Tuning Hole-Injection in Organic-Light Emitting Diodes with Self-Assembled Monolayers
Despoina Gkeka, Iain Hamilton, Thalis Stavridis, Zhongzhe Liu, Hendrik Faber, Dipti R. Naphade, Mantas Marčinskas, Tadas Malinauskas, George T. Harrison, Begimai Adilbekova, Temur Maksudov, Yue Yuan, Dimitrios Kaltsas, Leonidas Tsetseris, Vytautas Getautis, Mario Lanza, P. Patsalas, Shadi Fatayer, Thomas D. Anthopoulos
Abstract
Improving hole injection through the surface modification of indium tin oxide (ITO) with self-assembled monolayers (SAMs) is a promising method for modulating the carrier injection in organic light-emitting diodes (OLEDs). However, developing SAMs with the required characteristics remains a daunting challenge. Herein, we functionalize ITO with various phosphonic acid SAMs and evaluate the SAM-modified anodes in terms of their work function (WF), molecular distribution, coverage, and electrical conductivity. We fabricate and characterize green phosphorescent SAM-based OLEDs and compared their performance against devices based on the conventional poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) hole-injection layer. We find that the usage of [2-(3,6-diiodo-9H-carbazol-9-yl)ethyl]phosphonic acid (I-2PACz) SAM yields devices with superior performance characteristics, including a maximum luminance of ∼57,300 cd m –2 and external quantum efficiency of up to ∼17%. This improvement is attributed to synergistic factors, including the deep WF of ITO/I-2PACz (5.47 eV), the formation of larger I-2PACz molecular clusters, and the intrinsic I-2PACz dipole, that collectively enhance hole-injection.