Charge conduction study of phosphorescent iridium compounds for organic light-emitting diodes application

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 x 10(-6)-2 x 10(-5) cm(2) 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. (C) 2015 Elsevier B.V. All rights reserved.