The Solution-processed Films And Devices Of Organic Fluorescent Small Molecules

Organic optoelectronics functional materials and devices have made a greatbreakthrough in the design and synthesis of materials and preparation technology ofelectroluminescent device after nearly30years rapid development. At present, fullcolor display screen, prepared via the organic small molecular vacuum evaporationtechnology, begins to move toward the market. But, this preparation method hassome intrinsic disadvantages such as complex fabrication process, high cost, seriousmaterial waste, not easy to realize the large area display. Solution processingpreparation process (such as coating, ink jet printing and so on) can make up theabove deficiencies in vacuum evaporation process, thus it attracts more and moreattentions form researchers. Usually solution processing technology is based onorganic conjugated polymers, but low purity, structure defect and other seriousproblems affect the device efficiency greatly. So, nowadays the researchersintroduce small molecules with clear structure and high purity into the solutionprocessing technology, and a new class of solution processable material system aredesigned and developed based on organic small molecule. So far, deviceperformance based on solution processable small molecular organic light-emittingmaterials, cannot meet requirements for full color display. Since single devicestructure, low device luminous efficiency, and spin-coating film preparation are themain origins. Thus how to enhance and improve the comprehensive performance of light emitting device based on existing solution processable organic small molecularmaterials, become a fashion for a device physicists currently. This paper mainlyrevolves the film preparation of solution processing small organic molecules andtheir performance of organic electroluminescent devices, and explores how toprepare high quality spin coating film of small organic molecules, build fullysolution-processed organic small molecular multilayer electroluminescent devices,and achieve a new way to prepare solution-process full color LED display and whitelight illumination device.In the second chapter, we focus on the preparation of solution processing singlethin films of fluorescent organic small molecule TCBzC and its single layerelectroluminescent device performance. Through the optimization on choice ofsolvent and film thickness of luminous layer, a series of electroluminescent deviceswere prepared, and their performances were compared as well. The results indicatethat, the single-layer device performance has no relationship with solvent selection,except for the luminous layer film thickness. In order to obtain high quality film andhigh electroluminescence efficiency of TCBzC, heat treatment was carried, and thussingle layer device reaches6.9cd/A of maximum luminous efficiency, double of thatof the previous device (3.1cd/A). Combined studies with structural feature andsingle carrier device performance of TCBzC material, we find that, different heattreatment temperature can make TCBzC material be of different movement degrees.When the heat treatment temperature is at40℃, which is very close to the meltingpoint of single alkyl carbazole in the suspension chain, free movement andrearrangement happen in the alkyl chain carbazole section. This results in a morebalanced electron-hole transport, and thus greatly improves the electroluminescentdevice performance based on solution-process fluorescent organic small molecules.Meanwhile, we also extended heat treatment method to other organicelectroluminescent molecules, and high light emitting device performances wereobtained as well.In the third chapter, in order to obtain efficient and stable solution-processingwhite light device of organic fluorescent small molecule, white light devices with TCSoC and TCPC behaving different charge transport properties as host materialswere prepared and studied. We found that the one with the bipolar TCSoC as hostshowed better luminescence properties, with a maximum luminous efficiency andpower efficiency of6.64cd/A and3.52lm/W respectively at the current density of44.53mA/cm~2, and a maximum brightness of2192cd/m2as well. And with theincrease of the current density, color is very stable, with the color resolution index(CRI) of92, indicating a very good white light device. Electrochemical and time offlight (TOF) methods were used to test the carrier mobility of materials, whichshowed that TCSoC possessed a higher electron injection and transport capacity thanTCPC, although their hole transport mobility were similar. Therefore, TCSoCmaterial exhibits more balanced electron-hole transport ability, making its whitelight electroluminescent device perform high efficiency and stability. Experimentalresults show that, in a solution processing fluorescent organic small molecular whitelight device, bipolar host material having a more balanced carrier transport propertycan improve the performance and stability of white light device effectively.In the fourth chapter, through the building of multilayer device structure and thepreparation of composite film, we obtained a highly efficient and stable full solutionprocessing fluorescent molecular electroluminescent devices. Here we firstlyintroduced alcohol soluble conjugated polymer PFN into the solution processingfluorescent organic small molecular electroluminescent devices, and fabricated themonochromatic light and white light solution processing electroluminescent deviceswith multilayer structure. Based on optimization of PFN film thickness, we got ablue OCFSO-based monochromatic light multilayer device with the luminousefficiency of3.25cd/A, and a white light multilayer device based on blue OCPC,green OCBzC and red OCNzC mixed emissive layer, which reached9.2cd/A ofmaximum luminous efficiency. Meanwhile, the PFN layer did not affect the colorstability of the white light device. These results show that, introducing PFN layerinto the device structure brings increased maximum luminous efficiency in bothmonochromatic and white light case, twice those of the original device, which is thehighest value among reported results of similar solution processed organic fluorescent molecular electroluminescent device. Through photovoltaic testing, wefound PFN layer can effectively reduce electron injection barrier from cathode tolight-emitting layer, which improves electron injection capacity and makes electronsinject from cathode to light emitting layer more effectively, thus the device canachieve more balanced charge transport properties and exhibit the excellentperformance. This kind of preparation of highly efficient solution processing organicfluorescent small molecular light emitting device, can realize simple devicepreparation, low cost, and large area display, which greatly prompts the applicationsof solution-processed organic fluorescent small molecule light emitting devices infull color flat panel display and white light illumination.