Carbon Allotropes As ITO Electrode Replacement Materials in Liquid Crystal Devices

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 being indium 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 process viable for of ITO,industry roughly to be 90% recycled. of the materialHowever, actually the recycling remains of in indium 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). In contrast to popular belief, it is neither the liquid crystal indiumnor the of indium the ITO-coated of the ITO glass-coated sheets glass that sheets carry that the carry main the cost main of a costdisplay. of a The display. most The expensive most componentsexpensive components of a flat panel of a display flat panel are dis inplay fact theare backlight,in fact the andbacklight, the colour and the filter, colour if one filter, ignores if one the indirect,ignores personnel, the indirect, and personnel, sales costs and (Figure sales costs2). From (Figure this 2). point From of this view, point it becomes of view, clearit becomes that indium clear recyclingthat indium from recycling end-of-life from displays end-of- cannotlife displays be a cannot viable solutionsbe a viablefor solutions industry, for industry, but thatonly but that an overallonly recyclingan overall solution recycling can solution be strived can for, be whichstrived includes for, which glass, includes polarisers, glass, polarisers, and electronics. and electronics. This would This also bewould desirable also for be environmentaldesirable for environmental reasons. reasons.

FigureFigure 2. 2.Cost Cost of of the the components components of of a a liquid liquid crystal display display (LCD). (LCD). The The inset inset depi depictscts the the actual actual constructionconstruction of of a standarda standard sandwich sandwich cell cell forfor displaydisplay devices. [Reproduced [Reproduced by by permission permission after after [8]]. [8 ]].

AtAt the the present, present, end-of-life end-of-lifeproducts products dodo notnot play a significant significant role role for for recycling. recycling. Nevertheless, Nevertheless, muchmuch research research is is being being done, done, and and limitations limitations in primary indium indium production production as as well well as as environmental environmental aspectsaspects can can increase increase thethe amount of of recycled recycled flat flat screen screen TVs, TVs, computer computer monitors, monitors, laptop laptop displays, displays, and andmobile mobile phones phones in the in the future. future. Currently,Currently, the the country country with with thethe largestlargest recyclingrecycling capability of of indium indium (secondary (secondary production production mainlymainly from from waste waste products products during during manufacturing) manufacturing) is Japan, followed followed by by China, China, Belgium, Belgium, and and South South KoreaKorea (Figure (Figure3a) 3a) [ 5 [5].]. Naturally, Naturally, the the largestlargest amountamount of indium indium is is recycled recycled from from liquid liquid crystal crystal display display (LCD)(LCD) TVs TVs and and computer computer monitors, monitors, while while tablets,tablets, smartphones, and and mobile mobile phones phones play play a much a much less less significant role. This is due to the obviously much smaller display size of the product, but also significant role. This is due to the obviously much smaller display size of the product, but also because because these devices are often not simply disposed of, but often kept in cupboards. The overall these devices are often not simply disposed of, but often kept in cupboards. The overall amount of amount of indium recycled from end-of-life display devices is of the order of 80 tons, representing indium recycled from end-of-life display devices is of the order of 80 tons, representing approximately approximately 10% of the secondary indium production [8]. The amount of indium retrieved from 10% of the secondary indium production [8]. The amount of indium retrieved from one ton of electronic one ton of electronic display waste varies between about 100 and 1400 g, depending on respective displaydevice waste products varies and between treatment about methods 100 [9]. and 1400 g, depending on respective device products and treatment methods [9]. C 2020, 6, 80 4 of 28

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Figure 3. (a)Figure Main 3. countries (a) Main countries with awith recycling a recycling capability capability for for indium. indium. (b) Worldwide (b) Worldwide recycled mass recycled of mass of indium for different liquid crystal display technologies 2013 (note the logarithmic scale) (Reproduced indium for different liquid crystal display technologies 2013 (note the logarithmic scale) [Reproduced by by permission from ref. [8]]. (c) General block diagram of a simple recycling process for LCD devices. permission from[Reproduced ref. [8 by]]. permission (c) General from [10]). block diagram of a simple recycling process for LCD devices. [Reproduced by permission from [10]]. A typical recycling process is schematically depicted in Figure 3c. It starts with the collection of end-of-life liquid crystal display devices, which are then manually disassembled into a number of A typicaldifferent recycling fractions, process including is schematicallyplastic housing, metal depicted pieces, cables, in Figure printed3 c.circuit It starts boards with(PCB), thecold collection of end-of-life liquidcathode crystalfluorescent display lamps (CCFL) devices, of the backlight, which optical are then foils, and manually light guides. disassembled Once disassembled, into a number of differentthe fractions, polariser from including the actual LCD plastic panel housing,is often removed metal by thermal pieces, [11] or cables, chemicalprinted methods [12]. circuit boards In some processes, polariser foils are also recovered after a thermal shock treatment [13]. Then, the (PCB), coldliquid cathode crystal fluorescent (LC) is removed lamps in a chemical (CCFL) process, of generally the backlight, by dissolution optical with an organic foils, solvent. and light guides. Once disassembled,This leaves the the ITO polariser-covered glass, from which the needs actual to be mechanically LCD panel crushed is often and milled removed to reduce by the thermal [11] or chemical methodssize to a powder [12]. of In approximately some processes, 10–20 μ polariserm grain size foils [14]. are Different also millingrecovered processes after can a bethermal shock employed, leading to variations in indium recovery efficiency. To separate the ITO and therefore treatment [13indium]. Then, from the the liquid glass, two crystal principle (LC) process is removed may be employed: in a chemical a thermal process, process generallyand the more by dissolution with an organicwidely solvent. researched This hydrometallurgical leaves the ITO-covered processes. The former glass, consists which of needsvacuum tochlorination be mechanically [15] or crushed and milled tovacuum reduce carbon the size reduction to a powder[16], while of the approximately latter uses leaching 10–20 with mainlyµm grain HCl, size H2SO [414, or]. HNO Diff3erent milling [11,17] to separate the glass from the leachate, which contains different metals such as In, Se, Fe, and processes canAl. be Then, employed, these are further leading separated to variations by solvent extraction in indium [18,19] recovery or cementation efficiency. [20]. To separate the ITO and therefore indiumThe topic from of indium the glass, recycling, two different principle strategies process and applied may bemethods, employed: is obviously a thermal too process and the moreextended widely to be researched treated comprehensively hydrometallurgical in this review, which processes. is rather concerned The former about the consists possible of vacuum replacement methodologies of ITO in the liquid crystal technology sector based on carbon allotropes. chlorinationThus, [15] we or refer vacuum the interested carbon reader reduction to some excellent [16], while reviews the on the latter aspects uses of indium leaching recycling. with A mainly HCl, H2SO4, or HNOfirst summary3 [11,17 of] tothe separate recovery of the valuable glass materials from thefrom leachate,waste liquid which crystal display contains panels di hasfferent been metals such as In, Se, Fe, andgiven Al.by Li Then, et al. [21] these already are more further than a separateddecade ago. A by more solvent recent review extraction of recycling [18,19 indium] or cementationfrom [20]. waste LCDs was published by Zhang et al. [22]. The recent advances of indium metallurgy were The topicreviewed of indium by Pradhan recycling, and co di-workersfferent strategies[23], with a and particular applied view methods, on comparing is obviously mining and too extended to be treatedsecondary comprehensively indium production in this fromreview, LCDs. which The potential is rather and concerned strategies for about recycling the LCDspossible was replacement methodologiesdiscussed of ITO by in Ueberschaar the liquid et al. crystal [8], wh technologyile a very recent sector review basedwith a particular on carbon focus allotropes. on indium was Thus, we refer published by Fontana et al. [10]. An enlightening analysis of the techno-economic aspects of waste the interestedLCDs reader as a source to some of value excellent for electronic reviews waste ontreatment the aspects centres can of be indium found in recycling. a work by D’A Adamo first summary of the recovery ofet al. valuable [24], while materials Amato and from Beolchini waste [25] liquidprovide crystala recent patent display review panels for recycling has been of end given-of-life by Li et al. [21] already moreliquid than crystal a decade displays. ago. A more recent review of recycling indium from waste LCDs was published by2. Zhang Non-Carbon et al. ITO [22 Replacement]. The recent Technologies advances of indium metallurgy were reviewed by Pradhan and co-workers [23], with a particular view on comparing mining and secondary indium production from LCDs. The potential and strategies for recycling LCDs was discussed by Ueberschaar et al. [8], while a very recent review with a particular focus on indium was published by Fontana et al. [10]. An enlightening analysis of the techno-economic aspects of waste LCDs as a source of value for electronic waste treatment centres can be found in a work by D’Adamo et al. [24], while Amato and Beolchini [25] provide a recent patent review for recycling of end-of-life liquid crystal displays.

