Harnessing bipolar acceptors for highly efficient exciplex-forming systems

Abstract

Two bipolar molecules CzT2.1 and CzT2.2 are examined as electron acceptors to form exciplexes with electron donors 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA), respectively. The bipolar structural feature endows 1-(4-(9H-carbazol-9-yl)phenyl)-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole (CzT2.1) and 1-(4-(9H-carbazol-9-yl)phenyl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (CzT2.2) with hole- and electron-transporting properties, as well as the possibility of forming charge transfer emissive states. An aggregation-induced emission (AIE) protocol was employed to quickly screen the feasibility of exciplex formation of four donor (D):acceptor (A) blends as nanoparticles dispersed in tetrahydrofuran (THF) solution containing a high portion of water. Then, the vacuum-deposited D:A blended films were analyzed with steady and dynamic photophysical characterization studies. The observed results indicate that the emissions of the D:A blends are contributed by the bipolar acceptor as well as the exciplex system. The proportion of each contribution depends on the exciplex formation efficiency and the intrinsic relaxation behavior of the bipolar acceptor. The D:A blends employing a stronger donor (TAPC) give a higher propensity of forming exciplexes as compared to those of their counterparts with a weaker donor TCTA. The green device (EL λ_max = 537 nm) with the TAPC:CzT2.1 exciplex-forming blend as the emitting layer exhibits a high maximum external quantum efficiency (EQE) of 12.5% (39.2 cd A⁻¹, 41.4 lm W⁻¹) and a limited efficiency roll-off (12.5%, 34.8 cd A⁻¹, 39.1 lm W⁻¹ at 100 cd m⁻²) owing to the fast decay lifetimes of the exciplex. The green exciplex-based device (EL λ_max = 514 nm) adopting the TAPC:CzT2.2 blend as the emitting layer offers an even higher EQE of 15.0% (45.7 cd A⁻¹, 50.0 lm W⁻¹), yet suffers a limited efficiency roll-off (14.3%, 43.6 cd A⁻¹, 45.0 lm W⁻¹ at 100 cd m⁻²) due to the prolonged decay lifetimes of the emissive components. This work highlights the use of emissive bipolar acceptors to create exciplex emission channels working together with the inherent acceptor emission for enhancing the organic light-emitting diode (OLED) device performance.