Synthesis And Application In OLEDs Of Phenanthroimidazole Derivatives

The first thin-film organic light-emitting devices (OLEDs), i.e. multilayered devices based on organic semiconducting films sandwiched between electrodes, were described by Tang and VanSlyke in 1987. For the device of [ITO/NPB/Alq₃/LiF/Al], tris (8-hydroxyquinoline) aluminium (Alq3) and 4, 4-bis (N-(l-naphthyl)-N-phenylamino) biphenyl (NPB) are used as the electron and hole transporting media, respectively. However, the electron and hole mobilities in Alq₃ and NPB are the level of 10⁵ cm² V^-1 s^-1 and 10³ cm2 V^-1 s^<sup>-1 respectively, in an electric field of 2x10⁶ V cm^-1, which gives rise to an accumulation of excess holes at theNPB/Alq₃.Great efforts have been made to improved balance carrier injection into emitting layer to achieve high-efficiency devices.In this thesis, we investigate the significant enhancement of the EL performance by inserting an organic buffer layer based on series of phenanthroimidazole derivatives with the thickness of tens of nanometers between the NPB and Alq3 layer. Four phenanthroimidazole derivatives (a:1,2-diphenyl-1 H-phenanthro[9,10-d]imidazole,b:2-phenyl-1-p- tolyl-1 H-phenanthro[9,10-d]imidazole,c:1-phenyl-2-p-tolyl-1 H-phena nthro [9,10-d] imidazole and d:1,2-dip-tolyl-1 H-phenanthro [9,10-d]imidazole) were synthesized and their single crystal structures, photo physical, electrochemical and mobility properties were carefully studied. Taking advantage of the thermal stability and the hole transporting ability, the highly efficient Alq3-based organic light-emitting diodes (OLEDs) have been achieved by employing the compounds a-d as a functional layer between NPB and Alq3. The devices structures of [ITO/NPB/buffer layer/Alq3/LiF/Al] were fabricated, which presented high brightness (> 60000 cd m-2) and high efficiency (> 8 cd A-1). To further affirm the hole transport ability, time of flight (TOF) carrier-mobility measurement based on compounds a-d d with devices configuration of [ITO/a-d/Al] was carried out at room temperature.The results indicate that the highly efficient Alq3-based EL devices have been achieved by employing compounds a-d with easily prepared and reproduced thickness as an organic buffer layer inserted between NPB and Alq3 layers. This improvement is due to the weak HT ability of the phenanthroimidazole derivatives, which can decrease the amount of holes from the HTL transporting into the EML, thus improving the balance of the hole and electron and realizing a higher efficiency. The results of our study not only demonstrated an alternative idea to design novel HT materials, but also provided a convenient way to improve the performance of the NPB/Alq3 based devices by inserting a suitable organic buffer layer.