Department of Chemistry; Department of Physics
The charge conduction properties of a series of iridium-based compounds for phosphorescent organic light-emitting diodes (OLEDs) have been investigated by thin-film transistor (TFT) technique. These compounds include four homoleptic compounds: Ir(ppy)3, Ir(piq)3, Ir(Tpa-py)3, Ir(Cz-py)3, and two heteroleptic compounds Ir(Cz-py)2(acac) and FIrpic. Ir(ppy)3, Ir(piq)3 and FIrpic are commercially available compounds, while Ir(Tpa-py)3, Ir(Cz-py)3 and Ir(Cz-py)2(acac) are specially designed to test their conductivities with respect to the commercial compounds. In neat films, with the exception of FIrpic, all Ir-compounds possess significant hole transporting capabilities, with hole mobilities in the range of about 5 × 10−6–2 × 10−5 cm2 V−1 s−1. FIrpic, however, is non-conducting as revealed by TFT measurements. We further investigate how Ir-compounds modify carrier transport as dopants when they are doped into a phosphorescent host material CBP. The commercial compounds are chosen for the investigation. Small amounts of Ir(ppy)3 and Ir(piq)3 ( < 10%) behave as hole traps when they are doped into CBP. The hole conduction of the doped CBP films can be reduced by as much as 4 orders of magnitude. Percolating conduction of Ir-compounds occurs when the doping concentrations of the Ir-compounds exceed 10%, and the hole mobilities gradually increase as their values reach those of the neat Ir films. In contrast to Ir(ppy)3 and Ir(piq)3, FIrpic does not participate in hole conduction when it is doped into CBP. The hole mobility decreases monotonically as the concentration of FIrpic increases due to the increase of the average charge hopping distance in CBP.
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Supports of this research under the Research Grant Council under Grant #12201914 and the Research Committee of Hong Kong Baptist University under Grant #FRG2/12-13/080 are gratefully acknowledged. Prof. W.-Y. Wong thanks the National Basic Research Program of China (973 Project 2013CB834702), Hong Kong Baptist University (FRG2/12-13/083 and FRG1/13-14/053), Hong Kong Research Grants Council (HKBU203313) and Areas of Excellence Scheme, University Grants Committee of HKSAR, China (Project No. AoE/P-03/08) for financial support. Dr. Guiping Tan acknowledges the finance support from the Department of Education of Guangdong Province (No. 2014KQNCX219).
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Copyright © 2015 Elsevier B.V. All rights reserved.
Iridium, Organic light-emitting diodes, Charge transport, Organic thin film transistors, Doping mechanism
Choi, Wing-Hong, Guiping Tan, Wai-Yu Sit, Cheuk Lam Ho, Cyrus Yiu-Him Chan, Wenwei Xu, Wai Yeung Wong, and Shu Kong So. "Charge conduction study of phosphorescent iridium compounds for organic light-emitting diodes application." Organic Electronics 24 (2015): 7-11.