Consideration about precursor and anion for low-temperature decomposition of precursor

1. Consideration about precursor and anion for low-temperature decomposition of precursor

2. Introduction Nitrate-based recipe Zinc nitrate hexahydrate Indium nitrate hydrate Ethylene glycol + 2-methoxyethanol Requirement for low-temperature process 1) Lower ion valency 2) Large anion → easy to decompose - + r F~q/r2

3. Introduction Anion effect (Top) Magnesium acetate - Strong ion pairing - Hydrophobic Me group in acetate ions is driven to the surface (Bottom) Magnesium nitrate - Lack of ion pairing - Mg2+are strongly repelled from the surface Higher surface tension of nitrate solution → easy to dewet on surface

4. Cationvalency • Low-temperature decomposable precursor • 1) Lower valency • InSn(OtBu)3 precursor • Highly soluble in hydrocarbon • Low-temperature decomposition (thermal decomposition, TGA was performed in dry air) • Lower sheet resistance vs. commercially available ITO powder

5. Cationvalency • Low-temperature decomposable precursor • 1) Lower valency • In(η5-C5H5) precursor • Synthesized by InCl + LiCp • Room-temperature self decomposition • In NP synthesized in anisole with PVP Thermal oxidation

6. Cationvalency Low-temperature decomposable precursor 1) Lower valency In case of [Me3SnOZnR]4, Polymeric precursor has alkyl chain attached on metal atom decompose at low temperature of 177˚C Poor coating property limits its application on TFT use ※ Not only thermal decomposition, but solubility and coating property should be considered

7. Anion effect Low-temperature decomposable precursor 2) Anion effect In case of copper ethylenediamine complex and copper trimethylamine complex, oxalate anion starts decomposition at lower temperature - Oxalates do not hydrolyze, increases process temperature

8. Anion effect • Low-temperature decomposable precursor • 3) Heterometallic precursor • Fluoride precursors • In aqueous solution, fluorides hydrolyzes and • Forms Sn-O-Zn bonding • Homocondensation: high energy is required • F-Zn-OH + HO-Zn-F → F-Zn-O-Zn-F • F-Sn-OH + HO-Sn-F → F-Sn-O-Sn-F • Heterocondensation: low temperature • F-Sn-OH + HO-Zn-F → F-Sn-O-Zn-F • Result of the electronegativity difference • Mechanism of low-temperature • decomposition of remaining fluorides? • still question

9. Anion effect Low-temperature decomposable precursor 3) Heterometallic precursor Fluoride precursors - High mobility and stability positive △Vth - photo-adsorption of O2 – prevented by passivation M-F bonds at the channel/GI interface block oxygen vacancy diffusion → disturbing the trapping of photo-generated electron F plays as a dopant – high on-current – high µFE

10. Anion effect Low-temperature decomposable precursor 3) Heterometallic precursor Fluorine effect – in situ dehydroxylation PL of oxide phosphors – quenched by hydroxides In situ dehydroxylation by F in triflate ligands

11. Anion effect Aqueous zinc tin oxide transistor Sn hydroxide has high decomposition temperature but, ZTO processed at 300C Heterocondensation can help low temperature condensation

12. Conclusion • Thermal decomposition of anion is key issue for the metal salt-based solution process • Generally, decomposition temperature is increases when ions strongly bind together • Coulombic force between ions are determined by charge strength and distance • Nitrate is generally low-temperature decomposable. • It is difficult to tune the ions’ valency or bond distance • When dimer or higher polymer is used, precursor selection can be widen • Fluoride route enables low-temperature process, but limited solubility has to be overcomed

13. Future works • Investigation about heterometallic precursors are required • Easy tryout: fluoride precursors for gate dielectrics • ex) ATO gate dielectric with Al and Ti fluorides +

14. Low-temperature spin-coating Solution-processed aluminum oxide Nitrate precursor for low-temperature process Al nitrate and Ce nitrate are available Leakage current density of aluminum oxide EG 10% exhibited lowest leakage current Thickness Thickness is decreased from 30.9nm to 27.4nm per coating Refractive index n was increased but values were anomalous (1.391 -> 1.410)

15. Low-temperature spin-coating Surface tension Temperature dependency of 0.23M aqueous zinc nitrate solution σ = 1102 – 1.263 T (mN/m) 726 mN/m for 25˚C 752 mN/m for 4˚C

16. Viscosity Solvent’s viscosity increases fast near solvent’s melting point And EG’s MP is -13˚C (-85˚C for 2-methoxyethanol)

17. Viscosity Effect if solution viscosity on sol-gel spin-coated films Solution viscosity ↑ = Film density ↑ < 10 cP ~15 cP Solution viscosity was controlled by: sol aging time ※ Primary particle size is constant > 40 cP ~35 cP

18. Other issues Degradation under 4˚C In the solution, ethylene glycol replaces hydration shell of metal salts and omits water Subzero temperature spin-coating can prevents water evaporation and increases amount of residual water inside as-deposited film Still questions: In2O3 with O2/O3 annealing (JACS, 2011) InCl3 in EG/Acetonitrile: Heating substrate with halogen lamp during spin-coating

19. Low-temperature IZO 150C 5min 300C annealed IZO (after dry annealing) Dry annealing decreases mobility Film morphology was degraded (porosity) Annealing should be conducted under dry synthetic air By dry pre-annealing, On-current decreases 250C 1hr 150C 1hr 200C 1hr 300C 1hr

20. Supplement Stages of spin-coating Deposited mass ~ Films were thinned at lower temperature 1) Lower evaporation rate 2) Less porosity How high vapor pressure thicken deposited film? Evaporation covers all stages Solution concentrates during spin-off stage (~15s)