TRANSPARENT ELECTRONICS

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TABLE OF CONTENTS

Chapter 1 Introduction 5

Chapter 2 Pre- History 7

2.1. Transparent Conductive Oxides (TCOs) 7

2.2. Thin-Film Transistors (TFTs) 7

Chapter 3 How transparent electronics devices work? 9

3.1 Oxides play key role 11

Chapter 4 Advancements made in Transparent Electronics 13

Chapter 5 Applications of Transparent Electronics 18

5.1 Imaginative Examples of use of Transparent Electronics 19

Chapter 6 Market of Transparent Electronics 20

6.1 Three Factors That Can Lead to the Commercial 22 Awakening of Transparent Electronics

6.1.1 Aesthetics 22

6.1.2 Integration 22

6.1.3 Improved Economics 23

6.1.4 Non-transparent aspects of transparent materials 23

Chapter 7 Future Scope 24

Chapter 8 Conclusion 25

Chapter 9 References 26

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LIST OF FIGURES, TABLES AND GRAPHS

Fig 1.1 Transparent Computer (Artist’s Imagination) 5

Fig 1.2 Transparent iPhone 6

Table 2.1 Electrical properties of common transparent conducting 7

oxides (TCOs).

Fig 2.1 Fabrication of a bottom-gate TFT with a SnO2 channel layer. 8

Fig 2.2 Structure of layered TFT 8

Fig 3.1 Typical ZnO-TFT characteristics 10

Fig 3.2 Development of ZnO and a-IGZO Semiconductors 11

Fig 3.3 Graph showing variation of transmittance and wavelength of 12 Substrate.

Fig 4.1 Characteristics other than Transparency. 14

Fig 4.2 Fabrication of fully transparent aligned SWNT transistors. 15

Fig 4.3 Generation of Transparent Electronics 16

Fig 5.1 Examples of Transparent Electronics Devices (Illustrative) 19

Fig 6.1 Forecast of Transparent Electronics Products by Application 20

Fig 7.1Graphene is transparent and can be used as material. 24

Fig 8.1 Usage of Transparent Electronics devices in future 25

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Chapter-1

INTRODUCTION

Transparent electronics (also called as invisible electronics) is an emerging technology that employs wide band-gap semiconductors for the realization of invisible circuits.  This monograph provides the first roadmap for transparent electronics, identifying where the field is, where it is going, and what needs to happen to move it forward.  Although the central focus of this monograph involves transparent electronics, many of the materials, devices, circuits, and process-integration strategies discussed herein will be of great interest to researchers working in other emerging fields of optoelectronics and electronics involving printing, large areas, low cost, flexibility, wearability, and fashion and design.

Fig 1.1 Transparent Computer (Artist’s Imagination)

Transparent electronics is an emerging science and technology field focused on producing ‘invisible’ electronic circuitry and opto-electronic devices. Applications include consumer electronics, new energy sources, and transportation; for example, automobile windshields could transmit visual information to the driver. Glass in almost any setting could also double as an electronic device, possibly improving security systems or offering transparent

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displays. In a similar vein, windows could be used to produce electrical power. Other civilian and military applications in this research field include real time wearable displays. As for conventional Si/III–V-based electronics, the basic device structure is based on semiconductor junctions and transistors. However, the device building block materials, the semiconductor, the electric contacts, and the dielectric/passivation layers, must now be transparent in the visible –a true challenge! Therefore, the first scientific goal of this technology must be to discover, understand, and implement transparent high-performance electronic materials. The second goal is their implementation and evaluation in transistor and circuit structures. The third goal relates to achieving application-specific properties since transistor performance and materials property requirements vary, depending on the final product device specifications. Consequently, to enable this revolutionary technology requires bringing together expertise from various pure and applied sciences, including materials science, chemistry, physics, electrical/electronic/circuit engineering, and display science.

During the past 10 years, the classes of materials available for transparent electronics applications have grown dramatically. Historically, this area was dominated by transparent conducting oxides (oxide materials that are both electrically conductive and optically transparent) because of their wide use in antistatic coatings, touch display panels, solar cells, flat panel displays, heaters, defrosters, ‘smart windows’ and optical coatings.

