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 δ -hole density σ OSC is approximately 1 × 10 12 cm − 2 , for which the current-voltage characteristics indicate an Ohmic contact. However at 30 K, σ OSC falls to 2 × 10 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.