Hole injection in tri-arylamine containing polyfluorene co-polymer devices with molybdenum oxide contacts

Abstract

We report spectroscopic and electrical measurements to explore hole injection and conduction in devices comprising a molybdenum sub-oxide (MoOx) hole injection layers and poly[(9,9-dioctylfluorenyl-2, 7-diyl)-co-(4,4’(N-(4-sec-butylphenyl))) diphenylamine](TFB) hole transporting polymer. We report improvements in device conductivity over benchmark structures incorporating an ITO electrode and polyethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS) hole injection layers and furthermore achieve injection from MoOx to TFB that is efficient even with an underlying low workfunction Al electrode. XPS spectroscopy has been used to investigate the electronic structure of the interfaces and we find discrete energy alignment regimes consistent with recent surface science studies by Tengstedt et al. [Appl. Phys. Lett. 88, 053502 (2006)], corresponding to Fermi level pinning for MoOx/TFB and vacuum level pinning in the case of Al/TFB. While the energetic alignment regime is measured to be independent of MoOx thickness, the device conductivity continuously varies with MoOx thickness; an observation that can be qualitatively explained by considering two independent charge injection mechanisms from molybdenum oxide sites having different stoicheometry.