Fig 1.2 Transparent iPhone All these applications use transparent conductive oxides as passive

electrical or optical coatings. Oxide semiconductors are very interesting materials because they combine simultaneously high/low conductivity with high visual transparency. The field of transparent conducting oxide (TCO) materials has been reviewed and many treatises on the topic are available. However, more recently there have been tremendous efforts to develop new active materials for functional transparent electronics. These new technologies will require new materials sets, in addition to the TCO component, including conducting, dielectric and semiconducting materials, as well as passive components for full device fabrication.

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Chapter-2 PRE- HISTORY

The two technologies which preceded and underlie transparent electronics are Transparent Conductive Oxides (TCOs) and Thin- Film Transistors (TFTs).

2.1 Transparent Conductive Oxides (TCOs)

TCOs constitute an unusual class of materials possessing two physical properties- high optical transparency and high electrical conductivity. They are generally considered to be mutually exclusive (Hartnagel et al 1995). This peculiar combination of physical properties is only achievable if a material has a sufficiently large energy band gap so that it is non-absorbing or transparent or transparent to visible light, i.e., > ~3.1 eV and also possesses a high enough concentration > ~1019 cm-3, with a sufficiently large mobility > ~1 cm2V-1s-1, that the material can be considered to be a ‘good’ conductor of electricity.

The three most common TCOs are indium oxide In2O3, tin oxide SnO2 and zinc oxide ZnO2. All these materials have band gaps above that required for transparency across the full visible spectrum.

Table 2.1 Electrical properties of common transparent conducting oxides (TCOs). Conductivities reported are for best-case polycrystalline films

Material

Bandgap

(eV)

Conductivity (Scm-1)

Electron Concentratio

n (cm-3)

Mobility (cm2V-1s-1)

In2O3 3.75 10,000 >1021 35ZnO2 3.35 8,000 >1021 20SnO2 3.6 5,000 >1020 15

2.2 Thin-Film Transistors (TFTs)

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The thin-film transistor is another technology underlying transparent electronics, since it is a bridge between passive electrical and active electronic applications. Although TFTs were the subject of the earliest transistor patents, the first realization of a TFT was reported in 1961 by Weimer and fabricated via vacuum evaporation using CdS as a channel layer. None of these undertakings involved an attempt to realize a fully transparent TFT.

Fig 2.1 Fabrication of a bottom-gate TFT with a SnO2 channel layer. (a) Photo-resist is patterned by bottom exposure, using the aluminum

gate as a mask. (b) After photo resist development, a metal blanket coating is evaporated. (c) Final TFT device structure after lift-off.

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Fig 2.2 Structure of layered TFT

Chapter-3HOW TRANSPARENT ELECTRONIC

DEVICES WORK?The challenge for producing "invisible" electronic circuitry and opto-

electronic devices is that the transistor materials must be transparent to visible light yet have good carrier mobilities. This requires a special class of materials having "contra-indicated properties" because from the band structure point of view, the combination of transparency and conductivity is contradictory.

Transparent electronics are nowadays an emerging technology for the next generation of optoelectronic devices. Oxide semiconductors are very interesting materials because they combine simultaneously high/low conductivity with high visual transparency and have been widely used in a variety of applications (e.g. antistatic coatings, touch display panels, solar cells, flat panel displays, heaters, defrosters, optical coatings, among others) for more than a half-century.

Transparent oxide semiconductor based transistors have recently been proposed using as active channel intrinsic zinc oxide (ZnO). The main advantage of using ZnO deals with the fact that it is possible to growth at/near room temperature high quality polycrystalline ZnO, which is a particular advantage for electronic drivers, where the response speed is of major importance. Besides that, since ZnO is a wide band gap material (3.4 eV), it is transparent in the visible region of the spectra and therefore, also less light sensitive.

Transparent oxide semiconductor based transistors have recently been proposed using as active channel intrinsic zinc oxide (ZnO). The main advantage of using ZnO deals with the fact that it is possible to growth at/near room

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temperature high quality polycrystalline ZnO, which is a particular advantage for electronic drivers, where the response speed is of major importance. Besides that, since ZnO is a wide band gap material (3.4 eV), it is transparent in the visible region of the spectra and therefore, also less light sensitive.

(a)

(b)

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Fig 3.1 Typical ZnO-TFT characteristics (a) transfer and (b) output characteristics, with the channel layer deposited at room temperature by rf

magnetron sputtering produced at FCT-UNL.

