Prof Henry Snaith FRS

Abstract:

Improving the solar light harvesting and photon-to-electron conversion efficiency for hybrid, organic-inorganic photovoltaics are critical challenges. Titania based solid-state hybrid solar cells are moderately efficient at converting visible photons to electrons, but major electrical losses still remain. A material based paradigm shift is required to dramatically enhance the performance of these devices. Here, we present an investigation into solid-state dye-sensitized solar cells (SDSCs) incorporating a molecular hole-transporter and mesoporous tin oxide electrodes, in place of titania usually employed. We investigate the influence of treating the surface of the SnO(2) with different oxides and find that MgO "passivated" SnO(2) electrodes demonstrate an unprecedented absorbed photon-to-electron conversion efficiency of near unity across a broad spectral range. A dual surface treatment of TiO(2) followed by MgO enables tuning of the solar cell photovoltage, fill factor, and efficiency with visible light absorbing cells delivering 3% solar-to-electrical full sun power conversion efficiency.

Abstract:

This work is devoted to the development of hybrid bulk heterojunction solar cells based on porous zinc oxide (ZnO) electrodes and poly(3-hexylthiophene) (P3HT), using simple synthesis procedures and deposition techniques. Starting from ZnO nanocrystals with well-controlled properties, porous ZnO electrodes of suitable porosity are deposited by spin-coating, varying the main experimental parameters such as composition of the initial ZnO formulation and choice of the organic ligand. Significant charge transfer yields are observed in the corresponding solar cells, and the influence of processing conditions on device performance is investigated using conventional techniques as well as transient photovoltage/photocurrent decay measurements. The temperature used to sinter the ZnO electrode is found to be specifically crucial to ensure efficient charge transport in the device while avoiding a loss in interfacial area through nanocrystal coalescence. Using 8 × 13 nm ZnO nanorods, the best device exhibits a power conversion efficiency of 0.35% under 100 mW·cm ^-2 AM1.5G simulated solar emission. This strategy, using processing in air with simple deposition techniques, competes with related approaches based on nanostructured ZnO processed using more complex procedures. Moreover, device performance and photophysics are found to be greatly influenced by the morphology of the starting ZnO nanocrystals, illustrating that fine control of the inorganic component can effectively tune the performance of hybrid bulk heterojunction solar cells. © 2010 American Chemical Society.

Abstract:

We present a material and device based study on the fabrication of mesoporous TiO2 and its integration into dye-sensitized solar cells. Poly(isoprene-block-ethyleneoxide) (PI-b-PEO) copolymers were used as structure directing agents for the sol-gel based synthesis of nanoporous monolithic TiO2 which was subsequently ground down to small particles and processed into a paste. The TiO2 synthesis and the formation of tens of micrometre thick films from the paste is a scalable approach for the manufacture of dye sensitised solar cells (DSCs). In this study, we followed the self-assembly of the material through the various processing stages of DSC manufacture. Since this approach enables high annealing temperatures while maintaining porosity, excellent crystallinity was achieved. Internal TiO 2 structures ranging from the nanometre to micrometre scale combine a high internal surface area with the strong scattering of light, which results in high light absorption and an excellent full-sun power conversion efficiency of up to 6.4% in a robust, 3 μm thick dye-sensitized solar cell. © 2010 The Royal Society of Chemistry.

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.