Review of flexible and transparent thin-film transistors based on zinc oxide and related materials
Zhang Yong-Hui1, Mei Zeng-Xia1, †, Liang Hui-Li1, Du Xiao-Long1, 2, ‡
Key Laboratory for Renewable Energy, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China

 

† Corresponding author. E-mail: zxmei@iphy.ac.cn xldu@iphy.ac.cn

Abstract
Abstract

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.

1. Introduction

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.[221] 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. 1.

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 2.) The biocompatibilities of ZnO materials also make them perfectly desirable for medical and health-care applications as shown in Figs. 1(c)1(e).

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 1 are some of the state-of-the-art flexible transparent TFTs based on versatile ZnO and relevant materials in the past 6 years.

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 1. The limited reports can be categorized into 5 types as follows. (i) The pn heterojunction diode.[97102] As most of the wide-bandgap semiconductors are n-type conductive, a proper p-type wide-bandgap material must be chosen wisely to form a large built-in potential barrier. (ii) Schottky junction diode.[103106] The values of electron affinity (χ) of most wide-bandgap materials are all more than 4 eV, thus only a small Schottky barrier height (SBH) could be formed with non-noble metals. (iii) Metal-insulator-semiconductor (MIS) diode.[107] As in Schottky diode, a large difference between metal work function and semiconductor affinity (χ) is necessary to achieve a large rectification ratio. (iv) Metal–insulator–metal (MIM) diode.[108,109] Actually, it is not easy for MIM diodes to be applied to transparent circuits because of the difficulties in finding two kinds of transparent electrodes with large work function difference. (v) Self-switching diode (SSD).[110] Besides small rectification ratio, this kind of device involves nanofabrication process, which may bring high costs and challenges in technological compatibility. Zhang et al. recently demonstrated high-performance flexible fully transparent ZnO diodes with a high rectification ratio of 108 by using a diode-connected TFT architecture as shown in Fig. 2(a).[88] The device fabrication procedure is the same as that for standard TFTs (Fig. 2(b)). Both of the devices on polyethylene naphthalate (PEN) and quartz substrates are optically transparent (see the inset in Fig. 2(c)), with the whole devices (including the substrates) exhibiting a transmittance over 80% in full visible spectral range (Fig. 2(c)). Most importantly, the devices exhibit a high rectification ratio of (Fig. 2(d)) under flat and bent states (see the inset in Fig. 2(d)), which is 3–4 orders larger than those of conventional junction diodes. This work has broadened the application scope of flexible transparent TFTs and provided a solution to flexible fully transparent diodes which may be used for reference to flexible transparent TFTs based on other materials.

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 (IV) characteristics of field-effect diode with a high rectification ratio of while flat and bent. Insets show the photographs of device under test.[88]

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,[11,12,1721,113,114] 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.

2. Device fabrication

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 2.1. Afterwards, the device physics and technical process in preparing channel, dielectric and electrodes layers will be discussed in subSection 2.2, 2.3, and 2.4, respectively.

2.1. Flexible substrates

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. 3, according to the servicing temperature or glass transition temperature ( ), the polymer substrates can be categorized into three types: conventional polymers ( ), common high-temperature polymers ( ) and high-temperature polymers ( ).[117] The most commonly used polymer substrates in the literature include polyimide (PI), polyarylate (PAR), polyethylene terephthalate (PET), PEN, polyethersulfone (PES), polycarbonate (PC), polyetheretherketone (PEEK), polydimethylsiloxane (PDMS), etc.

Fig. 3. Classification of polymer optical films. According to the servicing temperature, polymer films can be categorized into 3 types: conventional polymers ( ), common high-temperature polymers ( ) and high-temperature polymers ( C).[117]

In Table 2 listed are the basic properties of PI, PEN, and PET for each of the three kinds of polymer substrates which are currently widely used.

Table 2.

Basic properties of four commonly used flexible transparent substrates.

.