3.1 Oxides Play Key Role:

One major reason why there has been such interest and activity in transparent electronics recently is that there has been a sharp jump in the carrier mobility of transparent semiconductors, which determines transparent TFT characteristics. This now exceeds the carrier mobility of materials such as low-temperature poly-Si (LTPS) and amorphous Si used in LCD panels.

Fig 3.2 Development of ZnO and a-IGZO Semiconductors Takes OffResearchers have been interested in ZnO and InGaZnO4 (a-IGZO) transparent amorphous oxide semiconductors in the last few years. Carrier mobility of ZnO transistors was 7cm2/Vs in 2003, rising to 70 cm2/Vs in 2006, and to 250 cm2/Vs in 2007. Several manufacturers have plans to use a-IGZO in products. While there are remaining problems, transparent oxide p-type semiconductors have also been in development.

Even better, it means lower cost. Transparent semiconductors such as GaN and diamond are already known, but they come at high cost (materials, manufacturing, etc) which makes them impossible to use in transparent electronic devices demanding relatively large screens, such as displays. The candidate materials attracting the most interest can be broadly divided into two oxide categories. The first group is zinc oxide (ZnO), and the second is amorphous oxides with heavy metal content, such as amorphous InGaZnO4 (a-IGZO). Both pass visible light and are almost completely transparent. The carrier mobility of a TFT made with ZnO is 250cm2/Vs, significantly higher than the 100cm2/Vs achieved by LTPS. A TFT made with a-IGZO ranges from 1cm2/Vs to 100cm2/Vs, again significantly higher than the 1cm2/Vs max that amorphous Si provides. The pace of R&D has been accelerating in the last few years, with growth in ZnO carrier mobility especially rapid and manufacturers actively

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developing applications based on a-IGZO. Announcements like that of LG Electronics at E-MRS 2007 are based on a-IGZO.

A comparison of ZnO and a-IGZO shows that ZnO has the lead when it comes to carrier mobility. At present, though, a-IGZO is the material of choice for large-area displays, electronic paper utilizing low-temperature processing, etc. There are even some organic transparent semiconductor materials, but even the best only achieve a carrier mobility of around 5cm2/Vs. Organic semiconductors are therefore limited to applications with larger area where the lower cost can be leveraged.

Fig 3.3 Graph showing variation of transmittance (denoting reflection) and wavelength of Substrate.

Chapter-4ADVANCEMENTS MADE IN

TRANSPARENT ELECTRONICS

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Significant advances in the emerging science of transparent electronics, creating transparent "p-type" semiconductors that have more than 200 times the conductivity of the best materials available for that purpose a few years ago. This basic research is opening the door to new types of electronic circuits that, when deposited onto glass, are literally invisible. The studies are so cutting edge that the products which could emerge from them haven't yet been invented, although they may find applications in everything from flat-panel displays to automobiles or invisible circuits on visors.

Most materials used to conduct electricity are opaque, but some invisible conductors of electricity are already in fairly common use, the scientists said. More complex types of transparent electronic devices, however, are a far different challenge - they require the conduction of electricity via both electrons and "holes," which are positively charged entities that can be thought of as missing electrons.

These "p-type" materials will be necessary for the diodes and transistors that are essential to more complex electronic devices. Only a few laboratories in the world are working in this area, mostly in Japan, the OSU scientists. As recently as 1997, the best transparent p-type transparent conductive materials could only conduct one Siemens/cm, which is a measure of electrical conductivity. The most sophisticated materials recently developed at OSU now conduct 220 Siemens/cm.

These are all copper oxide-based compounds that we're working with. Right now copper chromium oxide is the most successful. Researchers continue to work with these materials to achieve higher transparency and even greater conductivity.

Fig 4.1 Characteristics other than Transparency. Transparent semiconductors, inaddition to being transparent, have a number of useful characteristics, including a wide band gap, relatively high carrier mobility, low-temperature manufacturability, and low manufacturing costs thanks to the low-temperature process and inexpensive materials. As a result, R&D into properties other than transparency is also active.

