Synthesis And Electro-Optical Property Investigation Of Bipolar Host Materials Used For Solution-Processed Oleds

Solution-processed organic light-emitting diodes (OLEDs) have attracted much scientific and industrial attention due to the low cost manufacturing technology, the processability over large area size, and compatibility with flexible sustrates. Although phosphorescent OLEDs (PHOLEDs) can approach 100% internal quantum efficiency by harvesting both singlet and triplet excitons, the excited state quenching induced by the triplet-triplet annihilation should be minimized to achieve highly efficient electroluminance. Thus, polymer, dendrimer, and bipolar host materials were elaborately designed to improve the external quantum efficiency of the solution-processed PHOLEDs over the past decade. Nevertheless, the high operating voltage is still a major problem in solution-processed OLEDs, which simultaneously lead to the low power efficiency in large area application. Furthermore, the serious efficiency roll-off induced by the unbalanced charge transfer at high luminance also restricts its commercial prospects. Recently, developing host material with small triplet-singlet split (AEst) was demonstrated to be an effective way to reduce the operating voltage of PHOLEDs. However, the exploitation of bipolar host with sufficiently small △Est, especially for solution-processed PHOLEDs, is a rather difficult task because of the severe limitation of molecular design. Therefore, a new class of efficient small molecular co-host, which forms exciplex-state through the intermolecular interaction between donor and acceptor components, was developed to be an alternative strategy to achieve host material with intrinsically small △Est due to the complete frontier orbital separation. The main content of this dissertation is as follows:1. A series of bipolar hosts based on carbazole and phenyl benzimidazole (PBI) moieties collectively named xCz-nPBI, were designed and synthesized. On the basis of different number, ratio and link-configuration of the functional groups, the influence of substitution on the thermal, photophysical and electrochemical properties of the host materials were investigated in detail. Both DFT calculation and single carrier device demonstrate that the strategy of introducing more electron-withdrawing PBI groups in the molecules can effectively enhance the electron injection and transporting ability of the bipolar host, while the increased carbazole units endow the hosts with much smaller △Est for efficient hole injection at the cost of sacrificing the charge balance property. As a result, the solution-processed green-emitting PHOLEDs based on Cz-6PBI shows an extremely low turn on voltage of 2.9 V, and a highest current and power efficiency of 47.8 cd A-1 and 29.6 lm W-1, respectively.2. Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet-triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO-LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (Ex=3.0 eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2 cd A’1 and a power efficiency of 11.5 lm W-1, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving.3. Three solution-processable exciplex-type host materials were successfully designed and characterized by equal molar blending hole transporting molecules with a newly synthesized electron transporting material. The excited-state dynamics and the structure-property relationships were systematically investigated. The low energy electromer state, which only exists under the electroexcitation, was found as another possible channel for eneigy loss in exciplex-based phosphorescent organic light-emitting diodes (OLEDs). In particular, as quenching of the exciplex-state and the triplet exciton were largely eliminated, solution-processed blue phosphorescence OLEDs using the exciplex-type host achieved an extremely low turn-on voltage of 2.7 V and record-high power efficiency of 22.5 lm W-1, which were among the highest values in the devices with identical structure.4. A benzimidazole/phosphine oxide hybrid TPOB was newly designed and synthesized as the electron-transporting component to form an exciplex-type host with the conventional hole-transporting material TCTA. Due to the enhanced triplet energy and electron affinity of TPOB, the energy leakage from exciplex-state to the constituting molecule was eliminated. Using energy transfer from exciplex-state, solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) achieved an extremely low turn-on voltage of 2.8 V and impressively high power efficiency of 22 lm W-1. In addition, the efficiency roll-off was very small even at luminance up to 10000 cd m-2, which suggested the balanced charge transfer in the emission layer. This study demonstrated that molecular modulation was an effective way to develop efficient exciplex-type host for high performanced PHOLEDs.