The PI substrate has the highest , smallest CTE and surface roughness among all of the flexible substrates. In addition, it also shows good chemical stability in acid, alkali and organic solvents. Although the cut-off wavelength ( is in the visible range, which means that the PI substrates have deep color and poor optical transmittance as can be seen in Fig. 4, the colorless and optically transparent polyimide (CPI) films have been developed recently[117] and used in IGZO TFT fabrication.[118] In the case that the cost is not a problem, the CPI substrates will be the best choice for flexible transparent electronics. The PET substrate has excellent optical transparency and short (Fig. 4), and is currently widely used as the protection layers in various liquid crystal displayers (LCDs), such as televisions, computers and cellphones. The major disadvantage of PET for its use in flexible transparent TFTs is its relatively low servicing temperature ( ), which may bring challenges to device fabrication, integration and operation. The PEN substrate has higher servicing temperature ( ) than PET, but, as a compromise, it appears slightly white color as can be seen in Fig. 4. The PDMS substrate was recently used in flexible TFTs.[27,61,81,92,119] Besides the high optical transmittance and short , PMDS also features small elastic modulus ( MPa), which makes PDMS the perfect substrate for the emerging stretchable electronics.[17,27,61,92,120122] It is worth noting that ultra-thin (25 m–100 m) flexible glass, such as Willow by Corning company,[123] is also regarded as a promising flexible substrate for flexible transparent electronics with considering its higher servicing temperature ( ), good resistance to scratch and higher optical transmittance. However, up to now, few researches on flexible glass have been reported in the literature.[62,74]

Fig. 4. (color online) Optical transmittance spectra of four commonly used flexible transparent substrates (PI, PET, PEN & PDMS) in comparison with rigid quartz. Insets shows the actual photographs of the substrates

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]

2.2. Channel layers

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 2.1, limited by the utilized flexible substrate, the processing temperature cannot exceed , which could be as low as 80 °C. At such a low synthesis temperature, the materials usually appear to be polycrystalline or amorphous. In conventional semiconductors, such as silicon whose conduction band minimum (CBM) and valence band maximum (VBM) are composed of anti-bonding ( ) and bonding ( ) states of Si sp3 orbitals and whose band gap is formed by splitting the σ energy levels (Fig. 5(a)), the carrier transport properties depend critically on the chemical bonding direction (Fig. 5(b)). This is the reason for the degradation of electron mobility in amorphous silicon ( compared with crystalline silicon ( . In contrast to silicon, ZnO has very strong ionicity and electrons transfer from zinc to oxygen atoms. The electronic structure is formed by rasing the electronic level in zinc and lowering the electronic level in oxygen through the Madelung potential as shown in Fig. 5(c). As a result, the CBM in ZnO is primarily contributed to by the unoccupied spherically symmetric Zn 4s orbitals and the VBM is primarily determioned by the fully occupied axial symmetry O 2p orbitals.[4,9,128] Owing to the s orbitals being contributed to CBM, the electron transport is not affected significantly by the chemical bond direction and crystal structure randomness. That is the reason why high electron mobility occurs in ZnO and relevant materials in their polycrystalline and even amorphous states, and also why ZnO and relevant materials are suitable for low temperature process, such as flexible transparent electronics on polymer substrates.

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 and Sn have more broad-spreading 5s orbitals than Zn 4s. Ternary alloys such as IZO[52,129] and ZTO[130,131] do show higher mobility than pure ZnO. However, due to the scarcity and toxicity, In is not suitable for the large demands in commercial applications as shown in Figs. 1(a) and 1(b) and the safety use in the healthcare or medical areas as shown in Figs. 1(c) and 1(d). Sn has been regarded as a more promising candidate for increasing the electron mobility in ZnO. At low process temperature, lots of point defects which may act as scattering centers and electron traps within the channel layer, are unavoidably induced into ZnO.[132134] To suppress the concentration of VO, cations, which has a stronger bonding with oxygen, such as gallium (Ga), Sn, aluminum (Al), magnesium (Mg), hafnium (Hf), zirconium (Zr), and Si have been incorporated into ZnO. Quarternary alloys, for example, InGaZnO,[2,4,135] InSnZnO,[96,136] MgSnZnO,[137] AlSnZnO,[138,139] HfInZnO,[140,141] ZrInZnO,[142] and SiInZnO,[143,144] have been reported to have improved their performances and stabilities.

