Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High-Efficiency Exciplex Light-Emitting Diodes

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light-emitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D+center dot -> A(-center dot)), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (Delta E-ST) in 9,9 ' -diphenyl-9H,9 ' H-3,3 ' -bicarbazole (BCzPh):3 ' ,3 ' '' ,3 ' '' ''-(1,3,5-triazine-2,4,6-triyl)tris(([1,1 ' -biphenyl]-3-carbonitrile)) (CN-T2T) (Delta E-ST = 30 meV) and BCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (Delta E-ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE(max) (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, Delta E-ST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D+center dot -> A(-center dot)) size, enhanced SOC, increased orbital polarization, and decreased Delta E-ST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.