Photovoltaic and optoelectronic device group

Abstract:

We have used optical-pump terahertz-probe spectroscopy to explore the photoinduced conductivity dynamics in mesoporous anatase TiO2 films, commonly employed as the electron-transporting electrode in dye-sensitized solar cells. We find an intrinsic mobility value of 0.1 cm2/(V s) and diffusion length of ∼20 nm for electron motion through the TiO2 matrix. The photoconductivity dynamics in TiO2 films, both before and after sensitization with a ruthenium bypyridyl complex termed Z907, were examined in order to study the charge injection, trapping, and recombination time scales. We observe a biphasic charge injection from Z907, with a fast sub-500 fs component, followed by a slower 70-200 ps component. This is followed by photoconductivity decay over the first few nanoseconds, predominantly reflecting charge carrier trapping. In addition, we have utilized terahertz spectroscopy to investigate the influence of treating the titania surface with TiCl4 on early-time charge dynamics. In the solar cells, surface treatment of the mesoporous TiO2 with TiCl4 is critical to enable efficient operation. Here, we find that neither early-time charge mobility nor charge injection rate or decay times are significantly affected by the treatment, which suggests that it may, instead, have an impact on phenomena occurring on longer time scales. © 2010 American Chemical Society.

Abstract:

We report on the synthesis and spectroscopic study of a novel highly ordered nanohybrid structure consisting of a single-walled carbon nanotube (SWNT) coated with highly crystalline regio-regular poly(3-hexylthiophene) (rrP3HT) and discuss the applicability of the nanohybrids in organic photovoltaics. The use of a solvent extraction technique allows the nanohybrids to be produced with a high yield and high purity. We find evidence that the crystallinity of rrP3HT is enhanced in the presence of SWNTs, which introduces a reduced optical band gap and increased carrier mobility in the polymer. Study of the photoluminescence excitation spectra of the SWNTs reveals an efficient energy transfer of excitons created on the rrP3HT to the SWNTs. This energy transfer is expected to limit our ability to use the nanohybrids as a charge separating interface and can therefore explain the low efficiency of P3HT-SWNT solar cells produced to date. In addition, careful consideration of the energy transfer is necessary when attempting to improve state of the art polymer-fullerene photovoltaic devices with SWNTs in order to make use of their high charge carrier mobilities and increased rrP3HT crystallinity.

Abstract:

For an ideal solar cell, a maximum solar-to-electrical power conversion efficiency of just over 30% is achievable by harvesting UV to near IR photons up to 1.1eV. Dye-sensitized solar cells (DSCs) are, however, not ideal. Here, the electrical and optical losses in the dye-sensitized system are reviewed, and the main losses in potential from the conversion of an absorbed photon at the optical bandgap of the sensitizer to the open-circuit voltage generated by the solar cell are specifically highlighted. In the first instance, the maximum power conversion efficiency attainable as a function of optical bandgap of the sensitizer and the "loss-in-potential" from the optical bandgap to the open-circuit voltage is estimated. For the best performing DSCs with current technology, the loss-in-potential is -0.75eV, which leads to a maximum power-conversion efficiency of 13.4% with an optical bandgap of 1.48 eV (840 nm absorption onset). Means by which the loss-in-potential could be reduced to 0.4 eV are discussed; a maximum efficiency of 20.25% with an optical bandgap of 1.31 eV (940 nm) is possible if this is achieved © 2010 WILEY-VCH Verlag GmbH & Co. KCaA.

Abstract:

Recently ZnO nanowire films have been used in very promising and inexpensive dye-sensitized solar cells (DSSC). It was found that the performance of the devices can be enhanced by functionalising the nanowires with a thin metal oxide coating. This nm-scale shell is believed to tailor the electronic structure of the nanowire, and help the absorption of the dye. Core-shell ZnO nanowire structures are synthesised at low temperature (below 120°C) by consecutive hydrothermal growth steps. Different materials are investigated for the coating, including Mg, Al, Cs and Zr oxides. High resolution TEM is used to characterise the quality of both the nanowire core and the shell, and to monitor the thickness and the degree of crystallisation of the oxide coating. The interface between the nanowire core and the outer shell is investigated in order to understand the adhesion of the coating, and give valuable feedback for the synthesis process. Nanowire films are packaged into dye-sensitised solar cell prototypes; samples coated with ZrO2 and MgO show the largest enhancement in the photocurrent and open-circuit voltage and look very promising for further improvement. © 2010 IOP Publishing Ltd.