Redox-active molecules as electrical dopants for OLED transport materials (Conference Presentation)

Abstract

Electrical doping of organic semiconductors increases conductivity and reduces injection barriers from electrode materials, both of which effects can improve the performance of organic light-emitting diodes (OLEDs). However, the low electron affinities of typical OLED electron-transport materials make the identification of suitable n-dopants particularly challenging; electropositive metals such as the alkali metals are not easily handled and form monoatomic ions that are rather mobile in host materials, whereas molecular dopants that operate as simple one-electron reductants must have low ionization energies, which leads to severe air sensitivity. This presentation will discuss approaches to circumventing this issue by coupling electron transfer to other chemical reactivity. In particular, dimers formed by certain highly reducing organometallic sandwich compounds and organic radicals can be handled in air, yet have effective reducing potentials, corresponding to formation of the corresponding monomeric cations and contribution of two electrons to the semiconductor, of ca. –2.0 V vs. ferrocene. These values fall a little short of what is required for typical OLED materials; approaches to further extending the doping reach of these dimers will be described. One such approach involving photoirradiation of a dimer:semiconductor blend leads to metastable doping of a material with a redox potential of –2.24 V, which allows the fabrication of efficient OLEDs in which even high-workfunction electrodes, such as indium tin oxide, can be used as electron-injection contacts.