Chapter 1 - Transparent conductors Flashcards
Name typical classes of transparent electron conductors.
Oxides: binary (ITO or FTO), ternary and quaternary compounds exists.
Conducting polymers: polyaniline
Composites: insulating polymers as matrix for conducting metal nanoparticles.
Using energy diagrams, explain why oxide materials can have high electronic conductivity but remain transparent. Name state-of-the-art materials.
Need oxide material that is intrinsically insulating (have band gap above that of energy of photons in visible range). Make them degenerately doped, so that we get doped electrons in the conduction band and raise the Fermi level to be inside the conduction band. This give metallic conduction, but still have band-gap.
State-of-the-art materials are for example indium doped tin oxide and fluorine doped tin oxide.
Analyze pros and cons for the applications of ITO and FTO in solar cells.
ITO: has better transmittance of visible light than FTO, and conducts better.
FTO: chemically inert, high-temperature resistance, mechanically stable. Also less expensive than ITO (due to cost and low abundance of indium).
Explain why polymers can be good electron conductors. What are the mechanisms of conductivity?
Polymers can be good electron conductors if they have a so-called conjugated system of overlapping π-orbitals. The conduction can then happen through this delocalized system. Electron transport through hopping (interchain or between defects) also operative.
Explain the concept how to construct transparent electronically conducting composites. What is the minimum amount of metal particles to be used in these composites and why?
By using an insulating polymer as a matrix to host conducting metal nanoparticles. The minimum amount of metal particles needed is the amount where there is formed a continuous conducting path through the material.
Name some applications for transparent conductors?
Electrodes is thin-film photovoltaics, in optoelectronics, in touch-screen displays, in electrochromic windows.
What is the Burstein-Moss shift?
The increase in band gap of a transparent conducting oxide that occurs when the degenerately doped electrons fill part of the conduction band. This means that the transition from VB to CB will be larger, since VB-electrons can’t be excited into the bottom of the CB, since these states are already filled with doped electrons.
How does the visual appearance of ITO depend on In/Sn and O content?
It will have a optimum composition where it is clear. Too much of either compounds, and the thin film will be colored.
Name two conducting polymers.
Polyaniline and polypyrrole.
How can one regular the electrical conductivity and color of polyaniline?
By controlling oxidation state, doping level and morphology.
What is the percolation threshold when talking about composite transparent conductors?
It is the point above which a continuous net of conducting particles is formed throughout the transparent matrix.
