2020 Fiscal Year Annual Research Report
Low Temperature Photochemical Sol-Gel Processing of Functional Thin Films and their Applications
| Project/Area Number |
20J40137
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| Research Institution | Yamagata University |
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Principal Investigator |
孫 麗娜 山形大学, 有機エレクトロニクスイノベーションセンター, 特別研究員(RPD)
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| Project Period (FY) |
2020-04-24 – 2023-03-31
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| Keywords | vacuum ultraviolet / photochemical conversion / solution processing / metal oxide thin films / photoluminescence / thin film encapsulation / light emitting diode / low temperature |
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| Outline of Annual Research Achievements |
Photochemical conversion of metal-organic precursors to metal oxide thin films using Vacuum Ultraviolet (VUV) radiation at low temperature is an attractive approach for realization of roll-to-roll fabrication of flexible electronics. Selection of metal precursors, solvent, tuning of VUV conditions, as well as designing architectures of multilayers are considered to achieve various functionalities.
We have developed photochemical conversion of metal-organic precursors into a highly compact and transparent ZnO thin films for the purpose of encapsulation of OLEDs. Coatings were composed of extremely small ZnO NCs with less than 5 nm size, for which quantum size effect could certainly be expected. In 2020, the work specially focused on the photoluminescent (PL) properties of thus obtained ZnO. Indeed, we have discovered rather strong and stable PL from ZnO NCs, which is shown to produce color-tunable PL from violet (389 nm) to bluish-green (486 nm) by controlling the period of vacuum ultraviolet (VUV) irradiation. Systematic characterization show that the surface of the ZnO NCs is covered with a highly luminescent Zn－MEA complex with characteristic charge transfer absorption/emission that is responsible for the observed variable PL. This presents a hitherto unknown system of color-tunable PL in which only the “skin” emits, but neither the quantum size effect of the NC core nor the defect states in ZnO contribute. Combined X-ray, IR, and PL spectroscopic techniques suggest that preferential decomposition of acetate is responsible for the color shift during VUV irradiation.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
In 2020, we examined the luminescent properties of VUV-processed ZnO dry gel coatings with the primary goal of identifying the origin of the color-tunable PL. A careful examination of PL,PLE, and UV－vis spectra, combined with those of the precursor Zn－monoethanolamine (MEA) complex, has confirmed no size effect of ZnO for the color tuning. Rather the charge transfer (CT) absorption/emission from Zn－MEA surface complex appears to be the primary mechanism. Interestingly, the ZnO NC core is still necessary for the formation of the luminescent complex.
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| Strategy for Future Research Activity |
Based on the knowledge from previously established method to obtain highly photoluminescent ZnO thin films, which were composed of extremely small ZnO nanocrystals with less than 5 nm size, application of the thin film materials will be the main subject in this year. Thin film diodes and bottom-gate TFTs will be fabricated to reveal their semiconducting properties and tested. Effect of metal precursors, ligands, solvents, coating conditions, VUV irradiation time, atmosphere and temperature on crystallization are to be investigated. Especially interesting are those of In(III) and Ga(III) to envision high mobility IGZO thin films on their blend with Zn(II).
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