Design,Synthesis,and Electroluminescent Property Of Host Materials Based On N-type Unit Engineering

Organic light-emitting diodes(OLEDs)have progressed rapidly in terms of solid state lighting and flat panel display.Phosphorescent OLEDs(PhOLEDs)that possess a theoretical internal quantum efficiency uplimit of unity are usually fabricated with a host-guest doping structure to suppress exciton quenching.The host matrix which occupies the majority of the emitting layer controls the charge injection at interfaces,charge transportation in the device,and even the fabrication method and the whole performance of the OLEDs.Hence,designing and developing excellent host materials is one of the key tasks to achieve high performance OLEDs.Currently,the design strategies of host materials are not mature enough,such as the improper selection of n-type unit and thus the high driving voltage and power consumption,the serious efficiency roll-off in PhOLEDs caused by the imbalanced positive and negative charge transportation and thus the severe exciton quenching,and so on.In this dissertation several series of novel "dual n-type" bipolar host materials have been designed and prepared with the emphasis on the modulation of n-type units.By means of the molecular engineering strategies including the struture of n-type group,n/p molar ratio,substitution site and molecular conformation,and molecular stacking style,the optoelectronic parameters of the novel host materials were systematically investigated and optimized.High performance blue and green PhOLEDs have been obtained using these hosts.The main achievements are as following:(1)A group of host materials were prepared using a saturated carbon atom linking the p-and n-type units,which breaks the ?-conjugation and intramolecular charge transfer between p-and n-groups,finally leading to high triplet energies of 2.95 eV for these hosts.(2)Two novel "dual n-type" bipolar host materials,PyTzDCz and PyTzSCz,were developed using 5-(1H-triazol-1-yl)pyridine(PyTz)as "dual n-type unit".From the parent mCP and the reference PyDCz to PyTzDCz and PyTzSCz,the n/p ratio was regularly increased in the order of 0:2<1:2<2:2<2:1.Following the same order,the LUMO levels of these hosts were regularly pulled down and the turn-on voltage of their sky-blue and green PhOLEDs were regularly reduced.Particually the PyTzSCz hosted sky-blue PhOLED turned on at 2.6 V with a power efficiency of 48.1 lm/W(26.4%and 45.9 cd/A),which is the lowest turn-on voltage reported so far and one of the highest power efficiencies for FIrpic devices containing a single host.PyTzSCz hosted green devices also exhibited high efficiencies of 28.0%(85.0 cd/A,97.2 1m/W).(3)Bipyridine was used as the "dual n-type unit" to design and prepare a series of novel bipolar host materials m-BPyDCz,p-BPyDCz,m-BPySCz and p-BPySCz.By regularly tuning the n/p ratio and the substitution position of the second pyridine on the original pyridine,these four hosts show the scorpion-,Y-,Z-,and L-shape conformations,respectively,and have different molecular stacking patterns in these single crystals driven by intermolecular hydrogen bonding.Particularly,the Z-shaped m-BPySCz achieves 3D gridding packing mode,in which the regularly packed carbazoles and pyridines as hole and electron channels facilitate charge transportation and balance,while the relatively intermolecular distances due to the 3D gridding packing prevent intermolecular interactions and suppress exciton quenching.As a result the m-BPySCz-hosted sky-blue and green PhOLEDs achieved high efficiencies of 27.3%(50.3 cd/A,43.5 lm/W)and 28.0%(97.9 cd/A,102.5 lm/W)and low efficiency roll-offs of 8.1%and 4.6%at practical brightness of 1000 cd/m2.Even at extremely high brightness of 10000 cd/m2,the green one still exhibited high efficiency of 24.0%.It is demonstrated that the molecular stacking style of the host material plays essential role to determine the performance of PhOLEDs.(4)By selecting bulky oligocarbazole as p-type unit,bipyridine or pyridine-DPPO as"dual n-type unit",a series of bipolar hosts materials were designed and prepared.The incorporation of the second n-type unit successfully pulled down the LUMO levels and facilitated the electron injection and transportation,resulting in lower driving voltages.The bulky DPPO further enhand the three-dimensional noncoplanar character of host material,prevented the overlarge electron current and facilitated charge balance in the present device configuration,hence m-POPyTCz exhibited excellent performance with 27.0%(51.9 cd/A,46.5 lm/W)in sky-blue PhOLED.(5)A group of polymeric host materials were prepared using non-conjugated polyethylene as main chain and carbazole derivatives as side chains.They are characterized by high triplet energies and solution processibility.PLEDs were fabricated by solution spin coating using these polymer hosts and FIrpic or Ir(ppy)3 as doped emitters.