|Making Contact to Organic Light-Emitting Diodes |
Laboratory for Surface Modification
|Dr. Paul Lane, Optical Sciences Division, Naval Research Laboratory|
12:00 PM, Chem 260
Interfaces play a key role in organic opto-electronic devices. From charge separation at heterojunctions in organic solar cells to energetic offsets in organic light-emitting diodes (OLEDs), understanding and controlling interfaces is fundamental to understanding and improving the performance of such devices. This talk focuses on the interface between the anode and both polymer and molecular semiconductors. Indium tin oxide (ITO) is the standard material used for the anode of organic displays owing to its high transparency and moderate resistance. Unfortunately, the work function of ITO is much lower than the ionization potential of many organic semiconductors. This problem has been surmounted by coating ITO with the conducting polymer blend, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). This polymer blend has been shown to be an excellent hole injection material and is consequently widely used, especially in polymer-based OLEDs.
The origin of efficient hole injection from PEDOT:PSS is, however, poorly understood. The work function of PEDOT:PSS is 5.1 ± 0.1 eV, yet efficient hole injection from PEDOT:PSS into materials with ionization potentials as high as 5.8 eV has been demonstrated. Such a large injection barrier should preclude the use of PEDOT:PSS as an anode contact. The inconsistency between materials properties and device performance motivated studies of OLED physics. I present spectroscopic studies of polyfluroene OLEDs that show the Fermi level at the anode contact is pinned to the highest occupied molecular orbital. Such pinning eliminates the hole injection barrier, allowing efficient hole injection, even into materials with much higher ionization potentials.
A novel architecture for molecular OLEDs was demonstrated based on this new understanding of PEDOT:PSS. Molecular OLEDs are generally multi-layer structures that incorporate separate layers for hole and electron transport and for light emission. We demonstrate that PEDOT:PSS can inject holes directly into the archetypical molecular semiconductor tris(quinolin-8-olato) aluminum (III). Eliminating the hole transport layers simplifies device design, resulting in OLEDs with significantly lower operating bias than conventional bi-layer structures.