22-08-2014, 02:56 PM
A Seminar report on Transparent Electronics
[attachment=66898]
ABSTRACT
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 displays. In a similar vein, windows could be used to produce electrical power. Other civilian and military applications in this research field include realtime 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
INTRODUCTION
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 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. All these applications use transparent conductive oxides as passive electrical or optical coatings. 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.
COMBINING OPTICAL TRANSPARENCY WITH ELECTRICAL CONDUCTIVITY
Transparent conductors are neither 100% optically transparent nor metallically conductive. From the band structure point of view, the combination of the two properties in the same material is contradictory: a transparent material is an insulator which possesses completely filled valence and empty conduction bands; whereas metallic conductivity appears when the Fermi level lies within a band with a large density of states to provide high carrier concentration.
Efficient transparent conductors find their niche in a compromise between a sufficient transmission within the visible spectral range and a moderate but useful in practice electrical conductivity. This combination is achieved in several commonly used oxides – In2O3, SnO2, ZnO and CdO. In the undoped stoichiometric state, these materials are insulators with optical band gap of about 3 eV. To become a transparent conducting oxide (TCO), these TCO hosts must be degenerately doped to displace the Fermi level up into the conduction band. The key attribute of any conventional n-type TCO host is a highly dispersed single freeelectron- like conduction band (Figure 1). Degenerate doping then provides both (i) the high mobility of extra carriers (electrons) due to their small effective mass and (ii) lowoptical absorption due to the lowdensity of states in the conduction band. The high energy dispersion of the conduction band also ensures a pronounced Fermi energy displacement up above the conduction band minimum, the Burstein–Moss (BM) shift. The shift helps to broaden the optical transparency window and to keep the intense optical transitions from the valence band out of the visible range. This is critical in oxides which are not transparent throughout the entire visible spectrum, for example, in CdO where the optical (direct) band gap is 2.3 eV.
Fig.1: (a) Schematic electronic band structure of aTCOhost – an insulator with a band gap Eg and a dispersed parabolic conduction band which originates from interactions between metal s and oxygen p states. (b) and © Schematic band structure and density of states of a TCO, where a degenerate doping displaces the Fermi level (EF) via a Burstein-Moss shift, EBM, making the system conducting. The shift gives rise to inter-band optical transitions from the valence band,
Electronic Properties of Conventional TCO Hosts
Conventional n-type TCO hosts (In2O3, SnO2, CdO and ZnO) share similar chemical, structural and electronic properties. Exclusively oxides of the post transition metals with (n-1)d10ns2 electronic configurations, they have densely packed structures with four- or six-coordinate metal ions. Strong interactions between the oxygen 2p and metal ns orbitals give rise to electronic band
structures qualitatively similar for all these oxides (Figures 1 and 2): the bonding and nonbonding O 2p states form the valence band while the conduction band arises from the antibonding Ms–Op interactions. The empty p states of the metal ion form the following band at a higher energy. The partial density of states plots (Figure 2), reveal that the oxygen 2p and metal ns states make similar contributions to the conduction band. This provides a three dimensional Ms–Op network for charge transport once extra carriers fill the band
SUBSTITUTIONAL DOPING
Substitutional doping with aliovalent ions is the most widely used approach togenerate free carriers in TCO hosts. Compared with native point defects, it allowsa better control over the resulting optical and transport properties as well as betterenvironmental stability of the TCO films. Traditionally, same-period, next-rowelements, e.g, Sn4+ for In3+ and In3+ for Cd2+, are thought to provide bettercompatibility and, thus, less disturbance in the host crystal and electronicstructure.However, other dopants may prove beneficial for optimizing theproperties for a specific application. For example, transparent conducting ZnOfilms have been prepared by doping with Group III (Al, Ga, In and B), Group IV (Si,Ge, Ti, Zr and Hf) and a Group VII element (F substituted at an oxygen site),giving rise to a wide range of electrical conductivities.Here we will give a detailed consideration to rocksalt CdO, where the high crystalsymmetry and the densely packed structure ensures the most uniform chargedensity distribution via the isotropicMs–Op network. Compared with morecomplex In2O3 or SnO2, one can expect fewer ionized and neutral scatteringcenters and, hence, longer relaxation times. At the same time, introduction ofdopants into the densely packed structure may significantly influence the Cds–2phybridization and, therefore, alter the structural, electronic and optical propertiesof the host. A systematic comparison of CdO doped with In, Ga, Sc or Y, whoseionic radius and electronic configuration differ from those of the host cation, hasrevealed that i) Substitutional dopants with smaller ionic radii compared with that of Cdshrink the lattice. The shrinkage, however, is not as large as expectedfrom the Vegard’s law weighted
OXYGEN REDUCTION
Removal of an oxygen atom from a metal oxide leaves two extra electrons in the crystal. Whether one or both of these electrons become free carriers or remainlocalized at the vacancy site correlates with the oxide free energy of formation. Inlight metal oxides, such as CaO or Al2O3, where the formation energy is high,oxygen vacancies create deep charge localized states within the electronic bandgap known as color or F centers. A relatively low formation energy of the conventional TCOs favors large oxygen deficiencies even under equilibriumgrowth conditions, giving rise to the free-carrier densities of forIn2O3 and ZnO.Electronic band structure investigations of oxygen deficient oxides showed thatthe oxygen defect (in notation the superscript stands for effectivepositive charge) corresponds to a non-conducting state associated with the fillingof the lowest single conduction band by the two vacancy-induced electrons. Only ifthe vacancy is excited, e.g. via a photoexcitation , or partially compensatedto ,does the single conduction band become half-occupied and conductingbehavior may occur.In oxygen deficient TCOs, the conduction band wave function resembles the onein the corresponding hosts, i.e. it is derived from the M s and O p states (Figure 1).A relatively uniform charge density distribution suggests that the vacancy-inducedelectrons are delocalized
TRANSPARENT ELECTRONICS DEVICES
In order to produce a transparent-electronics-based system, appropriatematerials must be selected, synthesized, processed, and integrated together inorder to fabricate a variety of different types of devices. In turn, these devicesmust be chosen, designed, fabricated, and interconnected in order to constructcircuits, each of which has to be designed, simulated, and built in such a waythat they appropriately function when combined together with other circuit andancillary non-circuit subsystems. Thus, this product flow path involves materials→ devices → circuits → systems, with each level of the flow more than likelyinvolving multi-feedback iterations of selection, design, simulation, fabrication,integration, characterization, and optimization.From this perspective, devices constitute a second level of the product flow path.The multiplicity, performance, cost, manufacturability, and reliability of availabledevice types will dictate the commercial product space in which transparentelectronics technology will be able to compete. Thus, an assessment of thedevice toolset available to transparent electronics is of fundamental interest, andis the central theme of this chapter.Passive, linear devices - resistors, capacitors, and inductors – comprise the firsttopic discussed. Passive devices are usually not perceived to be as glamorousas active devices, but they can be enabling from a circuit system perspective,and they are also the simplest device types from an operational point-of-view.Together, these two factors provide the rationale for considering this topicinitially.Next, two-terminal electronic devices - pn junctions, Schottky barriers,heterojunctions, and metal-insulator-semiconductor (MIS) capacitors - constitutethe second major topic. The motivation for this topical ordering is againassociated with their relative operational complexity, rather than their utility. Thethird and final major topic addressed is transistors. This is the most importantmatter considered in this chapter. Most of this discussion focuses on TTFTs,since they are perceived to be the most useful type of transistor for transparentelectronics. Additionally, a very brief overview of alternative transistor types -static-induction transistors, vertical TFTs, hot electron transistors, and nanowiretransistors - is included.
