Doping of Hole Conducting Polymers Utilized to Enhance Polymer Electronics

Abstract

For this thesis the system �poly(3,4-ethylenedioxythiophene) (PEDOT) utilized as electrochemically adjusted anodic material for organic devices� has been studied in-depth. PEDOT films were polymerized electrochemically being subsequently adjusted to a certain electrochemical potential. Afterwards the resulting work function of the adjusted oxidation level was determined by Kelvin Probe measurements. This thesis provides unambiguous evidence that the work function of the PEDOT film surface is directly (linearly) correlated to the adjusted electrochemical potential. This finding has been utilized for optimising electronic properties of organic devices. Organic semiconducting polymeric devices of the general structure indium tin oxide (ITO) / electrochemically deposited and doped PEDOT / electroactive polymer / metal electrode have been prepared and characterized. By means of electrochemical doping the PEDOT layer was adjusted to a desired potential and its influence on the respective devices was studied. The adjusted doping level of the PEDOT layer could be directly correlated to its work function. This was demonstrated by Kelvin probe measurements above the semi-freestanding film and by photovoltaic measurements in the finished devices. Thereafter this discovery was utilized to optimise organic light emitting devices (OLED) by adjusting their hole injection barrier. This barrier is given by the difference in work function between the anodic contact (PEDOT) and the highest occupied molecular orbital (HOMO) level of the adjoining polymer film. If this barrier equals the barrier on the cathodic (metal) side of the device an optimized efficiency can be expected in zero order approximation. Experiments verified this assumption. If current contributions of holes and electrons are balanced, in principle each charge carrier could find an opposite charge and decay radiatively with the highest efficiency. Although this consideration neglects other influences like different charge carrier mobilities, it is of high interest to gain a possibility which enables current determination for both sorts of charge carriers rather than just the measurement of an overall current. This thesis presents an approach which enables the separation of electron and hole currents for OLED�s. The utilization of the results of hole only devices (electrons are blocked) allowed predictions about the ratio of current contributions in devices with hole and electron currents. These current contributions also confirmed enhanced efficiencies caused by tuned barriers. Investigations of up-to-date bulk-heterojunction solar cells (OSC) allowed a deeper insight in physical properties which govern these devices. By using a completely undoped PEDOT film the work function of the initial cathodic (metal) electrode could be passed and the PEDOT took over the cathodic function. Slightly doped PEDOT films could be adjusted to the same level as the metal contact causing zero-built-in-field devices. These investigations enabled a correlation between the anodic and cathodic energy levels and a rough estimation of the adjustable absolute range of PEDOT work functions