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

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 delta-hole density sigma(OSC) is approximately 1x10(12) cm(-2), for which the current-voltage characteristics indicate an Ohmic contact. However at 30 K, sigma(OSC) falls to 2x10(11) 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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3467469]