INDUCTORS
An ideal inductor is a magnetic field energy storage device possessing linearvoltage-current derivative (v-di/dt) characteristics. Important ideal inductorequations are collected in the first two entries of Table 6.In contrast to a TTFR and a TTFC, a transparent thin-film inductor (TTFI) andrelated transparent magnetically-coupled devices are expected to behave in anon-ideal manner. Two main reasons underlie this expectation. First, because ofthe relatively poor conductance of TCOs compared to metals, TTFIs will possessa significant amount of parasitic resistance. Second, efficient magnetic fieldcoupling is strongly facilitated by the use of a magnetically-permeable insulator.However, we are not aware of a transparent, magnetically-permeable insulatormaterial
TRANSPARENT THIN-FILM TRANSISTORS(TTFTs)
TTFTs constitute the heart of transparent electronics. The first two sectionsfocus on ideal and non-ideal behavior of n-channel TTFTs. Next, n-channelTTFT stability is considered. Finally, issues related to alternative devicestructures – double-gate TTFTs and the realization of p-channel TTFTs - arediscussed.Fig.7: Two possible transparent thin-film transistor (TTFT) device structures,(a) astaggered, bottom-gate, and (b) a coplanar, top-gate.Figure 7. illustrates two of four possible TTFT device structures. The first one,considered in Fig. 7a, is denoted as a staggered, bottom-gate since source-drainand gate contacts are located at the top and bottom of the device, respectively.Figure 7b shows a coplanar, top-gate structure in which the source-drain and thegate are all positioned on the top side of the TTFT. The remaining two TTFTdevice structures, not shown, are the staggered, top-gate and coplanar, bottomgateconfigurations. Although in a conventional TFT, the source, drain, and gatecontact materials would be metals, a highly conductive TCO, such as ITO, isused in a TTFT. Additionally, while the channel layer of a conventional TFTemploys a narrow band gap, opaque semiconductor, a highly insulating, wideband gap transparent semiconductor is used in a TTFT.
APPLICATIONS
As the oxide semiconductors are wide band gap materials, transparent TFTs canbe easily realized by the combination of transparent electrodes and insulators.Transparency is one of the most significant features of TAOS TFTs. As the bandgap 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 orlow process temperature have attracted a lot of attention. However, transparencyhas been underestimated or even neglected in the research and development ofTAOSs. Few examples of actual applications have been reported exploiting thetransparency of TAOSs until now [25, 26]. Transparent circuits will haveunprecedented applications in flat panel displays and other electronic devices,such as seethrough display or novel display structures. Here, practical examplestaking advantage of the transparency of TAOS TFTs are: Reversible Display,‘Front Drive’ Structure for Color Electronic Paper, Color MicroencapsulatedElectrophoretic Display, Novel Display Structure – Front Drive Structure. Indiumoxide nanowire mesh as well as indium oxide thin films were used to detectdifferent chemicals, includingCWA simulants.
FUTURE SCOPE
It should be apparent from the discussion that although much progress has beenmade in developing new materials and devices for high performance transparentsolar cells, there is still plenty of opportunity to study and improve deviceperformance and fabrication techniques compared with the nontransparent solarcell devices. In particular, the stability of transparency solar cells has not beenstudied yet. Solution-processable transparent PSCs have become a promisingemerging technology for tandem solar cell application to increase energyconversion efficiency. The transparency of solar cells at a specific light band willalso lead to newapplications such as solar windows. The field of energyharvesting is gaining momentum by the increases in gasoline price andenvironment pollution caused by traditional techniques. Continuedbreakthroughs in materials and device performance, accelerate and establishindustrial applications. It is likely that new scientific discoveries and technologicaladvances will continue to crossfertilize each other for the foreseeable future
CONCLUSION AND REMARKS
Oxides represent a relatively newclass of semiconductor materials applied toactive devices, such as TFTs. The combination of high field effect mobility andlowprocessing temperature for oxide semiconductors makes them attractive forhigh performance electronics on flexible plastic substrates. The marriage of tworapidly evolving areas of research, OLEDs and transparent electronics, enablesthe realization of novel transparent OLED displays. This appealing class of seethroughdevices will have great impact on the human–machine interaction in thenear future. EC device technology for the built environment may emerge as oneof the keys to combating the effects of global warming, and this novel technologymay also serve as an example of the business opportunities arising from thechallenges caused by climate changes The transparency of solar cells at aspecific light band will also lead to newapplications such as solar windows. Thefield of energy harvesting is gaining momentum by the increases in gasolineprice and environment pollution caused by traditional techniques.
