The energy level tuning effect, observed when mixing halogenated versions of similar molecules, has recently been identified as a new way to modify the bulk ionisation energy and electron affinity of an organic semiconducting thin film, and increase the open-circuit voltage in organic solar cells. In this thesis, the energy level tuning effect is extended to organic field-effect transistors (OFETs). A method for the fabricating ambipolar devices with balanced hole and electron transport is...