The spin sublevel dynamics of the excited triplet state in thermally activated delayed fluorescence (TADF) molecules have not been investigated for high-intensity organic light-emitting diode materials. Understanding the mechanism for intersystem crossing (ISC) is thus important for designing novel TADF materials. We report the first study on the ISC dynamics of the lowest excited triplet state from the lowest excited singlet state with charge-transfer (CT) character of TADF molecules with different external quantum efficiencies (EQEs) using time-resolved electron paramagnetic resonance methods. Analysis of the observed spin polarization indicates a strong correlation of the EQE with the population rate due to ISC induced by hyperfine coupling with the magnetic nuclei. It is concluded that molecules with high EQE have an extremely small energy gap between the (1)CT and (3)CT states, which allows an additional ISC channel due to the hyperfine interactions.
The relationship between molecular shape of fluorescent blue dopants (BDs)
IntroductionSome companies have started mass production of OLED TVs or OLED lighting. One of the most critical issues for OLED application is the reduction of power consumption due to the revision of power consumption regulation by "Energy Star" program or other commissions. Regarding phosphorescent devices, the performance of red or green devices has been dramatically improved and started to be used in practical applications. On the other hand, pure blue phosphorescent material with long lifetime has not yet been found. Therefore the improvement of fluorescent pure blue performance has been continuously needed. In 2008, we developed a technology to enhance the efficiency of blue fluorescence, "Triplet-triplet fusion (TTF)", however the efficiency was not so high under 10% of EQE [1].Recently molecular orientation attracts much attention to promote high efficiency of OLED. Specific molecular structures which show horizontal orientation to a substrate has been reported [2]. In addition, specific host and dopant materials show molecular orientation in a blue fluorescence device and show high EQE in doped and un-doped emitting systems [3][4][5].The objective of this paper is to reveal the relationship between molecular orientation property of applicative fluorescent blue dopants (BDs) and EQE of OLEDs. We composed four kinds of different fluorescent blue dopants with different molecular shape (ovality) and examined the influence of molecular orientation of BDs on the efficiency improvement of OLEDs. We found that BDs with larger ovality bring larger orientation factor and higher EQE in blue OLEDs with "Efficiency enhancement layer (EEL)" to enhance TTF phenomenon. Furthermore, as a result of analyzing IVL characteristic, impedance spectroscopy and transient EL [6][7][8], it was clarified that the effect of the EQE improvement depends only on the orientation of BD because they showed almost same carrier balance and TTF efficiency.
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