Journal of C Carbon Research Review Carbon Allotropes as ITO Electrode Replacement Materials in Liquid Crystal Devices Ingo Dierking Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M139PL, UK; [email protected]; Tel.: +44-161-275-4067 Received: 21 November 2020; Accepted: 8 December 2020; Published: 10 December 2020 Abstract: Indium tin oxide (ITO)-free optoelectronic devices have been discussed for a number of years in the light of a possible indium shortage as demand rises. In particular, this is due to the largely increased number of flat panel displays and especially liquid crystal displays (LCDs) being produced for home entertainment TV and mobile technologies. While a shortage of primary indium seems far on the horizon, nevertheless, recycling has become an important issue, as has the development of ITO-free electrode materials, especially for flexible liquid crystal devices. The main contenders for new electrode technologies are discussed with an emphasis placed on carbon-based materials for LCDs, including composite approaches. At present, these already fulfil the technical specifications demanded from ITO with respect to transmittance and sheet resistance, albeit not in relation to cost and large-scale production. Advantages and disadvantages of ITO-free technologies are discussed, with application examples given. An outlook into the future suggests no immediate transition to carbon-based electrodes in the area of LCDs, while this may change in the future once flexible displays and environmentally friendly smart window solutions or energy harvesting building coverings become available. Keywords: ITO; electrode; nanotubes; graphene; liquid crystals; metal nanowires; conducting polymers 1. Introduction Indium was discovered in 1863 by Reich and Richter, who were two chemists studying the ores from the mines of the region around Freiberg in Germany. It is one of the critical metals for the modern, technologically oriented industry [1]. Mined from zinc sulphide ores, it is a minor component and produced during the refinement process of the major component, zinc. In technology, it finds its main uses in the production of transparent electrodes as an indium–tin alloy, ITO, or indium tin oxide, consisting of indium oxide (In2O3) and tin oxide (SnO2) at a mass ratio of 9:1, or 74% In, 8% Sn, and 18% O2 [2]. The primary indium production is mainly located in China, South Korea, Japan, and Canada (Figure1a), which contribute nearly all of the world’s current annual indium production of roughly 800 tons per year [3]. This amount has been relatively stable and constant over the last decade (Figure1b) [ 4], while during the three decades before (1980–2010), the production has increased more than tenfold. The constant primary indium production should not disguise an increasing demand of the metal, even over the last decade. This has several reasons; firstly, there has been an increase in secondary indium production through recycling, and secondly, stored overproduction in China has been released onto the world market. It is thought that the secondary indium production has by now reached a similar level as the primary production, fulfiling the current total world demand of indium, which is in the order of about 1300 tons per year. While prices had been steeply increasing during the beginning of the 21st century, they have been dropping recently from about $600 per kg to roughly half of that value (Figure1c) [ 4] due to reasons mentioned above. The main end-use of indium by far is found in the display technology, having shifted nearly exclusively from CRTs (cathode ray tubes) C 2020, 6, 80; doi:10.3390/c6040080 www.mdpi.com/journal/carbon C 2020, 6, x FOR PEER REVIEW 2 of 30 C 2020, 6, 80 2 of 28 half of that value (Figure 1c) [4] due to reasons mentioned above. The main end-use of indium by far is found in the display technology, having shifted nearly exclusively from CRTs (cathode ray tubes) to flat panel displays (FPDs) over the last two decades. This is followed by the use as solder and in to flat panel displays (FPDs) over the last two decades. This is followed by the use as solder and in photovoltaics and solar cells (Figure1d) [ 5]. Since the display market is by a large margin the main photovoltaics and solar cells (Figure 1d) [5]. Since the display market is by a large margin the main consumerconsumer of of indium indium in in the the form form ofof ITO,ITO, thisthis paper will will place place an an emphasis emphasis on on this this technology. technology. FigureFigure 1. 1. (a) Proportion Proportion of of primary primary indium indium production production for the for main the producer main producer countries, countries, China, South China, SouthKorea, Korea, Japan, Japan, and Canada and Canada in 2018. in 2018.(b) Quantity (b) Quantity in tons in of tons worldwide of worldwide primary primary indium indium production production per peryear year for for the the last last decade decade (2010 (2010–2019).