The Effects of Excited States and Energetic Traps on Charge Transport in Disordered Organic Small Molecule Semiconductors.

Abstract

Charge transport is the one the most fundamental concepts in organic semiconductors. The key quantity that characterises this transport behaviour is carrier mobility. The ability to transport carriers in a fast and unimpeded nature in organic devices such as Organic Photovoltaics (OPV) or Organic Light Emitting Diodes (OLED) is a key parameter for building more efficient devices. Significant steps have so far been taken to understand and model this phenomenon, however there are still many questions that need to be answered. One such fundamental question is the role of excited states on the charge transport properties of organic materials which historically has been ignored. This thesis aims to investigate the transport properties of two of the most widely used organic materials, N,N′-bis-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPB) and are N,N’-diphenyl-N,N’-bis 3-methylphenyl-1,1’-biphenyl-4,4’-diamine (TPD). We demonstrate how excitons are generated in a single organic layer OLED devices and how traditionally hole transport materials are capable of fast long range electron transport. We provide a comprehensive analysis of the charge transport properties of both materials with respect to the Gaussian Disorder Model (GDM) and demonstrate how both types of carriers can easily be transported in these materials. We then investigate the effects of exciton generation on the transport properties of the materials and propose some numerical modeling to analyse the effects of such excited states and the distribution of energetic traps in our system. We show that the swing of carrier mobility in either direction depends on the interplay and dominance of each mechanism (triplet/carrier interaction and trap filling). We also investigate the effects of 5 removing excited states from our device by deliberately introducing impurities via doping of a phosphorescent molecule to alter their concentration. Finally we propose some future direction that one can take to model charge transport behaviour in disordered organics based on the experimental work discussed in thesis.