Synthesis And Characterization Of Solution-processable Bipolar Small-molecular Light-emitting Materials

Intensive attention has been paid to developing solution-processable small-molecularoptoelectronic materials. Small-molecular active compounds have the advantages ofunequivocal chemical structure, better reproducibility and simple purification. Andsolution-based device-processing technology can afford low-cost and large-area functionaldevices.Materials with bipolar transporting characters for OLEDs exhibit attractive merits. Onthe one hand, bipolar materials can balance the hole and electron injection and transport,which will improve the device efficiency and reduce the efficiency roll-off. On the anotherhand, because of their potential to function both as light emitting and charge transport layers,device processing can be simplified.This thesis was about the design, preparation and characterization ofsolution-processable bipolar small-molecular light-emitting materials. Four compounds weredesigned focusing on their solubility, thermal stability, carrier transport andelectroluminescent efficiency.In the first part, two small molecules named M1and M2containing2,1,3-benzothiadiazole were synthesized. The spiro[dibenzo[c,h]xanthene-7,9-fluorene] wasintroduced to improve thermal stability, and3,5-bis(4-alkoxyphenyl)phenyl block wasintroduced on the two sides to enhance the solubility. Thermogravimetric analysis indicatedthat the thermal decomposition temperature of the two compounds was above400°C.Differential scanning calorimetry measurements indicated that Tgof M1and M2was about211°C and178°C, respectively. The OLED device (ITO/PEDOT:PSS(40nm)/M1(45nm)/TPBI(30nm)/CsF(1.5nm)/Al(90nm)) exhibited a maximal current efficiency of11.04cd/A.In the second part, another two small molecules, namely C1and C2, were synthesized.Both C1and C2contained dibenzothiophene-S,S-dioxide as electron-withdrawing unit andtriphenylamine as electron-donating unit.3,5-bis(4-alkoxyphenyl)phenyl block wasintroduced on one side to enhance the solubility. The differences between them were that C1 was based on2,7-disubstituted dibenzothiophene-S,S-dioxide, while C2was based on3,6-disubstituted dibenzothiophene-S,S-dioxide. The compounds were characterized bythermogravimetric analysis, differential scanning calorimetry, and ultraviolet and visiblespectrophotometry. The C1-based nondoped single-layer device ITO/PEDOT:PSS/C1/CsF/Alshowed a luminous efficiency of2.92cd/A. The C2-based nondoped double-layer deviceshowed a luminous efficiency of3.3cd/A. Further optimization of the molecular structure isbeing explored.