–2019). (c) (Pricec) Price of indium of indium in US$ in US$ per per kg kgfor forthe the last last decade decade (2010 (2010–2019).–2019). (d)(d The) The main main uses uses of of indium indium cancan bebe foundfound in the production production of of flat flat panel panel displays, displays, as assolder solder and and in in photovoltaicsphotovoltaics/solar/solar cells, cells, 2015. 2015. [Data[Data collectedcollected from references references [1 [1––5]].5]]. InIn recent recent years, years, there there has has been been a controversial a controversial discussion discussion about about the exhaustion the exhaustion of indium of indium supplies forsu thepplies technology for the technology industry. Whileindustry. first While claims first of claims the supply of the of supply indium of beingindium at being threat at already threat in thealready mid 2020s in the havemid 2020s proven have to proven be incorrect to be incorrect [6], it is [6], estimated it is estimated that imminent that imminent shortages shortages may may occur overoccur the over next the one next or one two or generations, two generations, approximately approximately around around the the year year 2050 2050 [7 ].[7]. Of Of course, course, suchsuch an estimatean estimate is quite is quite hard hard to make,to make, as as it doesit does not not only only relyrely onon an estimation estimation of of the the remaining remaining primary primary indium deposits and the rate of increased consumption—one also has to take into account increasing indium deposits and the rate of increased consumption—one also has to take into account increasing incentives for recycling through policy changes, the price development of indium, and therefore the incentives for recycling through policy changes, the price development of indium, and therefore the viability of different recycling processes and technological developments. This is just naming some viability of different recycling processes and technological developments. This is just naming some of the factors, which have a relatively large margin of error for prediction. For example, during the of the factors, which have a relatively large margin of error for prediction. For example, during the sputtering process of ITO, roughly 90% of the material actually remains in the production machine, sputteringwhich is processviable for of ITO,industry roughly to be 90% recycled. of the materialHowever, actually the recycling remains of inindium the production from end- machine,of-life whichdisplays is viable is not for yet industry viable. Th tois be may recycled. change However, as the resources the recycling become of increasingly indium from scarce end-of-life and the displaysprice isof not indium yet viable. increases. This mayYet, the change recycling as the of resources display components become increasingly in general scarce may also and be the brought price of about indium increases.through political Yet, the measures recycling and of display the drive components for greener technologies. in general may also be brought about through politicalThis measures can illustrat andively the drive be seen for by greener the real technologies. cost of the different components installed in a liquid crystalThis flat can panel illustratively display. be The seen actual by the display real cost sandwich of the di ffcellerent is constructed components of installed two ITO in-coated a liquid glass crystal flatsubstrates panel display. onto which The actual polarizer display films sandwich are laminated. cell is constructedThe ITO films of act two as ITO-coated transparent, glass conductive substrates onto which polarizer films are laminated. The ITO films act as transparent, conductive electrodes. The substrates are glued together, being kept at a distance of approximately 5 µm through spacers. C 2020, 6, 80 3 of 28 C 2020, 6, x FOR PEER REVIEW 3 of 30 Alignmentelectrodes. layers, The substrates often rubbed are glued polyimide, together, assure being a kept uniform at a distance orientation of approximately of the liquid crystal,5 μm through which is spacers. Alignment layers, often rubbed polyimide, assure a uniform orientation of the liquid crystal, filled in the narrow sandwich cell gap. One of the glass substrates hosts an arrays of thin film which is filled in the narrow sandwich cell gap. One of the glass substrates hosts an arrays of thin transistors (TFT) to address individual pixels and produce grey-scale levels, while the other contains film transistors (TFT) to address individual pixels and produce grey-scale levels, while the other red–green–blue (RGB) colour filters providing the overall video rate colour images of, for example, contains red–green–blue (RGB) colour filters providing the overall video rate colour images of, for a movie (see inset of Figure2). In contrast to popular belief, it is neither the liquid crystal nor the example, a movie (see inset of Figure 2).