for achieving efficient solarto-electric energy conversion with very inexpensive materials and manufacturing processes. These cells feature dye molecules chemisorbed on the surface of TiO 2 nanocrystals that have been sintered into a highly porous thin film. The pores of the film are filled with a hole-conducting electrolyte. Photoexcited dye molecules inject electrons into the conduction band of TiO 2 , and species in the electrolyte transport the holes from the oxidized dye molecules to the counter electrode. While J SC is mostly controlled by the lightharvesting and charge-injection efficiencies of the dye, V OC is determined by the difference between the quasiFermi level of electrons in the TiO 2 film and the energy of the redox couple in the electrolyte.Over the past decade, numerous studies have reported that chemically treating the TiO 2 surface with certain organic molecules influences the photovoltage and/or the photocurrent. Passivation or shielding of recombination centers by adsorbed molecules is generally given as the explanation for the observed effect. However, there is evidence that adsorbed molecules can improve the cell performance by inducing band-edge movement.2 In most studies, the proposed mechanism for the improved cell performance is based on measurements that do not distinguish between shielding and band-edge movement. To devise more effective surface treatment strategies, it is critical to understand the mechanism by which the surface treatment affects cell performance. In this paper, we examine the basic physical processes by which the adsorbent, chenodeoxycholate, affects V OC and J SC in dyesensitized solar cells. Chenodeoxycholate is a cholesterol-based molecule that has been shown to improve J SC and V OC .Understanding the fundamental phenomena governing cell performance is critical for underpinning the development of this new solar cell technology. Of programmatic importance is that sensitized nanoparticle solar cells represent the distinct technological promise for achieving mass-produced, ultra-low-co...

The mechanism by which the adsorbent guanidinium affects the open-circuit photovoltage of dye-sensitized TiO2 nanocrystalline solar cells was investigated. The influence of the guanidinium cation on the rate of recombination and band-edge movement was measured by transient photovoltage. When guanidinium is present in the electrolyte recombination becomes slower by a factor of about 20. At the same time, the adsorbent causes the band edges to move downward, toward positive electrochemical potentials, by 100 mV. The collective effect of both a downward shift of the band edges and slower recombination, owing to the presence of guanidinium, results in an overall improvement in the open-circuit photovoltage.