2. Non-Carbon ITO Replacement Technologies Above, we have shortly discussed indium (In) and indium tin oxide (ITO), as well as some recycling aspects of that critical metal, because ITO is still the by far most used material for transparent electrodes, not only in liquid crystal display (LCD) technology, although this makes up most of its C 2020, 6, 80 5 of 28 global use. There are several reasons behind this fact. Firstly, it simply is the major established technology in industry, which is reluctant to change when devices work perfectly fine. With a resistivity 1 of approximately 10–30 Ω− for a film of approximately 200 nm thickness and a transmission of >90% for optical wavelengths, ITO outperforms most of its rivals. Secondly, its properties rival most replacement methods with respect to transmission, which is a key factor in display devices. A third C 2020, 6, x FOR PEER REVIEW 5 of 30 reason is its environmental stability against degradation. LCDs based on liquid crystal-filled ITO sandwich cells orAbove, panels we arehave UV shortly stable discussed and indiumcan last (In) for and several indium decades.tin oxide (ITO), Nevertheless, as well as some the use of ITO recycling aspects of that critical metal, because ITO is still the by far most used material for also has its drawbacks.transparent electrodes, The first not only lies in in liquid the crystal production display (LCD) process technology, of ITO although coated this glass, makes whichup is based on sputteringmost techniques. of its global Theseuse. There are are slow several in reasons production behind andthis fact. relatively Firstly, it expensive simply is the due major to losses and the high temperaturesestablished technology and vacuum in industry, equipment which is reluctant needed. to change Solution-based when devices work processes perfectly as fine. used for other With a resistivity of approximately 10–30 Ω□−1 for a film of approximately 200 nm thickness and a transparent electrodetransmission materials of >90% for are optical much wavelengths, faster and ITO outperforms mostly cheaper. most of its Another rivals. Secondly, disadvantage its of ITO is a certain brittleness,properties rival which most replacement makes other methods materials with respect better to transmission, suited for which flexible is a key substrates, factor in which is a display developmentdisplay devices. that A is third predicted reason is toits stronglyenvironmental increase stability in against thefuture. degradation. For LCDs these based reasons, on as well as liquid crystal-filled ITO sandwich cells or panels are UV stable and can last for several decades. the discussedNevertheless, limited natural the use of resources ITO also has of its indium, drawbacks. a rangeThe first of lies other in the materials production areprocess currently of ITO developed for the use ascoated flexible, glass, which transparent is based on electrodes. sputtering techniques. These These include are slow doped in production metal and oxides relatively other than ITO, metal grids, silverexpensive nanowires due to losses and and meshes, the high temperatures conductive and polymers, vacuum equipment ultrathin needed. metal Solution films,-based carbon nanotube processes as used for other transparent electrode materials are much faster and mostly cheaper. films and graphene,Another disadvantage as reviewed of ITO by is Caoa certain and brittleness, co-workers which inmakes reference other materials [26]. Inbetter the suited following, for we will shortly introduceflexible some substrates, of these which materialsis a display development before concentrating that is predicted in to morestrongly detail increase on in thethe future. carbon allotropes as well as carbon-basedFor these reasons, composites. as well as the discussed limited natural resources of indium, a range of other materials are currently developed for the use as flexible, transparent electrodes. These include doped metal oxides other than ITO, metal grids, silver nanowires and meshes, conductive polymers, 2.1. Metal Nanowires ultrathin metal films, carbon nanotube films and graphene, as reviewed by Cao and co-workers in One of thereference most [26]. promising In the following, materials we that will may shortly be introduce used to some replace of these ITO materials as transparent before electrodes concentrating in more detail on the carbon allotropes as well as carbon-based composites. in future technologies are most likely meshes or thin films of metal nanowire networks [27,28]. Two different2.1. metals Metal Nanowires are commonly used, Cu [29,30], and the even more popular Ag nanowires [31–40], due to their veryOne high of the conductivity. most promising Amaterials comparative that may be study used to of replace yet another ITO as transparent few materials electrodes used (Cu, Ag, Al, and Au) canin future be foundtechnologies in reference are most likely [41 meshes]. or thin films of metal nanowire networks [27,28]. Two different metals are commonly used, Cu [29,30], and the even more popular Ag nanowires [31–40], The metaldue nanowiresto their very high have conductivity. a quite polydisperseA comparative study length of yet ofanother the orderfew materials of 100 usedµm (Cu, and Ag,are very thin with a diameterAl, and of Au) approximately can be found in reference20 nm [41]. (Figure 4). One of the main advantages of nanowires is their ≈ processing possibilities,The metal nanowires which includes have a quite solution polydisperse processing length of the and order electro-spinning, of 100 μm and are very which thin makes them with a diameter of approximately ≈20 nm (Figure 4). One of the main advantages of nanowires is most suitabletheir for processing flexible possibilities, polymer which substrates. includes solution They can processing be produced and electro as-spinning, random which thin makes film networks, meshes, andthem oriented most s structures,uitable for flexible which polymer can also substrates. be aligned. They can be produced as random thin film networks, meshes, and oriented structures, which can also be aligned.

Figure 4. (a) Polydisperse size distribution of a metal nanowire made from Cu. (b) Scanning electron Figure 4. (a)m Polydisperseicrograph of a thin size film distribution of a random Cu of nanowire a metal mesh. nanowire (c) Performance made of from Cu and Cu. Ag( nanowiresb) Scanning electron micrograph of a thin film of a random Cu nanowire mesh. (c) Performance of Cu and Ag nanowires as transparent electrode materials in comparison to indium tin oxide (ITO), as well as other materials such as carbon nanotubes (CNT), graphene, or organic conductive polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) and its additive poly(styrene sulfonate) (PSS), that forms the transparent, conductive polyelectrolyte complex PEDOT:PSS. [Figures reproduced by permission: (a) from [42]; (b) from [43], (c) from [27]]. C 2020, 6, x FOR PEER REVIEW 6 of 30 C 2020, 6, 80 6 of 28 as transparent electrode materials in comparison to indium tin oxide (ITO), as well as other materials such as carbon nanotubes (CNT), graphene, or organic conductive polymers such as poly(3,4- ethylenedioxythiophene) (PEDOT) and its additive poly(styrene sulfonate) (PSS), that forms the With respecttransparent, to performance conductive aspolyelectrolyte transparent complex electrode PEDOT:PSS. material, [Figures the reproduced main parameters by permission: are (a sheet) resistance R, which shouldfrom be [42]; low, (b) from and [43], transmittance (c) from [27]]. T, which should be high. The industry standard for ITO is approximately R 10 Ω 1 and T 93%. As can be seen from Figure4c, silver nanowire electrodes With≈ respect to− performance≈ as transparent electrode material, the main parameters are sheet achieve a similarresistance performance R, which should as be ITO, low, and while transmittance copper nanowiresT, which should lag be behind high. The in industry sheet resistancestandard for the required transmittance,for ITO is approximately or alternatively, R ≈10 Ω□−1 if and they T ≈ achieve 93%. As thecan requiredbe seen from sheet Figure resistance, 4c, silver nanowire the transmittance strongly decreaseselectrodes to achieve values anot similar acceptable performance or optical as ITO, applications, while copper such nanowires as liquid lag crystal behind displays. in sheet Currently, resistance for the required transmittance, or alternatively, if they achieve the required sheet it appears that the only real alternative to ITO with regard to the physical performance side is Ag nanowires, resistance, the transmittance strongly decreases to values not acceptable or optical applications, such which alsoas explains liquid crystal why displays. the main Currently, effort in it metalappears nanowire that the only research real alternative lies on to these ITO materials,with regard asto the is evidenced in the numberphysical of review performance articles side specifically is Ag nanowires, written which about also explains silver nanowires. why the main Langley effort in et metal al. [44] have reviewed thenanowire fabrication, research properties lies on these ofmaterials, Ag nanowires, as is evidenced and in applications the number of employing review articles the specifically latter as electrode written about silver nanowires. Langley et al. [44] have reviewed the fabrication, properties of Ag material. Two much more recent summaries of the current state-of-the-art with respect to Ag nanowire nanowires, and applications employing the latter as electrode material. Two much more recent networks havesummaries been writtenof the current by Dongchen state-of-the- Tanart with et al. respect [45] andto Ag by nanowire Wenting networks Li et al. have [46 been]. written by Dongchen Tan et al. [45] and by Wenting Li et al. [46]. 2.2. Conductive Polymers, PEDOT:PSS 2.2. Conductive Polymers, PEDOT:PSS Of all the conductive polymers [47,48], it is poly(3,4-ethylenedioxythiophene) (PEDOT) and its Of all the conductive polymers [47,48], it is poly(3,4-ethylenedioxythiophene) (PEDOT) and its additive poly(styreneadditive poly(styrene sulfonate) sulfonate) (PSS), which (PSS), which form formthe transparent, the transparent, conductive conductive polyelectrolyte polyelectrolyte complex PEDOT:PSScomplex [49–55 PEDOT:PSS], which are [49 by–55], far which the mostare by prominentfar the most prominent to be used to as be electrode used as electrode materials. materials. The chemical structure ofThe PEDOT:PSS chemical structure is shown of PEDOT:PSS in Figure is5 showna, together in Figure with 5a, together a schematic with a schematic representation representation of the polymer film formedof fromthe polymer gel particles film formed in dispersion from gel particles with in water. dispersion The with grey water. areas The of grey the filmareas areof the PSS film rich, are while the PSS rich, while the blue areas are PEDOT: PSS rich, with crystalline regions that enhance the blue areasconductivity. are PEDOT: PSS rich, with crystalline regions that enhance the conductivity.

Figure 5. Figure(a) Chemical 5. (a) Chemical structure structure of of PEDOT:PSS, PEDOT:PSS, which which is solution is processed solution in processedwater via the formation in water via the of gel particles. The resulting polymer film of PEDOT:PSS-rich regions and PSS-rich regions on a formation of gel particles. The resulting polymer ﬁlm of PEDOT:PSS-rich regions and PSS-rich substrate forms a transparent conductive layer. The conductivity is enhanced by the crystalline regions on a substrate forms a transparent conductive layer. The conductivity is enhanced by the crystalline PEDOT:PSS regions. (Reproduced by permission from ref. [56]). (b–d) show the optical and electro-optical performance of a liquid crystal cell compared between an ITO and polyimide coated glass substrate versus a PEDOT:PSS electrode layer. While the transmittance performance of the PEDOT:PSS electrode is outperformed in the visible optical region by the ITO electrode (b), the switching characteristics (c), as well as the switching dynamics (d) of the respective liquid crystal devices are quite similar. (Reproduced by permission from ref. [57]).

The performance of PEDOT:PSS as an electrode material in comparison to ITO reveals that the transmittance of up to T 95% can in fact be similar to that of ITO, while the sheet resistance is ≈ 1 usually substantially larger, in the order of several 100 Ω− . As with most of the electrode materials, C 2020, 6, 80 7 of 28 transmittance versus sheet resistance is a trade-off in performance. For PEDOT:PSS with a sheet resistance comparable to that of poor ITO (R 60 Ω 1), the transmittance reduces to below 80%, ≈ − which is not acceptable for high-performance liquid crystal devices. Nevertheless, some liquid crystal applications have been demonstrated in which the ITO electrodes were replaced by PEDOT:PSS. These were mainly applications where solution processing is desirable, for example in devices with flexible substrates. One of such applications is polymer-dispersed liquid crystals (PDLC), where liquid crystalline droplets are dispersed in a polymer matrix. These devices switch from a scattering mode at zero-applied electric field to a clear, transmissive mode for applied voltages. An exemplary device was presented with post-treated PEDOT:PSS electrodes [58], where it was shown that the electro-optic performance was comparable to that with ITO electrodes. Similarly, such a technology has been used to demonstrate privacy windows on the basis of electrodes combining Ag nanowires with PEDOT:PSS, which exhibited a performance slightly diminished to that using ITO electrodes [59]. A different application of PEDOT:PSS electrodes are flexible liquid crystal contact lenses with a variable focal length [60]. However, also, more traditional display designs were accomplished with PEDOT:PSS electrodes, a normal twisted nematic (TN) cell [57], vertically aligned nematic (VAN) cells [61], or TN devices for phase shifters in the THz frequency regime [62]. The chemistry, structure, and electronic transport properties have been reviewed in detail in some recent publications by Xia et al. [63], Kayser and Lipomi [64], and very recently by Gueye et al. [65] and by Huseynova et al. [66].

