Transparent Conductive Oxides: What are they?

What is a TCO?

The acronym TCO stands for transparent conducting oxide, which as the name suggests, is a metal oxide material which conducts electricity and remains optically transparent. This subset of novel materials combines some of the most desirable attributes of materials such as metals for their relative conductivity, as well the transparency which comes with insulative materials such as glass. TCOs have applications throughout the electronics industry in fully transparent electrical devices such as transparent displays and lighting, and optoelectrical and photovoltaic devices.

How do they work?

TCO materials are semiconductor materials, meaning there is an energy gap (known as a band gap) between the occupied electronic states in the material (known as the valence band) and unoccupied electronic states in the material (known as the conduction band) and have conductivity values between those of a pure metal and an insulator. What separates TCO materials from standard semiconductors is that the optical band gap energy (the difference in energy between the energy levels of allowed optical electron transitions) is greater than the energy of visible light. This means that the optical band gap for the TCO material must be larger than the energy of violet light which has a wavelength of ~ 400 nm. This corresponds to an energy of 3.1 eV. What this means is that a band gap bigger than 3.1 eV will make the material ‘see through’.

In order to ensure we have lots of charge carriers ready to move around and conduct within the material, impurities in the form of different atoms are added into the material which either have additional electrons (n-type doping) or fewer electrons (p-type doping) than the atoms already in the material. In this way, additional charge carriers are added into the unoccupied states in the material, allowing for greater conductivity. This also can increase the transparency of the material, by making the band gap bigger. One can alter the parent material, its structure and the type of extra atoms added to create a whole host of TCO materials.

Some desirable design attributes for TCO materials are:

1. We want high mobility in the conduction/valence band for charge carriers to move and create current.
2. We want large numbers of charge carriers within the material to move and create current.
3. We want to have an optical band gap which is greater in energy than visible light so that the material appears see through. This can be improved by adding different atoms into the material (Moss-Burstein shift).
4. The gap between the conduction band minimum and the next higher conduction band minimum (known as CBM and CBM + 1) must be greater than 3.1 eV to ensure transparency.

So, what’s the big deal?

As we strive to become ever greener in way we live and consume energy, the need to create novel devices and electronics becomes greater. One major application of both n-type and p-type TCOs is in photovoltaics and in the improvement of solar panels. When light is incident on a solar panel, the individual photovoltaic cells (which are usually silicon-based p-n junctions) generate electron-hole pairs which are able to move around and generate current within the circuit. The theoretical efficiency of a multi-junction solar panel with an infinite number of layers using sunlight as the incident light is 68.7%^[3] and 86.8%^[4] using focused concentrated light. In reality, this efficiency is a lot lower, with William Shockley and Hans-Joachim Queisser estimating that the maximum efficiency for a single p-n junction solar cell was 33.2%^[5] with a band gap of 1.34 eV^[5]. Commercially available silicon-based p-n junctions’ solar cells having an efficiency of 24.4%^[6]. At the front of the solar cells, TCO layers are used and act as the optically transparent electrode that allows photons into the solar cell and transports the photo-generated electrons to the external device terminals. It has been shown that the use of new TCO materials can be optimised by tuning the refractive index and matching materials to minimise losses due to internal reflections at interfaces, increasing the efficiency of standard solar cells by 15%^[7], allowing for greater conversion of light into electricity by the solar cells.

Another use of TCO materials is in optically transparent displays. The TCO material functions as a transparent electrode which can form one of the layers in organic light emissive devices (OLEDs) used in touch panels, flat panel displays and other future devices. The most commonly used TCO for these applications is n-type indium tin oxide (ITO) which features very high conductivity and high levels of transparency, excellent for display electronics. Similar to their application in solar cells, tuning the way the TCO interacts with other layers in the display greatly influences the performance of the devices, improving the efficiency and stability of the displays.

References:

[1] Electan.com. (2022). ITO (Indium Tin Oxide) Coated PET Plastic – 100mm x 200mm. Available at: https://www.electan.com/ito-indium-tin-oxide-coated-pet-plastic-100mm-200mm-p-8272-en.html

[2] Dixon, S.C., Scanlon, D.O., Carmalt, C.J. and Parkin, I.P. (2016). n-Type doped transparent conducting binary oxides: an overview. Journal of Materials Chemistry C, [online] 4(29), pp.6946–6961. doi:10.1039/c6tc01881e.

[3] A. De Vos & H. Pauwels (1981). “On the Thermodynamic Limit of Photovoltaic Energy Conversion”. Appl. Phys. 25 (2): 119–125. Bibcode:1981ApPhy..25..119D. doi:10.1007/BF00901283. S2CID 119693148.

[4] De Vos, A. (1980). “Detailed balance limit of the efficiency of tandem solar cells”. Journal of Physics D: Applied Physics. 13 (5): 839–846. Bibcode:1980JPhD…13..839D. doi:10.1088/0022-3727/13/5/018.

[5] William Shockley and Hans J. Queisser (March 1961). “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells” (PDF). Journal of Applied Physics. 32 (3): 510–519. Bibcode:1961JAP….32..510S. doi:10.1063/1.1736034.

[6] G. Watson (2022). Solid State Materials and Modelling: Topic 7 Doped Semiconductors. https://tcd.blackboard.com/bbcswebdav/pid-2127782-dt-content-rid-12902411_1/courses/CHU33307-202122/CHU33307-Solid%20State%20Materials%20and%20Modelling-Topic%207%20Doped%20semiconductors.pdf

[7] K. Fleischer, E. Arca, I.V. Shvets, Improving solar cell efficiency with optically optimised TCO layers, Solar Energy Materials and Solar Cells, Volume 101, 2012, Pages 262-269, ISSN 0927-0248, https://doi.org/10.1016/j.solmat.2012.01.037. (https://www.sciencedirect.com/science/article/pii/S0927024812000530)