| Summary: | 博士 === 國立清華大學 === 化學系 === 95 === The electroluminescent and electric-optical properties of PLED devices are highly dependent on the film thickness of conjugated polymers, therefore it’s very important to establish an efficient and rapid method to obtain the true thickness of polymer film. We have successfully designed and demonstrated an unique and practical way of “non-destructive measurement method for polymer thickness” to measure the average thickness and to monitor the uniformality of the coated conjugated polymer film used in the device. As long as the thickness and absorbance relationship, expressed as a regression equation, for the employed polymer is established, we can directly obtain the precise information about the thickness and uniformality of polymer film by simply measuring its UV-vis spectrum.
Based on the “non-destructive measurement method” mentioned above, we have developed methods to obtain the coating with the desired thickness of EL polymers, and used the methods to prepare multi-layer PLED (ML-PLED) devices. In this dissertation, we have established a general guide for gaining the device with the desired thickness ratio having the optimum device performance for a given electrode material. In the case of MEH-PPV/Alq, where the polymer and small molecule materials are both EL active layers, we have found that the EL color is also sensitive to the thickness ratio of MEH-PPV/Alq layers and the employed cathode material. By using optimal thickness ratio, plus an additional hole blocking layer, together with an electrical annealing treatment, one of our ML-PLED devices with the structure of ITO/MEH-PPV(~45 nm)/TPBI(10 nm)/Alq(30 nm)/MgAg has gained a high luminescence of ~22,000 cd/m2, and an external quantum efficiency of ~1.2 ％.
We have further applied an ultrathin layer of Pan-MEA, a very unique derivative of polyaniline synthized via the concurrent reduction and substitution method, to the optimal device mentioned above for modifying the surface of the ITO electrode. With the presence of the Pan-MEA thin layer, the performance of OLED devices were much improved, while the improvement on PLED devices were not as significant. We believe that it is due to the partial dissolution of Pan-MEA film happened during the spin-casting process of the subsequent EL polymer solution, e.g., MEH-PPV/p-xylene solution. The treatment of Pan-MEA layer with the crosslinking agent thioacetic acid helps undo the “partial dissolution problem” of Pan-MEA, and meanwhile maintains the 3-D π-conjugation network of Pan-MEA. Here, we have obtained a highly efficient ML-PLED device with the structure of ITO/Pan-MEA(~2 nm)/MEH-PPV(~45 nm)/TPBI(10 nm)/Alq(30 nm)/MgAg/Ag, having a high luminescence of >35,000 cd/m2 and a high external quantum efficiency of 1.88 ％To our knowledge, this is the brightest PLED device of MEH-PPV.
In this dissertation, we have also developed a reliable method for measuring the work function data of both metals and organic compounds based on a high resolution XPS (PHI Quantera), with a measurement error of less than 0.03 eV.
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