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Research On Solution-Processed Organic Photoelectric Materials And Devices

Posted on:2017-05-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:C YaoFull Text:PDF
GTID:1108330485450040Subject:Materials Science and Engineering
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Benefitting from low cost, simple process, diverse materials, mechanical flexibility and easy to prepare large-area samples and so on, solution-processed organic optoelectronic materials and devices have been widely concerned in academic and commercial fields. The photoelectric conversion device is more popular in this field, especially for the organic light emitting diode (OLED) which converts electric energy into light, and the organic solar cell (OSC) which converts light into electric energy. In the past decades, the luminescence efficiency of organic light emitting diodes have been improved significantly, especially for the device prepared by vacuum evaporation technology, and the internal quantum efficiency has reached 100%. In recent years, the research on organic solar cells has yet made great progress, and the photoelectric conversion efficiency is over 10%. Although OLED and OSC have made great progress, there are still many aspects that need to be improved. The development of red and green OLED devices has been more mature, while the development of blue OLED devices is still far from enough. Compared with the inorganic solar cells, the conversion efficiency of OSC is still lagging behind. At the same time, the lifetime of solution-processed OLED and OSC devices is generally low, and it is farfrom the commercial applications. This thesis is mainly focus on improving the efficiency or lifetime of solution-processed blue organic light emitting diodes or organic solar cells.1. Two kinds of host materials, namely DTPAGe and DCzGe, for blue phosphorescent organic light-emitting diodes are designed by incorporating electron-donating groups (carbazole and triphenylamine) into tetraphenylgermane, which is a new type of core moiety that had never been studied in this field. This molecular structure endows the new compounds with wide energy bandgap, high thermal/morphological stability and good solution processability. Combined with this new compound, all-solution-processed blue-emitting phosphoprescent OLED were fabricated with a maximum luminance (Lmax) of 10000 cd m-2, a maximum external quantum efficiency (EQEmax) of 6.9% and a maximum current efficiency (CEmax) of 15.2 cd A-1. Which are higher than the silicon based materials. Moreover, the devices show a very low current efficiency roll-off. Therefore, tetraphenylgermane is an efficient core moiety for designing wide energy bandgap and high performace phosphorescent host materials.2. A new dumb-bell and efficient blue phosphorescent dye Cz-C8-Flrpic was designed and synthesised by incorporating 9-phenyl-9//-carbazole into a commonly used blue emissive iridium complex bis(4,6-(difluorophenyl)pyridine-N,C2’)picolinate (FIrpic) via an alkyl chain linkage. As being incompatible with host materials in a physically blended emitting layer, if phosphorescent dye is incompatible with host materials, it is prone to form aggregation induced by Joule heat in devices under work and decreace the device performance. We hope this dumb-bell structure can efficiently decrease the aggregation and improve the device performance. The Cz-C8-Flrpic and the reference material Flrpic doped emissive layers were investigated by AFM, STEM-EDS, transient photoluminescence decay curve and molecular dynamics simulations. The results show that in the Cz-C8-Flrpic doped film the phase aggregation of Flrpic units is less severe than that in the typically used Flrpic film. In addition, the optimized Cz-C8-Flrpic based device achieved a maximum luminance of 25142 cd m 2. a maximum EQE of 8.5% and a maximum current efficiency of 22.5 cd A-1 which is about 15% higher than that of the control device based on Flrpic. We conclude that this new dumb-bell structure, grafting a typically used dye to functional groups with alkyl chains, is useful to restrict phase separation in physically blended emitting layers, and thus can achieve high electroluminescence performances.3. In order to improve the device lifetime, a new method, called combustion processing, was reported to prepare solution-processed molybdenum oxide (MoO3) for hole selective layer (HSL) of OSC with low annealing temperature. The combustion precursor solution using ammonium heptamolybdate as the metal source, acetylacetone as a’fuel’, and nitric acid as an oxidizer can largely reduce the temperature for transformation of the polyoxomolybdate to a-phase MoO3. Finally, the MoO3 HSL exhibits a high charge-transporting performance which is similar to the widely used material poly-(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). When this MoO3 was used as HSL for OSC, the device lifetime has been greatly increased. The simplicity, rapidness, and effectiveness of this method, combined with the low annealing temperature, make it promising for the roll-to-roll manufacture of long-lifetime flexible organic solar cells.
Keywords/Search Tags:Organic Light-emitting Diodes, Organic Solar Cells, Phosphorescent Host, Phosphorescent Emitter
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