We have investigated the barrier height for electron injection at the
cathode / polyflu-orene derivatives interface by the internal photoemission
(IPE) spectroscopy techniques using the “electron only device” structure
consisting of TiO2, electron transporting polyimide inter-layer
(IL), and polyfluorene derivatives. We also estimated the barrier height by
the current analysis based on the Schottky thermal emission current model,
and it coincides well to the threshold energy of IPE result only when the
energy is lower than 1.1eV. The measured barrier height obtained by IPE
linearly increases with both the work-function of cathode materials.
However, the slope parameter becomes less than 1 (~0.6) for poly
(9,9-dioctylfluorene) (F8) probably due to the interfacial gap states. On
the other hand, the slope parameter becomes very small (~0.18) for the poly
(9,9-dioctylfluorene)-co- benzo- thiadiazole) (F8BT) probably due to the
electron pinning at the cathode/ acceptor interface.