Selective Passivation of Electrical Defects in Metal Oxides
Synthesis and Properties of Functional Materials
Final Report Abstract
Large area and transparent electronics require as active layer transparent semiconductors, like metal oxides, which can be deposited by a large area deposition technique, like spray pyrolysis. The necessary process requires high temperatures, i.e. several hundred degrees, for metal oxide (e.g. ZnO) synthesis from precursor molecules. Also the spray deposition process requires high temperatures to achieve thin and high quality layers needed for transistor fabrication. High temperatures cause high energy consumption and restrict possible substrate materials, e.g. for plastic substrates temperatures below 250 °C are typically needed. In this project novel fluorinated molecules were synthesized and tested, which are able to passivate defects caused by imperfections in the material, which occur at increasing number at lower temperatures. Best results were found after screening various classes of molecules for a fluorinated 1,3-diketone molecule with modified phenyl side group. A deposited layer with defects improves dramatically after room temperature treatment with the passivation molecule and results in stable transistor operation. A further approach followed was to develop Zn precursor molecules which allow ZnO formation already at lower temperatures. By fluorinated derivatives of the standard precursor Zinc acetate the ZnO formation temperature was found to be lowered by ~100°C, i.e. from 360 °C to 250 °C. Beside optimization of molecular binding properties by molecule variation also the deposition process itself needed to be tuned to lower temperatures. In spray pyrolysis deposition the material is dissolved in a solvent, e.g. water, and sprayed against a heated substrate where the solvent evaporates and the material is deposited. A principal problem occurs if droplets land at the heated substrate and deposit material statically at the landing site only, resulting in a very inhomogeneous layer. This is avoided if the temperature is high enough so that evaporation is so fast, that the droplet is lifted by its evaporating gas above the surface (Leidenfrost effect) and remains mobile. For water this temperature is around 260 °C. Usage of substrate temperatures beyond the Leidenfrost temperature is the established approach to achieve thin homogeneous layers, since the deposition from a droplet is not restricted to a static landing site. The Leidenfrost temperature depends on the solvent used. Several suitable solvents and their mixture were tested for a reduced Leidenfrost temperature. Best results were found for an ethanol-water mixture resulting in a relatively small temperature lowering of 10 °C. Therefore in this project it was decided to follow a more rigorous approach, i.e. to study the droplet behavior at the surface on the individual droplet level in detail by visualization using a high-speed camera. The analysis allowed to identify precisely conditions for landing droplets on the surface and mobile droplets hovering just above the surface. Surprisingly this approach allowed to find deposition conditions which allow for mobile water droplets at the surface even below 100 °C and to deposit thin homogeneous layers at those dramatically lowered temperatures.
Publications
- "Effect of β-diketone passivation on solution processed zinc oxide nano-layers used in thin film transistors" International Conference on the Formation of Semiconductor Interfaces (ICFSI-16), 02.-07.07.2017, Hannover, Germany, Talk ASS I T5, (2017)
Jonas Köhling, Marlis Ortel, Nataliya Kalinovich, Gerd-Volker Röschenthaler, Veit Wagner
- "Influence of long chain carboxylates as precursor on the performance of ZnO TFTs" Spring Meeting German Physical Society, 19.-24.03.2017, Dresden, Germany, HL 64.8, (2017)
Jonas Köhling, Veit Wagner
- "Influence of the pH value of the precursor solution on ZnO TFTs" Spring Meeting German Physical Society, 19.-24.03.2017, Dresden, Germany, HL 64.7, (2017)
Cristian Telescu, Jonas Köhling, Veit Wagner
- "Polymer passivated zinc oxide thin film transistors" Spring Meeting German Physical Society, 19.-24.03.2017, Dresden, Germany, HL 38.3, (2017)
Torsten Balster, Jonas Köhling, Marlis Ortel, Veit Wagner
- "Thin Film Transistor Active Layer Deposition by Spray Pyrolysis" MRS fall meeting, Symposium EM10: Solution-Processed Inorganics for Electronic and Photonic Device Applications, 26.11. - 01.12.2017, Boston, USA, invited talk, (2017)
Veit Wagner
- "ß-diketone passivated solution processed zinc oxide nano-layers for application in thin film transistors" International Meeting on Information Display (IMID), Seoul, Korea, 2017
Jonas Köhling, Marlis Ortel, Nataliya Kalinovich, Gerd-Volker Röschenthaler, Veit Wagner
- "DFT assisted tailoring of fluorine-containing molecules for passivation of zinc oxide layers in thin film transistors" Spring Meeting German Physical Society together with Condensed Matter Division of European Physical Society, 11. - 16. 03. 2018, Berlin, Germany, HL 40.2, (2018)
Jonas Köhling, Nataliya Kalinovich, Gerd-Volker Röschenthaler, Veit Wagner
- "Binding mechanism of fluorine-containing ketones on zinc oxide surfaces for thin film transistor passivation" Spring Meeting German Physical Society, 31.03. - 05.04.2019, Regensburg, Germany, DS 3.8, (2019)
Jonas Köhling, Nataliya Kalinovich, Gerd-Volker Röschenthaler, Veit Wagner
- "High Speed Real Time Single Droplet Analysis to Improve Spray Pyrolysis Deposition Process" MRS fall meeting, Symposium: FF04: Crystal Engineering of Functional Materials—Solution- Based Strategies, 01. - 06.12.2019, Boston, USA, FF04.03.04 (2019)
Jonas Köhling, Nataliya Kalinovich, Gerd-Volker Röschenthaler, Veit Wagner
- "Novel hovering droplet state at warm and hot surfaces for small droplets" Erfindungsmeldung (2020) DE 10 2020 133 536.9
Veit Wagner, Jonas Köhling
- "Tailored ß-diketones as effective surface passivation for solution processed zinc oxide thin film transistors" Organic Electronics v86 (2020) 105906
Jonas Köhling, Vladislav Jovanov, Nataliya Kalinovich, Gerd-Volker Röschenthaler, Veit Wagner
(See online at https://doi.org/10.1016/j.orgel.2020.105906)
