Understanding transport processes of organic semiconductors via magnetic resonance methods

The last decade has witnessed a huge development of organic semiconducting materials, including the boost in the discoveries of new materials for outstanding optoelectronic performance, as well as the rise of new application scenarios such as flexible electronics, biocompatible sensors, and inkjet printing electronics. The fundamental research focuses on understanding the excitation and charge transport processes and searching for novel material systems with high mobility together with sufficient carrier densities; however, due to the non-periodic arrangements of molecules and the softness of structure rising from the weakly van der Waals intermolecular interactions, the details of the charge transport processes are still not well-understood, in particular for the solution-processed conjugated polymers. Spins of the mobile electrons, as another degree of freedom aside from charge, are able to serve as a probe to understand the transport processes of the carriers in microscopic scales. The electron spin resonance method excites the precession of spins under microwaves and explores the possibility of obtaining information from spin by measuring the microwave power absorption/emission. However, the applicability of this method on organic devices is greatly limited by the relatively low number of carriers in organics and the microwave coupling to the devices due to the device dimensions restricted by the fabrication steps. This dissertation further explores spin resonance methods to understand the details of charge transport physics in organic semiconducting materials, as well as developing a new in-plane rotation platform for ferromagnetic resonances to study the spin-pumping effects between organic/inorganic interfaces. Chapter 1 introduces the development of research activities in organic semiconductors and reviews well-established theories for understanding charge transport physics. Chapter 2 describes the experimental methods used to investigate the charge transport processes, including electrical measurements of OFETs, field-induced ESR, and vertical bilayer devices in ferromagnetic / detection configuration for spin current injections. Chapter 3 applies the field-induced ESR methods to triclinic phase rubrene- a bench-marking organic molecular crystals – with p-type doping, to examine the spin relaxation dynamics of the induced carriers. Chapter 4 reports continuous-wave electrical-detected magnetic resonance measurement to organic field-effect transistors (OFETs) based on solution-processed conjugated polymers. By widely tuning the FET biasing conditions, a second peak caused by the spin blockade effects is able to be separated from the widely acknowledged contribution from electron-hole recombination. Chapter 5 presents the experimental results on the electrical detection of spin current generated from spin pumping of molecular-based ferrimagentic V[TCNE]x based on the inverse spin Hall effect (ISHE), free from the influence of possible electrical and thermal artefacts. This dissertation enriches the toolbox for the spin resonance methods and provides more details about the impact of the disorder in molecular structures on the electrical transport properties in bulk materials.

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

• Wang, Zichen

Advisor dc:contributor.advisor

• Sirringhaus, Henning

• organic semiconductors
• spin
• magnetic resonance
• transport

Licence

• https://www.repository.cam.ac.uk/bitstreams/a06ab5cf-badd-423b-825f-e1dcd155bebd/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.120883

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/388620