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New electroluminescent bipolar compounds for balanced charge-transport and tuneable colour in organic light emitting diodes: triphenylamine oxadiazole-fluorene triad molecules

Kamtekar, K.T.; Wang, C.S.; Bettington, S.; Batsanov, A.S.; Perepichka, I.F.; Bryce, M.R.; Ahn, J.H.; Rabinal, M.; Petty, M.C.

Authors

K.T. Kamtekar

C.S. Wang

S. Bettington

I.F. Perepichka

J.H. Ahn

M. Rabinal

M.C. Petty



Abstract

This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids which incorporate triphenylamine or carbazole units within the π-electron system, viz. compounds 7, 8, 14 and 16. A related bipolar bis(oxadiazolyl)pyridine system 20 is reported. The syntheses of these five new materials are discussed, along with their optoelectronic absorption and emission properties, and their solution electrochemical redox properties. Anodic electropolymerisation of 20 was observed. Calculations using DFT (density functional theory) establish that they all possess a significantly higher HOMO energy level (by 0.60–1.02 eV) than 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7) due to the presence of electron-rich amine moieties and increased conjugation lengths, thereby leading to more balanced charge-transport characteristics. Devices were fabricated by spin-coating techniques using the bipolar compounds as the emitters in the simple device architecture ITO:PEDOT-PSS:X:Ca/Al (X = 7, 8, 14, 16 or 20). The turn-on voltages were 2.9, 5.5, 3.6, 4.5 and 3.4 V for the devices incorporating 7, 8, 14, 16 and 20, respectively. The highest external quantum efficiency (EQE) was observed for compound 7: viz. EQE 0.36%; current efficiency 1.00 cd A−1; power efficiency 0.56 lm W−1 at 5.7 V. The EQE of the device fabricated from 8 was considerably lower than for devices using other materials due to low light emission. The EL emission peaked at λmax 430, 487, 487 and 521 nm for 8, 14 and 16, and 7, respectively. For the 20 device λmax = 521 nm and 564 nm. Thus the HOMO–LUMO gap has been modified, allowing the colour of the emitted light to vary from light blue through to green by the systematic chemical modification of the molecular subunits. The high chemical and thermal durability of these materials combined with the simplicity of the device structure and low turn-on voltages offers considerable potential for OLED applications.

Citation

Kamtekar, K., Wang, C., Bettington, S., Batsanov, A., Perepichka, I., Bryce, M., …Petty, M. (2006). New electroluminescent bipolar compounds for balanced charge-transport and tuneable colour in organic light emitting diodes: triphenylamine oxadiazole-fluorene triad molecules. Journal of materials chemistry, 16(39), 3823-3835. https://doi.org/10.1039/b604543j

Journal Article Type Article
Acceptance Date Jul 25, 2006
Online Publication Date Aug 24, 2006
Publication Date 2006
Journal Journal of Materials Chemistry
Print ISSN 0959-9428
Electronic ISSN 1364-5501
Publisher Royal Society of Chemistry
Peer Reviewed Peer Reviewed
Volume 16
Issue 39
Pages 3823-3835
DOI https://doi.org/10.1039/b604543j
Keywords ELECTROPHOSPHORESCENT DEVICES; CONJUGATED POLYMERS; 1,3,4-OXADIAZOLE; LUMINESCENT; PERFORMANCE; DERIVATIVES; EFFICIENCY; CARBAZOLE; SEGMENTS; DENSITY