The role of poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) as a hole injection layer in a blue-emitting polymer light-emitting diode

The authors study the role of the conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS) in determining the transient and steady-state operation of a blue-emitting polymer light-emitting diode. Combining the results from photoemission spectroscopy, time-of-flight photocurrent measurements, and studies on hole-only devices reveals a significant barrier for the injection of holes into the polymer. Simulations with a numerical drift-diffusion model, however, show that the injection currents determined from single-carrier devices cannot account for the rapid transient luminance onset and the efficient steady-state luminance output of the corresponding bipolar light-emitting devices. It is shown that the transient electroluminescence traces measured at different external bias can be well reproduced when assuming the presence of a weak barrier for electron extraction at this interface, which is attributed to electron accumulation at a thin phase-segregated PSS-rich layer at the surface of PEDOT:PSS. In addition, interface conditioning, presumably due to electron trapping near PEDOT:PSS, renders the anode-polymer interface nearly Ohmic. This conditioning, however, occurs on intermediate time scales normally not addressed by either transient or steady-state measurements.