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

Abstract

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, N₂,N₂,N₇,N₇-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.