Low-Temperature Cross-Linkable Hole Transport Materials for High-Performance Solution-Processed Quantum Dot and Organic Light-Emitting Diodes

Solution-Processed Quantum Dot Diodes (QLED) and Organic Light-Emitting Diodes (OLED) have seen significant progress in recent years, offering numerous advantages in the display technologies such as enhanced color purity, brightness, and and suitability for large-scale manufacturing. However, it is difficult to fabricate efficient solution-processed QLEDs and OLEDs in conventional device architectures (anode/hole injection layer (HIL)/hole transport layer (HTL)/emitting layer (EML)/electron transport layer (ETL)/cathode). The main obstacle to the fabrication of solution-processed QLEDs and OLEDs is the interface mixing/interfacial erosion of solution-based emitting materials in a small molecular HTL, degrading the performance of QLEDs and OLEDs.
Recently, various studies were conducted to develop thermal/photo cross-linkable organic molecular hole transport materials (HTMs) for efficient and stable solution-processed QLEDs and OLEDs. In particular, thermal cross-linking of organic molecular HTMs is the most promising technique for achieving efficient solution-processed QLEDs and OLEDs because it does not require the use of the photoinitiator. However, previously developed thermal cross-linkable HTMs possessed poor hole transport properties, high cross-linking temperatures, and long curing times.
To achieve efficient cross-linkable HTMs with high mobility, low cross-linking temperature, and short curing time, we designed and synthesized a series of low-temperature cross-linkable HTMs comprising dibenzofuran (DBF) and 4-divinyltriphenylamine (TPA) segments for highly efficient solution-processed QLEDs and OLEDs. The introduction of divinyl-functionalized TPA in various positions of the DBF core remarkably affected their chemical, physical, and electrochemical properties. Interestingly, cross-linked 4-(dibenzo[b,d]furan-3-yl)-N,N-bis(4-vinylphenyl)aniline (3-CDTPA) showed a deep highest occupied molecular orbital energy level (5.50 eV), excellent thermal stability (T_d , 427 °C ), high hole mobility (2.44 × 10^–4 cm² V^–1 s^–1), uniform surface morphology (RMS, 0.95 nm), low cross-linking temperature (150 °C), and short curing time (30 min). Furthermore, a green QLED with 3-CDTPA as the HTL exhibited an impressively high maximum external quantum efficiency (EQE_max) of 18.59% with a high maximum current efficiency (CE_max) of 78.48 cd A^–1. In addition, solution-processed green OLEDs with 3-CDTPA showed excellent device performance with an EQE_max of 15.61%, a CE_max of 52.51 cd A^–1. To the best of our knowledge, this is the first report on green solution-processed QLEDs and phosphorescent OLEDs showing high EQE, luminescence using DBF as the core and divinyl-functionalized TPA as the cross-linked HTL. These results reveal that TPA-functionalized divinyl moieties at suitable positions in the DBF core provide a new strategy to achieve high hole mobility, low cross-linking temperature, and short curing time in solution-processed QLEDs and phosphorescent OLEDs.