Modern organic light-emitting diodes (OLEDs) comprise multi-layer stacks of several different functional materials with well-defined thicknesses and morphologies. The fabrication of such structures is normally done using small molecular organic semiconductors and high-cost slow-vacuum processes, which results in expensive OLED devices. Alternatively, solution-processing of soluble molecular or polymeric semiconductors can enable low-cost high-throughput OLED fabrication.
Several obstacles have to be overcome for a multi-layer stack preparation from solution; particularly, avoiding resolubilization and wash-off of subsequently deposited layers is a challenge. Orthogonal solvent strategies or stamping technics can circumvent this problem, but inherently limit the range of applicable materials or solvents. Thermally activated cross-linking of materials is a feasible alternative towards solvent-stable layers and subsequent solution-processing of multiple layers using same or similar solvents. However, most thermal cross-linking approaches still require high activation temperatures and long curing periods.[1]
Here, we present a new cross-linking approach based on a thermally activated one-component system with ynol ethers, which can fully cross-link within a few minutes at moderate temperatures without requiring any catalysts or co-reactants. This system is chemically inert towards a variety of reaction conditions (e.g., radical polymerization, strong bases, etc.), making it applicable to a wide range of organic materials for electronic devices. The cross-linkable materials are bench-stable over more than a year.
This new system was incorporated into several polystyrene-based side-chain hole-transporting polymers via copolymerization. The cross-linking reaction was analyzed with DSC, FT-IR and UV measurements. Solution-stable hole transport layers have been prepared as parts of multi-layered solution-processed OLEDs.
[1] Zuniga, C. A., Barlow, S. & Marder, S. R. Approaches to Solution-Processed Multilayer Organic Light-Emitting Diodes Based on Cross-Linking. Chem. Mater. 23, 658–681 (2011).