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Researchers at Oregon State University and Hewlett Packard have reported their first example of an entirely new class of materials which could be used to make transparent transistors that are inexpensive, stable, and environmentally benign. This could lead to new industries and a broad range of new consumer products, scientists say.  The possibilities include electronic devices produced so cheaply they could almost be one-time "throw away" products, better large-area electronics such as flat panel screens or flexible electronics that could be folded up for ease of transport. Findings about this new class of "thin-film" materials, which are called amorphous heavy-metal cation multi component oxides, were just published in a professional journal, Applied Physics Letters. The research was funded by the National Science Foundation and Army Research Office.

This is a significant breakthrough in the emerging field of transparent electronics, experts say. The new transistors are not only transparent, but they work extremely well and could have other advantages that will help them transcend carbon-based transistor materials, such as organics and polymers, that have been the focus of hundreds of millions of dollars of research around the world.

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Fig 4.2 Fabrication of fully transparent aligned SWNT transistors. (a) Schematic diagram of aligned SWNT transfer and a device structure consisting of a substrate (glass or PET), ITO as back gate, SU8 as dielectric, aligned SWNTs as channel, and ITO as source and drain. (b) SEM image of transferred aligned SWNTs on SU8 on a glass substrate. (c) SEM image of devices showing the ITO source and drain electrodes fabricated on glass. Inset: SEM image of aligned nanotubes bridging ITO electrodes. (d) Optical micrograph of fully transparent aligned SWNT transistors on a 4 in. glass wafer. (e) Optical micrograph of fully transparent aligned SWNT transistors on a PET sheet of 3 in. - 4 in.

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"Compared to organic or polymer transistor materials, these new inorganic oxides have higher mobility, better chemical stability, ease of manufacture, and are physically more robust," said John Wager, a professor of electrical and computer engineering at OSU. "Oxide-based transistors in many respects are already further along than organics or polymers are after many years of research, and this may blow some of them right out of the water."

"Frankly, until now no one ever believed we could get this type of electronic performance out of transparent oxide transistors processed at low temperatures," Wager said. "They may be so effective that there will be many uses which don't even require transparency, they are just a better type of transistor, cheap and easy to produce."

Fig 4.3 Generation of Transparent Electronics

The newest devices are zinc-tin-oxide thin film transistors, according to collaborating researchers in the OSU College of Engineering, OSU College of Science and at Hewlett Packard. They are an evolution of zinc oxide transistors, which gained attention as the world's first see-through transistor when OSU scientists announced their discovery last year. But this new material combines the characteristics of different elements to give levels of electronic performance and

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"mobility" – in electronics, an observation about how fast electrons can move within a material – that are an order of magnitude faster than the earlier transparent transistors, Wager said.

They are amorphous, meaning they have no long range crystalline order, which helps to keep processing costs a great deal lower. They are also physically robust – hard to scratch, chemically stable, resist etching, and have a very smooth surface. They are made from low cost, readily-available elements such as zinc and tin, which raise no environmental concerns.

From material and design advancements to new innovative processing methods, there have been significant recent achievements in the area of transparent electronics. Materials & Performance advancements in transparent wide band gap electronic materials are described in the articles reporting on metal oxide, GaN, and rare earth systems. Material enhancements focusing on lowering resistivity and increasing mobility are described. Attention is given to both experimental and modeling and simulation efforts. These papers discuss measured material properties, modeling results, and performance of structures up to the complexity of TFTs. Along with material progress; advancements in Fabrication Techniques are required to enable new device designs and new applications. The full benefits of transparent electronics are seen in the final device design and performance. Transparent electronics enable advancements in device technologies and open the opportunity for new applications. Application articles focused on the benefits of transparent electronics include display and organic light-emitting diode devices.

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Chapter-5APPLICATIONS OF

TRANSPARENT ELECTRONICSAs the oxide semiconductors are wide band gap materials, transparent TFTs can be easily realized by the combination of transparent electrodes and insulators. Transparency is one of the most significant features of TAOS TFTs. As the band gap of a-Si is 1.7 eV and that of crystalline-Si is 1.1 eV, ‘transparent electronics’ cannot be realized in Si technology. In TAOS TFTs, features of high mobility or low process temperature have attracted a lot of attention. However, transparency has been underestimated or even neglected in the research and development of TAOSs. Few examples of actual applications have been reported exploiting the transparency of TAOSs until now [25, 26]. Transparent circuits will have unprecedented applications in flat panel displays and other electronic devices, such as see through display or novel display structures. Here, practical examples taking advantage of the transparency of TAOS TFTs are: Reversible Display, ‘Front Drive’ Structure for Color Electronic Paper, Color Microencapsulated Electrophoretic Display, and Novel Display Structure – Front Drive Structure. Indium oxide nanowire mesh as well as indium oxide thin films were used to detect different chemicals, including CWA simulants.

