Nanoscale Covalent Self-Assembly Approach to Enhancing Anode/Hole-Transport Layer Interfacial Stability and Charge Injection Efficiency in Organic Light-Emitting Diodes

The integrity of electrode/organic interfacial contact is shown to be crucial to the performance and stability of organic light-emitting diodes (OLEDs). In this contribution, vapor-deposited lipophilic, hole-transporting N,N‘-diphenyl-N,N‘-bis(3-methylphenyl)[1,1‘-biphenyl]-4,4‘-diamine (TPD) films are shown to undergo dewetting on indium tin oxide (ITO) anode surfaces under mild heating. An effective approach to minimize this interfacial decohesion effect is by small hole transport molecule self-assembly on the anode surface. Thus, conformal N(p-C6H4CH2CH2CH2SiCl3)3 (TAA)-based monolayers can be covalently self-assembled from solution onto hydroxylated ITO surfaces. The resulting nanometer-scale-thick TAA layers (∼11 Å/layer) prevent dewetting of the vapor-deposited TPD hole transport layers. Furthermore, ITO/(TAA) n /TPD/Alq/Al structured OLED devices fabricated with these modified ITO−TPD interfaces exhibit, as a function of n, reduced turn-on voltages as well as considerably higher luminous intensities and thermal stabilities compared to bare ITO-based devices.