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Title: Experimental studies and modelling of charge transport and defect states in disordered organic semiconductors
Author: Shi, Xingyuan
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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Understanding and optimising charge carrier transport in conjugated polymers is an important challenge for the realisation of organic electronic devices. The transport properties are complicated by the intrinsic disorder in these soft solids, that leads to disorder in the energies and spatial extent of electronic states and in the strength of electronic coupling between them. Traditionally, different disorder models have been used to describe charge transport but the models tend to be descriptive and experimental measurements are often contradictory. The complex origins of disorder have made it difficult to correlate any transport property uniquely with specific defects. This thesis addresses the specific relationship between hole transport and microstructure for selected conjugated polymers. In the first results chapter, we show that the effect of introducing a known conformational defect, the β-phase, into poly(9,9)dioctylfluorene (PFO) is to reduce the time-of-flight hole mobility by two orders of magnitude and the steady state hole mobility by less than one order of magnitude. We develop a high-dynamic-range photocurrent measurement to probe the density of states and show that the β-defect introduces an additional sharp feature into the hole density-of-states (DoS) ~0.3 eV below the transport level. The effect on DoS is confirmed with complementary electrochemical technique and agrees with theory. We then reconcile the different observations from transient and steady-state measurements using an energy resolved device model and conclude that a non-thermal-equilibrium picture is needed to interpret transient measurements in such disordered materials. The second results chapter shows that hole mobility can be controlled and in some cases increased by use of nucleating agents and crystallisation kinetics to promote growth of large crystallites in PFO films; further optimisation may lead to percolation through the crystalline phase. We also show that the effect of a chemical defect (keto defect) on hole transport is relatively minor, despite major impact on emission properties, and in contrast with prior reports. Finally, we show that hole transport in a high mobility indacenodithiophene-altbenzothiadiazole copolymer is indeed fast but not, as previously reported, trap free. Nevertheless, further efforts to improve hole mobility should address inter-chain coupling rather than intra-chain transport.
Supervisor: Nelson, Jenny Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral