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

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 cm 2 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. Solution-processed light-emitting device was feasibly achieved by a new exciplex-forming BCz3Ph : PO-T2T blend as the emitting layer casted on top of a thermally polymerized hole-transporting layer formed by a dicarbazole-based monomer BCzC4Sy .