Design, Synthesis, And Electroluminescent Property Of Novel Bipolar Host Materials

Phosphorescent organic light-emitting diodes (PhOLEDs) and thermally activated delayed fluorescence (TADF) OLEDs have attracted extensive research attention since they can reach a theoretical maximum internal quantum efficiency of unity by utilizing both singlet and triplet excitons for light emission. To avoid any unwanted quenching pathway for triplet excitons in neat films, the phosphors and TADF emitters are usually doped in certain host matrixs at a certain concentration to fabricate OLEDs. Therefore, the host materials are important as well to determine the overall performance of the whole devices. In this thesis, series of "single n-type unit" and "dual n-type units" bipolar host materials were designed, synthesized, and used in PhOLEDs and TADF-OLEDs. The structure-property-electroluminescence performance correlation of these host materials was emphasized to discuss. The details are as follows:(1) CN-containing "single n-type" bipolar host materials:By using cyanophenyl as n-type unit and N-phenyl-carbazole as p-type unit, a series of isomers,o-CzCN, m-CzCN, and p-CzCN, were developed for use as bipolar host materials for blue PhOLEDs and TADF-OLEDs. By varying the ortho-, meta-, and para-linking style of the functional groups in these isomers, the triplet energy, charge mobility, and amorphous stability were greatly tuned. The FIrpic device with m-CzCN host exhibited high efficiencies of 46.81 cd A-1 and 23.14%, and it was characterized by slow efficiency roll-off. The 2CzPN based blue TADF device with the o-CzCN host achieved high efficiencies of 29.23 cd A-1 and 14.98%, which are higher than the highest literature value of 13.6% for 2CzPN devices with other hosts.(2) Pyrazole-containing "single n-type unit" bipolar host materials:A series of high-triplet-energy bipolar materials,o-CzDPz, m-CzDPz,3-CzDPz, and mCPDPz, were developed as universal hosts for both phosphorescent and delayed fluorescence OLEDs of various emitting colors. The charge transporting mobilities were well tuned by varying the molar ratio of n-type pyrazole unit to p-type carbazole group. The maximum efficiencies of 26.8% and 29.0% were achieved for m-CzDPz hosted blue and 3-CzDPz hosted green phosphorescent OLEDs, respectively. More importantly, the green TADF devices using these pyrazole-containing hosts were characterized by rather slow efficiency roll-off.(3) Cyanopyridine-containing "dual n-type units" bipolar host materials:By employing "dual n-type units" cyanopyridine as an electron-transporting moiety, two novel bipolar host materials, namely m-PyCNmCP and 3-PyCNmCP, were developed for application in green PhOLEDs. Extremely low turn-on voltages of 2.01 V and 2.27 V (corresponding to brightness of 1 cd m2) and high power efficiency of 101.4 lm W-1 and 119.3 lm W-1 were achieved for m-PyCNmCP and 3-PyCNmCP hosted green devices, respectively. These turn-on voltages are much lower than the theoretical limit (2.4 V) of the emitter Ir(ppy)3, and the 2.01 V is comparable to the lowest record of 1.97 V reported so far for green PhOLEDs.(4) Pyrazol-pyridine-containing "dual n-type units" bipolar host materials:Three bipolar host materials o-CzPyPz,m-CzPyPz, and p-CzPyPz with simple structures were developed using 3-(1H-pyrazol-1-yl)pyridine as the electron-transporting unit. By varying the linking mode of n-type and p-type units on the phenylene bridge, the molecular conformation and thus optoelectronic parameters of hosts were tuned to large extent. The meta-substituted isomer m-CzPyPz was proved to be the optimal host structure for the blue PhOLED that exhibited highest efficiency of 49.1 cd A-1 (24.5%) and low-efficiency roll-off, while the para-substituted isomer p-CzPyPz was proved to be the optimal host structure for the green PhOLED that exhibited low turn-on voltage (2.8 V) and achieved the highest efficiency of 91.8 cd A-1 (27.3%) among the three isomers. Two-component white OLEDs using p-CzPyPz as common host for blue and orange layers also exhibited high efficiencies of 60.7 cd A-1 and 23.1%.(5) Diphenylphosphine oxide (DPPO)-pyridine-containing "dual n-type units" bipolar host materials:DPPO-pyridine was designed as "dual n-type units" and incorporated onto the skeleton of the traditional uni-polar host mCP to constructed novel bipolar host materials, namely m-PyPOmCP and p-PyPOmCP, for application in blue and green PhOLEDs. In comparison with the reference compound PymCP containing pyridines as the single n-type unit and the literature cases where DPPO was attached onto the p-type moieties, the direct linking of dual n-type units in m-PyPOmCP and p-PyPOmCP was proved to pull down the LUMO levels but simuteneously keep the HOMO as shallow, which finally facilitate both hole- and electron-injections in OLEDs. High efficiency of 55.6 cd A-1 (25.3%) and 98.2 cd A-1 (28.2%) were achieved in m-PyPOmCP hosted blue and p-PyPOmCP hosted green PhOLEDs, respectively.