Dresden 2011 – scientific programme
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HL: Fachverband Halbleiterphysik
HL 67: OLEDs and OFETs
HL 67.1: Talk
Wednesday, March 16, 2011, 18:00–18:15, FOE Anorg
Band bending and energy-level alignment in organic semiconductors — •Ilja Lange1, James Blakesley1, Johannes Frisch2, Norbert Koch2, and Dieter Neher1 — 1Universität Potsdam — 2Humboldt Universität Berlin
Energy level alignment at organic semiconductor / electrode interfaces has been the subject of intensive debate in recent years. In particular, the existence of band bending in undoped organic semiconductors is disputed. It has been proposed that strong band bending should be present due to electronic states within the energy gap of a disordered material. It is also known that such states dominate some crucial properties of organic semiconductor devices, such as charge injection and charge transport. Thus the elucidation of the distribution of these tail states provides a key to understanding fundamental processes in such devices and hence to increasing device efficiency. Unfortunately, the densities of the relevant states are often so low that they are difficult to detect directly. We use a Kelvin probe (KP) to study the energy level alignment of four undoped conjugated polymers deposited on various electrodes. Band bending is observed in all polymers when the substrate work function exceeds certain critical values. Through modeling, we show that the band bending is caused by charge transfer into a low-density population of states that extends several hundred meV into the band gap. KP can therefore be used as a tool to study the energetic distribution of such states. The energetic spread of these states is correlated with charge transport properties, suggesting that these states also determine relevant device properties.