Pyridine-Based Bipolar Host Materials for Phosphorescent Organic Light-emitting Diodes

doi:10.6023/A25080281

Abstract

Achieving an excellent device performance with low efficiency roll-off is still a crucial requirement for organic light-emitting diodes (OLED) in displays and white lighting applications. Therefore, we selected carbazole as the hole transporting group (p-type unit), phenylene as the linkage unit and pyridine as the electron transporting unit (n-type unit), to construct two bipolar host materials o-2CzPy and m-2CzPy, by varying the ortho- and meta-linking modes. Their chemical structures were confirmed by mass spectrometry (TOF-MS-EI), ¹H and ¹³C NMR spectroscopy, and elemental analysis. These two materials have good thermal stability with the thermal decomposition temperatures of than 360 ℃ and efficient hole- and electron-transporting properties. FIrpic and Ir(ppy)₃ based blue and green PhOLEDs incorporating o-2CzPy and m-2CzPy as hosts show pretty color purity, high efficiencies and low efficiency roll-off. Among them, the maximum current efficiency of 45.16 cd•A⁻¹ and external quantum efficiency of 24.44% were achieved for o-2CzPy based blue device, which are close to the best values for FIrpic-based blue PhOLEDs with single bipolar host reported in recent literatures. Furthermore, the o-2CzPy based green device achieved the high efficiencies of 28.58% and 96.91 cd•A⁻¹, and stable emission color of ΔCIEy≤0.0001. Even at the practical brightness of 1000 cd•m⁻² and 6000 cd•m⁻², the external quantum efficiencies of o-2CzPy hosted green device still keep as high as 28.03% and 24.70%, with the slight efficiency reduction of 1.9% and 13.6%. These excellent results indicate that these bipolar host materials may find practical applications in highly efficient and stable OLED displays.

Key words: phosphorescent organic light-emitting diodes (PhOLEDs), bipolar host materials, pyridine, low efficiency roll-off, carbazole

Cite this article

Dongqing Xu, Haishan Dong, Luyan Shi, Ting Zhang. Pyridine-Based Bipolar Host Materials for Phosphorescent Organic Light-emitting Diodes[J]. Acta Chimica Sinica, 2026, 84(2): 240-247.

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Fig. & Tab. 10

Figure 1. The chemical structures and synthetic protocol of o-2CzPy and m-2CzPy

Figure 2. DSC curves of the second heating cycle for o-2CzPy and m-2CzPy

Figure 3. (a) Absorption and fluorescence spectra of o-2CzPy and m-2CzPy in CH2Cl2 at ambient temperature; (b) Low-temperature phosphorescence spectra of o-2CzPy and m-2CzPy in 2-MeTHF at 77 K

Compd.	$ \lambda_{\max }^{\mathrm{abs}}$/nm	$ \lambda_{\text {max }}^{\mathrm{em}}$/nm	E_g^a/eV	E_T^b/eV	HOMO/eV	T_g/℃	T_d/℃
Compd.	CH₂Cl₂		E_g^a/eV	E_T^b/eV	HOMO/eV	T_g/℃	T_d/℃
o-2CzPy	293, 324, 338	417	3.48	2.74	－5.59	71	365
m-2CzPy	293, 326, 340	431	3.47	2.64	－5.61	95	417

Table 1. Summary of physical parameters for o-2CzPy and m-2CzPy

Figure 4. Cyclic voltammograms of o-2CzPy and m-2CzPy at ambient temperature

Figure 5. Theoretical calculated HOMO and LUMO orbitals, and molecular structures of o-2CzPy and m-2CzPy

Figure 6. The current density versus voltage curves for o-2CzPy and m-2CzPy based (a) hole-only and (b) electron-only devices

Figure 7. (a) The J-V-B characteristics and (b) efficiency curves for o-2CzPy hosted blue device B1

Device	Host	Dopant	V_on^a/V	L_max/(cd•m⁻²)	η_c^b/(cd•A⁻¹)	η_p,max/(lm•W⁻¹)	η_ext^b/%	FWHM/nm	CIE (x, y) at 6 V	Φ_PLQY^c/%	τ^c/μs
B1	o-2CzPy	FIrpic	2.9	19880	45.16, 40.54, 28.62	37.60	24.44, 21.94, 15.49	46	(0.14, 0.28)	92.0	2.21
G1	o-2CzPy	Ir(ppy)₃	2.9	108810	96.91, 95.05, 83.74	88.22	28.58, 28.03, 24.70	56	(0.28, 0.63)	95.4	0.92
G2	m-2CzPy	Ir(ppy)₃	3.1	59386	83.84, 78.99, 62.77	73.56	24.48, 23.06, 18.33	60	(0.29, 0.63)	80.8	1.14

Table 2. Electroluminescent performance for o-2CzPy and m-2CzPy based blue and green devices

Figure 8. (a) The J-V-B characteristics and (b) efficiency curves for o-2CzPy and m-2CzPy hosted green devices G1 and G2

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