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Title: Solution-processed lead zirconate titanate as an interfacial modifier in hybrid oxide/polymer solar cells
Author: Hewlett, Robert
ISNI:       0000 0004 5349 6353
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Due to its excellent optoelectronic properties and ease of processing. ZnO has been considered a promising acceptor material in oxide:polymer hybrid photovoltaics (hPVs); however, devices have consistently underperformed for a number of reasons including poor charge generation and high recombination rates. The characteristics of the oxide:polymer interface have been cited as crucial in determining device performance: to this end, modification of the interface with both molecular and inorganic species has been shown to yield significant improvement. In this study, growth of the perovskite ferroelectric material lead zirconate titanate (PZT) is carried out on ZnO using sol-gel deposition. Subsequently, the PZT layers are used to modify the donor-acceptor interface in ZnO:poly(3-hexylthiophene) (P3HT) hPV devices: this study is carried out in order to lay the foundations for future investigations into the effect of ferroelectric properties on organic-based photovoltaics using inorganic ferroelectric materials. PZT was confirmed to crystallise on ZnO in the tetragonal phase and microstructural characterisation by atomic force microscopy revealed that the ZnO-PZT films consist of small, tightly packed PZT grains, indicating that the ZnO substrates provide a high seeding density for PZT. The ZnO-PZT interface was studied to determine the way by which growth proceeds in these films: whilst it was found that intermixing between the constituent elements of the two materials does not occur, transmission electron microscopy of the interface revealed the presence of a secondary phase thought to be orthorhombic PbO - it is hypothesised that this phase plays a role in the crystallisation of PZT. Growth and characterisation of low thickness PZT (< 15 nm) was conducted, confirming the crystallisation of tetragonal PZT for a film thickness of ~ 11 nm. Finally, the modification of bilayer ZnO:P3HT hPV devices with PZT was shown to have a positive effect on device performance: using transient absorption spectroscopy, the presence of the PZT layer was seen to increase the hole polaron lifetime from 15 μs to 2 ms, indicating a suppression of charge recombination. The open-circuit voltage (Voc) of the hPV devices was observed to increase from 0.162 to 0.734 V; moreover, prolonged illumination of up to 30 minutes led to further increases in Voc, reaching a maximum of 0.947 V - this was attributed to the progressive filling of electron traps with photoexcited charges, leading to a reduction in trap-assisted charge recombination.
Supervisor: McLachlan, Martyn; Stingelin, Natalie Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available