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Manipulating Energy Levels in Organic Solar Cell Materials

Shaikh, Jordan; (2021) Manipulating Energy Levels in Organic Solar Cell Materials. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

Organic solar cells offer the prospect of an entire new class of renewable energy resource. This thesis aims at addressing the issue of triplet state formation in these devices, which can contribute to both a reduction in device efficiency and operational lifetime. Furthermore, triplet states can be exploited to enhance device efficiency via photophysical processes such as triplet-triplet annihilation upconversion and thermally activated delayed fluorescence. Transient absorption spectroscopy on a timescale of ps-ms is employed as the primary investigative technique throughout this thesis. In chapter 3 we explore the dynamics of a prototypical small molecule DPP/fullerene system, which was found to elegantly demonstrate the issue of triplet state formation stemming from recombination. It was found that ultrafast spin mixing, followed by subsequent geminate recombination yielded the donor-triplet exciton, the lowest energy state of the system. In chapters 4 and 5 novel polymeric materials were investigated, which were designed with the intention of manipulating/harvesting triplet states such that they no longer constitute a loss mechanism. In chapter 4 a zinc porphyrin-F8BT hybrid copolymer solution, F8BT-HAPAPP, was found to undergo a dual energy-transfer mechanism, whereby the fate of each photogenerated F8BT singlet exciton was found to depend upon its distance from the porphyrin unit. Intriguingly, the F8BT-HAPAPP triplets generated were found to possess a lifetime intermediate between the two pristine materials. In chapter 5 a donor-orthogonal acceptor low band-gap polymer, Thiro, was found to exhibit facile triplet formation on a <200 fs timescale. This rapid triplet population was thought to derive from a spin-orbit charge transfer interaction, afforded by CT states deriving from the perpendicular substituted spirobifluorene groups. Most peculiarly, free charge formation was observed in both the solution and pristine Thiro film, as evidenced via the observation of a radical cation in the μs-TA spectra. This assignment was further validated via comparison with a Thiro:PC60BM μs-TA spectrum, which is the first time a D-oA conjugated polymer has been investigated in a bulk heterojunction.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Manipulating Energy Levels in Organic Solar Cell Materials
Event: UCL
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10129211
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