Charge transport and excited states in organic semiconductors

Abstract

Organic semiconductors are of increasing technological interest in applications such as light emitting diodes, field effect transistors, and photovoltaic devices In order to reveal the basic principles behind these organic semiconductors, charge transport theory in these organic materials has been introduced and has been receiving increasing attention over the last few years. Although excitons are known to interact with free charges, the effect that excited states may have on the charge transport is not generally considered in the field of organic electronics. This occurs even though organic light emitting diodes (OLEDs) are known to contain large numbers of triplet states during operation. Indeed, it is quite possible that the mobility in working devices may well be a function of drive current, as the excited state population will change with operating conditions. This work is thus motivated by both technological and fundamental scientific interest. In this thesis, the hole mobilities in both poly-(3-hexylthiophene) (P3TH) and N,N’- diphenyl-N,N’-bis(3-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD) devices ( P HT cm Vs Hole m 3 » 5.0´10-5 2 / , TPD cm Vs Hole m » 5.0´10-4 2 / ) have been measured, and observed a remarkable mobility reduction (～15%) in ambipolar samples (in both P3HT and TPD) after applying a small DC offset bias. This correlated to the turn-on voltage in I-V characterization, and the luminescence in the ambipolar TPD sample. In the unipolar sample, however, there is no such behaviour. This strongly suggests that the reduction of the hole mobility is due to site blocking/interacting caused by the excited triplet states. In further experiments in the presence of a magnetic field (500 mT), results an increase in the mobility ( ～ 5%) and steady state current density in ambipolar samples only, this is consistent with magnetically mediated inter-conversion of 4 (blocking/interacting) triplet states to the singlet states. The correlation between the magnetic mobility increase and the steady state current increase offers direct evidence for a microscopic mechanism behind organic magneto resistance (OMR). Given the experimental evidence, we conclude that excitons (specifically triplet states play a critical role in charge transport in organic semiconductors.