2.3. Ultrathin Metal Films Another alternative to the use of ITO as electrode materials that has been proposed is to employ metals of larger availability, for example silver, gold, copper, or aluminium, as ultrathin layered transparent ﬁlms [67,68]. One of the problems with the production of ultrathin metal layers, which exhibit a thickness of approximately 3–10 nm, is the growth of islands [69,70], which is shown in Figure6a for an ultrathin silver ﬁlm. This leads to a percolation threshold behaviour for the conductivity (and thus sheet resistance). This island growth may be reduced or even avoided through the additionC 2020, 6, x of FOR seed PEER layers REVIEW [71 –78], which are often Ge, TiO2, Al, Au, Cr, or Ni. 8 of 30

FigureFigure 6. (a ) 6. Island (a) Island formation formation of an of ultrathin an ultrathin Ag transparent Ag transparent conducting conducting film film leading leading to a to rough a rough surface surface morphology (bottom), which can be avoided by doping with Al (top) (Reproduced by morphology (bottom), which can be avoided by doping with Al (top) (Reproduced by permission from permission from ref. [79]). (b) Transmission versus sheet resistance performance of a range of ref. [79]). (b) Transmission versus sheet resistance performance of a range of ultrathin Ag, Au, and Cu ultrathin Ag, Au, and Cu films produced with seed layers and/or doping. Ultrathin metal films can filmsoutperform produced withITO in seed terms layers of sheet and resistance,/or doping. but Ultrathin they generally metal have films lower can transmittance. outperform ITO in terms of sheet resistance, but they generally have lower transmittance. The wetting behaviour of the metal on the substrate is one of the key factors for reducing the percolation threshold, as well as the surface roughness [80]. Seed layers of thickness 1–2 nm [81] and doping [82–85] of the ultrathin metal films often with Al, N, or O, are the main parameters to adjust the wetting behaviour. Figure 6b summarises the T-R performance of a collection of data on ultrathin metal films of Ag, Au, and Cu, produced by a variety of methods including seed layers and doping. The data can be found in tabular form in the excellent recent review on ultrathin metal films as transparent electrodes by Yan-Gang et al. [86]. From the graph, it is apparent that ultrathin metal films can outperform ITO in terms of sheet resistance, which can be decreased to well below 10–20 Ω□−1. Problems in the replacement of ITO electrodes are related to the transmittance values, which are quite clearly below 90% in nearly all cases, representing the barrier threshold for any electrooptic applications utilising liquid crystal technology. Furthermore, besides the optical performance, also the production process of ultrathin metal films represents no strategic improvement over standard ITO electrodes, as discussed in the review by Yun [87].

C 2020, 6, 80 8 of 28

The wetting behaviour of the metal on the substrate is one of the key factors for reducing the percolation threshold, as well as the surface roughness [80]. Seed layers of thickness 1–2 nm [81] and doping [82–85] of the ultrathin metal films often with Al, N, or O, are the main parameters to adjust the wetting behaviour. Figure6b summarises the T-R performance of a collection of data on ultrathin metal films of Ag, Au, and Cu, produced by a variety of methods including seed layers and doping. The data can be found in tabular form in the excellent recent review on ultrathin metal films as transparent electrodes by Yan-Gang et al. [86]. From the graph, it is apparent that ultrathin metal films can outperform 1 ITO in terms of sheet resistance, which can be decreased to well below 10–20 Ω− . Problems in the replacement of ITO electrodes are related to the transmittance values, which are quite clearly below 90% in nearly all cases, representing the barrier threshold for any electrooptic applications utilising liquid crystal technology. Furthermore, besides the optical performance, also the production process of ultrathin metal films represents no strategic improvement over standard ITO electrodes, as discussed Cin 20 the20, 6 review, x FOR PEER by Yun REVIEW [87]. 9 of 30

3. Carbon Allotropes as Electrode Materials Due to the the possibility possibility of of large large conductivities conductivities and and charge charge mobility, mobility, carbon carbon nanotubes nanotubes as aswell well as grapheneas graphene have have been been discussed discussed as potential as potential materials materials for transparent for transparent electrodes electrodes for a number for a number of years of years[88–90]. [88 The–90]. clear The clearadvantage advantage of these of these materials materials over over ITO, ITO, but but also also other other methods methods such such as as ultrathin ultrathin metal films, films, lies in the possibility for solution processing, which enablesenables the production of flexibleflexible electronics and displays in conjunction with polymer substrates. One One of of the the problems is the poor solubility, which has has partially partially been been resolved resolved by by the the use use of ofsurfactants surfactants [91,92]. [91,92 On]. the On other the other hand, hand, this usethis of use surfactants of surfactants or eve or evenn covalent covalent bonding bonding of ofdispersion dispersion agents agents negatively negatively affects affects the the conductivity. conductivity. Nevertheless, in recent years, some enhanced solutions havehave emerged.emerged.

3.1. Single Single-Wall-Wall and Multi Multi-Wall-Wall Carbon Nanotubes The main main issue issue of ofusing using carbon carbon nanotubes nanotubes as materials as materials to form to thin form network thin sheets network as transparent sheets as transparentelectrodes is againelectrodes the trade-off is again between the trade transmittance-off between at transmittance optical wavelengths at optical and wavelengths sheet resistance and [93 sheet–99], resistanceas shown in [93 Figure–99], 7as[100 shown]. in Figure 7 [100].

Figure 7. ((a)) Transmittance Transmittance versus versus sheet sheet resistance resistance for for single-walled single-walled carbon carbon nanotube nanotube films films processed processed from fromeither either an organic an organic or an aqueousor an aqueous solution. solution. At acceptable At acceptable transmittances transmittances for liquid forcrystal-based liquid crystal devices-based 1 (Tdevices> 90%), (T the > 90%), sheet resistance the sheet of resistance approximately of approximately R 200–300 Ω R −≈200falls–300 far Ω□ short−1 offalls the far requirements short of the of 1 ≈ R 10–20 Ω .(b) SEM image−1 of a typical carbon nanotube network film, which demonstrates a quite requirements≈ −of R ≈10–20 Ω□ . (b) SEM image of a typical carbon nanotube network film, which demonstrateslarge roughness a (Reproducedquite large roughness by permission (Reproduced from [100 by]). permission from [100]).

Due to the poor interconnectivity interconnectivity between nanotubes and nanotube bundles in the film, film, the sheet resistance is increased. Sur Surfactantsfactants or or dispersing dispersing agents agents further further reduce the conductivity and thus increase the sheet resistance. The sheet or network conductivity can be described by a percolation mechanism [36,101], which already indicates that poor interconnectivity between carbon nanotubes is one of the major problems, and that an understanding of the conductivity mechanism involves a fragile balance between carbon nanotube quality, solubility, dispersion agent, and processing conditions. This is essential for a reproducible and thus commercially viable source of carbon nanotube network films for transparent electrodes. Further, the sheet resistance of those electrodes at approximately R > 200 Ω□−1 for acceptable transmittance values of T > 90% is still too large. Any solution based on carbon nanotubes alone, which reduces the sheet resistance, decreases the transmittance to unacceptable values (Figure 7a). A further problem, specifically also with respect to liquid crystal applications, is the roughness of the carbon nanotube films (Figure 7b). These issues of interconnectivity and roughness have been addressed by post-treatment with strong acids, such as HNO3 [102]. Thin nanotube network films [103–106] and treated films for increased conductivity [107–109] may be used as electrodes for liquid crystal devices. In addition to the feature of acting as an electrode, one would like to incorporate further functionality, for example, the action as a patterned substrate [103] or an alignment layer [110–112]. Most often, this would be a planar alignment of the liquid C 2020, 6, 80 9 of 28 increase the sheet resistance. The sheet or network conductivity can be described by a percolation mechanism [36,101], which already indicates that poor interconnectivity between carbon nanotubes is one of the major problems, and that an understanding of the conductivity mechanism involves a fragile balance between carbon nanotube quality, solubility, dispersion agent, and processing conditions. This is essential for a reproducible and thus commercially viable source of carbon nanotube network films for transparent electrodes. Further, the sheet resistance of those electrodes at approximately 1 R > 200 Ω− for acceptable transmittance values of T > 90% is still too large. Any solution based on carbon nanotubes alone, which reduces the sheet resistance, decreases the transmittance to unacceptable values (Figure7a). A further problem, specifically also with respect to liquid crystal applications, is the roughness of the carbon nanotube films (Figure7b). These issues of interconnectivity and roughness have been addressed by post-treatment with strong acids, such as HNO3 [102]. Thin nanotube network films [103–106] and treated films for increased conductivity [107–109] may be used as electrodes for liquid crystal devices. In addition to the feature of acting as an electrode, one would like to incorporate further functionality, for example, the action as a patterned substrate [103] or an alignment layer [110–112]. Most often, this would be a planar alignment of the liquid crystal, for example,C 20 in20, a6, x TN FOR cell, PEER asREVIEW shown in Figure8a. This can be achieved by aligned nanotubes10 of or 30 nanotube bundles, whichcrystal, for are example, depicted in a atTN di cell,fferent as shown magnifications in Figure 8a. This in can Figure be achieved8b. The by liquidaligned nanotubes crystal molecules will align inor nanotube such structures bundles, which with are their depicted long at molecular different magnifications axis parallel in toFigure the 8b. natural The liquid grooves, crystal which are otherwisemolecules generated will through align in su rubbingch structures a polymer with their layer long thatmolecular is spin-coated axis parallel onto to the the natural ITO grooves, electrode. In the case of thewhich aligned are otherwise nanotube generated films, thethrough latter rubbing fulfils a polymer both functions layer that atis once:spin-coated that onto of the the electrodeITO and electrode. In the case of the aligned nanotube films, the latter fulfils both functions at once: that of the that of theelectrode alignment and that layer. of the alignment layer.

