Achieving high efficiency and stability in blue OLEDs: Role of wide-gap hosts and emitter interactions

Qi Zhu; Mingjian Zhang

doi:10.1515/nleng-2025-0107

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Achieving high efficiency and stability in blue OLEDs: Role of wide-gap hosts and emitter interactions

Qi Zhu and Mingjian Zhang

Published/Copyright: April 25, 2025

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From the journal Nonlinear Engineering Volume 14 Issue 1

Abstract

The influence of host materials on the electroluminescent (EL) properties of blue thermally activated delayed fluorescence (TADF) emitters were lacking. In this investigation, we systematically studied the effect by doping blue emitter into various host materials with differing polarities as the emitting layer (EML), and revealed the choice of host-impacted EL performance significantly. By delving deeper into the light-emission mechanisms of TADF devices, we identified that the device performance variations were predominantly governed by both energy transfer and charge trapping processes within the EML(s). Notably, stable blue TADF device exhibiting peak efficiencies and brightness values of 23.74 cd/A and 5,695 cd/m², respectively, were achieved.

Keywords: EL performance; host-emitter influence; blue device

1 Introduction

Since Adachi’s pioneering work, thermally activated delayed fluorescence (TADF) emitters have been seen as the next big thing in organic light-emitting diodes (OLEDs) due to their potential for 100% internal quantum efficiency [1,2,3,4,5,6]. Unlike heavy metal-based phosphors, TADF uses pure organic elements, promising cost savings and reduced environmental impact. While TADF OLEDs have seen improvements in efficiency and brightness, blue TADF devices still face challenges [3,7,8]. Their wide energy gaps, high triplet energy, and long delayed component lifetimes make it hard to find suitable host materials, which leads to high driving voltages, poor efficiencies, and low stability. These issues stem from easy exciton quenching, insufficient exciton utilization due to mismatched host-emitter interactions, and large charge carrier injection barriers.

To address these issues, researchers have been optimizing blue TADF OLEDs. Khan et al. created a novel blue TADF emitter, SpiroAC-TRZ [9], with a large planar structure and high horizontal dipole ratio. This emitter achieved an external quantum efficiency of 37% and a current efficiency near 100 cd/A. Its short delayed fluorescence lifetime and small ΔE _ST resulted in high photoluminescence quantum yields. Despite these advancements, there is still limited research on host-emitter interactions in TADF emitters, unlike the extensive studies on host-phosphor interactions in phosphorescent OLEDs.

In addition to exploring novel emitters, developing advanced host materials to enhance device efficiency is another critical approach. For example, wide-gap hosts with high triplet energy (T ₁), such as bis{2-[diACHTUNGTRENUNG(phenyl)phosphino]-phenyl}ether oxide, can suppress exciton quenching and balance charge distribution by optimizing energy-level alignment and carrier mobility, thereby significantly improving device efficiency and stability. However, there is a lack of literature that thoroughly investigates the interaction mechanisms between host and guest materials.

In this study, we investigated how different host materials affected the electroluminescent (EL) properties of blue TADF emitter 2,7-Bis(9,9-dimethylacridin-10(9H)-yl)-9,9-dimethyl-9H-thioxanthene 10,10-dioxide (DMTDAc) [10], and explored the interaction between wide-gap hosts and yellow/green TADF emitters. Our findings led to a high-performance blue emission device with DMTDAc and DPEPO, achieving a maximum current efficiency of 23.74 cd/A and EQE of 21.9%.

2 Methodology

Organic materials were used without purification. Indium tin oxide (ITO)-coated glass served as the anode. The substrates were cleaned and dried before film deposition under high vacuum. Organic layers were deposited at 0.1 nm/s, with emitting layers (EMLs) prepared by co-evaporating emitters and host material. The hole transport layer (HTL) was also co-evaporated. LiF and Al were deposited in another vacuum chamber. Layer thicknesses and evaporation rates were monitored with quartz crystal monitors. A shadow mask created eight emitting dots on each substrate. J-B-V characteristics were measured with a Keithley unit and photodiode. EL spectra were measured with a Hitachi spectrophotometer. Measurements were conducted at 25°C and <30% humidity, following ASTM E2307-20 standards. EQE was calculated based on photodiode measurements, EL spectrum, and device current.

3 Results and discussion

Figure 1 shows the optimized OLED structure and energy levels. 1,4,5,8,9,11-Hexaazatriphenylene-hexacarbonitrile enhances hole injection [11], di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) serves as HTL/electron block layer, and Tm3PyP26PyB is the electron transport material. DMTDAc is the blue emitter with pure blue, high photo-luminescence quantum yield (PLQY), small ΔE _ST, and short delayed lifetime. TXO-TPA and TXO-PhCz are yellow and green emitters, chosen for high PLQY and stable emissions [12]. Molecular structures are also shown.

Figure 1

The optimized energy levels and molecule structure used in this study.

