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Title: Ultrafast spectroscopy of charge separation, transport and recombination processes in functional materials for thin-film photovoltaics
Author: Wehrenfennig, Christian
ISNI:       0000 0004 5367 7488
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2014
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Dye-sensitized solar cells (DSSCs) and perovskite solar cells are emerging as promising potential low-cost alternatives to established crystalline silicon photovoltaics. Of the employed functional materials, however, many fundamental optoelectronic properties governing photovoltaic device operation are not sufficiently well understood. This thesis reports on a series of studies using ultrafast THz and photoluminescence spectroscopy on two classes of such materials, providing insight into the dynamics of charge-transport and recombination processes following photoexcitation. For TiO2-nanotubes, which have been proposed as easy-to-fabricate electron transporters for DSSCs, fast, shallow electron trapping is identified as a limiting factor for efficient charge collection. Trapping lifetimes are found to be about an order of magnitude shorter than in the prevalently employed sintered nanoparticles under similar excitation conditions and trap saturation effects are not observed, even at very high excitation densities. In organo-lead halide perovskites - specifically CH3NH3PbI3 and CH3NH3PbI3-xClx, which have only recently emerged as highly efficient absorbers and charge transporters for thin-film solar cells, carrier mobilities and fundamental recombination dynamics are revealed. Extremely low bi-molecular recombination rates at least four orders of magnitude below the prediction of Langevin's model are found as well as relatively high charge-carrier mobilities in comparison to other solution-processable materials. Furthermore a very low influence of trap-mediated recombination channels was observed. Due to a combination of these factors, diffusion lengths reach hundreds of nanometres for CH3NH3PbI3 and several microns for CH3NH3PbI3-xClx. These results are shown to hold for both, solution processed and vapour-deposited CH3NH3PbI3-xClx and underline the superb suitability of the materials as absorbers in solar cells, even in planar heterojunction architectures. The THz-frequency spectrum of the conductivity of the investigated perovskites is consistent with Drude-like charge transport additionally exhibiting weak signatures of phonon coupling. These coupling effects are also reflected in the luminescence of CH3NH3PbI3-xClx, where they are believed to be the cause of the observed homogeneous spectral broadening. Further photoluminescence measurements were performed at temperatures between 4 K and room temperature to study the nature of recombination pathways in the material.
Supervisor: Herz, Laura M. Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Condensed Matter Physics ; terahertz ; THz ; photoluminescence ; perovskites ; TiO2 ; nanotubes ; dye-sensitized ; solar cells ; charge carrier dynamics ; CH3NH3PbI3 ; CH3NH3PbI(3-x)Clx