Abstract

Transparent electronics is the technology that the world is eagerly awaiting to make it possible to create undetectable optical and electrical components. Through the introduction of several applications, this breakthrough has the potential to completely transform our everyday lives. Consider transparent windows that can identify intruders and notify the homeowner, or windshield displays that are seamlessly connected to technology. Bose Frames, Ray-Ban Stories, and Vuzix Blade are examples of electronic eyewear that only scratches the surface of what is possible. The materials required for this technique must be both transparent and conductive, despite the seeming contradiction. The standard for total transparency in both optical and metallic dimensions is not fulfilled by conventionally translucent conductors. Ongoing investigations, however, continue to identify compounds displaying these two features at a sufficient degree, propelling the search for materials ideal for transparent electronics. The two main goals of transparent thin film transistors (TTFTs) are to enable lower temperature production and to achieve increased device mobility. A wider range of possible applications is made possible by increasing the mobility of these devices and improving their operational speed while lowering their overall power consumption. Furthermore, low-temperature manufacturing is needed to create transparent electronics on flexible substrates, which is crucial for both creative applications and economic viability. Though significant progress has been made, more needs to be done, especially in terms of increasing the devices' stated mobility, which is now inferior to that of their opaque counterparts.

1. Introduction

There are limits to how much a conductor may do in terms of both metallic conductivity and optical transparency. Gaining insight into the reasons for the transparency of some materials and not of others helps one to appreciate this constraint. Remarkably, the many kinds of inter- or intramolecular bonding (van der Waals, ionic, covalent, metallic) have no discernible effect on a material's transparency. Furthermore, for bonding types such as ionic or electronic polarization, a solid's reflectivity, transparency, and absorption with respect to particular light wavelengths are theoretically controlled by the dielectric function.

A solid's band structure, the location of the Fermi energy, and the existence of free electrons are some of the main factors that affect a solid's transparency. Solids are made up of electron bands divided by what is known as the HOMO/LUMO gap. The electron states between the Lowest Unoccupied Molecular Orbital (LUMO) and the Highest Occupied Molecular Orbital (HOMO) are represented by this gap. All electrons in the same band have the same energy. The configuration of electrons at various energies has a significant influence on the transparency and opacity of a material. Electrons can move to different bands in proportion to the band gap when they absorb or release energy. Photons of visible light are absorbed by the electrons when the band gap is in line with their energy range, making the material opaque. In contrast, the material exhibits transparency if the band gap is either smaller or larger than the visible light energy range. This is because it permits light to pass through the material. The optical properties of a solid cannot be determined solely by the particular bonding types present in a semiconductor, even though these types of bonding aid in defining the Fermi energy and band structure.

Grain boundaries and a material's porosity both have a major effect on how transparent it is. Nevertheless, some materials remain transparent in spite of these factors, such as rubies and other Al2O3 gemstones. The intersections of the many tiny crystals that make up these gemstones create grain boundaries that scatter light photons. This material would eventually become transparent if it were heated to high temperatures and allowed these tiny crystals to grow into larger ones. Because rubies are single crystals without boundaries or porosity (like trapped air bubbles), they don't scatter light. On the other hand, porosity and grain boundaries cause glass that has been powdered and heated without enough fusion to remain opaque. Transparency in the material is only achieved when the particles are sufficiently melted together to remove porosity and grain boundaries.

Both the substance and how it interacts with photons are involved in transparency. Metals are primarily grey because of the way that their electrons behave. This means that they are good at absorbing light radiation. Ge is added to fiber optic glasses in order to increase the transmission of infrared (IR) photons because it interacts with these photons less than silicon does. As a result of processing techniques, impurities, and surface quality, structural composition and bonding properties do in fact have a big impact. A material can become translucent when it gets thin, which gives its optical qualities another dimension.

In contrast to metallic conductivity, where the Fermi level is located in a band with a high density of states producing a significant carrier concentration, insulators are characterized by valence bands that are completely occupied and conduction bands that are empty. Transparent Conducting Oxides (TCOs) are widely used materials with an optical band gap of about 3 eV. These materials are mainly In2O3, SnO2, ZnO, and CdO. Degenerate doping techniques provide transparency and conductivity to these materials by raising the Fermi level into the conduction band. Since the majority of electrons are now in the conduction state, there is less optical absorption by new carriers, which raises the mobility of the carriers. Strong optical transitions from the valence band are effectively prevented from entering the visible spectrum by the Burstein-Moss (BM) shift, which increases the optical transparency range. This is an extremely important phenomenon, particularly for oxides such as CdO, which have an optical band gap of 2.3 eV and are completely opaque in the visible range.