Figure 8. Figure(a) Schematic8. (a) Schematic illustration illustration of of a twisted a twisted nematic nematic (TN) cell (TN) using cella film using of oriented a film carbon of oriented nanotubes as electrodes and at the same time as an alignment layer for the liquid crystal molecules. carbon nanotubes as electrodes and at the same time as an alignment layer for the liquid crystal (b) SEM images at different magnification of such an aligned carbon nanotube film. (c) Electro-optic molecules.switching (b) SEM curve images of a TN cell at diwithfferent carbon magnification nanotube electrode/alignment of such an films, aligned switching carbon from bright nanotube film. (c) Electro-opticat zero applied switching voltage curve to dark ofat applied a TN electric cell with field carbonabove a threshold nanotube voltage. electrode (Reproduced/alignment by films, switchingpermission from bright from atreference zero applied[111]). voltage to dark at applied electric field above a threshold voltage. (Reproduced by permission from reference [111]). For a TN cell, the orientation direction of both substrates is chosen such that the bottom and top alignment are at an angle of 90 degrees. Incoming linear polarised light is guided by the twisted For astructure TN cell, and the can orientation pass the crossed direction polariser. of Theboth device substrates appears is transmissive, chosen such or bright that with theout bottom and top alignmentapplied are voltage, at an as angle shown of in 90 Figure degrees. 8c. Application Incoming of an linear electric polarised field above lightthe threshold is guided causes by the the twisted structure andpositively can dielectric pass the liquid crossed crystal polariser. to reorient the The long device molecular appears axis, which transmissive, is at the same or time bright the without applied voltage,optic axis, as along shown the infield Figure direction,8c. Applicationperpendicular to of the an substrates. electric field Incoming above linear the polarised threshold light causes the is not affected by the LC and is absorbed by the crossed polariser. The device appears black. A typical positivelytransmission dielectric liquidcurve of crystala TN device to reorientas a function the of long applied molecular voltage via axis,the nanotube which electrod is at thees is same also time the optic axis,shown along inthe Figure field direction,8c. Although perpendicular most often employed, to the substrates. not only single Incoming-wall carbon linear nanotubes polarised light is not affected(SWCNTs) by the LCcan be and used is for absorbed applications by as the electrodes crossed in polariser. liquid crystal The devices, device but appearsalso devices black. with A typical transmissionmulti curve-wall carbon of a TN nanotubes device (MWCNTs) as a function are feasible of applied [113–115], voltage albeit at via even the higher nanotube sheet resistance. electrodes is also Similarly, it was very recently demonstrated that also a vertically aligned liquid crystal device shown in Figurecan be constructed8c. Although with mostnanotube often electrodes employed, and alignment not only [116]. single-wall Nanotubes carbon are (nearly) nanotubes vertically (SWCNTs) grown on the substrate, and the liquid crystal orients perpendicular with its long molecular axis to the substrates on both sides. Here, the long molecular axis represents the director and thus the optic axis. When viewed at zero applied voltage between two crossed polarisers, the device appears black in the off state, as there is no birefringence when the direction of light propagation is along the optic axis. Incoming linearly polarised light is unaffected by the LC and absorbed by the crossed polariser. In the case of such a vertically aligned cell, one employs a liquid crystal with a negative dielectric anisotropy. This will switch from homeotropic to planar for an applied voltage above a threshold voltage, leading to an electro-optic response of the cell, as shown in Figure 9a. The corresponding C 2020, 6, 80 10 of 28 can be used for applications as electrodes in liquid crystal devices, but also devices with multi-wall carbon nanotubes (MWCNTs) are feasible [113–115], albeit at even higher sheet resistance. Similarly, it was very recently demonstrated that also a vertically aligned liquid crystal device can be constructed with nanotube electrodes and alignment [116]. Nanotubes are (nearly) vertically grown on the substrate, and the liquid crystal orients perpendicular with its long molecular axis to the substrates on both sides. Here, the long molecular axis represents the director and thus the optic axis. When viewed at zero applied voltage between two crossed polarisers, the device appears black in the off state, as there is no birefringence when the direction of light propagation is along the optic axis. Incoming linearly polarised light is unaffected by the LC and absorbed by the crossed polariser. In the case of such a vertically aligned cell, one employs a liquid crystal with a negative dielectric anisotropy. This will switch from homeotropic to planar for an applied voltage above a threshold voltage,C 2020 leading, 6, x FORto PEER an REVIEW electro-optic response of the cell, as shown in Figure9a. The corresponding11 of 30 schematic device with nanotube alignment electrodes (grey) and liquid crystal orientation (yellow) is shownschematic for the device electric with field nanotube on- and alignment off-state electrodes in Figure (grey)9b,c, respectively.and liquid crystal orientation (yellow) is shown for the electric field on- and off-state in Figure 9b,c, respectively.

FigureFigure 9. Electrooptic 9. Electrooptic response response of of a verticallya vertically aligned aligned liquid crystal crystal device device with with nanotube nanotube electrodes electrodes and negativeand negative dielectric dielectric liquid liquid crystal crystal (LC). (LC). ( a(a)) Typical Typical transmissiontransmission versus versus voltage voltage curve, curve, due dueto the to the birefringence changes between crossed polarisers. The device is dark in the “off-state” and bright in birefringence changes between crossed polarisers. The device is dark in the “off-state” and bright in the “on-state”. (b,c) Schematic orientation of the liquid crystal in the on- and the off-state, respectively. the “on-state”. (b,c) Schematic orientation of the liquid crystal in the on- and the off-state, respectively. (d) The change in transmission between crossed polarisers as the negative dielectric liquid crystal (d) Theresponds change to inthe transmission applied increasin betweeng voltage. crossed (e) Dynamic polarisers switching as the response negative of the dielectric cell with liquid response crystal respondstimes to in the the applied ms range increasing for the electric voltage. field ( edriven) Dynamic switching switching into the response on-state, ofand the in cell the with10s of response ms timesrange in the for ms the range elastically for the-driven electric switching field into driven the off switching-state. The into response the on-state, is similar and for the in thenanotube 10s of ms rangeelectrodes for the elastically-driven as compared to a commercial switching cell. into (Reproduced the off-state. by The permission response from is similar[116]). for the nanotube electrodes as compared to a commercial cell. (Reproduced by permission from [116]). When viewing the device between crossed polarisers, one thus observes an increasing Whentransmission viewing from the the device dark between off-state crossedto the bright polarisers, on-state, one as thusthe vo observesltage across anincreasing the cell is increased transmission from(Figure the dark 9d). o ffThe-state dynamic to the response bright on-state,of this device as the is depicted voltage in across Figure the 9e. cellThe isresponse increased time (Figure for the 9d). The dynamicdark off- responseto the bright of thison-state device is dependent is depicted on inthe Figure applied9e. electric The response field and timeof the for order the of dark a few o ff - to the brightmilliseconds. on-state That is dependent of the transition on thefrom applied the bright electric on- to fieldthe dark and off of-state the is order longer, of as a it few is not milliseconds. driven by the electric field but dominated by elastic interactions between the liquid crystal and the That of the transition from the bright on- to the dark off-state is longer, as it is not driven by the electric homeotropic substrate. One should also note that the very slight offset angle of the nanotube field butalignment dominated from by theelastic right angleinteractions to the betweensubstrate the(Figure liquid 9a) crystal is in fact and intentional the homeotropic and of substrate.great One shouldadvantage, also as note it thatprevents the very the formation slight off set of angle so-called of the reverse nanotube tilt domains, alignment which from degrade the right the angle to theperformance substrate (Figure of any 9verticallya) is in fact aligned intentional nematic LC and device. of great advantage, as it prevents the formation of so-calledWe reverse should tiltmention domains, at this which point that degrade also other the performance display devices of based any vertically on liquid crystals aligned have nematic LC device.been realised with nanotube film electrodes, such as polymer-dispersed liquid crystals (PDLC) [117]. WeThese should are scatteringmention at devices this point without that also the useother ofdisplay polarisers. devices Liquid based crystal on liquid droplets crystals of the have low been realisedmicrometre with nanotube-size range film are dispersed electrodes, and such phase as separated polymer-dispersed within a continuous liquid polymer crystals matrix. (PDLC) This [ 117]. Theseleads are to scattering light scattering devices as the without LC director the useis randomly of polarisers. oriented. LiquidApplication crystal of an droplets electric field of thevia low nanotube film electrodes causes the director to align with the field direction. The refractive index of micrometre-size range are dispersed and phase separated within a continuous polymer matrix. the surrounding polymer matrix is matched to that of the long molecular axis and scattering is This leadsdiminished, to light the scattering device becomes as the LCtransparent. director isPDLCs randomly have their oriented. application Application as privacy of an windows, electric and field via nanotubenanotube film electrodeselectrodes can causes be produced the director as flexible to align foils. with the field direction. The refractive index of the Other applications of conductive, transparent nanotube film electrodes in liquid crystal devices are mostly found as hybrid materials with other technologies, such as Ag nanowires, PEDOT:PSS, or graphene, and these will be discussed in more detail below. A review of the fabrication, properties, and applications of carbon nanotube thin films was published by Liangbing Hu et al. [118] and more recently by Hirotani and Ohno [119], concentrating on flexible electronics applications.

