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On the Relation Between Exciton Dynamics and Nano-Morphology in Organic Semiconductor Blends / Drew Riley

Swansea University Author: Drew Riley

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DOI (Published version): 10.23889/SUthesis.62403

Abstract

Optoelectronic processes in semiconductor-based devices are widely understood through the constructs of highly-symmetric crystalline inorganic systems, where the lattice periodicity allows significant simplification. Emerging technologies, such as organic semiconductor-based devices, share many qual...

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Published: Swansea 2023
Institution: Swansea University
Degree level: Doctoral
Degree name: Ph.D
Supervisor: Armin, Ardalan ; Sandberg, Oskar J. ; Meredith, Paul
URI: https://cronfa.swan.ac.uk/Record/cronfa62403
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Abstract: Optoelectronic processes in semiconductor-based devices are widely understood through the constructs of highly-symmetric crystalline inorganic systems, where the lattice periodicity allows significant simplification. Emerging technologies, such as organic semiconductor-based devices, share many qualities with crys-talline inorganic semiconductors; however, they diverge in subtle yet important ways. Optical absorption in organic semiconductors gives rise to long-lived and tightly-bound excitons, which migrate in manner often disregarded in the un-derstanding of highly-symmetric crystalline inorganic semiconductors. Further, ‘free’ charge carriers in organic semiconductors ‘hop’ between organic molecules in a disordered film rather than the undergo band transport that delocalised car-riers in periodic lattices do. This hopping transport leads to lower charge carrier mobilities which have far-reaching ramifications to device operation.In the work summarised in this thesis the effect that excitonic and charge transport have on device performance of solar cells based on organic photo-voltaics will be explored. Experimental techniques are designed and developed to gain insight into the efficiency of exciton transport, the nanostructure of organic-semiconductor blends, and the relation between charge injection and extraction. Utilising these techniques, various state-of-the art systems are examined in detail and various pathways for improving device performance are voiced.Specifically, a technique to measure the exciton diffusion length in organic semiconductors is developed and shown to have many advantages over established techniques while improving the accuracy of the measurement. This technique is expanded to blends of organic semiconductors to quantify the efficiency of dif-fusion and quenching occurring between semiconductors in blends. This, along with a developed theoretical understanding, allows for the size of the phase sepa-rated domains to be quantified. Relationships between the excitons generated in organic semiconductors, charge carriers created in the blends, and the transport of charges to and extraction at the electrodes is considered in detail. Finally, a technique to distinguish between the nonradiative recombination occurring within an organic semiconductor blend and at the interface between the blend and the larger device structure is introduced. This technique utilises the well-establish reciprocity theory to reconcile the imbalance between charge injection and ex-traction unique to low-mobility organic semiconductors.
Item Description: ORCiD identifier: https://orcid.org/0000-0001-6688-0694
Keywords: Organic semiconductors, solar cells, physics, photoluminescence
College: Faculty of Science and Engineering