Color-stable non-doped white phosphorescent organic light-emitting diodes based on ultrathin emissive layers

Abstract

Color-stable white organic light-emitting diode (WOLED) with bipolar mixed spacer based on ultrathin non-doped phosphorescent emitting layers was investigated. We find that the location and thickness of Bis(3,5-difluoro-2-(2-pyridyl) phenyl-(2-carboxypyridyl)iridium(III) (FIrPic) play a critical role on the spectral stability. Transient PL lifetime results show that the spectral variations with the driving voltage can be effectively controlled by enhanced energy transfer or weakened energy transfer respectively depending on different location of FIrPic. Meanwhile, green layer, located between blue layer and orange layer, acts an indispensable role in keeping the color stability by alleviating triplet–triplet annihilation and maintaining sequential energy transfer. The most stable spectrum of WOLED can be obtained with the change in Commission Internationale de L’Eclairage coordinates less than (0.001, 0.002) as the luminance increase from 697 cd m−2 to about 16 550 cd m−2 when FIrPic is on anode side. This discovery provides a significant method for achieving high-quality WOLEDs with superior color stability.

Recent progress on organic light-emitting diodes with phosphorescent ultrathin (<1nm) light-emitting layers

“…In these devices, it is demonstrated that the direction of energy transfer is from FIrpic UEML on the anode side to Ir(ppy) 2 (acac) UEML and then to PO-01 UEML on the cathode side, which is just opposite to the direction of the drift of carrier recombination zone, shown in Figure 6 A. Increasing the thickness of FIrpic UEML on the anode side can increase the efficiency of energy transfer, which can offset the effect of the drift of recombination zone and then obtain stable spectra, as shown in Figure 6 B ( Dai et al., 2019 ).

Figure 6 Operational mechanism and EL spectra for UEMLs-based three-color white OLEDs Operational mechanism (A) and normalized EL spectra (B) for three-color white OLEDs with complementary phosphorescent UEMLs incorporated into TCTA: TmPyPB-based mixed spacer.

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“…But compared with conventional doped-EML-based white OLEDs, above adverse result can be easily suppressed by adjusting the thickness and sequence of phosphorescent UEML inserted in mixed spacer layer. ( Dai and Cao, 2020 ; Dai et al., 2019 ). For example, Cao et al.…”

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Figure 6 Operational mechanism and EL spectra for UEMLs-based three-color white OLEDs Operational mechanism (A) and normalized EL spectra (B) for three-color white OLEDs with complementary phosphorescent UEMLs incorporated into TCTA: TmPyPB-based mixed spacer. Reproduced with permission ( Dai et al., 2019 ), Copyright, IOP Publishing.

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“…But compared with conventional doped-EML-based white OLEDs, above adverse result can be easily suppressed by adjusting the thickness and sequence of phosphorescent UEML inserted in mixed spacer layer. (Dai and Cao, 2020;Dai et al, 2019). For example, Cao et al fabricated three-color white OLEDs by incorporating complementary phosphorescent UEMLs into TCTA:TmPyPB-based mixed spacer with the structure of ITO/MoO 3 :TCTA(2:3, 35 nm)/TCTA(18 nm)/FIrpic(x nm)/TCTA:TmPyPB(1:1, 3 nm)/ Ir(ppy) 2 (acac)(0.15 nm)/ TCTA:TmPyPB(1:1, 3 nm)/ PO-01 (0.05 nm)/ TmPyPB(40 nm)/ LiF(1 nm)/ Al(100 nm), where x = 0.1, 0.2, and 05, respectively.…”

“…(2015b) PO-01/CBP:TPBi/TPBi:8%FIrPic 38.3 40.1 – – Yu et al. (2018) Ir mphmq) 2 (acac)/TCTA: Tm 3 PyPB/PO-01/TCTA: Tm 3 PyPB/Ir(ppy) 2 (acac)/TCTA: Tm 3 PyPB/FIrpic 44.9 42.5 16.1 – Dai et al. (2019) FIrPic/TCTA: TmPyPB/Ir ppy) 2 (acac)/TCTA: TmPyPB/PO-01 55.2 57.6 16.0 – Liu et al.…”