Solution-processed high efficiency OLED harnessing a thermally cross-linked hole-transporting layer and exciplex-forming emission layer

Abstract

A new dicarbazole-based donor, BCz3Ph, for exciplex formation was synthesized and characterized. The new green (PL λ_max = 527 nm) exciplex BCz3Ph : PO-T2T (2 : 1) blend with a photoluminescence quantum yield (PLQY) of 43% and thermally activated delayed fluorescence (TADF) character was utilized as the emitting layer (EML) of solution-processed OLED devices. To facilitate hole injection into the EML, a new cross-linkable monomer, BCzC4Sy, adopting a dicarbazole core linked to a styrene group by a butyl chain was designed to realize a solvent resistant hole-transporting layer (HTL) after thermal polymerization. The flexible butyl (C4) bridge accounts for a lower polymerization temperature as compared to that of the methyl (C1)-bridged counterpart, BCzC1Sy, resulting in the formation of amorphous films with better solvent resistance as well as smoother morphology. The choice of dicarbazole as the HTL core not only suppresses the HTL-to-EML energy barrier, but also prevents the emission color variation stemming from the possible exciplex emission at the HTL/EML interface. The best OLED device with EQE_max = 9.2%, CE_max = 27.94 cd A⁻¹ and PE_max = 28.7 lm W⁻¹ was obtained with a thermally polymerized BCzC4Sy film as the HTL. The device achieved a maximum luminance up to 22 000 cd m⁻² together with a very low efficiency roll-off, retaining 97% efficiency at 1000 cd m⁻² (EQE_1000cd = 8.9%). More significantly, a 2 × 2 cm² device fabricated by slot-die coating gave good color homogeneity and rather high brightness and promising efficiency (EQE 5.0%), manifesting the potential of employing a thermally cross-linkable HTM and exciplex-based EML to produce high efficiency solution-processed OLEDs.