† Corresponding author. E-mail:
Flexible and transparent electronics enters into a new era of electronic technologies. Ubiquitous applications involve wearable electronics, biosensors, flexible transparent displays, radio-frequency identifications (RFIDs), etc. Zinc oxide (ZnO) and relevant materials are the most commonly used inorganic semiconductors in flexible and transparent devices, owing to their high electrical performances, together with low processing temperatures and good optical transparencies. In this paper, we review recent advances in flexible and transparent thin-film transistors (TFTs) based on ZnO and relevant materials. After a brief introduction, the main progress of the preparation of each component (substrate, electrodes, channel and dielectrics) is summarized and discussed. Then, the effect of mechanical bending on electrical performance is highlighted. Finally, we suggest the challenges and opportunities in future investigations.
The last thirteen years have witnessed the rise of flexible and transparent electronics. Since Hoffman et al. demonstrated the first fully transparent zinc oxide thin-film transistor (ZnO TFT) in 2003,[1] numerous important researches have been reported.[2–21] The typical applications involve active-matrix flexible or transparent displays, logic circuits, electronic skins, bio-sensors, and wearable devices. Owing to their mechanical flexibilities, optical transparencies, light weights, low production costs, low power consumptions and, above all, high electrical performances, these devices have drawn broad interest in both academy and industrial circles. A wide range of diverse applications of flexible and transparent TFTs based on ZnO related materials are illustrated in Fig.
![]() | Fig. 1. (color online) Versatile applications of ZnO related TFTs: (a) 6.5-in (1 in = 2.54 cm) flexible full-color display driven by indium gallium zinc oxide (IGZO) TFTs.[6] (b) Flexible transparent IGZO circuits.[7] (c) Electronic skin based on IGZO TFT.[22] (d) Smart contact lens based on IGZO TFTs.[23] (e) Biomimetic neuronal microelectronics based on IGZO TFT.[16] |
Organic and hydrogenated amorphous silicon (a-Si:H) TFTs have also been demonstrated but their applications are limited by the low mobility of the conductive channel. ZnO and relevant materials have thus emerged as promising candidates for channel materials in flexible and transparent TFTs since the advent of this subject field in 2003–2004.[1,4] Compared with other inorganic wide bandgap semiconductors, such as gallium nitride (GaN) and silicon carbide (SiC), the ZnO materials for flexible devices have a great advantage, i.e., their low synthesis temperatures, which is exactly the most important requirement in flexible device fabrication process.[24] (For details, see Section
In addition, the feasibility of modulating the electrical properties via doping or alloying with other elements offers the opportunities to adjust device performances and thus their own diverse functionalities. The most commonly used alloys are indium zinc oxide (IZO), IGZO, zinc tin oxide (ZTO), zinc indium tin oxide (ZITO), and magnesium zinc oxide (MZO). Listed in Table
![]() | Table 1.
Some of the state-of-the-art flexible transparent TFTs based on ZnO and relevant materials in the period of 2010–2016. (“–” means not mentioned or not clear in the literature). . |
Besides TFTs, the operation of a flexible transparent circuit also needs high-performance thin film diodes. However, research on flexible transparent diodes is quite limited despite the great progress made in TFTs as shown in Table
![]() | Fig. 2. (color online) Device structure and performance of flexible transparent ZnO diode. (a) Conceptualized structure diagram, which features two-terminal configuration. (b) Fabrication procedure of field-effect diode. (c) Optical transmittance spectra of devices on glass and PEN substrates with transmittance over 80% in a visible spectral range. The insets show the photographs of these two devices. (d) Current–voltage (I–V) characteristics of field-effect diode with a high rectification ratio of ![]() |
Owing to the rapid developments of science and technology in flexible transparent electronics, many wonderful products are very close to achieving commercial-productions.[111,112] In fact, IGZO panels have already been used in iPad Air and iPad Pro products, and Apple is considering IGZO panel for its new iPhone in late 2017. As for flexible transparent display and other more applications, there are still difficulties before desirable products come into use. In this regard, the present review aims at summarizing recent advances in flexible and transparent TFTs based on ZnO and relevant materials, and discussing the major challenges in device fabrication and mechanical strain effects. Finally, we propose several issues to be considered for further investigations. Since there are plenty of reviews on oxide semiconductor TFTs,[11,12,17–21,113,114] to avoid repetition, this review will specifically focus on ZnO and relevant materials and emphasize the novel device physics and technical problems which are only present in flexible and transparent ZnO field-effect transistors.
