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Title: Bulk and surface recombination limitations to high voltage solution processed solar cells
Author: Wheeler, Scot Graeme McQueen
ISNI:       0000 0004 7657 625X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The focus of this thesis is an experimental study of how material energetics and non-geminate recombination dynamics combine to define device performance, in two solution processed solar cell technologies; organic and perovskite photovoltaics. The work utilises transient optoelectronic measurements to elucidate the impact of non-geminate recombination losses on open-circuit voltage. The importance of both bulk recombination at the internal active layer heterojunction and surface recombination at the electrode interfaces is considered, particularly in high voltage systems. The first experimental chapter concerns the impact of blend morphology on recombination in OPV. Transient optoelectronic techniques are used as an in situ probe of energetics and recombination kinetics for different morphologies. The impact of polymer:fullerene blend ratio is studied for a range of donor polymers pBTTT, DPPTT-T and PBDTTT-C-T; while the increase in efficiency, despite a decrease in voltage with the use of the solvent additive DIO is studied for the PBDTTT-C-T system. The second experimental chapter reports a study of recombination in OPV blends utilising the recently developed rhodanine flanked non-fullerene acceptor family, particularly FBR. When blended with high performing polymers Pff4TBT-2OD and Pff4TBT-2DT, these acceptors demonstrate unusually high open-circuit voltages (> 1V) compared to PCBM while maintaining high current. If field-dependent geminate recombination, present with FBR but not with PCBM, can be reduced, NFA efficiencies beyond 10 % are achievable. In the final two experimental chapters, the methodology is extended to the new field of perovskite photovoltaics. The importance of charge accumulation and recombination in determining the voltage is investigated. A simple kinetic model allows the voltage behaviour to be reconstructed over a range of operational light intensities. Finally, the origin of voltage is investigated further by varying the halide composition and interlayer choice which impacts both energetics and recombination.
Supervisor: Durrant, James ; Nelson, Jenny Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral