Carrier transfer and luminescence characteristics of concentration-dependent phosphorescent Ir(ppy)3 doped CBP film

doi:10.1016/j.optlastec.2013.07.011

Optics & Laser Technology

Volume 56, March 2014, Pages 20-24

https://doi.org/10.1016/j.optlastec.2013.07.011 Get rights and content

Highlights

•
Superior hole-transporting capacity of Ir(ppy)₃:CBP film.
•
High efficiency and low roll-off phosphorescent OLEDs with 10wt% Ir(ppy)₃ doped CBP being EML.
•
Bathochromic-shift PL and EL emission due to concentration quenching effect.
•
A reduced fluorescent lifetime of excitons at high doping concentration.

Abstract

The carrier transfer and luminescence characteristic of concentration-dependent tris(2-phenylpyridine) iridium (Ir(ppy)₃) doped in 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) film of 40 nm have been investigated. Based on the charge-only devices with different concentration of Ir(ppy)₃:CBP, we have found that an increasing dopant concentration can facilitate hole transfer, whereas suppress electrons transfer reversely. The ratio of Ir(ppy)₃ in CBP plays a significant role in energy transfer process. Results have shown that excess Ir(ppy)₃ possessed a shortened fluorescence lifetime, indicating a concentration quenching effect in Ir(ppy)₃:CBP film. The performance of the device decreased rapidly with increasing of doping concentration of Ir(ppy)₃ in CBP. A high efficiency of 65.2 cd/A with low efficiency roll-off phosphorescent OLED was achieved, in which 10 wt% Ir(ppy)₃ in CBP was light-emitting layer.

Introduction

Since tris(8-hydroxyquinolate) aluminum (Alq₃) was first demonstrated for green organic light-emitting diodes (OLEDs) [1], which have received considerable attention for energy-saving solid-state lighting and eco-friendly flat panel display applications [2], [3], [4], [5]. However, the main bottleneck for the realization of OLED applications is the organic material, which should ideally combine high fluorescence quantum yield, suitable energy level alignment, high thermal stability, and good thin-film morphology. Utilization of phosphorescence material in OLED is a good way to satisfy above requirements for advanced commercial application of OLED, owing to emission from both triplet and singlet excitons in phosphorescence material. Phosphor complexes based on heavy metal showed the characteristic of strong spin–orbit coupling, which induced nearly 100% of internal quantum yield [6], [7], [8], [9].

Particularly, cyclometalated Ir(III) complexes are promising candidates for room-temperature phosphorescence owing to their highly efficient emission derived from mixed metal-to-ligand charge transfer (MLCT) [10]. To date, dopant of Ir(ppy)₃ in 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) is very classic doping system reported widely. The lowest unoccupied molecular orbital (LUMO) energy level of Ir(ppy)₃ is approximately equal to that of CBP, resulting in efficient electrons trapping on Ir(ppy)₃. Next, the triplet excitons of Ir(ppy)₃ were confined in CBP due to the lower triplet energy level relative to CBP [11]. Finally, there is great large spectral overlap between the fluorescence band of CBP and the excitation band of Ir(ppy)₃ [12], [13], indicating the sufficient Förster energy transfer from CBP to Ir(ppy)₃.

However, little effort has been directed toward understanding of the nature of carrier transfer characteristic of concentration-dependent Ir(ppy)₃. Previous studies have demonstrated that small amount of phosphorescent dopant in hole-transporting material can not only help to lower turn-on voltage, but also contribute to fabricate white OLED with a high efficiency and a high color rendering index [14], [15], [16]. In addition, excitons generated from Ir(ppy)₃ were quenched by triplet–triplet annihilation easily, which due to relatively longer phosphorescent lifetime [17], [18], [19]. Therefore, it still have a room to study carrier transfer and luminescence properties of concentration-dependent in Ir(ppy)₃:CBP film. In this paper, charge-only devices and OLEDs with different doping concentrations of Ir(ppy)₃ in CBP film were fabricated. The carriers transfer capacity and luminescence property of concentration-dependent in Ir(ppy)₃:CBP film were also studied in detail.

Section snippets

Experiments

The CBP:Ir(ppy)₃ film were fabricated to verify carrier transfer capacity as follows, ITO/NPB (30 nm)/CBP:Ir(ppy)₃ (x wt%, 40 nm)/Ag (10 nm) /Al (200 nm) and ITO/TPBi (20 nm)/CBP:Ir(ppy)₃ (x wt%, 40 nm)/TPBi (20 nm)/Al (200 nm) for hole-only and electron-only devices, respectively. Furthermore, in order to determine the optimum doping concentration of Ir(ppy)₃, three devices were fabricated by varying doping concentration of Ir(ppy)₃ from x=5, to x=10, to x=15 in CBP as emissive layer (EML) for Device A, B,

Carrier transfer property

It has been widely known that CBP has versatile function in OLED devices, which can act as host material of plenty of phosphorescence dopants in OLED and a bipolar transfer material. Nevertheless, the carrier transfer property of Ir(ppy)₃:CBP is rarely mentioned. We constructed charge-only devices based on Ir(ppy)₃:CBP film to investigate the carrier transfer property. The current density–voltage curves of hole-only and electron-only devices are plotted in Fig. 2(a) and (b), respectively. The

Conclusion

In summary, the characteristic of concentration-dependent Ir(ppy)₃:CBP film has been investigated. The doping level of Ir(ppy)₃ molecules in CBP matrix can not only change carrier transfer characteristics, but also affect its luminescence properties. Improving in hole transfer was obtained using 10 wt% Ir(ppy)₃ doped in CBP film, which can effectively improved the performance of OLED.

The improvement in hole transfer characteristics of Ir(ppy)₃:CBP film indicates that the ppy group in Ir(ppy)₃

Acknowledgment

This work was financially supported by International Science & Technology Cooperation Program of China (2012DFR50460), National Natural Scientific Foundation of China (21071108, 60976018, 21101111, 61136003, and 61275041) and Innovation Group Project from Shanghai Education Commission.

References (30)

S.-Y. Kim et al.
A study on the characteristics of OLED using Ir complex for blue phosphorescence
Current Applied Physics
(2007)
S. Tokito et al.
High-efficiency blue and white phosphorescent organic light-emitting devices
Current Applied Physics
(2005)
Y.F. Lv et al.
Improved hole-transporting properties of Ir complex-doped organic layer for high-efficiency organic light-emitting diodes
Organic Electronics
(2013)
J.Y. Lee
Relationship between doping concentration and recombination zone in green phosphorescent light-emitting diodes
Sensors and Actuators A: Physical
(2009)
J. Kalinowski et al.
Triplet energy exchange between fluorescent and phosphorescent organic molecules in a solid state matrix
Chemical Physics
(2004)
M. Makowska-Janusik et al.
Absorption spectra of new synthesized electroluminescent materials of poly-N-vinylcarbazole derivatives
Materials Letters
(2004)
H. Tang et al.
Green organic light emitting diodes with improved stability and efficiency utilizing a wide band gap material as the host
Displays
(2008)
C.W. Tang et al.
Organic electroluminescent diodes
Applied Physics Letters
(1987)
B.W. D’Andrade et al.
White organic light-emitting devices for solid-state lighting
Advanced Materials
(2004)
J. Brodrick
Next-generation lighting initiative at the U.S. department of energy: catalyzing science into the marketplace
Journal of Display Technology
(2007)

F. So et al.

Organic light-emitting devices for solid-state lighting

MRS Bulletin

(2008)

B. Geffroy et al.

Review organic light-emitting diode (OLED) technology: materials, devices and display technologies

Polymer International

(2006)

M.C. Gather et al.

White organic light-emitting diodes

Advanced Materials

(2011)

S.-Y. Ku et al.

High-luminescence non-doped green OLEDs based on a 9,9-diarylfluorene- terminated 2,1,3-benzothiadiazole derivative

Journal of Materials Chemistry

(2009)

Y.Q. Zhang et al.

Concentration quenching of electroluminescence in neat Ir(ppy)₃ organic light-emitting diodes

Journal of Applied Physics

(2010)

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View full text

Carrier transfer and luminescence characteristics of concentration-dependent phosphorescent Ir(ppy)3 doped CBP film

Highlights

Abstract

Introduction

Section snippets

Experiments

Carrier transfer property

Conclusion

Acknowledgment

Current Applied Physics

Current Applied Physics

Organic Electronics

Sensors and Actuators A: Physical

Chemical Physics

Materials Letters

Displays

Organic electroluminescent diodes

Applied Physics Letters

White organic light-emitting devices for solid-state lighting

Advanced Materials

Next-generation lighting initiative at the U.S. department of energy: catalyzing science into the marketplace

Journal of Display Technology

Organic light-emitting devices for solid-state lighting

MRS Bulletin

Review organic light-emitting diode (OLED) technology: materials, devices and display technologies

Polymer International

White organic light-emitting diodes

Advanced Materials

High-luminescence non-doped green OLEDs based on a 9,9-diarylfluorene- terminated 2,1,3-benzothiadiazole derivative

Journal of Materials Chemistry

Concentration quenching of electroluminescence in neat Ir(ppy)3 organic light-emitting diodes

Journal of Applied Physics

Carrier transfer and luminescence characteristics of concentration-dependent phosphorescent Ir(ppy)₃ doped CBP film

Concentration quenching of electroluminescence in neat Ir(ppy)₃ organic light-emitting diodes