A star-shaped 1,3,5-triazine/cyano hybrid molecule CN-T2T was designed and synthesized as a new electron acceptor for efficient exciplex-based OLED emitter by mixing with a suitable electron donor (Tris-PCz). The CN-T2T/Tris-PCz exciplex emission shows a high ΦPL of 0.53 and a small ΔET-S = -0.59 kcal/mol, affording intrinsically efficient fluorescence and highly efficient exciton up-conversion. The large energy level offsets between Tris-PCz and CN-T2T and the balanced hole and electron mobility of Tris-PCz and CN-T2T, respectively, ensuring sufficient carrier density accumulated in the interface for efficient generation of exciplex excitons. Employing a facile device structure composed as ITO/4% ReO3:Tris-PCz (60 nm)/Tris-PCz (15 nm)/Tris-PCz:CN-T2T(1:1) (25 nm)/CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm), in which the electron-hole capture is efficient without additional carrier injection barrier from donor (or acceptor) molecule and carriers mobilities are balanced in the emitting layer, leads to a highly efficient green exciplex OLED with external quantum efficiency (EQE) of 11.9%. The obtained EQE is 18% higher than that of a comparison device using an exciplex exhibiting a comparable ΦPL (0.50), in which TCTA shows similar energy levels but higher hole mobility as compared with Tris-PCz. Our results clearly indicate the significance of mobility balance in governing the efficiency of exciplex-based OLED. Exploiting the Tris-PCz:CN-T2T exciplex as the host, we further demonstrated highly efficient yellow and red fluorescent OLEDs by doping 1 wt % Rubrene and DCJTB as emitter, achieving high EQE of 6.9 and 9.7%, respectively.

This work reports a new strategy of introducing remote steric effect onto the electron donor for giving the better performance of the exciplex-based organic light-emitting device (OLED). The bulky triphenylsilyl group (SiPh) was introduced onto the fluorene bridge of 4,4'-(9H-fluorene-9,9-diyl)bis(N,N-di-p-tolylaniline) (DTAF) to create remote steric interactions for increasing the possibility of effective contacts between electron-donating chromophores and acceptor molecules, rendering the resulting exciplex to have a higher photoluminescence quantum yield (PLQY). The green exciplex device based on DSDTAF:3N-T2T (1:1) as an emitting layer exhibits a low turn-on voltage of 2.0 V, high maximum efficiencies (13.2%, 42.9 cd A, 45.5 lm W), which are higher than the device employed DTAF (without SiPh groups) (11.6%, 35.3 cd A, 41.3 lm W) as donor under the same device structure. This strategy was further examined for blue exciplex, where the EQE was enhanced from 9.5% to 12.5% as the electron acceptor PO-T2T mixed with a tert-butyl group substituted carbazole-based donor (CPTBF) as the emitting exciplex in device. This strategy is simple and useful for developing high performance exciplex OLEDs.

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm−1 was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the original CN-Cz2:PO-T2T configuration within ~14 µs.

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