Better conductivity but less transparency result from an oxide with more carriers. The complicated link between the electrical and optical properties makes it difficult to strike a balance between transparency and conductivity.

Our research is limited to post-transition metal oxides, which are distinguished by their tightly packed structures with metal ions coupled with four or six nearby atoms and display (n-1) d10ns2 electronic configurations. The electrical band structures of these oxides are significantly influenced by the strong interactions between the metal ns orbitals and oxygen 2p orbitals. The occupied 2p antibonding bands in the highest valence band and the unoccupied cations bonding bands in the lowest conduction band make up most of the band gaps. Bonding O 2p states make up the majority of the valence band, whereas nonbonding O 2p states make up the conduction band. The valence and conduction bands are clearly separated by this interaction. The gap is direct for ZnO, but indirect for CdO, In2O3, and SnO3.

The oxide hosts must be doped in order to introduce carriers into them. This process turns the oxide hosts into conductivity- and transparency-rich oxides, which are essential for carrier generation. Two techniques are available for accomplishing doping: substitution and oxygen reduction.

2. Fabrication of Ideal TCO

Inter-band transitions beyond the visible spectrum can be preserved by introducing a deeper impurity band within the band structure of the host insulating material, thus opening up a wider range of underlying states. The band gap of the host material must be greater than 6.2 eV for this to happen. In addition, the impurity band width should not be greater than 1.8 eV in order to maintain the plasma frequency and intra-band transitions below the visible spectrum. High conductivity requires a high enough concentration of impurities to create a band in which the impurities' electronic wave functions significantly overlap.

High impurity concentration materials require dense packing, which tends to exacerbate ionized impurity scattering and impede electron transport. As a result, materials that are closely packed might not be the best for producing Transparent Conducting Oxides (TCOs). Zeolites are thought to be good TCO candidates because of their large interconnected pores and broad band gaps, which are among their structural and optical characteristics. These materials have the capacity to contain required guest atoms in nanoscale cavities, which presents an opportunity to adjust the material's transport characteristics.

3. Applications

A glass substrate with an electrode made of a transparent conducting oxide (TCO), such as indium tin oxide (ITO), is typically used by OLEDs to emit light. In the meantime, an opaque metal layer with a low work function is usually used to create the upper electrode, also known as the cathode. An OLED's top electrode needs to be transparent as well, as shown in Figure 1, to guarantee transparency. Initially, ultra-thin semi-transparent top electrodes made of materials like Mg: Ag or Ca were used in transparent OLED prototypes. But these metal layers caused significant optical losses, which limited these devices' transparency to about 60–70% in the visible spectral range of 450–750 nm.

