Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) ratio: Structural, physical and hole injection properties in organic light emitting diodes

We report on the employ of several kinds of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) dispersions as a hole injection layer to increase the stability and the charge injection in organic light emitting diodes (OLEDs). The PEDOT:PSS dispersions have been characterized in solution and thin film by means of dynamic viscosity, Dynamic Light Scattering, profilometer, contact angle measurement, UV–Vis–NIR transmittance and current–voltage characteristics. Then, the dispersions have been employed to manufacture OLEDs with structure: indium tin oxide (ITO)–PEDOT:PSS–poly(9,9-dihexyl-9H-fluorene-2,7-diyl–Tris (8-hydroxy)quinoline aluminium–Al. Device electrical and optical properties have been extensively investigated and discussed in function of PEDOT:PSS ratio. We have found that the hole barrier at the ITO–PEDOT:PSS interface plays a key role in electrical transport and in setting the external quantum efficiency.