[attachment=66898]
ABSTRACT
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 displays. In a similar vein, windows could be used to produce electrical power. Other civilian and military applications in this research field include realtime 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
INTRODUCTION
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 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. All these applications use transparent conductive oxides as passive electrical or optical coatings. 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.
COMBINING OPTICAL TRANSPARENCY WITH ELECTRICAL CONDUCTIVITY
Transparent conductors are neither 100% optically transparent nor metallically conductive. From the band structure point of view, the combination of the two properties in the same material is contradictory: a transparent material is an insulator which possesses completely filled valence and empty conduction bands; whereas metallic conductivity appears when the Fermi level lies within a band with a large density of states to provide high carrier concentration.
Efficient transparent conductors find their niche in a compromise between a sufficient transmission within the visible spectral range and a moderate but useful in practice electrical conductivity. This combination is achieved in several commonly used oxides – In2O3, SnO2, ZnO and CdO. In the undoped stoichiometric state, these materials are insulators with optical band gap of about 3 eV. To become a transparent conducting oxide (TCO), these TCO hosts must be degenerately doped to displace the Fermi level up into the conduction band. The key attribute of any conventional n-type TCO host is a highly dispersed single freeelectron- like conduction band (Figure 1). Degenerate doping then provides both (i) the high mobility of extra carriers (electrons) due to their small effective mass and (ii) lowoptical absorption due to the lowdensity of states in the conduction band. The high energy dispersion of the conduction band also ensures a pronounced Fermi energy displacement up above the conduction band minimum, the Burstein–Moss (BM) shift. The shift helps to broaden the optical transparency window and to keep the intense optical transitions from the valence band out of the visible range. This is critical in oxides which are not transparent throughout the entire visible spectrum, for example, in CdO where the optical (direct) band gap is 2.3 eV.
Fig.1: (a) Schematic electronic band structure of aTCOhost – an insulator with a band gap Eg and a dispersed parabolic conduction band which originates from interactions between metal s and oxygen p states. (b) and © Schematic band structure and density of states of a TCO, where a degenerate doping displaces the Fermi level (EF) via a Burstein-Moss shift, EBM, making the system conducting. The shift gives rise to inter-band optical transitions from the valence band,
Electronic Properties of Conventional TCO Hosts
Conventional n-type TCO hosts (In2O3, SnO2, CdO and ZnO) share similar chemical, structural and electronic properties. Exclusively oxides of the post transition metals with (n-1)d10ns2 electronic configurations, they have densely packed structures with four- or six-coordinate metal ions. Strong interactions between the oxygen 2p and metal ns orbitals give rise to electronic band
structures qualitatively similar for all these oxides (Figures 1 and 2): the bonding and nonbonding O 2p states form the valence band while the conduction band arises from the antibonding Ms–Op interactions. The empty p states of the metal ion form the following band at a higher energy. The partial density of states plots (Figure 2), reveal that the oxygen 2p and metal ns states make similar contributions to the conduction band. This provides a three dimensional Ms–Op network for charge transport once extra carriers fill the band
SUBSTITUTIONAL DOPING
Substitutional doping with aliovalent ions is the most widely used approach togenerate free carriers in TCO hosts. Compared with native point defects, it allowsa better control over the resulting optical and transport properties as well as betterenvironmental stability of the TCO films. Traditionally, same-period, next-rowelements, e.g, Sn4+ for In3+ and In3+ for Cd2+, are thought to provide bettercompatibility and, thus, less disturbance in the host crystal and electronicstructure.However, other dopants may prove beneficial for optimizing theproperties for a specific application. For example, transparent conducting ZnOfilms have been prepared by doping with Group III (Al, Ga, In and B), Group IV (Si,Ge, Ti, Zr and Hf) and a Group VII element (F substituted at an oxygen