We made OLEDs with different host materials to study their effect on EL performance. The devices had ITO/HAT-CN (2 nm)/TAPC (40 nm)/emitter-host (20 nm)/Tm3PyP26PyB (60 nm)/LiF (1 nm)/Al (100 nm) structure. Hosts included CzSi, DPEPO, Tm3PyP26PyB, TcTa, and 26DCzPPy. TcTa, a hole-transport host, had moderate brightness but low efficiency. Tm3PyP26PyB, an electron-transport host, performed poorly. DPEPO, another electron-transport host, achieved high current efficiency (10.51 cd/A) and EQE (8.8%). At the same voltage, DPEPO-based devices maintained the highest EL efficiencies and brightness as shown in Figure 2.

Figure 2

The EL performances of devices with different hosts.

To understand EL differences, we looked at DMTDAc’s charge trapping in various hosts. In TcTa and CzSi, DMTDAc was a poor hole trapper due to its lower HOMO (−3.35 eV). This led to unbalanced charge trapping and low efficiencies. In Tm3PyP26PyB, DMTDAc was a good trapper but caused charge imbalance, resulting in low efficiency. 26DCzPPy’s low T ₁ (2.7 eV) energy hindered exciton confinement. However, DPEPO, with matched energy levels (−6.1/−2.0 eV of HOMO/LUMO) and high T ₁ (3.1 eV), allowed efficient host-emitter interactions, leading to high EL efficiencies.

We also tested two TADF emitters, TXO-TPA and TXO-PhCz, doped into DPEPO with a similar device structure but different doping concentrations. The HOMO/LUMO levels of TXO-TPA and TXO-PhCz were −5.78/−3.58 eV and −5.37/−3.49 eV, respectively. DPEPO’s low hole mobility and poor hole transport characteristics hindered hole injection and accumulation, even at high current densities. Most electrons were trapped by the emitters due to their lower LUMO levels compared to DPEPO, leading to exciton quenching. Both yellow and green devices showed low EL performances and intense peaks at short wavelengths due to unexpected energy loss as listed in Figure 3(a) and (b). Suitable host materials can solve these issues, highlighting the importance of considering host-emitter interactions in designing high-performance devices.

Figure 3

(a) Detailed EL performance of TXO-TPA/PhCz doped devices. (b) EL spectra of TXO-TPA/PhCz doped devices.

Based on extensive research, we fabricated high-efficiency blue OLEDs using DMTDAc and DPEPO. Initially, we explored single-EML devices with DMTDAc doped into DPEPO, observing a sharp rise in current density and brightness with increased doping, attributed to DMTDAc’s deep LUMO level facilitating direct electron injection as shown in Figure 4(a). However, high doping led to concentration quenching, reducing EL efficiencies. The optimal device, Device A, achieved a maximum current efficiency of 11.56 cd/A and an EQE of 9.7% at a brightness of 3,815 cd/m². Yet, efficiency roll-off at high brightness persisted due to DPEPO’s unbalanced charge mobility.

Figure 4

(a) Detailed EL performance of single-EML device. (b) Detailed EL performance of double-EML device. (c) Detailed EL performance of single-EML device with TcTa ladder layer.

To address this, we designed two strategies: A double-EMLs device with TcTa and DPEPO as hosts, and a single-EML device with a non-doped TcTa film as a hole transport ladder layer. Both methods mitigated efficiency roll-off, boosting brightness and EL efficiencies. The optimized double-EMLs device, Device B, reached maximum efficiencies of 16.84 cd/A, 16.52 lm/W, and 13.7%, with a brightness of 6,526 cd/m². At 4.1 V, it maintained a brightness of 1,000 cd/m² with efficiencies of 12.60 cd/A and 10.2% EQE as carved in Figure 4(b).

The optimized device with the TcTa ladder layer, Device C, also demonstrated impressive performance in Figure 4(c), achieving maximum efficiencies of 23.74 cd/A, 24.02 lm/W, and 21.9%. Even at 1,000 cd/m², it retained efficiencies of 16.79 cd/A and 14.7% EQE. Detailed EL performances of all devices are summarized in Table 1.

Table 1

EL properties of devices A, B, and C

Device	Turn-on voltage (V)	Brightness^MAX (cd/m²)	Current efficiency (CE)^MAX	Power efficiency^MAX	CE (EQE)@1,000 (cd/m²)	CIE_x,y@10 (mA/cm²)
A	2.8	3,815	11.56 (9.7%)	10.27	6.34 (5.3%, 4.7 V)	(0.136, 0.152)
B	3.0	6,529	16.84 (13.7%)	16.52	12.60 (10.2%, 4.1 V)	(0.134, 0.160)
C	3.0	5,695	23.74 (21.9%)	24.05	16.79 (14.7%, 3.7 V)	(0.138, 0.141)

To understand these mechanisms, we analyzed charge distribution in Devices A, B, and C in Figure 5. In Device A, DPEPO’s unipolarity led to imbalanced hole and electron distribution on DMTDAc molecules, causing efficiency roll-off. Device B’s double-EMLs design broadened the recombination zone, enhancing efficiencies. Device C’s TcTa ladder layer facilitated hole injection, balancing hole and electron distribution on emitters, achieving the highest EL efficiencies among the three. While Device C’s brightness was slightly lower due to a narrowed recombination zone, its brightness and efficiencies surpassed Device A’s. This confirms our device design strategy’s effectiveness in creating high-performance OLEDs with minimal efficiency roll-off.

