Determination of the interface δ-hole density in a blue-emitting organic semiconductor diode by electromodulated absorption spectroscopy

Abstract

The hole density at the interface of a poly(fluorene-alt-triarylamine) (TFB) with p-doped poly(3,4-ethylenedioxythiophene) (PEDT) anodes has been determined from electromodulation of the TFB subgap polaron band. At 295 K, this δ-hole density σOSC is approximately 1× 1012 cm-2, for which the current-voltage characteristics indicate an Ohmic contact. However at 30 K, σOSC falls to 2× 1011 cm-2, and the contact characteristics approach the injecting-blocking boundary. The PEDT/TFB vacuum offset inside the device is inferred to be 0.1 eV, and so the Fermi level is not as deeply pinned in the TFB gap as suggested by ultraviolet photoemission measurements. © 2010 American Institute of Physics.