The Hall Effect for Probing Conjugated Polymer Charge Transport in High Carrier Density Regimes

Abstract

Conjugated polymer semiconductors hold much promise when it comes to their potential for novel applications. Their ability to form uniform films from solution is generally thought to be of high value to industry, opening up the possibilities promised by large area electronics manufacturing. Probing charge transport in these materials often proves difficult. Hall effect mea- surements, a mainstay of semiconductor characterisation in more traditional materials, provide anomalous results when used on polymers. This often leads to inaccurate claims being made off the back of erroneously interpreted Hall data and, in some cases, missing out on interesting physics that gets “screened out” of the Hall effect in these materials. It is for these reasons that a means of both efficiently acquiring and properly interpreting Hall data in polymers was sought. This thesis recounts the creation of a new AC system for measuring the Hall effect in shorter time spans than those required by superconducting electromagnet systems. This is followed by a series of measurements and analysis, leading to the construction of a new model for describing how the Hall coefficient in these materials varies with temperature, and allowing for multiple interesting parameters to be determined. This model is built around a central concept of different carriers in a polymer system having different degrees of average coupling, g, to a magnetic field. This is assumed to be described by a statistical distribution characterised by its average value, ⟨g⟩. This, and other parameters that can be extracted from this modelling, are an exciting prospect for areas of research involving material optimisation. Insights into the average degree of delocalisation of carriers, relative levels of energetic disorder as well as hopping dimensionality can all theoretically be determined. This is in addition to the traditional quantities typically extracted from Hall measurements: mobile charge carrier density and mobility. By performing these analyses on data measured from ion-exchange doped PBTTT, P3HT and IDT-BT, as well as electrochemically gated DPP-BTz, several interesting results were determined. These include values, such as the hopping temperature coefficient for PBTTT, that indicated these polymer systems exhibit a greater amount of energetic order when they are doped. Similarly, for PBTTT, it has been shown that the overall degree of delocalisation of carriers increases for greater doping levels. The most highly-doped PBTTT device was also found to exhibit diminishing returns on conductivity enhancement with greater carrier densities, owing to decreasing mobilities. This suggests that it approaches a limit on conductivity through increased doping alone, while independent, spectroscopic carrier-density measurements suggest this is occurring at a near-100% level of doping efficiency. Measurements on the less conductive systems proved to be more difficult, and led to many of them not being able to be fully fit. However, some useful insights were still gained. DPP-BTz was observed to change from p-type to n-type transport when doped highly enough. Beyond this, its conductivity would also start to decrease the further into this n-type regime the doping went. It was therefore concluded that the limit of one carrier per monomer unit must have been reached and exceeded, causing the band to no-longer be limited by hole transport. This similarly suggested that at high gate voltages, DPP-BTz Organic Electrochemical Transistor (OECT) devices were also capable of achieving near-100% doping efficiencies. Perhaps the most intriguing result of all is the promise the model and analyses in this thesis hold. While much analysis was limited by the number of data points available in many cases, as well as the quality of the data in some cases, it nonetheless showed that it is possible to extract useful information from the Hall effect in these materials when treated carefully. Future work can therefore focus on using these tools to analyse better and denser data, enabling it to use the findings in this thesis as a tool in the drive forward to optimise these polymer systems, thus perhaps enabling their use across their many potential applications.