Inkjet-printed alloy-like cross-linked hole-transport layer for high-performance solution-processed green phosphorescent OLEDs

The coffee-ring effect and material aggregation are main obstacles to obtaining high-performance small-molecule hole-transport layers by non-bank inkjet printing for organic light-emitting diodes (OLEDs). In this paper, a binary solvent ink system of cyclohexane-dipropylene glycol methyl ether (CYC-DGME) was constructed for the cross-linkable hole-transport material N,N '-di-p-tolyl-N,N '-bis(4-vinylphenyl)-[1,1 '-biphenyl]-4,4 '-diamine (V-p-TPD). The coffee-ring effect during film formation was eliminated by adjusting the solvent ratio and printing conditions. Furthermore, N-2,N-2,N-7,N-7-tetrakis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)-phenyl)-9,9-dimethyl-9H-fluoren-2,7-diamine (p-BCz-F) was used as the auxiliary hole-transport component. With V-p-TPD/p-BCz-F = 8 : 2 and CYC/DGME = 7 : 3, non-bank inkjet printing and thermal cross-linking produced an alloy-like hole-transport layer film with a low surface roughness (RMS = 1.23 nm) and excellent resistance to solvents. The target green phosphorescent OLEDs were qualified with an enhanced maximum current efficiency, external quantum efficiency and lowered turn-on voltage of 55.47 cd A(-1), 15.44% and 3.18 V, respectively, which represents a state-of-the-art record among OLEDs based on inkjet-printed HTLs. This result provides an effective and unique approach for the facile preparation of highly efficient OLED display devices by inkjet printing.

Automated summary

The binary solvent ink system and the application of auxiliary hole-transport component successfully alleviated the coffee-ring effect and material aggregation, achieving the goal of obtaining high-performance small-molecule hole-transport layers via non-bank inkjet printing. The resulting green phosphorescent OLEDs demonstrated enhanced current efficiency, quantum efficiency, and lowered turn-on voltage, representing a new milestone in OLED technology development.