Improving CRI of white phosphorescence organic light-emitting diodes by controlling exciton energy transfer in the planar heterojunction

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

“…In general, holes and electrons mainly recombine in the organic layer between HTL (hole transporting layer) and ETL (electron transporting layer), which is structured by doping emitters into hosts in conventional host-guest system-based OLEDs. For UEMLs-based OLEDs, instead of doping emitter, phosphorescent UEML is directly inserted into common host materials such as TCTA ( Zhao et al., 2013a ; Tao et al., 2017 ; Zhang et al., 2017a ), CBP ( Liu et al., 2013b ; Xue et al., 2015a , 2015b ), and mCP ( Tan et al., 2015 ; Xue et al., 2015c ; Wang et al., 2019a , 2020 ; Yu et al., 2011 ; Yang et al., 2018 ) etc. Some work have been carried out for investigating the spatial distribution of excitons, and further revealed the effects of the thickness and position of UEMLs on the device performance in UEMLs-based OLEDs ( Tan et al., 2015 ; Zhao et al., 2018b ).…”

“…Few literature have reported that OLEDs using pure blue phosphor film as conventional EML achieved the satisfactory device performance because of severe exciton quenching induced by dense phosphorescent molecules. Thus, blue phosphors are usually doped into hosts (single material or mixed materials) to prepare doped film as blue phosphorescent EMLs, and the relevant blue phosphorescent OLEDs have achieved the extremely high EQE beyond the theoretical upper limit ( Wang et al., 2016 , 2020 ; Shin et al., 2014 ; Kang et al., 2016 ).…”

“…They found that the thickness ratio of the planar heterojunction spacer (NPB/mCP or NPB/TPBi) between green and yellow UEMLs can change the direction of exciton energy transfer, as shown in Figure 14 D. The fabricated white device with NPB(1.5 nm)/TPBi(1.5 nm) spacer displays a gradient enhanced EL spectra from blue light at short wavelengths to red light at long wavelengths, obtaining maximum CRI of 94 at 7.5V ( Figure 14 E). By reducing the doping concentration of FIrpic in blue EML (close to ETL) to 3.3 wt%, the corresponding device achieves the maximum luminance of 36,230 cd m −2 , and device show a slight CIE coordinates shift of (0.0440, 0.0029) from 5.5 V to 8.5 V. ( Wang et al., 2020 ).

Figure 14 Light-emitting mechanism and EL spectra diagrams The light-emitting mechanism diagram (A) and normalized EL spectra (B and C) of the fabricated four color white OLEDs with mCP as spacer layer; Reproduced with permission ( Wang et al., 2019a ), Copyright, Elsevier.

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“…For example, in 2016, an efficient exciplex of mCP: B3PYMPM was applied as the host of blue FIrpic by Ma et al., and they successfully fabricated white OLEDs by simply inserting an orange UEML within the blue emissive zone with the structure of ITO/MoO 3 (10 nm)/TAPC: 10 wt%MoO 3 (50 nm)/TAPC(20 nm)/PO-01 (0.06 nm)/mCP:B3PYMPM:FIrpic 1:1:0.4, 10 nm)/B3PYMPM(15 nm)/B3PYMPM: 3 wt%Li 2 CO 3 (40 nm)/Li 2 CO 3 (1 nm)/Al. ( Wang et al., 2020 ). The target device achieves the maximum forward-viewing CE, PE, and EQE of 64.5 cd/A, 75.3 lm/W, and 20.0%, respectively, and remains 62.8 cd/A, 63.1 lm/W, and 19.5% at a luminance of 1000 cd/m 2 .…”

“…(2014) TCTA: FIrpic/Ir(bt) 2 (acac) /26DCzPPy: FIrpic 59.3 63.2 23.1 – Wang et al. (2019b) mCP: FIrpic/Ir piq) 2 (acac)/mCP/PO-01/mCP/Ir(ppy) 3 /mCP: FIrpic 31.9 33.4 – 23,730 Wang et al. (2020) mCP:FIrpic/Ir piq) 2 acac)/mCP/PO-01/NPB/TPBi/Ir(ppy) 3 /mCP:FIrpic 20.4 19.7 15.0 36,230 Phosphorescent UEML inserted into mixed-host forming blue phosphorescent EML Chen et al.…”