Nearly Ohmic injection contacts from PEDOT: PSS to phenylamine compounds with high ionization potentials

We demonstrate that poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) can act as an excellent hole injection material for small organic charge transporters. With PEDOT:PSS as a conducting anode, it is possible to achieve nearly Ohmic hole injection contacts to phenlyamine-based materials with HOMO values of up to 5.5 eV. In current-voltage experiment, the PEDOT:PSS anode can achieve nearly Ohmic hole injection to NPB (N,N'diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine), and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl) (1,1'-biphenyl)-4,4'diamine). Meanwhile, dark-injection space charge limited current (DI-SCLC) transients are clearly observed and are used to evaluate the charge-carrier mobility of these phenylamine compounds. The carrier mobilities extracted by DI-SCLC are in excellent agreement with independent time-of-flight (TOF) technique. It is conceivable that PEDOT:PSS can be used as a general conducting anode for the electrical characterizations of organic materials that require Ohmic hole contacts.