Role of electron injection in polyfluorene-based light emitting diodes containing PEDOT:PSS

We report electromodulation (EM) studies of polyfluorene-based light-emitting diodes containing poly(3,4-ethylene-dioxythiophene)-poly(styrene-sulfonate) (PEDOT:PSS), in which the barrier to hole injection is large $(\sim 0.7\mathrm{eV})$. Measurements are reported on devices fabricated with aluminium and barium cathodes to provide respectively poor and efficient electron injection into the active layer. The Al devices exhibit low currents, indicating low rates of electron and hole injection, whereas the Ba devices exhibit high currents and high electroluminescence efficiencies, implying efficient injection of both electrons and holes despite the large hole injection barrier. The Al devices show conventional field-induced EM behavior consistent with the Stark effect (SE). The Ba devices show conventional SE behavior for low applied biases but, above turn-on, the (field-induced) SE features vanish, indicating suppression of the internal field, and are replaced by charge-induced bleaching and absorption features. The behavior of the devices is attributed to the presence of electron traps close to the PEDOT:PSS/organic interface. The experimental findings are consistent with earlier findings by Murata et al., Van Woudenbergh et al., Poplavskyy et al., and Lane et al.