Anode Interfacial Engineering Approaches to Enhancing Anode/Hole Transport Layer Interfacial Stability and Charge Injection Efficiency in Organic Light-Emitting Diodes

Abstract

The integrity of anode/organic interfacial contact is shown to be crucial to the performance and stability of archetypical small molecule organic light-emitting diodes (OLEDs). In this contribution, vapor-deposited lipophilic, hole-transporting 1,4-bis(phenyl-m-tolylamino)biphenyl (TPD) and 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) thin films are shown to undergo decohesion on ITO anode surfaces under mild heating. An effective approach to ameliorate such interfacial decohesion is introduction, via self-assembly or spin-coating, of covalently bound N(p-C6H4CH2CH2CH2SiCl3)3 (TAA)- and 4,4‘-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPD-Si2)-derived adhesion/injection layers at the anode/hole transport layer interface. The resulting angstrom-scale hole transport layers prevent decohesion of vapor-deposited hole transport layers and significantly enhance OLED hole injection fluence. OLEDs fabricated with these modified interfaces exhibit appreciably reduced turn-on voltages, considerably higher luminous intensities, and enhanced thermal robustness versus bare ITO-based control devices. Spin-coated, cross-linked TPD-Si2 films, in particular, prove to be superior to conventional ITO functionalization interlayers, including copper phthalocyanine, in this regard. The present ITO-functionalized devices achieve maximum external forward quantum efficiencies as high as 1.2% and a luminous level of 15 000 cd/m2 in simple ITO/interlayer/HTL/Alq/Al heterostructures. We also show that Cu(Pc) interlayers actually suppress, rather than enhance, hole injection and template crystallization of vapor-deposited TPD and NPB at modest temperatures, resulting in poor OLED thermal stability.

Molecular Self‐Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin‐Film Transistor Applications

“…[122][123][124][125][126] In the case of SiO 2 surfaces, the driving force for self-assembly is the in situ formation of siloxanes, which connect the precursor silane to the surface silanol (-Si-OH) groups via very strong Si-O-Si bonds (Fig. 3).…”

Modification of Transparent Conducting Oxide ( TCO ) Electrodes through Silanization and Chemisorption of Small Molecules

“…Marks and coworkers have shown how extensively cross-linked silane monolayers and multilayers can be used to stabilize an electrode and regulate charge injection in a condensed OLED phase [14,[53][54][55]. Their approach has been to start with silanes functionalized with various redox active groups and to terminate by functional groups, which can subsequently be converted to new reactive silanes, to produce a layer-by-layer ''self-limiting'' chemistry.…”

High‐Brightness Blue Light‐Emitting Polymer Diodes via Anode Modification Using a Self‐Assembled Monolayer

“…Similar interfacial energy modulation of hole injection into a conjugated polymer was achieved by Yu et al [6] using vapordeposited copper phthalocyanine (CuPc) as a hole-transport layer for blue PLEDs. Here, however, only reduced turn-on voltage without significant improvement in maximum light output or external quantum efficiency was observed, presumably due to the weaker hole-injection capacity of CuPc versus TPD-Si 2 [25,26] and PEDOT±PSS, supported by the lower CuPc IP energy (4.8 eV) [6] and relevant hole-only device data.…”