Another structure that transparent OLEDs can choose from is an inverted layer arrangement where the anode is at the top. A semi-transparent Au anode has been used in a few cases. Transparent Conducting Oxide (TCO), a bottom electrode that effectively injected electrons, was made possible by the novel idea of electrochemically doping organic electron transport layers. But even with this improvement, losses related to the Au electrode meant that the device still had an overall opacity. These results highlight the need for using a TCO as the top electrode because highly transparent systems cannot use even an incredibly thin metal top layer. The optimal ITO layer for OLED applications is thought to be around 100 nm thick, with a sheet resistance of 20 ohms per square and less than 20% opacity throughout the visible spectrum. Sputtering is a common industry technique for coating large areas with TCO, but the high-energy particle emission from this process has been shown to seriously harm the organic layers underneath. Therefore, it becomes essential to use a protective buffer layer to shield organic components while the TCO top electrode is being deposited. High transparency in the visible spectrum, sufficient electrical conductivity, effective protection against incoming particles, ease of charged carrier injection into adjacent organic charge transport materials from the ITO, and use of a low-impact, non-reactive deposition process without negatively impacting the underlying organic layers are just a few of the strict requirements that must be met for this buffer layer. It is rare, though, for a single material to fully satisfy these requirements. For example, although thin metal coatings provide effective shielding and make it easier to introduce charges into organic systems, even at lower thicknesses, their transparency is restricted, with alloys such as Mg:Ag having a total transmittance of no more than 50%. Certain organic buffer layers, such as copper phthalocyanine (CuPc) or pentacene, can meet the minimum total transparency requirement of 75% for safety requirements for transparent displays, such as those found in automobile windshields. However, planar molecules with grain sizes larger than 5 mm, like pentacene or CuPc, frequently form polycrystalline structures that affect the conductivity and integrity of the ITO film on top of these layers. In order to address this problem, pentacene's surface was coated with poly 3,4-ethylenedioxythiophene & poly styrenesulfonate (PEDOT:PSS), which smoothed the material's surface and produced a strong hybrid buffer that facilitated ohmic hole injection. However, the requirement for vacuum interruption and moisture introduction through the PEDOT:PSS wet process limits the practicality of this hybrid buffer technique for mass production. Combining an ultra-thin layer of Li with a clear organic buffer consisting of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is another method for creating devices that are more transparent. Interestingly, there is still substantial sputter-induced damage to the BCP layer. OLED applications have resurrected the use of transition metal oxides (TMOs), such as NiO, MoO3, V2O5, WO3, and ReO3, because of their enhanced stability, increased transparency, and powerful hole injection capabilities. Although these compounds show potential as efficient sputter protectants, when used in charge generation layers, they also cause significant sputter-induced damage to the BCP layer. The OLED arrangement shown in Figure 2 is an inverted bottom electrode 14 cathode 20 configuration in which Li is exposed to electron injection after doping the bathophenanthroline (BPhen) layer. The emission zone is made up of a TPBi layer doped with 7% volume phenylpyridine iridium, which uses 4,40,400-tris(N-carbazolyl) triphenylamine (TCTA) as a hole transport agent. The tungsten oxide (WO3) layer on p that has been thermally evaporated performs two functions: it acts as a buffer layer and it injects holes. The ITO top electrode is 60 nm in size and was created by RF (Radio frequency) magnetron sputtering.

Multiple-layered OLED devices have been investigated recently as an intriguing way to increase OLED current efficiency. n vertically stacked OLEDs are connected in series to produce an n-fold increase in operating voltage as well as a corresponding n-fold increase in current efficiency. Each injected electron-hole pair in these configurations should ideally produce n photons. Charge-generating layers (CGLs), also known as thin metal interlayers, are used to join more than two OLEDs to form stacked OLEDs. Doped organic/inorganic layer sequences or doped organic/organic p-n junctions are examples of alternative CGL designs. The behavior of these organic p-n heterostructures is similar to that of tunnel diodes, which are frequently observed in the field of inorganic semiconductor devices. An electron is tunnel transferred from the Lowest Unoccupied Molecular Orbital (LUMO) of the n-doped layer to the Highest Occupied Molecular Orbital (HOMO) of the p-doped material within the reverse-biased CGL structure, creating an electron-hole pair. This transfer happens because of a large drop in electric field across the CGL heterostructure, which is only a few nanometers thick.

Stacking designs have become very popular recently for transparent OLEDs, particularly in situations where high overall brightness levels or improved daylight readability are required. The device architecture of a stacked transparent OLED with three emissive units (EMUs). The Charge Generating Layers (CGLs) in this design are made up of electrochemically doped layers such as Li-doped 1,3,5-tri (phenyl-2-benzimidazole) benzene and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), which is doped with 4,40,400-tris (N-1-naphtyl-N-phenylamino) triphenylamine (1-TNATA).

In contrast to earlier assertions, the efficiency increases noticeably rather than proportionately with each extra Emissive Unit (EMU). For instance, at 1000 cd/m2 brightness, a single EMU can reach an efficiency of 12 cd/A; two EMUs can produce values of 37.5 cd/A, and three EMUs can reach 70 cd/A. The maximum power efficiency peaks at 11 lm/W at 1000 cd/m2 brightness. Sputter damage on the uppermost device, resulting from a sputter protection layer that is not optimally optimized (a 60nm pentacene layer), is responsible for the super-linear increase in efficiency. It is interesting to note that devices with one EMU are more affected by this problem than devices with two or three EMUs. Moreover, the stacking idea offers prospects for producing mixed-color OLEDs, with each EMU providing unique spectral properties. This breakthrough makes it possible to incorporate transparent white light emitting devices into architectural windows, which will become a popular light source in the future.