TFTs fabricated on flexible substrates are lightweight, low costed, rugged, flexible, foldable, twistable or even stretchable. However, they are also vulnerable to ambient environment. Therefore, device fabrication and characterization processes may be quite different compared with conventional case on rigid substrates, such as glass and silicon.[115,116] The most serious problem associated with flexible and transparent (polymer) substrates is the changes of their dimensions, which would bring difficulties to sequential alignment. Besides, the mismatch between substrates and films during dimension change would cause strain in the film and thus degrade the material quality, or even cracks and delamination which would cause permanent failure. This undesirable dimension change comes from the large differences in coefficient of thermal expansion (CTE), elastic modulus and toughness between the polymer substrates and functional films on them. Other issues with polymer substrates are surface roughness, chemical stability and gas permeability, which will be discussed in subSection
The properties of polymer substrates will affect material quality and carrier transportation behavior and limit maximum fabrication temperature, and thus are of great importance for the flexible transparent devices. As shown in Fig.
![]() | Fig. 3. Classification of polymer optical films. According to the servicing temperature, polymer films can be categorized into 3 types: conventional polymers (![]() ![]() ![]() |
In Table
![]() | Table 2.
Basic properties of four commonly used flexible transparent substrates. . |
The PI substrate has the highest
To reduce the surface roughness and gas permeability, and to increase the chemical resistance and adhesion to the film, a barrier layer or encapsulation is often involved onto the substrate.[69,83,124] The commonly used encapsulation materials are Al2O3,[125,126] Si3N4,[58,127] and SiO2,[27,82] which are electrical insulating and easy to grow by chemical vapor deposition with perfect coverage ratio. Encapsulations of stacked layer have also been reported.[67,69,91]
Whatever the substrate is, the device performance usually depends on channel layer, especially within 1 nm–2 nm from the interfacial layer. The electron mobility, electron concentration, density of state and interfacial charge would directly influence the field-effect mobility, on/off ratio, sub-threshold swing and turn-on voltage. In this part, we summarize the properties of ZnO channel layer that only appears in flexible transparent device.
As described in subSection
![]() | Fig. 5. (color online) Different formation mechanisms of CBM, VBM, and bandgap in silicon (a) and ZnO (c) semiconductors, as well as crystal structures and CBM/VBM wave functions in silicon (b) and ZnO (d) semiconductors.[128] |
To further extend the s orbital of ZnO and thus to achieve higher electron mobility, indium (In) and tin (Sn) are often added into ZnO, because In
As listed in Table
On the other hand, roll-to-roll (R2R), sheet-to-sheet (S2S), and roll-to-sheet (R2S) printing technologies hold great promise and offer advantages over classic microfabrication.[61,145] It allows extremely low manufacture cost, large fabrication area, fast printing speed on the order of meters per second,[146] and fine feature size of sub-10
As the name indicates, the transistor means transfer + resistor, which is essentially a variable resistor whose resistance is determined by the external electric field, which is generated by the gate voltage (
Silicon dioxide (SiO2) and silicon nitride (Si3N4) are two kinds of inorganic dielectric materials adopted in a-Si:H and poly-Si TFTs.[24] However, the high deposition temperature above 300 °C for high-quality film by industrialized plasma enhanced chemical vapor deposition (PECVD) hinders their application to flexible substrates. Instead of SiO2[39] and Si3N4,[77] high-k dielectrics are more widely used in flexible transparent ZnO TFTs, because they can be synthesized at low temperature by atomic layer deposition (ALD)[151] or solution processes. We have deposited aluminum oxide (Al2O3), a typical high-k dielectric material, on rigid silicon, quartz, flexible PEN and PI substrates by ALD at low temperatures of 150 °C, 150 °C, 100 °C and 100 °C, respectively. The capacitance–voltage (C–V) measurements of ZnO/Al2O3/ITO MIS capacitors at a frequency of 50 kHz on different substrates are shown in Fig.
Inorganic dielectric, such as Al2O3, has exhibited low-temperature fabrication convenience and excellent device performance. However, mechanical failure may occur when the film is under tensile or compressive strain as shown in Fig.
