1. Development of Indium-free Transparent Conducting Oxides
via the Sol-gel Method
M. SKOF1,2, B. Fleming3, S. Rushworth3, A. Walker4, H. Wang2, N. Farmilo2, A. Hassan2
1NSIRC, UK, 2Materials & Engineering Research Institute, Sheffield Hallam University, UK, 3 EpiValence Ltd, UK 4TWI Ltd., UK
Introduction
Transparent Conductive Oxides (TCOs)
Optical transparent and conductive thin films
Doped metal oxides, exhibiting an energy bandgap of 3.2 eV and above
Used as the active material in photovoltaic cells, where the sunlight is absorbed to generate of charge carriers, or electrodes in optoelectronic devices, such as touchscreens, where a conductive object creates a change in capacitance.
Market demands are continuously rising, from £32.14 Billion USD in 2014 to an estimated £55.53 Billion USD in
Indium Tin Oxide (ITO)
ITO is the industrial standard for the production of TCOs.
It consists of up to 90 % indium2, which is listed as a critical technology material by the EU3, showing a volatile price and increasing demand by the industry.
Sputter coating as deposition process for ITO is costly and inefficient, with a yield of less than 50 % including recycling2. Also, it requires expensive vacuum equipment and post processing, resulting in a high environmental impact.
Methodology
Precursor
To develop of a precursor solution for transformation into a printable ink requires an active ingredient, a solvent and a stabilising agent.
Titanium alkoxide was chosen as the active ingredient and a functional alkoxide as both the solvent and stabilising agent to keep solution complexity to a minimum.
The thermal properties of the solution were assessed by thermogravimetric analysis (TGA) whilst the evaporation behaviour and atmosphere reactivity was evaluated using drop tests at two temperatures (30°C and 70°C).
Matrix
To development the active matrix, the sol-gel method was selected, as it allows for relatively easy control of composition and homogeneity in solution.
Drop tests have been performed to analyse the stability of varying precursor concentrations as well as to examine the effect of niobium doping on reaction kinetics.
The films were deposited via spin coating and sintered in air, allowing for a natural hydrolysation of the precursor.
The sintering has been controlled with XRD and the band gap was determined via UV/VIS. Light and electron microscopy have been used to study defects and EDX was used to analyse the elemental distribution of Ti and Nb in the coating.
Figure 3: Sol-gel precursor
Figure 1: Indium metal price (US$/kg)4
Figure 2: Indium metal production trends4
Problem: High demand for indium tin oxide (ITO), decreasing resources and increasing price
Aim: Finding an alternative as standard for transparent conductive coatings
Solution: Low cost sol-gel process, using widely available elements and nano-structured coatings
Potential Substitute: Titanium dioxide based material, doped with niobium
Desired Outcome: Process route to fabricate printable ink and low temperature sintering process
Benefits: Reduction of waste during process of manufacture and use of flexible substrates
Results
Precursor Development
Results
Matrix Development
Results
Film Formation
EDX shows agglomeration of Nb in the TiO2 film as well as inhomogeneities in the matrix itself
Films form defects, such as pores or cracks during the drying and sintering process
1st weight loss region
2nd weight loss region
Figure 4: TGA of the precursor titanium ispropoxiethoxide in ispropoxiethanol, showing two weight loss regions
The TGA analysis shows two weight loss regions
The first weight loss is due to the evaporation of the solvent
The second weight loss step indicates the decomposition and evaporation of the precursor
Figure 8: Reaction kinetics of precursor systems at ambient conditions with different titanium alkoxide concentrations, glass slides showing different stages of the drop
Figure 11: Distribution of Ti and Nb in the thin film
Figure 12: Micrographs of sintered coatings
Drop Test of Doped and Non-doped Samples
The stability of the drops shows a dependency on the concentration of the precursor
Nb doping strongly alters the onset of the hydrolysation reaction, which could potentially lead to agglomerates of Nb oxide in the active matrix
Reaction Finished
Onset Hydrolysation
As Deposited
Figure 5: Isothermal TGA of titanium ispropoxiethoxide in ispropoxiethanol
Analysis of the precursor via isothermal TGA at a temperature of 80°C shows a complete solvent removal after about 52 minutes
Conclusion
TGA results indicate two weight loss regions, with the onset of precursor decomposition at around 150°C. The isothermal TGA at 80°C shows a complete solvent removal after 52 minutes.
Drop tests reveal an influence of precursor concentration as well as an over-all decrease in stability in ambient atmosphere for doped samples.
Different reaction kinetics of matrix and dopant precursor influence homogeneity of the thin films and lead to the formation of agglomerates, as seen in the EDX graphs.
The XRD data show the formation of anatase above 500°C.
UV/VIS measurements show a low absorption of the coatings over the whole measurement range.
Light and electron microscopy show formation of defects and cracks in coatings.
Figure 9: XRD patterns of TiO2 thin films sintered at different temperatures
A(100)
XRD Analysis of Sintering Temperature
Coatings were deposited and sintered in air for 25 min
The XRD data indicates the formation of the desired TiO2 anatase structure in non-doped samples at temperatures above 500°C
Below this temperature only an amorphous structure could be detected.
Figure 6: Drop Test at 30°C Ti(OIPE)4 in iPEOH solution
Figure 7: Drop Test at 70°C Ti(OIPE)4 in iPEOH solution
Films were deposited and sintered in air at 500°C
The films show a low absorption over the whole measurement range, an influence of concentration can be detected
Low solvent evaporation rate due to low substrate temperature
No evidence of metal oxide powder formation
Elevated temperature improved drying
Drop shows apparent film formation at the end
Figure 10: Absorption spectra of doped and non-doped films
Acknowledgments
European Commission, EASME, Horizon 2020
Alec Gunner, TWI Ltd.
Anthony Bell, Sheffield Hallam University
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References
[1] 07/08/17.
[2] J. Mater. Chem., 2011, 21,
[3] 08/08/17.
[4] 08/08/17.