Role of δ-Hole-Doped Interfaces at Ohmic Contacts to Organic Semiconductors

An electromodulated absorption spectroscopy study of the contact between an organic semiconductor (OSC) poly(2,5-dialkoxy-p-phenylenevinylene) and p-doped poly(3,4-ethylenedioxythiophene) electrodes of different work functions (phi(vac)) reveals direct evidence for the formation of a hole-doped layer at the OSC interface in equilibrium with high-phi(vac) electrodes. When the hole density at this interface exceeds a few 10(11) cm(-2), degenerate bandlike polaron states emerge. This appears to be crucial to furnish efficient carrier injection into the bulk of the OSC to achieve Ohmic injection. The gap measured by ultraviolet photoemission between the electrode Fermi level and the OSC transport level (typically pinned at 0.6 eV) does not reflect the true injection barrier.