pyridyl)-phen-3-yl]benzene) is the electron-transporting and hole-blocking layer. And mCP was selected as the reference host for its
suitable HOMO-LUMO energy, higher S1 and T1 energy, but larger singlet-triplet splitting.
Fig. 3 represents the current density–voltage-luminance (J-V-L) and external quantum efficiency-voltage-luminance (EQE-L) of the
PHOLEDs. The turn-on voltage is 4.3 and 4.5 V for device A and B, respectively. They emitted red light with peaks centered at 625
and 632 nm with color coordinates of (0.68, 0.32). Although the electron injection barrier at the interface between the EML and ETL is
different for the two devices, the efficient energy transfer is reflected by the nearly overlapped peak position (633 nm for device A and
623 nm for device B) and the approximately equal PLQY (photoluminescent quantum yields) for the dopant films (51.3% for
DMBFTX: 5 ± 1 wt% (piq)2Ir(acac) and 47.6% for mCP: 5 ± 1 wt% (piq)2Ir(acac)). The highest luminances are 22674 cd/m2 at 16.5 V
and 27425 cd/m2 at 13.9 V for device A and B, respectively. The device A afforded a maximum current efficiency of 9.3 cd/A, a
maximum power efficiency of 6.0 lm/W, and a maximum external quantum efficiency of 12.9% without any light out-coupling
enhancement. The maximum current efficiency, power efficiency and maximum external quantum efficiency of device B, was
estimated to be 7.8 cd/A, 4.0 lm/W and 12.5%, respectively. By contrast, the titled host displays competitive or even preponderant
performance compared with commercially available mCP in sensitizing the red (piq)2Ir(acac) phosphor. It is noteworthy that the device
A showed lower efficiency roll-off under high current density and luminescence. For device A, the EQE can be up to 10.4% with low
efficiency roll-off of 19.4% at the luminescence up to 1000 cd/m2 and 7.9% with an efficiency roll-off of 38.8% at the luminescence up
to 10 000 cd/m2. However, for device B, the EQE is only 6.3% with an efficiency roll-off up to 49.6% at the luminescence up to 1000
cd/m2 and 3.6% with an efficiency roll-off of 71.2% at the luminescence up to 10 000 cd/m2. From the previous research, the efficiency
roll-off is mainly resulted from either the TTA mode or the TPQ mechanism for PHOLED [6], which implies that the abundent triplet
exciton is primarily responsible for the conventional efficiency decline. In our previous work [26], the cooperative effect of the reverse
ISC from triplet to singlet on the host and rapid Förster energy transfer process from host to guest reduced the density of free triplet
excitons on the host, and thereby diminishing the efficiency roll-off. To further illustrate the influence of the RISC from the T1 to S1 on
the energy transfer from host to guest, the PL spectrum and PLQY of the two different dopant films [DMBFTX: 5 ± 1 wt%
(piq)2Ir(acac) and mCP: 5±1 wt% (piq)2Ir(acac)] in vacuum and exposure to air were investigated. Since the concentration of the
dopant is as low as 5 ± 1 wt% and the number of adjacent host and guest is few, the Dexter energy transfer could be neglected. As
shown in Fig. S7 in Supporting information, the emission of the doped film is from the T1 of the phosphor via Förster energy transfer
(FRET) from host. The PLQY of DMBFTX: 5 ± 1 wt% (piq)2Ir(acac) is 51.3% in vacuum, and decreased to 34.8% and 23.1% after
exposure in air for 30 and 60 min. While the reduction of PLQY on the mCP: 5 ± 1% (piq)2Ir(acac) was comparatively small (47.6%,
38.5% and 31.4%). In the above mentioned mechanism, the process susceptible by the oxygen concentration was marked as red line in
Fig. S7. The main differences between the two films are the RISC from T1 to S1 and subsequently FRET for DMBFTX host, but direct
FRET for mCP host. The distinction of PLQY versus the air exposure time indirectly illustrated that RISC is closely related to the
energy transfer between host and guest. In this study, the compound DMBFTX is a TADF host material with small ΔEST, which is
composed of TX and fluorene moiety. Consequently, we ascribe the reduced efficiency roll-off of PHOLED to the evacuation of the
triplet population on the DMBFTX host, resulting from the fast RISC from T1 to S1. In fact, the efficiency roll-off for DMBFTX based
PHOLED (device A) is actually weakened when compared to mCP with no TADF and RISC, which indirectly reflecting the
superiority of TADF host in reducing the efficiency decline for PHOLED. Further investigation of the direct proof related to the
detailed energy transfer process of this TADF host is underway.
In conclusion, we have designed and synthesized a novel TADF host material, DMBFTX, for efficiency red PHOLED with low
efficiency roll-off at high luminance. The performance further proves that our strategy of employing the TADF host material with TX
moiety to reduce efficiency roll-off in PHOLED is validated and suggests the potential of applying small ΔEST host materials for high-
performance PHOLEDs.
Acknowledgments
This study was supported by NSFC (No. 61605158), the Science and Technology Department of Shaanxi Province (No.
2016JQ2028), and the Education Department of Shaanxi Province (No. 16JK1790).
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