Origin of efficient hole injection from conducting polymer anodes into organic light-emitting diodes

We studied hole injection from the conducting, polymer blend poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) by optical spectroscopy and characterization of organic light-emitting diodes (OLEDs). Electroabsorption (EA) spectroscopy was used to measure the built-in potential of polyfluorene-based OLEDs with indium tin oxide (ITO) or poly(3.4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOTTSS) anodes. Although the work function of PEDOT:PSS is 5.1 eV, the inferred anode work function matches the ionization potential of the emitting polymer. We conclude that the Fermi level at the PEDOT:PSS/polyfluorene interface is pinned to the highest-occupied molecular orbital (HOMO) of the emitting polymer, permitting efficient hole injection. To test this hypothesis, we fabricated OLEDs usin- the archetypical molecular semiconductor, tris(8-hydroxyquinoline) aluminum (III) (Alq(3)). Although the anticipated hole injection barrier is 0.7 cV, OLEDs with Alq3 deposited onto PEDOTTSS operate at a lower bias and higher power efficiency than OLEDs with a hole transport layer. The quantum efficiency of single layer Alq(3) and rubrene-doped Alq(3) devices is equal to that of multi-layer devices, showing that EL is not quenched by PEDOTTSS.