They have been widely used in a variety of applications like:1. Antistatic coatings2. Touch display panels3. Solar cells,4. Flat panel displays5. Heaters6. Defrosters7. Optical coatings etc

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5.1 Imaginative Examples of use of Transparent Electronics

You are travelling in a car and you want to watch a movie or video play. Now the glass shields i.e. window panels will turn into a television screen and this is possible with this technology. This is helpful when the driver can't take away his eyes from road but still want to watch out a map of route. Then front window panel acts display with the help of this tech.

Fig 5.1 Examples of Transparent Electronics Devices (Illustrative)

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Chapter-6MARKET OF TRANSPARENT

ELECTRONICSEventually the materials suite used by transparent electronics will stabilize

and the role of organic electronics materials and nanomaterials in transparent electronics will become clearer. But as we have explained above, the possible technical directions that these materials are likely to take are fairly well defined; although we should not exclude surprises entirely.

Fig 6.1 Forecast of Transparent Electronics Products by Application

Opportunities in the area of the transparent electronics products themselves can be somewhat difficult to pick out. This is not just because of the diversity of the possible products that can be built within the context of transparent electronics paradigm, but also because both the actual past of transparent electronics so far and the somewhat futuristic prognostications about transparent electronics that have

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been widely published are a distraction from understanding what can really be achieved in the next few years with transparent electronics:

Too cool to succeed: Transparent electronics suffers, we believe, from the fact that it is so cool that it virtually cries out to be built into highly futuristic scenarios. And this is exactly what has happened. Just a casual look at the literature on transparent electronics—even the formal technical literature—usually reveals quite quickly a slew of references to science fiction movies in which transparent electronics are featured. The favorite in this regard is the Tom Cruise movie “Minority Report,” but other movies are also referenced. This is all a lot of fun, but gives a false impression of the current state of the art in transparent electronics and what might be achieved using this technology. Watching Cruise in “Minority Report,” it is never quite clear just why he is using transparent displays in his work. In other words, these display are props not just in the sense that they are not physically real (they don’t actually function). They are also divorced from market realities.

Current apps for transparent electronics are quite primitive: Paradoxically, the other reason why systems opportunities in the transparent electronics space can be difficult to identify is the exact opposite of the over-optimism reported on in the previous bullet point. A quick examination of the current offerings that might reasonably be included under the heading of “transparent” electronics reveals not products that, with a little tinkering could make it into “Minority Report II,” as it were, but rather primitive niche products.

For example, in the display space, if one looks for transparent displays, what one will find are simple passive matrix LED and EL displays which represent a tiny niche within the digital signage business; they are displays with very limited functionality. Similarly, self-tinting smart windows have been around long enough to show that they cannot compete with a conventional window, when a customer is looking for something that enables good natural lighting and attractive views. Or where tinting is critical to the specific application, the difference between tinted and untinted offered by a smart window is just not great enough. Again, we are looking at products and concepts that are out of tune with market realities.

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6.1 Three Factors That Can Lead to the Commercial Awakening of Transparent Electronics

Given all this, the big question is can transparent electronics move beyond the fanciful on the one hand and low-performing niche products on the other? In our view, there are four critical aspects of “transparency” that the design and marketing of transparent electronics products needs to focus on for it to become a serious revenue earner. These factors are (1) aesthetics, (2) integration (3) improved economics and (4) (somewhat paradoxically), aspects of transparent materials that are not directly related to transparency:

Other relevant drivers for transparent electronics may be discovered over time, but these are the ones that seem to matter now.

As the transparent electronics materials suite that we discussed earlier improves, it seems reasonable to expect an increased ability of transparent electronics to compete over all and any of the three dimensions mentioned above.

6.1.1 Aesthetics

It is intrinsically hard to measure the impact of aesthetics on market response, but important to remember that aesthetics has always been a key factor in marketing glass products; the glass industry having a considerably longer history and deeper understanding of marketing transparent products than the emerging transparent electronics industry.

