Charge-transporting organic semiconductors are an important class of materials that play crucial roles in electronic and optoelectronic devices such as organic light-emitting devices (OLEDs), thin film transistors, and photovoltaic cells. OLEDs, that have organic hole- and electron-transport layer sandwiched between two electrodes, show low driving voltage and bright emission and are of importance for application to full color flat-panel displays and lighting. Since electron mobilities in organic materials, in general, are several orders of magnitude lower than hole mobilities, electron-transport materials (ETMs) with high electron mobility are required to further improve OLED performance. To achieve an effective electron injection and transport in an OLED, a high conductive n-doped electron-transport layer is developed by Kido’s group and Leo’s group by co evaporation of ETM and highly active alkaline metal of Li and Cs. In this case, a hole- and exciton-block buffer layer is indispensable to prevent exciton quenching in the emissive layer by the dopants in the electron-transport layer. This may induce more complexity of device structure and thus higher cost for applications to flat-panel displays and lighting.
Forrest and Thompson’s group has proposed a way to break through the efficiency limitation by using phosphorescent emitting materials. This renders possible harvesting both electro-generated singlet and triplet excitons for emission from OLEDs and realizing nearly 100% internal quantum efficiencies of electroluminescence. It is essential that the triplet excited states of all the corresponding materials should be higher than that of the phosphorescent emitter to confine the generated triplet excitons in the emissive layer. To match this request, the conjugation length of the material must be limited to achieve high triplet energy levels. However, it is difficult to meet these requirements because there is a tradeoff between increasing the band gap of the material to increase singlet and triplet energies and decreasing the p-conjugation system, which may adversely affect the charge transport properties. As such, it becomes particularly challenging to develop carrier transport materials with high triplet energy levels especially for a blue triplet emitter.
Two pyridine-containing triphenylbenzene derivatives of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene(TpPyPB), which possess both high electron mobilities and high triplet energy levels. High external quantum efficiency and power efficiency are realized for the iridium (III)bis(4,6-(di-fluorophenyl)pyridinato-N,C20) picolinate (FIrpic)-based blue phosphorescent OLED and the fac-tris(2-phenylpyridine) iridium (Ir(PPy)3)-based green phosphorescent OLED using TmPyPB and TpPyPB as the ETMs, respectively (Scheme 1).