The Synthesis,Characterization And Optoelectronic Properties Of Solution-Processible Small Molecules

Since the first electroluminescence (EL) device report by Tang and his coworkers in 1987, the exploring of various organic compounds and the device structure have attracted more and more attention in search for better performance. Organic conjugated polymer materials have been widely studied because of their high photoluminescence (PL) and electroluminescence (EL) efficiencies, high thermal, spectral stabilities and ease to process. However, polymers exhibit some instinct defects e.g. uncertain structure and molecules weight, which limit their application on OLEDs. Small molecular oligomers exhibit high PL and EL efficiencies and attract much attention because of their well-defined structures; furthermore, they can be functioned as model compounds for polymer, which are considered to be promising candidate for OLEDs and potential active materials for electronic devices. Small molecules are generally processed by vacuum evaporation. This method, whilst well proven, has potential commercial disadvantages for large area full color displays. It is desirable to design and synthesize solution-processible small molecules, which could open up the possibilities of fabricating small molecule OLEDs in a way similar to polymer light-emitting diodes (PLEDs) by low cost solution techniques. Otherwise, an imbalance injection and transport of electron and hole carriers is also becoming an important issue for a new breakthrough in OLED research. In order to reduce the imbalance and increase the efficiency of the devices, the charge balance of electron and hole carriers needs to be boosted significantly.In this thesis, based on achieving solution-processible small molecules, balancing the charge carriers and tuning the emission color, we designed a new oligomer TCBzC. The introduction of electron-deficient 2, 1, 3-benzothiadiazole unit into the backbone of oligomer can promote the injection and transport of electron and tune the emission color into green light. The introduction of hole-transporting group carbazole into the side chain of oligomer may promote the injection and transport of holes. Otherwise, because of introducing electroactive carbazole groups into the side chain, via this such design, TCBzC films can be electrochemical deposition onto the ITO electrode. The electrochemical investigation indicates that the compound has relatively higher highest occupied molecular orbital (HOMO) energy level of 5.43 eV and a lower lowest unoccupied molecular orbital (LUMO) energy level of 2.70 eV, suggesting a better charge carrier injection and transport capability. Efficient green photoluminescence (PL) both in solution and film can be observed; especially a high PL quantum efficiency in solid film up to 0.84 is obtained. The thermal results show that TCBzC exhibits amorphous morphology in the solid state with a glass transition temperature (Tg) of 85℃, and the decomposition temperature is 416℃, this is benefit for the EL devices.Because of choosing appropriate length alkyl chain and large molecular weight of TCBzC, thin films of the compound can be cast from solution by spin-coating. AFM images shows that the spin-coated films clearly have a fairly smooth surface morphology with a root mean square (RMS) roughness of 0.47 nm. Highly efficient OLEDs have been achieved by spin-coating films of TCBzC as the emitting layer. In the single-layer device with the configuration ITO/PEDOT/TCBzC/Ba/Al, the molecule emits green light with a maxmum brightness of 6901 cd m^-2, a luminous efficiency of 5.71 cd A^-1 and a power efficiency of 4.09 lm W^-1. In order to increase the efficiency of the device furtherly, the double-layer device with TPBi as the hole-blocking layer exhibits a maximum brightness of 9226 cd m^-2, luminous efficiency of 31.63 cd A^-1 and power efficiency of 20.69 lm W^-1. Otherwise, we also fabricated EL devices based on TCBzC electrochemical deposition films. The single-layer device exhibits green light emission with a maximum brightness of 5400 cd in^-2, luminous efficiency of 0.74 cd A^-1 and power efficiency of 0.31 lm W^-1. The double-layer device with TPBi as the hole-blocking layer exhibits a luminous efficiency of 5.72 cd A^-1 and power efficiency of 3.59 lm W^-1. All of the EL devices emit stable green light. High EL efficiencies and facilitating fabrication the light-emitting films indicated the oligomer to be promising candidate for green light-emitting diodes for display applications.Furthermore, based on the concept of designing diffenert color materials that can be electrochemical deposition, we synthesized a yellow fluorene-based oligomer TCFoC, the compound exhibits good thermal property, the decomposition temperature is 411℃. It also can be electrochemical deposition onto the electrode not affecting the emission of compound. It can provide the basis of designing new functional compounds.