Influence of poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate) in polyfluorene-based light-emitting diodes: Evidence of charge trapping at the organic interface

Although the improvement of the indium tin oxide (ITO) anode by PEDOT:PSS has often been reported in literature, the origin of it is not well known. In this paper, we investigate polyfluorene-based devices with a relatively high work function (Al) cathode by electroabsorption (EA) spectroscopy and thermally stimulated current (TSC) techniques. A 0.9 V increase of built-in voltage evidenced by EA in PFV-based devices was similar to that obtained previously in PFO-based devices. It was inconsistent with the work function difference between ITO (4.9 eV) and PEDOT:PSS (5.2 eV). The formation of new types of traps related to the presence of the PEDOT:PSS layer was detected by TSC in PF-N-Ph based devices, confirming that the PEDOT:PSS/PF contact layer acts as an electron trapping surface. We conclude that the TSC technique should allow a deeper characterization of charge traps created at the electrodes and a better modelling of charge injection in future studies.