Aesthetics seems to be key too much of the transparent electronics that has appeared to date. The simple transparent displays that is already available for use in advertising use “transparency” to gain extra attention. And transparent solar panels are being deployed in part because they look better than large framed solar panels installed in an all-too-visible fashion on a roof.

6.1.2 Integration

Because transparency enables visual access to multiple layers of a large-area panel it permits an additional level of integration. This is most obvious in the transparent overlay displays that are already being built in prototype by the display

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industry; but it is also part of the design story in the smart-window concepts that are being dreamed up that combine self-tinting windows, OLEDs and PV.

6.1.3 Improved economics

Obviously, in the end all of the advantages attributable to PV reduce to improved economics, but in some cases this is more obviously the case. One example of that is in the PV space again, where transparent solar panels represent an example of building integrated PV in which the cost of building materials and of PV can be distributed over a common substrate, thereby reducing total expenditures.

6.1.4 Non-transparent aspects of transparent materials

As mentioned above, in the case of transparent conductors, some transparent electronic materials have been developed without truly transparent electronics in mind as an application. However, it is possible that the converse could be true as well; that is that materials that are developed specifically with transparent electronics in mind could find a larger market.

The primary example—perhaps the only example, so far—of this kind of thing relates to the oxide TFTs that are being developed with transparent display backplanes in mind. There is also serious consideration being given to the possibility that these TFTs could be used in OLED displays more generally—that is, in non-transparent OLED displays—on price and performance grounds

Obviously, the business potential for transparent electronics is limited if all the work and all the press releases concerned just materials and research devices. This would suggest that the only market for the new materials would be the R&D community, which is a real market and one that is extremely interested in buying new materials; but in very small quantities.

Fortunately, there are also signs that the transparent electronics market is beginning to move beyond the niche products that are mentioned above. It is particularly gratifying that transparent displays are now moving from being the province of little signage firms to one that interests the likes of Apple, LG, Microsoft and Samsung. And when one digs down a little further it is possible to find interest in designing transparent solar panels from major PV firms.

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Chapter-7FUTURE SCOPE

It should be apparent from the discussion that although much progress has been made in developing new materials and devices for high performance transparent solar cells, there is still plenty of opportunity to study and improve device performance and fabrication techniques compared with the nontransparent solar cell devices. In particular, the stability of transparency solar cells has not been studied yet. Solution-processable transparent PSCs have become a promising emerging technology for tandem solar cell application to increase energy conversion efficiency. The transparency of solar cells at a specific light band will also lead to new applications such as solar windows. The field of energy harvesting is gaining momentum by the increases in gasoline price and environment pollution caused by traditional techniques. Continued breakthroughs in materials and device performance, accelerate and establish industrial applications. It is likely that new scientific discoveries and technological advances will continue to cross fertilize

each other for the foreseeable future.

Fig 7.1 Graphene is transparent and can be used as material

It would not be a complete surprise to find players in the smart window; sensor and lighting industries also begin to invest substantially in transparent electronics over the next few years.

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Chapter-8CONCLUSION

Oxides represent a relatively new class of semiconductor materials applied to active devices, such as TFTs. The combination of high field effect mobility and low processing temperature for oxide semiconductors makes them attractive for high performance electronics on flexible plastic substrates. The marriage of two rapidly evolving areas of research, OLEDs and transparent electronics, enables the realization of novel transparent OLED displays. This appealing class of see through devices will have great impact on the human–machine interaction in the near future. EC device technology for the built environment may emerge as one of the keys to combating the effects of global warming, and this novel technology may also serve as an example of the business opportunities arising from the challenges caused by climate changes The transparency of solar cells at a specific light band will also lead to new applications such as solar windows. The field of energy harvesting is gaining momentum by the increases in gasoline price and environment pollution caused by traditional techniques. Let us hope that we are soon going to see transparent technology being implemented in our lives.

Fig 8.1 Usage of Transparent Electronics devices in future

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Chapter-9REFERENCES

‘Transparent Electronics ’, Springer publications, J.F.Wager, D. A. Keszler, R. E. Presley.

‘Transparent electronics: from synthesis to applications’, Wiley publications: Antonio Facchetti, Tobin J. Marks.

www.wikipedia.org

www.ieee.org

www.alternative-energy-news.info/transparent-a-solar-energy-breakthrough/

www.nanomarkets.net

www.nikkeibp.co.jp

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