3.2. Graphene, Graphene Oxide, and Reduced Graphene Oxide Due to its high electric conductivity, transparency, and mechanical stability, graphene and its derivatives appear to be ideal materials for transparent, ITO-free electrodes that offer solution processibility and the potential for flexible devices [120–128]. In principle, for single-layer graphene, a transmittance of T = 97.7% can be achieved with a sheet resistance of approximately R 30 Ω 1. ≈ − These areC theoretical 2020, 6, x FOR PEER values REVIEW that have very closely been reached experimentally under12 of 30 laboratory conditions [129–132]. Under more realistic conditions, on the other hand, for real graphene thin films, a transmittance of T = 97.7% can be achieved with a sheet resistance of approximately R ≈ 30 Ω□−1. flake sizesThese imply are that theoretical multi-layers values that may have exist very andclosely that been there reached are experimentally grain boundaries under laboratory both affecting the transmittanceconditions and the[129– sheet132]. Under resistance. more realistic Therefore, conditions, as inon the other case hand, of carbon for real nanotubes,graphene thin films, there will be a trade-off betweenflake sizes conductivity, imply that multi transmittance,-layers may exist deviceand that lifetime,there are grain and boundaries cost [133, 134both]. affecting the Probably,transmittance the first and LC thedevice sheet resistance. made withTherefore, graphene as in the electrodecase of carbon is nanotubes, a cell shown there will schematically be a in trade-off between conductivity, transmittance, device lifetime, and cost [133,134]. Figure 10a, consistingProbably, the largely first LC of device graphene made onwith a glassgraphene substrate, electrode ais polyvinyla cell shown alcohol schematically alignment in layer, and the nematicFigure 10a, liquid consisting crystal, largely which of graphene is covered on a glass by substrate, a top substratea polyvinyl alcohol with analignment ITO electrode layer, [128]. The electro-opticand the nematic response liquid ascrysta a functionl, which is covered of applied by a top increasing substrate with voltage an ITO electrode is shown [128]. in The Figure 10b, switchingelectro the liquid-optic crystalresponse fromas a function a bright of applied zero volt increasing off-state voltage to a is dark shown on-state. in Figure 10b, switching the liquid crystal from a bright zero volt off-state to a dark on-state.

Figure 10.FigureLiquid 10. Liquid crystal crystal sandwich sandwich cell cell mademade of of a graphene a graphene layer layerelectrode electrode and an ITO and electrode. an ITO (a) electrode. The cell is constructed of ① a glass substrate, ② a graphene layer, ③ electrical contacts, ④ an (a) The cell is constructed of O1 a glass substrate, O2 a graphene layer, O3 electrical contacts, O4 an alignment alignment layer, ⑤ the liquid crystal layer, ⑥ another alignment layer, ⑦ an ITO electrode, and ⑧ layer, O5 thethe liquid top glass crystal substrate. layer, (b) DueO6 another to the positive alignment dielectric layer, anisotropyO7 an ofITO the LC, electrode, the optic axis and switchesO8 the top glass substrate. (fromb) Due planar to tothe homeotropic; positive dielectric thus, it switches anisotropy from bright of the to LC, dark the between optic axis crossed switches polarisers from for planar to homeotropic;increasing thus, voltage. it switches (c) Light from transmission brightto through dark the between 20-μm thick crossed LC device polarisers as a function for increasingof applied voltage. voltage. The inset depicts the low-voltage regime in more detail for monochromatic green light (505 (c) Light transmission through the 20-µm thick LC device as a function of applied voltage. The inset nm, blue curve) and for white light (red curve). (Reproduced by permission from ref. [128]). depicts the low-voltage regime in more detail for monochromatic green light (505 nm, blue curve) and for whiteThe lighttransmission (red curve). through (Reproduced the 20 μm thick by LC permission device is shown from ref. in Figure [128]). 10c for monochromatic green light with a wavelength of 505 nm (blue curve), as well as for white light (red curve in the The transmissioninset). The oscillatory through behaviour the 20 ofµ m the thick transmi LCssion device is due is to shown the change in Figure in birefringence, 10c for monochromatic as the green lightmolecular with a wavelength long axis reorients of 505 for nm increasing (blue curve),applied asvoltage, well which as for results white in light an additional (red curve phase in the inset). retardation of several multiples of π. The oscillatoryWe behaviour have seen above of the that transmission aligned nanotube is due films to can the be changeused not onl iny birefringence, as electrode material as the but molecular long axis reorientssimultaneously for increasing as an alignment applied layer for voltage, the liquid which crystal. results A similar in behaviour an additional can be phaseobserved retardation for of several multiplesgraphene ofdueπ .to the π–π interaction of the carbon honeycomb lattice with the phenyl-ring system of the liquid crystal core. This has been exploited in nematic devices [135,136] as well as a ferroelectric liquid crystal (FLC) cell with graphene electrodes and alignment [137]. Another advantage of graphene electrodes is the possibility to produce flexible substrates on polymer films, as shown in Figure 11. The graphene film is grown on a copper foil, which is laminated to the polymer film. In an etching step, the copper is removed, and the flexible graphene on polymer foil electrode is obtained. This has been shown to produce flexible substrates of sizes up to 30 inches[121], but it may be extended to even larger sizes, which is suitable for example for the use in the production of touch screen displays or PDLC privacy windows. The process can be repeated for a layer by layer increase of the graphene film. The successive addition of graphene layers reduces the C 2020, 6, 80 12 of 28

We have seen above that aligned nanotube films can be used not only as electrode material but simultaneously as an alignment layer for the liquid crystal. A similar behaviour can be observed for graphene due to the π–π interaction of the carbon honeycomb lattice with the phenyl-ring system of the liquid crystal core. This has been exploited in nematic devices [135,136] as well as a ferroelectric liquid crystal (FLC) cell with graphene electrodes and alignment [137]. Another advantage of graphene electrodes is the possibility to produce flexible substrates on polymer films, as shown in Figure 11. The graphene film is grown on a copper foil, which is laminated to the polymer film. In an etching step, the copper is removed, and the flexible graphene on polymer foil electrode is obtained. This has been shown to produce flexible substrates of sizes up to 30 inches [121], but it may be extended to even larger sizes, which is suitable for example for the use in the production of touch screen displays or PDLC privacy windows. The process can be repeated for a layer by layer increase of the graphene film. The successive addition of graphene layers reduces the transmittance in the opticalC 2020, wavelength 6, x FOR PEER REVIEW regime (Figure 11b), while it decreases the sheet resistance13 of (Figure 30 11c). Once again,transmittance a compromise in the optical needs wavelength to be made regime between (Figure 11b), high while transmittance it decreases the and sheet low resistance sheet resistance, although for(Figure 3-layer 11c). substrates,Once again, a thecompromise performance needs to comes be made close between to that highof transmit ITO,tance with and the low additional sheet benefit of flexibility,resistance, as shown although in Figure for 3-layer 11d substrates, or the touch the performance panel of Figure comes 11 closee. to that of ITO, with the additional benefit of flexibility, as shown in Figure 11d or the touch panel of Figure 11e.

Figure 11.Figure(a) Schematic11. (a) Schematic of the of the roll-to-roll roll-to-roll production production process process of 30” of flexible 30” flexible polymer foils polymer with a foils with graphene electrode. (b) The transmittance of the transparent and flexible electrode depends on the b a graphenenumber electrode. of graphene ( layers,) The decreasing transmittance for increasing of the graphene transparent film thickness. and flexible(c) At the same electrode time, depends on the numberthe sheet of resistance graphene decreases layers, for decreasingincreasing graphene for increasing film thickness, graphene illustrating film the thickness.trade-off that (c) At the same time,needs the to sheet be managed. resistance In the given decreases example, for a 3 increasing-layer film is probably graphene the best film compromise thickness, between illustrating the trade-off thathigh transmittance needs to be and managed. low sheet resistance, In the givenapproaching example, the performance a 3-layer of film ITO electrodes. is probably (d) the best Visual example of a transparent flexible assembled touch panel from graphene/PET, and (e) compromise between high transmittance and low sheet resistance, approaching the performance of graphene-based touch screen with computer control (Reproduced by permission from [121]). ITO electrodes. (d) Visual example of a transparent flexible assembled touch panel from graphene/PET, and (e) graphene-basedOther applications touch of screen ITO-free, with flexible, computer and transparent control (Reproduced graphene electrode by permission liquid from crystal [ 121]). devices can be found in the previously discussed Polymer-Dispersed Liquid Crystals (PDLC) for Otherexample applications as privacy of or ITO-free, smart windows flexible, [138,139]. and transparent These will be discussed graphene furt electrodeher in Section liquid 4. crystal devices can be found inFurthermore, the previously to reduce discussed costs, it has Polymer-Dispersedalso been attempted to exploit Liquid reduced Crystals graphene (PDLC) oxide (rGO) for example as as electrode material [140–144]. This can be obtained from graphene oxide (GO) through thermal privacy orreduction, smart windows i.e., heating [ 138to approximately,139]. These 160 will °C, bewhen discussed GO loses its further surface inand Section edge attached4. epoxide Furthermore,and hydroxide to reduce functional costs, groups. it has While also GO been is non attempted-conducting, torGO exploit regains reducedsome of the graphene conductivity oxide (rGO) as electrodeof graphene. material It should [140–144 be noted]. This though can that be the obtained sheet resistance from of graphene rGO films is oxide much (GO)larger than through that thermal −1 reduction,of i.e., graphene heating or to indeed approximately ITO, at values 160 ofC, whenapproximately GO loses RrGO its≈1– surface50 kΩ□ and. In edge addition, attached the epoxide transmittance is generally not quite acceptable◦ for high-end display applications, ranging at about and hydroxideTrGO ≈ 80%. functional This performance groups. can While be somewhat GO is non-conducting, improved by acid treat rGOments regains [145] or some the doping of the and conductivity of graphene.attachment It should of Ag be nanoparticles noted though [146]. that the sheet resistance of rGO films is much larger than that of graphene or indeedAs in the ITO, case atfor valuescarbon nanotubes, of approximately much research R is being1–50 done kΩ to increase1. In addition, the performance the transmittance of rGO≈ − is generallygraphene not quite or graphene acceptable-based materials for high-end as electrodes, display which applications, are to be exploited ranging effectively at fo aboutr electro T- 80%. optic and flexible electronic devices. Here, one may resort to hybrid materials with PEDOT:PSS orrGO ≈ This performancesilver nanowires can be or somewhat even nanotube/graphene improved hybrids by acid for treatments improvement, [145 as ]will or be the discussed doping in and more attachment of Ag nanoparticlesdetail in the next [146 section.]. Several reviews can be found on the topic of graphene-related materials as transparent, flexible electrodes, discussing different systems, preparation, and applications in detail [147–150].

