TRANSPARENT ELECTRONICS

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.

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 (c) 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,

Ev, and from the partially filled conduction band up into the next empty band, Ec, as well as to intraband transitions within the conduction band, Ei.

Achieving the optimal performance in a TCO is a challenging because of the complex interplay between the electronic and optical properties. The large carrier concentrations desired for a good conductivity may result in an increase of the optical absorption (i) at short wavelengths, due to inter-band transitions from the partially filled conduction band and (ii) at long wavelengths, due to intra-band transitions within this band. In addition, plasma oscillations may affect the optical properties by reflecting the electromagnetic waves of frequency below that of the plasmon. Furthermore, ionized impurity scattering on the electron donors (native point defects or substitutional dopants) have a detrimental effect on the charge transport, while the structural relaxation around the impurities may alter the electronic and optical properties of the host, leading to a nonrigid-band shift of the Fermi level. We demonstrate here that a thorough understanding of the microscopic properties of metal oxides provides an insight into the underlying phenomena and also suggests that the range of efficient TCO materials can be significantly broadened.

CONCLUSION AND REMARKS

Oxides represent a relatively newclass of semiconductor materials applied to active devices, such as TFTs. The combination of high field effect mobility and lowprocessing 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 seethrough 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 newapplications 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.