When used in passive matrix mode, transparent OLED displays provide a high pixel count on small screens with little information. However, an active matrix addressing system is required to achieve high resolution on larger-area OLED displays. Because of their opacity in the visible spectrum, conventional poly-Si TFT or a-Si:H backplanes are not suitable as drivers for transparent displays. The overall transparency and filling factor of the display are decreased significantly by the close proximity of driving electronics and pixels.

Oxide semiconductor channels are used in transparent thin film transistors (TTFTs), which have been the subject of much recent research. Even though devices made of single-crystalline materials are capable of achieving channel mobilities of up to 80 cm2/Vs, these devices frequently need high-temperature post-growth processing at 1400°C or deposition temperatures above 700°C in order to reach such levels. Regrettably, these circumstances limit the range of appropriate substrates that can be used, reduce uniformity over wider areas, and make mass production more expensive. On the other hand, room temperature fabrication of devices utilizing nanocrystalline ZnO or amorphous InGaZnO is possible, even on flexible plastic substrates. For instance, the amorphous semiconductor zinc tin oxide (ZTO) can be deposited as a channel material by using RF magnetron sputtering. Based on post-accumulation heating temperatures ranging from 300°C to 600°C, Chiang and colleagues achieved field effect mobilities ranging from 5 cm2/Vs to cm2/Vs. Despite the amorphous nature of the material, the incorporation of heavy metal cations, specifically (n-1) d10 ns0 (where n is 4), has been linked to relatively high electron mobilities. It is important to remember, though, that films degrade and show reduced carrier mobilities when they are deposited in environments deficient in oxygen, as demonstrated by Minami et al. An air or oxygen environment at a high temperature is used during post-deposition annealing as a common technique to improve the defect density and background carrier concentration of films.

The assembly of a standard bottom-gate thin-film transistor (TFT) device on a glass substrate is shown in Figure 2. To make the gate electrode, transparent conducting oxides (TCOs) such as ITO or AZO are used. The gate dielectric in this particular configuration is a superlattice with a thickness of 220 nm and a capacitance of 60 nF cm^2 per square centimeter. This dielectric was deposited using atomic layer deposition, or ALD. Because of the difficulties in structuring materials like ZrO2 and TiO2 (ATO) by means of a nanolaminate technique, well-defined Al2O3 dielectric layers created by ALD are frequently preferred for the production of transparent circuits. In this configuration, ZnO-based transparent oxide (ZTO), which can be deposited by sputtering or pulsed laser deposition (PLD), is the selected channel semiconductor. PLD targets are typically made by cold pressing mixtures of ZnO and SnO2 powders with the necessary molar ratios of Zn and Sn, followed by sintered at 1100°C for 12 hours in an oxygen environment.

In the visible spectrum, the resulting Thin-Film Transistors (TFTs) showed a transmissivity of more than 80%. The active ZTO layer was created using the PA-PLD method (oxygen plasma-assisted PLD), which called for the addition of radical oxygen species from an RF oxygen plasma source. By preventing oxygen deprivation and improving material quality overall, this deposition method removes needless doping from the films. In contrast to earlier discoveries, improving the electrical characteristics is possible without post-deposition annealing in an air or oxygen environment.

4. Conclusion

Transparent displays are a reality thanks to the convergence of quickly developing technologies like OLEDs, TFTs, and transparent electronics. In the near future, this fascinating class of see-through technologies is expected to significantly alter the human-machine interface. These discoveries have implications for consumer applications as well as possible uses in the defense and medical sectors. The development of high-end transparent displays is powered by transparent driver electronics, which rely on transparent Thin-Film Transistors (TFTs). Significant advancements have been made with transparent TFTs in terms of device technology and our understanding of oxide semiconductor materials. Among the many noteworthy benefits of TFTs are their exceptional stability against bias stress and their ability to be used to create devices with lower visible light sensitivity. Numerous industries are actively pushing for the practical application of see-through screens due to their multitude of potential applications.

References