Investigating Exciton-Polaron interactions and their effect on high magnetic field effects in organic semiconductors

Abstract

The research area of magnetic field effects (MFE) on organic systems has been intensively studied during the last decade. It has been revealed that there are processes that are subject to low fields (< 50 mT) and processes that are subject to high fields (> 50 mT). While the low field processes are widely accepted to be a result of the suppression of the spin mixing caused by random hyperfine fields and spin-orbit coupling within the devices, the origin of the high field processes is still not clear. Although several mechanisms, like triplet-charge carrier interaction (TCI) and triplet-triplet annihilation (TTA), were proposed to explain the high field MFEs, how these processes are affected by a magnetic field is not well understood. This thesis presents a study of the role of excitons on MFEs using aluminium tris(8-hydroxyquinolinate) (Alq3) based diodes, focusing on the behaviour of high field effects on electroluminescence (MEL). In order to investigate the role of excitons on high field MEL, devices with different structures were designed to modify the population of exciton and excess charge carriers in the devices via controlling the injection of charge carriers. In this way, the exciton population dependent TTA and TCI processes can be studied further and even distinguished, since the TTA depends mainly on the population of triplets while the TCI depends on the exciton to charge carrier ratio. Steady state MFE measurements were performed, and results show that significant high field MEL decay can be seen in a device with extremely low triplet concentration. This indicates that TTA cannot be the underlying mechanism of high field MEL decay. The gradual trend of high field MEL, changing from a moderate increase to significant decays upon adjusting the hole and electron injection from balanced to severely hole dominated, suggests that this high field decay is exciton-hole interaction dependent. To decompose the role of singlets and triplets on high MELs, transient MFE measurements were performed on the Alq3 based standard devices. Since in Alq3 layer singlets feature a lifetime of ~ns and triplets feature a lifetime of ~ms, the behaviour of MEL at the rising edge and the falling edge of a pulse can provide a useful tool. Results show that an extra component occurs in the transient MEL at large current density and high fields. This indicates that the high field MEL is related to triplet-charge carrier interaction.