Organic Small Molecule Films Via Inkjet Printing And Application In Light-Emitting Diodes

Organic light-emitting diode(OLED)has the features of excellent mechanical flexibility,thinness,self-luminescence,and low power consumption.It is a new generation of flat panel display technology applied to flexible displays and wearable devices.Because inkjet printing technology has the advantages of non-contact,maskless,high material utilization,flexible patterning,etc.,it is the most promising technology for preparing low-cost,high-precision,patterned,and large-area OLED devices.This thesis focuses on the organic small molecule films preparing by inkjet printing technology and the application in OLEDs.In order to prepare uniform organic small molecule films by the inkjet printing technology,it is essential to formulate inks to meet the requirements of the inkjet printing process.The solvent system not only needs to meet the solubility capacity for organic small molecule materials,but also needs to meet the parameters that the ink can be ejected uniformly and stably by the inkjet printer.First,we studied the influence of the solvent system on the quality of inkjet printed ligh-emitting layers(EMLs).The results show that the organic small molecule ink is formulated with a 2:8volume ratio of o-dichlorobenzene(o DCB)and chloronaphthalene(CN)binary solvents,and a co-hostsystemof4,4',4''-tris(carbazol-9-yl)triphenylamine(TCTA)and2,6-bis[3-(9H-carbazol-9-yl)phenyl]pyridine(26DCz PPy)for green phosphorescent dopant tris(2-phenylpyridine)iridium(Ir(ppy)₃)(9:9:2 in weight ratio).Furthermore,we studied the effect of hole-translorting layers and inkjet printing process parameters on inkjet printed EMLs,such as annealing temperature,driving voltage,dot pitch and driving voltage waveform.Eventually,the ink formula and process parameters were determined,leading to a stable inkjet process and a uniform light-emitting film.Then,we applied the inkjet printed EMLs in OLEDs.The EML was inkjet printed on different hole-transport films,and the surface morphology and photoluminescence tests were carried out.The results showed that the inkjet printed EML has a uniform surface morphology on poly(vinylcarbazole)(PVK)film withour re-dissolving problem.Further,the inkjet-printed EML was applied to fabricate green OLEDs.The PVK-based OLEDs obtained the best device performance with a maximum brightness(L_max),maximum current efficiency(CE_max)and maximum power efficiency(PE_max)of 1489 cd/m²,7.81 cd/A and 2.51 lm/W,respectively.We also utilized the ink formulation to prepare red,green and blue inks for OLEDs,indicating this ink formulation was universal applicable.Finally,in order to solve the interlayer mixing problem between EML and electron transpoting layers(ETLs),we developed an orthogonal alcohol solvent system for1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene(TPBi).We found that the combination of methanol(MA)and ethylene glycol(EG)with a volume ratio of 90:10 can meet the requirements of solubility and viscosity,and suppressed the formation of coffee ring.Furthermore,surfactant octoxynol(Triton X-100)was introduced to improve the spreadability of the ETL ink on the EML surface.It was found that 0.15 vol.%Triton X-100 performed the highest coverage and continuous ETL.Finally,OLED with the inkjet printed EML and ETL was prepared,which had a L_max,CE_maxand PE_max of 324 cd/m²,3.38 cd/A and 0.63 lm/W,respectively.