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Title: The photophysics of metal halide perovskites for next-generation solar cells
Author: Martin, Elizabeth S.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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Metal halide perovskites have emerged as a promising absorber material for efficient third generation solar cells. In this thesis, the photophysics of these materials are investigated by photoluminescence and optical absorption spectroscopy, focussing on the charge-carrier recombination dynamics and the band gap. The work primarily discusses tin-based perovskite materials, which allow the band gap to be tuned to lower values but currently display poor power conversion efficiencies due to rapid charge-carrier recombination. In addition, the effect of thickness on lead-based perovskites is investigated, allowing the band gap to reach higher values by quantum confinement. First, the temperature dependent photoluminescence of MASnI3 is examined. A dramatic reduction in PL spectral linewidth with a concomitant increase in PL lifetime is seen at the low temperature structural phase transition. This points to the deactivation of defects, thus reducing scattering and recombination. Defect tolerance may therefore be related to the structure of the perovskite and could be enhanced by a targeted structural change to increase the charge-carrier lifetime and improve solar cell efficiency. Further to this, the PL lifetime and relative radiative efficiency of FASnxPb1-xI3 are investigated to uncover how metal composition influences charge-carrier recombination mechanisms. The PL dynamics fall into two regimes, with the tin-rich samples displaying fast mono-exponential decay that is largely temperature-independent, and lead-rich samples having longer, more stretched decays dominated by multi-phonon recombination. Band gap bowing of the mixed metal FASnxPb1-xI3 perovskites is found to be parabolic at all temperatures, and is most likely due to local deformations in the lattice to accommodate the random placement of lead and tin ions. The benefits of a lower band gap may therefore be achieved whilst keeping the slower charge-carrier recombination of lead perovskites, thus enabling high efficiency bottom cells for all perovskite tandem solar cells. Finally, the prospect of band gap tuning is investigated from a different angle, by growing very thin films of MAPbI3 by vapour deposition which display quantum confinement. Films initially grow as islands, with blue-shifted photoluminescence, before forming uniform films still thin enough to exhibit quantum confinement. These films could be used to develop efficient LEDs, and the exposition of the growth mode could also lead the way to producing large-area vapour deposited solar cells with larger grain size which improves both efficiency and stability. Together, the results show that the metal composition, the structural phase (which can be adjusted by changing the organic and halide ions) and the dimensions of the material, can all impact the optoelectronic properties, allowing great scope for tuning the properties of these materials to optimize their photovoltaic performance.
Supervisor: Snaith, Henry ; Herz, Laura Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available