C 2020, 6, 80 13 of 28

As in the case for carbon nanotubes, much research is being done to increase the performance of graphene or graphene-based materials as electrodes, which are to be exploited effectively for electro-optic and flexible electronic devices. Here, one may resort to hybrid materials with PEDOT:PSS or silver nanowires or even nanotube/graphene hybrids for improvement, as will be discussed in more detail in the next section. Several reviews can be found on the topic of graphene-related materials as transparent, flexible electrodes, discussing different systems, preparation, and applications in detail [147–150].

4. Carbon Composite Technologies From the discussion of many of the options to replace ITO by other materials, we have so far concentratedC our 2020 attention, 6, x FOR PEER on REVIEW the use of single materials, such as metal nanowires, conductive14 of 30 polymers, ultrathin metal ﬁlm, and especially carbon nanotubes and graphene. While there are materials with 4. Carbon Composite Technologies properties approaching those of ITO in terms of high transmittance and low sheet resistivity, there is no competitor that isFrom clearly the discussion standing of out. many Thus, of the oneoptions can to tryreplace to combine ITO by other the materials, various we advantages have so far of diﬀerent concentrated our attention on the use of single materials, such as metal nanowires, conductive approaches inpolymers, such a ultrathin way that metal the film, shortcomings and especially carbon of one nanotubes material and may graphene. be compensated While there are by another through hybridmaterials approaches with properties or composite approaching materials.those of ITO in In terms the following,of high transmittance we will and focus low sheet on composites incorporatingresistivity, carbon there allotropes. is no competitor that is clearly standing out. Thus, one can try to combine the various advantages of different approaches in such a way that the shortcomings of one material may be compensated by another through hybrid approaches or composite materials. In the following, we 4.1. Nanotube-Basedwill focus Composites on composites incorporating carbon allotropes.

One of the4.1. Nanotube popular-Based combinations Composites is that of metal nanowires with carbon nanotubes [151–153]. An example is shownOne of in the Figure popular 12 combinations, where a network is that of metal of silver nanowires nanowires with carbon (Ag nanotubes NW) forms [151– a153]. composite with single-wall carbonAn example nanotubes is shown to in makeFigure a12, transparent where a network conducting of silver nanowires ﬁlm [152 (Ag]. NW) The forms actual a composite current conduction is achieved viawith di singleﬀerent-wall junctions, carbon nanotubes nanowire–nanowire, to make a transparent nanowire–nanotube, conducting film [152]. The and actual nanotube–nanotube current conduction is achieved via different junctions, nanowire–nanowire, nanowire–nanotube, and junctions, whilenanotube the composite–nanotube junctions, network while is the embedded composite network in a polymer is embedded matrix. in a polymer matrix.

Figure 12. (a) Schematic illustration of the silver nanowire (Ag NW)–single-wall carbon nanotube Figure 12. (a) Schematic illustration of the silver nanowire (Ag NW)–single-wall carbon nanotube (SWCNT) network and the conductive junctions of the composite. (b) SEM of the surface morphology (SWCNT) networkof the Ag and nanowire the conductive–nanotube composite. junctions (c) of Transmittance the composite. versus (b sheet) SEM resistance of the at surface different morphology of the Ag nanowire–nanotubenanotube loading, which composite. displays a ( performancec) Transmittance comparable versus to ITO. sheet (Repro resistanceduced by permissionat diﬀerent nanotube from ref. [152]). loading, which displays a performance comparable to ITO. (Reproduced by permission from ref. [152]). The sheet resistance of the composite at equal transmittance values is lower than that of the Ag nanowire network alone due to the additional contact junctions formed by the carbon nanotube C 2020, 6, 80 14 of 28

The sheet resistance of the composite at equal transmittance values is lower than that of the Ag nanowire network alone due to the additional contact junctions formed by the carbon nanotube network. 1 The electrode performance with a transmittance of 95% at a sheet resistance of 30 Ω− is comparable to that of standard ITO electrodes, while showing the excellent bending behaviour needed for flexible devices. Similar results are shown by solution-processed carbon nanotubes with self-assembled Ag nanoparticles, 1 which form a conductive nanoparticle network with sheet resistance < 10 Ω− and transmittance of 84% [151]. Shin et al. [153] investigated the composites of an Ag metal grid in combination with multi-wall carbon nanotube (MWCNT) networks as well as graphene films. They found that while the transmittance was acceptable in all cases of possible combinations, (i) MWCNT, (ii) MWCNT + Ag grid, (iii) graphene, (iv) graphene + Ag grid, and comparable to that of ITO (93%), the sheet resistance was clearly larger in 1 cases (i)–(iii) and only comparable to ITO (21 Ω− ) for the graphene film with an Ag grid composite. In addition, all-carbon composites of nanotubes and graphene-based materials have been produced. Double-layer composites of a MWCNT film on rGO [154] showed a drastic reduction of sheet resistance 1 when the nanotubes were added onto the reduced graphene oxide film from about 30 to 2 kΩ− before annealing. Even after annealing, the sheet resistance was still reduced by another factor of about 2 or 3. The transmittance was obviously also reduced as compared to single material solutions, but it remained at values > 90%, which is sufficient for devices. Similar attempts with multi-layer composites [155] produced 1 electrodes with a transmittance of 81% and sheet resistance of 8 kΩ− . In addition, the use of nanotube interlinked graphene films [156] did not reach a performance that could rival ITO. A possibility to increase the performance of nanotube-based composite electrodes is to embed the composite in a conductive polymer, mostly PEDOT:PSS [157–160]. Cho et al. [157] used multi-wall carbon nanotube networks for which the sheet resistance was reduced by the addition of gold nanoparticles anchored uniformly on the outside of the nanotubes. In a plasma treatment step, these nanoparticles fused individual MWCNTs to form conductive junctions and thus increase the percolation conductivity,leading to 1 a reduction in sheet resistance of the network from approximately 225 to 50 Ω− , which is a value close to that of ITO (Figure 13). The such connected nanotube network was embedded in a PEDOT:PSS matrix. Together with a transmittance close to 90%, flexible transparent electrodes were produced, which had a much-improved bending behaviour compared to that of ITO electrodes. A similar methodology was used by Shin et al. [158] to achieve a corresponding performance. Furthermore, hybrid composites combining rGO, MWCNTs, and PEDOT:PSS were realised [159], as well as Ag nanowire–nanotube–PEDOT:PSS 1 hybrids [160] with 86% transmittance and sheet resistance as low as 7 Ω− .

4.2. Graphene-Based Composites Most single graphene material approaches show that the transmittance of a one to four-layer graphene film lies above approximately 90%, which is comparable to ITO. Yet, due to the relatively small graphene flake sizes, there are numerous defects and grain boundaries in such films, which increase the sheet resistance to values far above those observed for the standard ITO material. This implies that a hybrid approach is favourable, and as for carbon nanotube films, also for the use of graphene in electrode materials, the most investigated composite appears to be a hybrid structure of metal nanowires with graphene-based films [161–166]. Chen and co-workers [161] proposed a co-percolating graphene–Ag nanowire structure in two different hybrid varieties: (1) Ag nanowire network on a flat film of graphene and (2) Ag nanowire network covered by a graphene film. While in both cases, the sheet resistance of the hybrid is strongly reduced from that of a graphene film alone, it is still not competitive with ITO for hybrid-1. On the other 1 hand, hybrid-2 does exhibit a sheet resistance of about 20 Ω− at a transmittance of approximately 90%. These are values as expected for good ITO electrodes. Given the relatively costly price of silver and the very much comparable conductivity of copper at much lower costs, it is well worth exploring the possibility of replacing the Ag nanowire with a Cu nanowire network [162]. Such a Cu nanowire–graphene hybrid electrode was shown to have a sheet resistance of 50 Ω 1 at a ≈ − transmittance of 90%, which is still quite close to the performance of ITO. A similar electrode hybrid ≈ C 2020, 6, 80 15 of 28 with Cu nanowires has also been proposed using reduced graphene oxide, rGO, albeit at increased 1 sheet resistance of 300 Ω− for a 90% transmittance [164]. A hybrid Ag nanowire–GO electrode has been demonstratedC 2020, 6, x FOR to PEER drive REVIEW a PDLC display [165]. 16 of 30

Figure 13. Figure(a) Multi-wall 13. (a) Multi carbon-wall carbon nanotubes nanotubes are are uniformly covered covered with withanchored anchored Au nanoparticles. Au nanoparticles. Through aThrough plasma a treatment,plasma treatment, these these nanoparticles nanoparticlesfuse fuse the the nanotubes nanotubes and create andcreate conductive conductive junctions. junctions. (b) SEM of a gold nanoparticle-covered MWCNT network. (c) The transmission of the composite (b) SEM of aembedded gold nanoparticle-covered in a PEDOT:PSS matrix MWCNT exhibits network. optical transmittance(c) The transmission values of of the the order composite 85–90% embedded in a PEDOT:PSSdepending matrix on exhibitscomposition. optical (d) The transmittance sheet resistance values is largely of thedecreased order through 85–90% the depending plasma treatment on composition. (d) The sheetof resistancethe Au nanoparticles is largely anchored decreased on the through MWCNTs. the (Reproduced plasma treatment by permission of the from Au nanoparticles ref. [157]). anchored on the MWCNTs. (Reproduced by permission from ref. [157]). 4.2. Graphene-Based Composites A somewhatMost disingleﬀerent graphene fabrication material approach approaches wasshow chosen that the bytransmittance Liu et al. of [166 a one], into depositingfour-layer an Ag nanowire–graphenegraphene film hybrid lies above material approximately on ﬂexible 90%, which polymer is comparable (PET) to substrates ITO. Yet, due through to the relatively electrophoresis, small graphene flake sizes, there are numerous defects and grain boundaries in such films, which as shown inincrease the scheme the sheet of resistance Figure to14 values. The far transmittance above those observed of the reportedfor the standard device ITO is material. practically This constant over the opticalimplies range that a hybrid of the approach visible spectrum,is favourable, and and it as depends for carbon on nanotube the applied films, also electrophoresis for the use of voltage, decreasinggraphene with increasing in electrode voltage materials, amplitude, the most investigated and generally composite being appears still tooto be low a hybrid at T

over the optical range of the visible spectrum, and it depends on the applied electrophoresis voltage, 95% for small numbersdecreasing with of bilayers,increasing voltage it naturally amplitude, continuouslyand generally being decreases still too low at with T < 65% increasing for high- the number of bilayers. end optical technology. On the other hand, the sheet resistance is in fact well below the normally required values for ITO-free electrodes.