site),giving rise to a wide range of electrical conductivities.Here we will give a detailed consideration to rocksalt CdO, where the high crystalsymmetry and the densely packed structure ensures the most uniform chargedensity distribution via the isotropicMs–Op network. Compared with morecomplex In2O3 or SnO2, one can expect fewer ionized and neutral scatteringcenters and, hence, longer relaxation times. At the same time, introduction ofdopants into the densely packed structure may significantly influence the Cds–2phybridization and, therefore, alter the structural, electronic and optical propertiesof the host. A systematic comparison of CdO doped with In, Ga, Sc or Y, whoseionic radius and electronic configuration differ from those of the host cation, hasrevealed that i) Substitutional dopants with smaller ionic radii compared with that of Cdshrink the lattice. The shrinkage, however, is not as large as expectedfrom the Vegard’s law weighted
OXYGEN REDUCTION
Removal of an oxygen atom from a metal oxide leaves two extra electrons in the crystal. Whether one or both of these electrons become free carriers or remainlocalized at the vacancy site correlates with the oxide free energy of formation. Inlight metal oxides, such as CaO or Al2O3, where the formation energy is high,oxygen vacancies create deep charge localized states within the electronic bandgap known as color or F centers. A relatively low formation energy of the conventional TCOs favors large oxygen deficiencies even under equilibriumgrowth conditions, giving rise to the free-carrier densities of forIn2O3 and ZnO.Electronic band structure investigations of oxygen deficient oxides showed thatthe oxygen defect (in notation the superscript stands for effectivepositive charge) corresponds to a non-conducting state associated with the fillingof the lowest single conduction band by the two vacancy-induced electrons. Only ifthe vacancy is excited, e.g. via a photoexcitation , or partially compensatedto ,does the single conduction band become half-occupied and conductingbehavior may occur.In oxygen deficient TCOs, the conduction band wave function resembles the onein the corresponding hosts, i.e. it is derived from the M s and O p states (Figure 1).A relatively uniform charge density distribution suggests that the vacancy-inducedelectrons are delocalized
TRANSPARENT ELECTRONICS DEVICES
In order to produce a transparent-electronics-based system, appropriatematerials must be selected, synthesized, processed, and integrated together inorder to fabricate a variety of different types of devices. In turn, these devicesmust be chosen, designed, fabricated, and interconnected in order to constructcircuits, each of which has to be designed, simulated, and built in such a waythat they appropriately function when combined together with other circuit andancillary non-circuit subsystems. Thus, this product flow path involves materials→ devices → circuits → systems, with each level of the flow more than likelyinvolving multi-feedback iterations of selection, design, simulation, fabrication,integration, characterization, and optimization.From this perspective, devices constitute a second level of the product flow path.The multiplicity, performance, cost, manufacturability, and reliability of availabledevice types will dictate the commercial product space in which transparentelectronics technology will be able to compete. Thus, an assessment of thedevice toolset available to transparent electronics is of fundamental interest, andis the central theme of this chapter.Passive, linear devices - resistors, capacitors, and inductors – comprise the firsttopic discussed. Passive devices are usually not perceived to be as glamorousas active devices, but they can be enabling from a circuit system perspective,and they are also the simplest device types from an operational point-of-view.Together, these two factors provide the rationale for considering this topicinitially.Next, two-terminal electronic devices - pn junctions, Schottky barriers,heterojunctions, and metal-insulator-semiconductor (MIS) capacitors - constitutethe second major topic. The motivation for this topical ordering is againassociated with their relative operational complexity, rather than their utility. Thethird and final major topic addressed is transistors. This is the most importantmatter considered in this chapter. Most of this discussion focuses on TTFTs,since they are perceived to be the most useful type of transistor for transparentelectronics. Additionally, a very brief overview of alternative transistor types -static-induction transistors, vertical TFTs, hot electron transistors, and nanowiretransistors - is included.
