Design, Synthesis And Properties Of Novel Ambipolar Transporting Materials

Small molecule ambipolar materialswhich help to facilitate the balance of electronsand holes,simplify the device structure, and control the exciton formationzone fororganic light emitting diaodes （OLEDs），have attracted a great of attention in recentyears. In this dissertation, two high performance ambipolar materials were designed andsynthesized, and the characteristics of materials and performance of devices werestudied. The main contents of this dissertation are as follows:1.4,9-bis（4-（benzo[d]thiazol-2-yl）phenyl）naphtha[2,3-c][1,2,5]thiadiazole（BBTPNTD）, which exhibited very high and close electron and hole mobilities of1.7×10^-3cm²/Vs and1.9×10^-3cm²/Vsat an electricfield of4.5×10⁵V/cm, respectively,was designed and synthesized. The study showed that the ambipolar property ofBBTPNTD is closely related to the molecular reorganization energy and the distributionof electrons on frontier orbitals. Pure red-emission non-doped OLED was obtained byusing BBTPNTD as the emitting layer. The maximum wavelength is646nm with theCIE at （0.64,0.36）.2.9,10-bis(3-(pyridin^-3-yl)phenyl)anthracene （DPyPA） with twistedconformationwhich benefits the thermal stability and thin film formation, was designedand synthesized. The electron and hole mobilities of DPyPA are0.87×10^-3cm²/Vs and1.16×10^-3cm²/Vs at an electricfield of3.6×10⁵V/cm, respectively. The single layerOLED with DPyPA emitted pure deep blue emission with the CIE of （0.17,0.08）.Red-emitting, green-emitting,and blue-emitting fluorescent OLEDs with DPyPA astheelectron transporting material （ETM） displayed lower turn-on voltages, higherefficiencies, and higher operational stability than the devices with the conventionaltris（8-quinolinolato）aluminum（Alq₃） as the ETM. The power efficiencies ofthe devicesbased on DPyPA are greater by84–143%relative to those of theAlq₃-based devices,and the half-life-time of the DPyPA based green-emitting device is6times longer thanthat of the Alq₃-based device.3. According to the fact that the HOMO and LUMO of the ambipolar materialswith non-donor-acceptor （non-D-A） structure are mainly located on the samechromophorein the molecules, charge transfer integral ratios of electron to hole （Vab-e/Vab-h） can be approximated to1. Thus the Marcus Equationwas simplified, and theratio of electron mobility to hole mobility at zero field can be obtained by calculatingthe reorganization-energy of electron and hole, and the ambipolar property of moleculecould be predictedbefore synthesis.4. Double-layer OLEDs with similar structures using unipolar Alq₃and ambipolar4,9-bis（4-（2,2-diphenylvinyl）phenyl）naphtho[2,3-c][1,2,5]thiadiazole （BDPNTD）, werefabricated and the life-times of these OLEDs were studied. The results demonstratedthat the stable cations and anions of ambipolar materials benefit the operational stabilityof OLEDs..5. Based on the results of TOF experiments at different temperatures, thetransporting characteristics of4,4’-N,N’-dicarbazole-biphenyl （CBP） withdonor-acceptor （D-A） structure and9,10-di-（2-naphthyl）anthracene （ADN） withnon-D-A structure were compared and analyzed. The study showed that at the sametemperature the electron and hole mobilities of CBP are quite different, while theelectron and hole mobilities of ADN are almost the same. And changes of the electronand hole mobilities of CBP showed different trends with temperature and ectric-fieldintensity,while the change trends of the electron and hole mobilities of ADN are almostthe same. The energetic disorders of the CBP film and the ADN film were calculatedthrough Gaussian Disorder Model （GDM）, and the CBP film shows higher energeticdisorder. It is disclosed that the relatively largedipole moment of D-A type ambipolarmaterials, which leads to high energetic disorder, is responsible for the the low chargemobilities of D-A type ambipolar materials.