![]() | Fig. 8. FESEM image of cracks in Al2O3 film with a thickness of 200 nm on PEN after 0.6% strain.[66] |
Nevertheless, organic dielectric materials,[153] such as poly(4-vinylphenol) (PVP), poly(methyl methacrylate) (PMMA) and polystyrene (PS), can sustain larger strain[91] because the molecules in them are linked through van der Waals bond and/or hydrogen bond, and they are weakly interacting. In addition, polymer dielectrics can be formed by simple and low-cost processes, such as spin-coating and printing. The characteristics of these materials can be tuned by designing the molecular precursors and polymerization reaction conditions, which offer more application opportunities in a wide range of electronic devices. Figure
![]() | Fig. 9. Chemical structures of some typical polymeric dielectrics.[154] |
Lai et al. fabricated ultra-flexible IGZO TFT on 125-
The stable and efficient operation of flexible transparent ZnO TFTs under mechanical stress requires all of the components to work stably and reliably. Lai et al. proposed that the polymeric gate dielectrics can also reduce the stress in the IGZO channel layer.[43] The Young’s modulus for polymer material is around several GPa. However, it is more than 100 GPa for oxide based inorganic semiconductor. This large difference enables the stress to be mainly located at the polymer side, leaving the IGZO layer less stressed as shown in Fig.
Although polymeric dielectrics sustain greater strain than their inorganic counterparts and can relax the stress in the channel layer, they have some drawbacks as follows.
i) Polymers are usually soft, and thus deposition of channel layer may induce damages inside polymer layer or at the polymer/channel interface, which will significantly influence the transport behaviors of field-modulated electrons.
ii) The values of dielectric constant (k) of most polymers are relatively low (
iii) The polymeric dielectrics are more hydrophobic than inorganic materials, which is undesirable for directly growing the channel semiconductors.
Besides utilizing stacked organic/inorganic hybrid dielectric gate,[156] these problems could also be solved by introducing inorganic nanoparticles into polymer matrices to form polymer nanocomposites. Lai et al. fabricated a nanocomposite dielectrics by incorporating high-k Al2O3 nanoparticles into polymer PVP films as shown in Fig.
![]() | Fig. 11. (color online) Organic/inorganic hybrid nanocomposite dielectrics. (a) Schematic diagram of IGZO TFT with nanocomposite dielectrics. (b) Capacitance of nanocomposite dielectrics, which increases with Al2O3 concentration increasing.[68] |
The gate and source/drain electrodes in flexible transparent ZnO TFTs should possess at least three characteristics: low conductive resistivity, high optical transmittance, and good mechanical stability. Yet the most widely used flexible transparent electrodes (FTEs) are transparent conductive oxides (TCOs), represented by ITO, FTO, AZO, GZO, IZO, ZTO, and IZTO, as they have wide bandgaps, low resistivities and can be deposited at low temperatures.[161,162] However, the mechanical stability of TCO electrode is still a tough issue to be solved for achieving stable and reliable device operation.[163–166]
Leterrier et al. investigated the effect of ITO thickness on the crack onset strain (COS), the critical strain at failure of the film.[163] The evolution of film mechanical failure under uniaxial strain was recorded as shown in Fig.
![]() | Fig. 12. Progressive cracking of a 100-nm thick ITO film on 100-![]() |
To seek for flexible transparent electrodes which can sustain higher mechanical strains, oxide–metal–oxide (OMO) stacked structure (Fig.
![]() | Fig. 13. (color online) OMO stacked electrode. (a) Schematic diagram of a typical OMO stacked electrode. (b) Sheet resistance, transmittance at 550 nm. (c) FOM of an IZO/Ag/IZO multilayer anodes on PET substrates as a function of the Ag thickness. (d) Normalized resistance change after repeatedly bending as a function of the number of cycles for IZO/Ag/IZO/PET and amorphous ITO/PET sample.[171] |
The random meshed Ag nanowire (Ag NW)[178–182] electrode is another promising candidate for flexible transparent ZnO TFTs, as Ag forms ohmic contact with ZnO.[104] The unique advantage of Ag nanowire electrodes is their mechanical robustness, because nano-materials can be bent to much smaller radii than conventional “3D” materials.[183] Figure
Operation of flexible transparent ZnO TFTs often involves the mechanical deforming of substrate, so the understanding of the evolution of device performance under stress is of fundamental importance for conducting the research in this area.[184,185] In fact, the requirements for flexibilities of devices are quite different when they are used in different areas. In the case of flexible display, the device may need a large strain tolerance when it was rolled up like a scroll.[6] In skin sensors and wearable electronics, devices may go through small but repeated strains.[22] Despite the differences, the mechanical processes all follow the same basic principles. So in this section, we will review the mechanical fundamentals on flexible transparent ZnO TFTs and focus on the test technique, strain calculation and characterization methods. Finally, the present reports on flexibility test of ZnO TFTs will be briefly depicted.