INDUCTORS
An ideal inductor is a magnetic field energy storage device possessing linearvoltage-current derivative (v-di/dt) characteristics. Important ideal inductorequations are collected in the first two entries of Table 6.In contrast to a TTFR and a TTFC, a transparent thin-film inductor (TTFI) andrelated transparent magnetically-coupled devices are expected to behave in anon-ideal manner. Two main reasons underlie this expectation. First, because ofthe relatively poor conductance of TCOs compared to metals, TTFIs will possessa significant amount of parasitic resistance. Second, efficient magnetic fieldcoupling is strongly facilitated by the use of a magnetically-permeable insulator.However, we are not aware of a transparent, magnetically-permeable insulatormaterial
TRANSPARENT THIN-FILM TRANSISTORS(TTFTs)
TTFTs constitute the heart of transparent electronics. The first two sectionsfocus on ideal and non-ideal behavior of n-channel TTFTs. Next, n-channelTTFT stability is considered. Finally, issues related to alternative devicestructures – double-gate TTFTs and the realization of p-channel TTFTs - arediscussed.Fig.7: Two possible transparent thin-film transistor (TTFT) device structures,(a) astaggered, bottom-gate, and (b) a coplanar, top-gate.Figure 7. illustrates two of four possible TTFT device structures. The first one,considered in Fig. 7a, is denoted as a staggered, bottom-gate since source-drainand gate contacts are located at the top and bottom of the device, respectively.Figure 7b shows a coplanar, top-gate structure in which the source-drain and thegate are all positioned on the top side of the TTFT. The remaining two TTFTdevice structures, not shown, are the staggered, top-gate and coplanar, bottomgateconfigurations. Although in a conventional TFT, the source, drain, and gatecontact materials would be metals, a highly conductive TCO, such as ITO, isused in a TTFT. Additionally, while the channel layer of a conventional TFTemploys a narrow band gap, opaque semiconductor, a highly insulating, wideband gap transparent semiconductor is used in a TTFT.
APPLICATIONS
As the oxide semiconductors are wide band gap materials, transparent TFTs canbe easily realized by the combination of transparent electrodes and insulators.Transparency is one of the most significant features of TAOS TFTs. As the bandgap 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 orlow process temperature have attracted a lot of attention. However, transparencyhas been underestimated or even neglected in the research and development ofTAOSs. Few examples of actual applications have been reported exploiting thetransparency of TAOSs until now [25, 26]. Transparent circuits will haveunprecedented applications in flat panel displays and other electronic devices,such as seethrough display or novel display structures. Here, practical examplestaking advantage of the transparency of TAOS TFTs are: Reversible Display,‘Front Drive’ Structure for Color Electronic Paper, Color MicroencapsulatedElectrophoretic Display, Novel Display Structure – Front Drive Structure. Indiumoxide nanowire mesh as well as indium oxide thin films were used to detectdifferent chemicals, includingCWA simulants.
FUTURE SCOPE
It should be apparent from the discussion that although much progress has beenmade in developing new materials and devices for high performance transparentsolar cells, there is still plenty of opportunity to study and improve deviceperformance and fabrication techniques compared with the nontransparent solarcell devices. In particular, the stability of transparency solar cells has not beenstudied yet. Solution-processable transparent PSCs have become a promisingemerging technology for tandem solar cell application to increase energyconversion efficiency. The transparency of solar cells at a specific light band willalso lead to newapplications such as solar windows. The field of energyharvesting is gaining momentum by the increases in gasoline price andenvironment pollution caused by traditional techniques. Continuedbreakthroughs in materials and device performance, accelerate and establishindustrial applications. It is likely that new scientific discoveries and technologicaladvances will continue to crossfertilize each other for the foreseeable future
CONCLUSION AND REMARKS
Oxides represent a relatively newclass of semiconductor materials applied toactive devices, such as TFTs. The combination of high field effect mobility andlowprocessing temperature for oxide semiconductors makes them attractive forhigh performance electronics on flexible plastic substrates. The marriage of tworapidly evolving areas of research, OLEDs and transparent electronics, enablesthe realization of novel transparent OLED displays. This appealing class of seethroughdevices will have great impact on the human–machine interaction in thenear future. EC device technology for the built environment may emerge as oneof the keys to combating the effects of global warming, and this novel technologymay also serve as an example of the business opportunities arising from thechallenges caused by climate changes The transparency of solar cells at aspecific light band will also lead to newapplications such as solar windows. Thefield of energy harvesting is gaining momentum by the increases in gasolineprice and environment pollution caused by traditional techniques.