Figure 14. (a) SchematicFigure 14. (a) Schematic illustration illustration of the of the mechanism mechanism to fabricate to fabricate Ag nanowire/graphene Ag nanowire material/graphene on material a polymer substrate by electrophoresis. (b) SEM of the obtained electrode morphology. (c) The on a polymertransmittance substrate of by the flexibleelectrophoresis. electrode depends ( onb) applied SEM electrophoresis of the obtained voltage but electrode is constant morphology. (c) The transmittancethroughout of the the optical ﬂexible regime. electrode(d) The sheet depends resistance ison well applied below that electrophoresis of ITO, while the voltage but is transmittance is still too low for state-of-the-art electro-optic devices. (Reproduced by permission constant throughout the optical regime. (d) The sheet resistance is well below that of ITO, while the from [166]). transmittance is still too low for state-of-the-art electro-optic devices. (Reproduced by permission from [166]). Another approach that we have briefly touched upon above is the combination of graphene and nanotubes in all-carbon based flexible and transparent electrodes [155,167]. It should be mentioned Cfrom 2020, the6, x FORbeginning PEER REVIEW that the performance of such electrodes with respect to sheet resistance is18 by of no30 standards comparable to ITO—rather in the kΩ□−1 than the low Ω□−1 range and thus far larger than desired by the liquid crystal display industry. Nevertheless, it is worthwhile to discuss these approaches because of the excellent flexibility, bending behaviour, processing, and lower costs of these hybrid materials. Furthermore, these graphene-based composites can in fact be made from the largely available graphene oxide or reduced graphene oxide. Figure 15 depicts one of such electrodes composed from multi-layer GO/rGO and MWCNTs.

Figure 15. (a) SchematicFigure 15. (a) Sc illustrationhematic illustration of the of the multi-layer multi-layer electrode electrode structure structure made from made GO/rGO from and GO/rGO and multi-wall carbon nanotubes. (b) Atomic force microscopy image of the film surface morphology. multi-wall carbonOne can nanotubes. see individual (GO/rGOb) Atomic flakes as force well as microscopy MWCNTs, and imagethe surface of roughnes the films is about surface 10 morphology. One can see individualnm on average. GO/ (rGOc) The flakestransmittance as well versus as sheet MWCNTs, resistance anddiagram the shows surface a relatively roughness high sheet is about 10 nm on resistance, which can be lowered by treatment processes. (d) The optical transmittance of the film c average. ( ) Theelectrode transmittance is quite high versus for a low sheet number resistance of bilayers, diagramand it is comparable shows ato relativelyITO, but it naturally high sheet resistance, which can be lowereddecreases with by an treatment increasing number processes. of bilayers. (d (Reproduced) The optical by permission transmittance from [155]). of the film electrode is quite high for a low number of bilayers, and it is comparable to ITO, but it naturally decreases with an By atomic force microscopy (AFM), one can clearly resolve the surface morphology of the multi- increasing numberlayer GO/rGO of bilayers.-MWCNT structures (Reproduced with GO/rGO by permission flakes visible from at an average [155]). size of approximately 1 μm and MWCNTs on top, leading to a surface roughness of about 10 nm on average. The latter strongly increase the conductivity, or lowers the sheet resistance, by bridging the grain boundaries and possible open spaces of the GO/rGO layers. Nevertheless, even after treatment procedures to further decrease the sheet resistance, the latter is still well in the kΩ□−1 range, much larger than for ITO and indeed other single material or hybrid electrode materials. While the transmittance is high at about 95% for small numbers of bilayers, it naturally continuously decreases with increasing the number of bilayers.

5. Applications of Carbon-Based ITO-Free Technologies We have discussed the possible need for replacement technologies of ITO electrodes, not simply in light of the limited supply of indium, but also due to reasons of an increasing need for flexible substrate technologies, which require different processing conditions. Several materials were shown to be candidates for a sufficiently transparent yet highly conductive ITO-free technology, including hybrid materials and composites. We will now specifically address some of the applications based on liquid crystals that incorporate these ITO-free electrodes, besides the already mentioned twisted nematic TN cells [103,136] or the vertically aligned switching devices with negative dielectric liquid crystal [116]. One of the promising ITO-free applications are flexible privacy windows, as demonstrated in Figure 16 [138]. Sub-micrometre nematic droplets are dispersed in a continuous polymer matrix with graphene electrodes for electric field application. In the electric field free off-state, the film is strongly C 2020, 6, 80 17 of 28

5. Applications of Carbon-Based ITO-Free Technologies We have discussed the possible need for replacement technologies of ITO electrodes, not simply in light of the limited supply of indium, but also due to reasons of an increasing need for flexible substrate technologies, which require different processing conditions. Several materials were shown to be candidates for a sufficiently transparent yet highly conductive ITO-free technology, including hybrid materials and composites. We will now specifically address some of the applications based on liquid crystals that incorporate these ITO-free electrodes, besides the already mentioned twisted nematic TN cells [103,136] or the vertically aligned switching devices with negative dielectric liquid crystal [116].

C 2020One, 6, x ofFOR the PEER promising REVIEW ITO-free applications are flexible privacy windows, as demonstrated19 ofin 30 Figure 16[ 138]. Sub-micrometre nematic droplets are dispersed in a continuous polymer matrix with graphenelight scattering, electrodes duefor to electrica random field orientation application. of director In the electric field of field the free LC odroplets,ff-state, the which film leads is strongly to an lightabruptly scattering, changing due refractiveto a random index orientation on length of director scales of field optical of the wavelengths LC droplets, which(Figure leads 16a). to The an abruptlyapplication changing of an electric refractive field index causes on length a director scales reorientation of optical wavelengths of the LC (Figurewithin the16a). droplet The application such that ofthe an long electric molecular field causesaxes are a all director pointing reorientation in the sameof direction. the LC withinLight experiences the droplet a suchrefractive that theindex long n‖, molecularwhich is index axes are matched all pointing to that in the of the same isotropic direction. polymer Light experiences matrix, npolymer a refractive≈n‖, and index the foil n , whichbecomes is k indextransparent matched (Figure to that 16b). of the It isotropic is worth polymer noting matrix, that these npolymer foils relyn , and on the light foil scattering, becomes transparent not on the ≈ k (Figuremodulation 16b). of It the is worth polarisation noting state that of these light, foils and rely thus, on they light do scattering, not have any not need on the for modulation polarisers, ofwhich the polarisationgenerally absorb state of a light, lot and of the thus, transmitted they do not light have any intensity need for already. polarisers, A similar which generally device was absorb also a lotdemonstrated of the transmitted by Kim light et al. intensity [139]. already. A similar device was also demonstrated by Kim et al. [139].

FigureFigure 16.16. Schematic (top) and device example (bottom) of a polymer-dispersedpolymer-dispersed liquid crystal (PDLC) flexibleflexible displaydisplay withwith graphenegraphene electrodeselectrodes inin ((aa)) thethe lightlight scatteringscattering ooffff-state-state andand ((bb)) thethe transparenttransparent on-state.on-state. SuchSuch switchable,switchable, flexibleflexible foilsfoils areare usedused withoutwithout polarisers,polarisers, forfor exampleexample inin smartsmart andand privacyprivacy windows.windows. (Reproduced(Reproduced byby permissionpermission fromfrom ref.ref. [[138]).138]).

THzTHz devicesdevices havehave increasinglyincreasingly becomebecome importantimportant inin thethe areaarea ofof non-destructivenon-destructive testingtesting andand forfor securitysecurity screening. screening. As As liquid liquid crystals crystals are switchable are switchable and thus and are suitable thus are to modulatesuitable to the modulate birefringence the ofbirefringe devices alsonce of in thedevices THz regime,also in the it is THz of beneﬁt regime, to construct it is of benefit LC-based to construct THz optical LC components.-based THz Oneoptical of thecomponents. problems isOne the of opaqueness the problems of ITOis the at opaqueness these frequencies, of ITO whichat these cause frequencies, the need which for THz-transparent cause the need electrodes.for THz-transparent A number electrodes. of phase shift A number and wave-plate of phase devices shift and have wave been-p reported,late devices which have utilise been graphenereported, ﬁlmwhich electrodes. utilise graphene film electrodes. Such ITO-free LC devices are for example employed as THz phase shifters [168] and polarisation phase controllers [169], THz quarter wave-plates [170], or broadband tuneable liquid crystal THz wave-plates [171]. Figure 17a schematically depicts such a THz device, which resembles a liquid crystal sandwich cell with planar boundary conditions and graphene electrodes. The application of an electric field switches the dielectrically positive liquid crystal from planar to homeotropic orientation, changing the THz refractive index. For the graphene electrodes, one is once again faced with the compromise between transmittance and sheet resistance. Ideally, the device would have a high transmittance > 90% while exhibiting a low sheet resistance < 50 Ω□−1. A suitable compromise would probably be a two-layer graphene film. The THz signal that is transmitted through the device is phase shifted depending on the applied voltage, as shown for several different applied voltages and frequencies in Figure 17d,e, respectively. C 2020, 6, 80 18 of 28

Such ITO-free LC devices are for example employed as THz phase shifters [168] and polarisation phase controllers [169], THz quarter wave-plates [170], or broadband tuneable liquid crystal THz wave-plates [171]. Figure 17a schematically depicts such a THz device, which resembles a liquid crystal sandwich cell with planar boundary conditions and graphene electrodes. The application of an electric field switches the dielectrically positive liquid crystal from planar to homeotropic orientation, changing the THz refractive index. For the graphene electrodes, one is once again faced with the compromise between transmittance and sheet resistance. Ideally, the device would have a high 1 transmittance > 90% while exhibiting a low sheet resistance < 50 Ω− . A suitable compromise would probably be a two-layer graphene film. The THz signal that is transmitted through the device is phase shifted depending on the applied voltage, as shown for several different applied voltages and frequenciesC 20 in20 Figure, 6, x FOR PEER17d,e, REVIEW respectively. 20 of 30