The bending test systems reported in the literature are all laboratory-made and can be roughly classified as two types according to how devices are bent: 1) substrate wrapped around a rigid rod (Fig.
In a simplified case (no Poisson ratio nor fabrication-induced strain), the strain (ε) within the film on bending substrate can be roughly obtained through Eq. (
![]() |
![]() |
![](cpb_26_4_047307/cpb_26_4_047307_ieqn50.gif)
![](cpb_26_4_047307/cpb_26_4_047307_ieqn51.gif)
For the case in which neither the premise nor the assumption holds, one could go to Eq. (
![]() |
![](cpb_26_4_047307/cpb_26_4_047307_ieqn52.gif)
![](cpb_26_4_047307/cpb_26_4_047307_ieqn53.gif)
The relationships between ε and
![]() | Fig. 16. (color online) Visualizations of relationships (a) between R and (![]() |
For a given film (fixed
![]() |
![](cpb_26_4_047307/cpb_26_4_047307_ieqn65.gif)
![](cpb_26_4_047307/cpb_26_4_047307_ieqn66.gif)
![](cpb_26_4_047307/cpb_26_4_047307_ieqn67.gif)
In practical applications, the flexible transparent ZnO TFTs might be bent into various forms, and thus the functional films would sustain various kinds of strains, such as tensile, compressive and twisting. Besides conventional electrical field, the mechanical stress field is also an important issue that must be included in the analysis of flexible electronics. For flexible IGZO TFT, the tensile strains parallel (Fig.
![]() | Fig. 17. (color online) Strain directions with respect to the channel direction. Schematic of tensile strains (a) parallel and (b) perpendicular to the channel direction in flexible TFT. (c) Normalized IGZO saturation mobility changes induced by mechanical strains parallel or perpendicular to channel.[42] |
Bending the TFTs outwards causes a tensile strain, while bending the TFTs inwards causes a compressive strain. Compared with the tensile strain, the compressive strain has a relatively small effect on the electrical performance.[192–194] The influence of strain on the electron transport mobility can be described as follows:
![]() |
![](cpb_26_4_047307/cpb_26_4_047307_ieqn73.gif)
![]() | Fig. 18. (color online) Strain types under inward or outward bending. Schematic description of (a) tensile and (b) compressive strains in flexible TFT. (c) Normalized IGZO TFT linear mobility varying with compressive and tensile strains.[194] |
The maximum operation strain of flexible transparent ZnO TFT depends on the mechanical property of each of the functional layers. Devices with high robustness that can sustain high intense and repeated bending are always desirable. In Table
![]() | Table 3.
Results of bending test of flexible transparent ZnO TFTs in the period from 2010 to 2016. . |
The flexible and transparent electronics has received particular attention in electronic material, device and circuit areas, especially in the last thirteen years. Mechanical deformation capability, light weight, low cost and other unique advantages make it better than conventional electronic circuits which are based on rigid substrates, like silicon and glass. The inorganic flexible transparent electronic devices and products are mainly based on ZnO and relevant materials, owing to their high electrical properties, good optical transparency and low-synthesis temperature. On the contrary, the requirement for uniform material growth in large area at low-temperature rules out other inorganic semiconductors, such as Si, GaN, and SiC, which needs elevated temperatures for good material quality. In this paper, we give a brief introduction of recent advances in flexible transparent TFTs based on ZnO and relevant materials, and discuss several important issues of device physics and fabrication technology relating to the substrate, electrodes, channel and dielectric layer in Section
The advent of flexible transparent electronics has also boosted the development of solution process techniques, especially roll-to-roll printing and other printing technics.[19,21,115,162,181] The inks used in printing techniques will need to dissolve the zinc precursors into solvents, such as dissolving zinc hydroxide (Zn(OH)2) into aqueous ammonia (NH4OH),[37,49] zinc acetate dihydrate [Zn(CH3COO)2] into 2-methoxyethanol (CH3OCH2CH2OH)[28,46,93,95,197–200] and zinc chloride (ZnCl2) into ethylene glycol (C2H6O2).[62] Thus, a high-temperature (
Despite the problems that are to be solved, due to the rapid development, we have plenty of reasons to keep optimistic for the coming era of flexible transparent technology.
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