The effects of strain on the electronic and optical properties of rubrene single crystals

Organic semiconductors are an emerging class of materials which show great potential for a wide range of applications yet, despite intense efforts to understand the physics at play in these systems, there remain many open questions. One leading theory for charge transport in these materials is that of transient localisation, where the charge carriers are in general delocalised but become spontaneously localised by dynamic disorder present in the crystal due to molecular vibrations. This theory has succeeded in explaining many phenomena seen in organic crystals but there are still uncertainties about how best to optimise materials for specific properties based on transient localisation theory. One important area of focus is that of how the crystal structure impacts the delocalisation and relevant vibrations, and hence the charge transport. Strain provides a useful tool for manipulating the structure in a controlled way without the more drastic changes of altering molecular structure. With this in mind rubrene was chosen as a model system due to the ease of growing high quality single crystals and a host of desirable properties including air and light stability, high mobility, bright luminescence and, crucially, flexibility. The outset of this project involved designing and testing the apparatus capable of straining single crystals and measuring the desired properties in situ. This involves a computer-controlled strain stage that bends the samples to a controlled bending radius and a way to make electrical connection to the crystal whilst under strain. Once developed, several properties were subject to investigation. As a starting point the strained mobility trend was measured and found to agree with several previous theoretical and experimental data, lending weight to the methods used. Three different device architectures were optimised and it was confirmed that this is not responsible for the large discrepancies seen in the literature, although the exact cause remains unclear. The trend of increasing mobility on compression is attributed to increases in orbital overlap that occur on straining rather than a decrease to the dynamic disorder present (and vice versa in tension), confirmed by Raman spectroscopy. Significantly, the first measurements of the Seebeck coefficient under strain have been performed, showing an order of magnitude smaller sensitivity to structural differences and opposite sign to the mobility changes. The Seebeck coefficient is determined by the position of the Fermi level and the energy dependence of the scattering times for various charge carriers, which can vary as strain impacts their localisation. This could provide a new avenue for improving the performance of thermoelectric devices by suggesting a way to boost electrical conductivity without compromising the Seebeck coefficient. Finally, a study on the optical properties of rubrene crystals has been performed. Here the trend in photoluminescence emission intensity is not linear but quadratic, with large decreases observed in both tension and compression despite a lack of change in the decay lifetimes seen. This is explained in terms of the singlet fission and triplet-triplet annihilation processes that are responsible for a majority of the emission in rubrene. By altering the proportion of singlets that undergo fission and subsequent fusion the overall intensity can be drastically changed. Again, the changes are attributed to differences in the coupling between singlet and triplet wavefunctions which are modulated by strain as the different packings change the relative position of molecules. In all of these studies a key result seems to be that the result of straining is mostly due to changes to the equilibrium geometry of the crystal packing, rather than the minor effects of altering the dynamic disorder. This insight helps to further understanding of transient localisation theory and can provide guidelines for future materials development that emphasise the crystal packing as an important parameter that can be tuned to optimise the properties of interest.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Goldberg, Elliot

Advisor dc:contributor.advisor

• Sirringhaus, Henning

• rubrene
• transient localisation
• organic semiconductors
• strain
• Seebeck coefficient
• mobility
• photoluminescence
• transient absorption spectroscopy
• transistors

Licence

• https://www.repository.cam.ac.uk/bitstreams/0cb854cb-5954-4561-a9be-72cb2b7ce0d5/download
• https://creativecommons.org/licenses/by-nc/4.0/

Language dc:language

eng

Author Identifier

0009-0007-3640-8699

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/398989