Investigating the AC Hall effect as a characterisation tool for organic semiconductors

Abstract

Organic semiconductors (OSCs) can be utilised in electronic devices to create a new range of electronic applications. These can be printed over large area flexible substrates, creating lightweight and low-cost alternatives to conventional silicon-based electronics. However, the production of high mobility OSCs, required for optimal device performance, is stunted by a shortage of electrical characterisation methods capable of directly measuring the intrinsic charge-transport properties. This problem can be resolved through the use of the Hall effect, however, conventional instruments do not possess the sensitivity required to probe the weak Hall signals that OSCs produce. This study investigates whether utilisation of the AC Hall effect method which contains an alternating current (AC) magnetic field along with a phase-sensitive lock-in amplifier could resolve the small Hall voltages that OSCs produce.

Initial experiments in this thesis focused on measuring the AC Hall effect at a low magnetic field (\textit{B\textsubscript{rms}} = 1.19 T) in a range of small molecule and polymeric semiconductors typically utilised in organic electronic devices. Despite the enhanced instrumental sensitivity, this method did not increase the poor signal-to-noise ratio produced by the low mobility samples and all were dominated by electrical noise. Further work then focused on measuring the AC Hall voltage in RR-P3HT through modifications to the instrumental set-up. Changes were made to sample and contact geometry alongside instrumental alterations to the phase-sensitive lock-in amplifier settings. Despite attempts to optimise the methodology, the small signal-to-noise ratio of the AC Hall output could not be resolved for this polymeric semiconductor.

However, AC Hall voltages of the polymeric semiconductors RR-P3HT and C14-PBTTT could be enhanced by molecular doping with the small molecular F4TCNQ. Subsequently, through the use of solution sequential and vapour methods, the onset of a clear AC Hall voltage could be detected. Therefore, the doping density, free carrier concentration and resulting charge carrier mobility at which the materials transitioned from incoherent to coherent band-like transport were measured. Through further work, the novel AC Hall method could become a powerful tool for exploring the effect of high molecular dopant concentrations in OSCs.