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

• Published in: Advanced materials (Weinheim) 2019-11, Vol.31 (46), p.e1904114-n/a
• Main Authors: Wang, Miaosheng, Huang, Yi‐Hsuan, Lin, Kai‐Siang, Yeh, Tzu‐Hung, Duan, Jiashun, Ko, Tzu‐Yu, Liu, Shun‐Wei, Wong, Ken‐Tsung, Hu, Bin
• Format: Article
• Language: English
• Subjects: Charge transfer; charge‐transfer states; delayed fluorescence; Diodes; Electric dipoles; Electron clouds; Electron spin; Energy conversion efficiency; exciplexes; Fluorescence; Magnetic properties; orbital polarization; Orbital resonances (celestial mechanics); Organic light emitting diodes; Parameters; Photoluminescence; Polarization (spin alignment); Quantum efficiency; spin–orbital coupling
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.201904114

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+•→A−•), providing the ionic polarization to generate SOC in exciplexes. By having different singlet‐triplet energy differences (ΔEST) 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) (ΔEST = 30 meV) and BCzPh:bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methyl‐pyrimidine (B3PYMPM) (ΔEST = 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, EQEmax (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, ΔEST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D+•→A−•) size, enhanced SOC, increased orbital polarization, and decreased ΔEST can simultaneously occur to cooperatively operate the triplet‐to‐singlet conversion. The cooperative relationship between spin, energy, and polarization parameters is revealed to maximize triplet‐to‐singlet conversion based on high‐efficiency exciplex organic light‐emitting diodes (OLEDs) with the EQEmax over 21%. This cooperative relationship provides a critical guideline to further advance the development of organic light‐emitting diodes.