In-situ switching from barrier-limited to ohmic anodes for efficient organic optoelectronics

Abstract

Injection and extraction of charges through ohmic contacts are required for efficient operation of semiconductor devices. Treatment using polar non-solvents switches polar anode surfaces, including PEDOT:PSS and ITO, from barrier-limited hole injection and extraction to ohmic behaviour. This is caused by an in-situ modification of the anode surface that is buried under a layer of organic semiconductor. The exposure to methanol removes polar hydroxyl groups from the buried anode interface, and permanently increases the work function by 0.2-0.3 eV. In the case of ITO/PEDOT:PSS/PBDTTT-CT:PC71BM/Al photovoltaic devices, the higher work function promotes charge transfer, leading to p-doping of the organic semiconductor at the interface. This results in a two-fold increase in hole extraction rates which raises both the fill factor and the open-circuit voltage, leading to high power conversion efficiency of 7.4%. In ITO/PEDOT:PSS/F8BT/Al polymer light-emitting diodes, where the organic semiconductor's HOMO level lies deeper than the anode Fermi level, the increased work function enhances hole injection efficiency and luminance intensity by 3 orders of magnitude. In particular, hole injection rates from PEDOT:PSS anodes are equivalent to those achievable using MoO3. These findings exemplify the importance of work function control as a tool for improved electrode design, and open new routes to device interfacial optimization using facile solvent processing techniques. Such simple, persistent, treatments pave the way towards low cost manufacturing of efficient organic optoelectronic devices. A polar non-solvent treatment switches anode surfaces from barrier-limited hole injection/extraction to ohmic behavior. This is caused by the removal of polar hydroxyl groups, increasing the anode work function. In photovoltaic devices, a two-fold enhancement in hole extraction raises the fill-factor and open-circuit voltage, leading to high power-conversion efficiency. In light-emitting diodes, hole injection and luminance intensity increase by 3 orders of magnitude. © 2014 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.