Figure 17. FigureLiquid 17. crystalLiquid crystal THz THz wave wave shifter shifter with with graphene graphene electrodes. electrodes. (a) Schematic (a) Schematicdevice with a deviceglass with a glass substrate,substrate, graphene grapheneelectrode, electrode, and and alignment alignment layer layersandwich sandwich cell, confining cell, confiningthe liquid crystal. the liquid The crystal. liquid crystal director is reoriented from planar (left) to homeotropic (right) on application of an The liquid crystal director is reoriented from planar (left) to homeotropic (right) on application of an electric field. (b) The transmittance of the device decreases with increasing graphene film thickness, electric field.while (b ()c) The the sheet transmittance resistance also of thedecreases. device (d) decreases THz signal withtransmitted increasing through graphene the phase shifter film thickness, while (c) thedevice sheet at resistancevaried applied also electric decreases. voltages. (d )(e THz) Phase signal shift transmitted as a function of through applied the bias phase voltage shifter for device at varied appliedseveral THz electric frequencies. voltages. (Reproduced (e) Phase by permission shift as a functionfrom [168]). of applied bias voltage for several THz frequencies. (Reproduced by permission from [168]). For all non-display LC devices that rely on curved, flexible surfaces, an ITO-free electrode on a polymer substrate is of great advantage. An illustrative example of such a device is switchable lenses For alland non-display variable focus LC lenses devices and especially that rely adaptive on curved, contact lenses flexible [60,172,17 surfaces,3]. an ITO-free electrode on a polymer substrateAn example is of great of a contact advantage. lens of size An suitable illustrative for the example human eye of is suchdepicted a device in Figure is 18, switchable which lenses and variableis based focus on lenses a flexible and polymer especially substrate adaptive and covered contact with lenses a transparent [60,172 graphene,173]. electrode on the An examplecurved active of a contactpart of the lens contact of size lens. suitable Above a threshold for the human voltage eyeof approximately is depicted 2 inV, Figurethe optical 18 , which is power changes continuously up to 0.7 D, which allows focussing on a text. based on a flexible polymer substrate and covered with a transparent graphene electrode on the curved active part of the contact lens. Above a threshold voltage of approximately 2 V, the optical power changes continuously up to 0.7 D, which allows focussing on a text. C 2020, 6, 80 19 of 28

C 2020, 6, x FOR PEER REVIEW 21 of 30

Figure 18. Figure(a) Schematic 18. (a) Schematic of a graphene of a graphene electrode electrode flexible flexible contact contact lens. lens. (b) Substrate (b) Substrate of the contact of the lens contact lens with deposited graphene film electrode, covering the whole curved active area. (c) Change of the with deposited graphene film electrode, covering the whole curved active area. (c) Change of the optical optical power of the lens as a function of applied voltage, allowing changes from 0–0.7 D. (d) power of theFocussing lens as a atext function by application of applied of a voltage. voltage, (Reproduced allowing by changespermission from from [172]). 0–0.7 D. (d) Focussing a text by application of a voltage. (Reproduced by permission from [172]). Other applications of ITO-free LC applications with carbon-based electrodes that have been Otherreported applications include cholesteric of ITO-free multi LC-domain applications structures for with energy carbon-based-saving smart windows electrodes [174], thatmid IR have been reported includespatial light cholesteric modulators multi-domain [175], micro-lens structures arrays [176], for and energy-saving tuneable scattering smart devices windows [177]. [174], mid IR spatial light6. Discussion modulators and [ 175Outlook], micro-lens arrays [176], and tuneable scattering devices [177]. In the last few years, it has emerged that the predicted shortage of indium to produce ITO 6. Discussion and Outlook electrodes on substrates is not as imminent as it was once feared. It now looks as though natural In theresources last few will years, last for itthe has next emergedthree decades that until the about predicted 2050. The shortageprice of indium of indiumhas not exploded; to produce ITO on the contrary, it remained on a more or less constant level for a decade, despite the strong increase electrodesof on flat substrates panel display is production not as imminent during this astime. it One was of once the reasons feared. is an It increased now looks indium as recovery though natural resources willduring last the for ITO the sputtering next three process, decades where untilmuch of about the material 2050. Theis in fact price not of being indium used up has on not the exploded; on the contrary,substrate. it remained Furthermore, on it a morecan be or expected less constant that more level indium for awill decade, be recovered despite from the advancing strong increase of flat panel displayrecycling processes. production These during will be not this so time.much driven One through of the reasonsthe lack ofis indium an increased resources but indium rather recovery through ecological considerations, as an increasing amount of electronic waste in landfill will bring during theabout ITO advanced sputtering technologies process, for where recycling much not only of the of indium, material but isalso in glass, fact notplastics being, metal, used andup on the substrate. Furthermore,electronic components it can of be end expected-of-life LCDs. that moreAt present, indium these will technologies be recovered are not from financially advancing viable, recycling processes.and These there will will be be not the so need much for financial driven incentives, through the such lack as a of blanket indium recycling resources fee for but electronic rather through ecologicalconsumable considerations, items, as as it is an already increasing done in some amount countries. of electronic waste in landfill will bring about Much research has been performed over the last decade to present technical alternatives to the advanced technologiesuse of ITO in displays for recycling and other optoelectronic not only of indium,applications. but Some also of glass, these were plastics, outlined metal, above, and and electronic componentsthe ofmost end-of-life promising appear LCDs. toAt be based present, on Ag these nanowire technologies networks and are meshes, not financially graphene, and viable, single- and there will be thewall need nanotube for financial network incentives, films, and such their as composite a blanket materials. recycling Materials fee for electronic are also available consumable at items, as it is alreadycomparable done inperformance some countries. to ITO (Figure 19a), yet at still higher cost and mostly not having been scaled up to sizes and the reproducibility of industrial standard (Figure 19b). The main problem with Much research has been performed over the last decade to present technical alternatives to the use of ITO in displays and other optoelectronic applications. Some of these were outlined above, and the most promising appear to be based on Ag nanowire networks and meshes, graphene, and single-wall nanotube network films, and their composite materials. Materials are also available at comparable performance to ITO (Figure 19a), yet at still higher cost and mostly not having been scaled up to sizes and the reproducibility of industrial standard (Figure 19b). The main problem with all alternative material solutions appears to be the trade-off between high transmittance versus low sheet resistance. Nevertheless, it is well conceivable that these issues will be solved over the next few years. C 2020, 6, x FOR PEER REVIEW 22 of 30

all alternative material solutions appears to be the trade-off between high transmittance versus low C 2020, 6, 80 sheet resistance. Nevertheless, it is well conceivable that these issues will be solved over the next few 20 of 28 years.

Figure 19.Figure(a) Transmittance 19. (a) Transmittance versus versus sheet sheet resistance resistance performance performance of a of range a range of single of alternative single alternative materials proposedmaterials proposed to replace to replace ITO; ITO; status status 2014. 2014. Composite Composite materials materials are not considered are not in consideredthis graph. in this (Reproduced by permission from [26]). (b) A rough placement of different material groups as graph. (Reproduced by permission from [26]). (b) A rough placement of different material groups as alternatives to ITO electrodes, showing conductivity versus cost. The higher the conductivity, the alternativeslower to ITO the sheet electrodes, resistance, showing but also conductivitythe lower the transmittance versus cost. generally The higher as well. the conductivity, the lower the sheet resistance, but also the lower the transmittance generally as well. Even once a performance equivalent replacement technology for ITO is found, it is unlikely that Evenwe once will a see performance ITO-free devices equivalent taking over replacement the market soon technology afterwards. for For ITO this isto found,happen, itthe is new unlikely that materials either need to be considerably better or clearly cheaper than ITO, because the latter is, and we will see ITO-free devices taking over the market soon afterwards. For this to happen, the new has been for many years, the established industry standard with all the necessary production materials eitherinfrastructure need and to R&D be considerably knowledge in place. better Moreover, or clearly a deviation cheaper from thanthis standard ITO, becausewill be a slow the latter is, and has beenprocess for that many can only years, be initia theted established via a considerable industry step-increase standard in performance with all the or cost necessary decrease, or production infrastructurefor example and R&D via legislation knowledge in the in form place. of environmental Moreover, decisions, a deviation or with from the thisdevelopment standard of novel will be a slow display technologies, which cannot be realised with conventional ITO electrode techniques. One of process that can only be initiated via a considerable step-increase in performance or cost decrease, such technologies that may be on the rise over the next years are flexible devices on polymer or for examplesubstrates, via legislation which cannot in be the processed form of by environmental ITO sputtering techniques. decisions, In this or withcase, solution the development processing of novel display technologies,would pave the which way for cannot electrodes be either realised of metal with nanowire conventional networks ITOor of electrodecarbon-based techniques. materials, One of such technologiesor indeed that composite may bematerials on the of rise both. over In this the case, next it yearsis likely are that flexible price will devices be the ondecisive polymer factor, substrates, which could favour carbon-based technologies. Especially, future large-area applications with which cannot be processed by ITO sputtering techniques. In this case, solution processing would pave environmental impact, such as energy saving or energy-harvesting windows, technologies which are the way foroften electrodes first seen on either office ofbuildings, metal could nanowire tip the networksbalance towards or of carbon carbon-based-based electrode materials, materials. or indeed compositeYet, materials it is not likely of both. that within In this the case, next itfew is years, likely a thatdrastic price move will towards be the ITO decisive-free technologies factor, will which could favour carbon-basedbe observed. technologies. Especially, future large-area applications with environmental impact, suchFunding: as energy This research saving received or energy-harvesting no external funding. windows, technologies which are often first seen on office buildings, could tip the balance towards carbon-based electrode materials. Yet, it is not likely Conflicts of Interest: The author declares no conflict of interest. that within the next few years, a drastic move towards ITO-free technologies will be observed.

Funding: This research received no external funding. Conﬂicts of Interest: The author declares no conﬂict of interest.

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