Figure 5

Carrier distribution of devices A, B, and C.

4 Conclusion

This study explored the impact of host materials on the EL performance of blue TADF OLEDs. We found that optimizing the host material to match energy levels and charge mobility with emitters reduces charge injection barriers, balances carrier trapping, and broadens recombination zones. Using DMTDAc and DPEPO, we designed optimized device, which achieved maximum brightness of 5,695 cd/m², current efficiency of 23.74 cd/A, and EQE of 21.9%, with a narrow blue emission. At 1,000 cd/m², the device maintained high current efficiency of 16.79 cd/A and EQE of 14.7%. This study does not address long-term device stability, which warrants future investigation to assess practical viability.

Acknowledgments

The authors acknowledge the key Research Project of Henan Provincial Department of Education (25B510001).

Funding information: Authors state no funding involved.
Author contributions: Qi Zhu and Mingjian Zhang conceived and designed the research study. Mingjian Zhang performed the experiments and data collection. Qi Zhu developed the theoretical framework and analytical models. Both authors contributed to data interpretation and participated in manuscript drafting. Qi Zhu supervised the research project, critically revised the manuscript, and approved the final version for submission. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

References

[1] Cai M, Qiu H, Chen X, Wang X, Cai Z. Extension of phosphorescence and delayed fluorescence lifetime of carbon dots using substrate-assisted method. Chem Eng J. 2025;504:158747.10.1016/j.cej.2024.158747Search in Google Scholar

[2] Jiang H, Ai D, Jin J, Xie Z, Wong WY. Recent progress in circularly polarized multi-resonance thermally activated delayed fluorescence materials for organic light-emitting diodes. Sci China Chem. 2024;67:1–16.10.1007/s11426-024-2355-0Search in Google Scholar

[3] Jiang HX, Yang YJ, Liu M, Ge SJ, Wu JR, Zheng Q, et al. Thermally activated delayed fluorescence emitters with oxygen-bridged triarylborane acceptors for pure blue organic light emitting diodes. Dye Pigment. 2025;235:112597.10.1016/j.dyepig.2024.112597Search in Google Scholar

[4] Li MK, Li ZZ, Peng XM, Liu DH, Chen ZJ, Xie WT, et al. Excited-state engineering of chalcogen-bridged chiral molecules for efficient OLEDs with diverse luminescence mechanisms. Angew Chem Int Ed. 2024;63(1):e202420474.10.1002/anie.202420474Search in Google Scholar PubMed

[5] Li S, Zhu CH, Mao LJ, Zhang X, Ye ZC, Li CJ, et al. Thermally activated delayed fluorescence enhanced by macrocyclization. Sci China Chem. 2025;68:1–7.10.1007/s11426-024-2427-ySearch in Google Scholar

[6] Xie X, Gao T, Shen S, Pang Z, Qin Y, Lv X, et al. Suppression of nonradiative decay in TADF emitters via a macrocyclic strategy: an experimental study. Org Lett. 2024;27(1):202–6.10.1021/acs.orglett.4c04202Search in Google Scholar PubMed

[7] Moraes ES, Germino JC, Pereira L. Approaches for white organic light-emitting diode via solution-processed blue and yellow TADF emitters: Charge balance and host-guest interactions in a single emission layer. Org Electron. 2025;137:107175.10.1016/j.orgel.2024.107175Search in Google Scholar

[8] Ren W, Raju TD, Yao C, Hong SH, Kim TG. Omnidirectional reflector based on Ta2O5 cylinder-filled IZO mesh structure for flexible and efficient blue TADF top-emission OLEDs. Int J Extrem Manuf. 2024;7(2):025502.10.1088/2631-7990/ad92caSearch in Google Scholar

[9] Khan A, Qiang W, Kumar S, Leydecker T, Wang Z. Two new thermally activated delayed fluorescent molecules incorporating spiro-acridine donors for sky-blue organic light emitting diode devices demonstrating an impressive 31% EQE. Dye Pigment. 2024;232:112478.10.1016/j.dyepig.2024.112478Search in Google Scholar

[10] Lee I, Lee JY. Molecular design of deep blue fluorescent emitters with 20% external quantum efficiency and narrow emission spectrum. Org Electron. 2016;29:160–4.10.1016/j.orgel.2015.12.001Search in Google Scholar

[11] Peng B, Wang P, Luo H, He G, Xie H, Liu Y, et al. Outstanding high-temperature capacitive performance in all-organic dielectrics enabled by synergistic optimization of molecular traps and aggregation structures. Mater Horiz. 2025;12(4):1223–33.10.1039/D4MH01407CSearch in Google Scholar

[12] Zhu Q, Duanmu Q. Highly efficient thermally activated delayed fluorescence devices using a phosphorescent dye as host. Opt Commun. 2021;488:126854.10.1016/j.optcom.2021.126854Search in Google Scholar

Received: 2025-01-06

Revised: 2025-02-27

Accepted: 2025-03-18

Published Online: 2025-04-25

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/nleng-2025-0107

Keywords for this article

EL performance; host-emitter influence; blue device

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