Properties of the hole-injection layer in organic semiconducting devices

Abstract

Since the 1990s, poly(3-4,-ethylenedioxythiophene) complexed with poly(styrenesulfonic acid) (PEDT:PSSH) has been the material-of-choice as a hole-conductor for organic devices. Here, we summarize the optical and electronic properties of PEDT:PSSH and its various roles in organic semiconductor devices. We also show that despite its known environmental stability, PEDT:PSSH exhibits an instability of its redox-state during charge transport which can be reversed with chemical re-doping. We also describe evidence for electromigration of doped PEDT chains in the PSSH matrix at high current densities which can be surppressed by suitable crosslinking. We show that contrary to conventional wisdom, the work-function of PEDT is strongly determined by the Madelung potential of the local ion structure in which the hole carriers are embedded. The work-function can be tuned by over 1 eV simply through control of the spectator ions. This opens new possibilities for the development of various work-function hole-injecting organic conductor materials.