As listed in Table 1, the most commonly used synthesis technique for growing ZnO channel layer on polymers is still sputtering because its simplicity and effectiveness. Another widely used technique is solution processing techniques, including spin-coating, ink-jet printing, chemical bath deposition, etc.[19,95,115] Although these synthesis technics are important and widely used, there is not much difference in the device fabrication technique between rigid and flexible substrates, and they have been described in detail in many reviews.[11,12,18,19,21,113,114] Therefore, they will not be discussed here.

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 m.[147] The above-mentioned unique advantages make it a perfect candidate for applications in cheap, portable, large-volume and disposable systems, such as radio-frequency identification (RFID) tags, flexible displays, and food packaging.[148150] Figure 6 shows the schemes of an R2R (Fig. 6(a)) and R2S (Fig. 6(b)) gravure printing system, which describes a typical flexible device fabrication procedure. In each of the R2R gravure printing units, one particular material based ink is selected to print the functional layers, i.e., buffer layers, electrodes, active layers, insulators, passivation, logos, etc. The cells are filled with inks from an ink reservoir and wiped using a doctor blade, afterthat, the inks are transferred from the roll to the flexible substrate. Patterns are defined by recessed cells that are engraved into a roll. After the transfer process, the individual aliquots of inks spread and dry to form the final pattern. After going through each gravure printing unit, multi-functional layers are formed, patterned and aligned to each other as shown in Fig. 6(c).

Fig. 6. (color online) Descriptive scheme for R2R/R2S gravure printing procedure. (a) R2R gravure printing system used to print RF sensor tags and smart packaging on a single line,[145] (b) overview of a gravure printing process, and (c) optical micrograph of a printed TFT.[147]
2.3. Dielectric layers

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 ( within a metal–insulator–semiconductor (MIS) capacitor. Therefore, the quality of the bulk dielectric and interface of semiconductor/dielectrics is of the vital importance. Currently, there are three types of dielectric materials, that is, inorganic dielectric, organic dielectric, and organic dielectric/inorganic hybrid dielectric dielectric, which are commonly used in the flexible ZnO TFTs in the literature.

2.3.1. Inorganic dielectrics

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 (CV) measurements of ZnO/Al2O3/ITO MIS capacitors at a frequency of 50 kHz on different substrates are shown in Fig. 7. The Al2O3 insulators show comparable insulating performances on rigid quartz substrate and flexible PEN, PI substrates. The 50-nm-thick Al2O3 on PEN exhibits a capacitance density of and dielectric constant of 8.47, which is basically equivalent to the dielectric property of the Al2O3 film fabricated on quartz.

Fig. 7. (color online) Capacitance–voltage (CV) measurements of Al2O3 dielectrics deposited at low temperatures on (a) p++-silicon, (b) quartz glass, (c) PEN, and (d) PI substrates.
2.3.2. Organic dielectrics

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. 8. Jen et al. reported that a 80-nm-thick ALD-grown Al2O3 film could only sustain a strain level of 0.52%.[152] Xu et al. claimed the critical strain, a critical point at which the film becomes useless, for 200-nm-thick anodized Al2O3 film on PEN is 0.6%.[66] If the thickness of the flexible substrate is 125 m, the critical bending radius is approximately 10 mm (See Section 3 for more details of mechanical bending), which is not suitable for some flexible, foldable or stretchable applications.

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 9 shows the chemical structures of typical polymeric gate dielectrics.

Fig. 9. Chemical structures of some typical polymeric dielectrics.[154]

Lai et al. fabricated ultra-flexible IGZO TFT on 125- m-thick PET substrate with PVP used as polymeric gate dielectrics, on which device performance shows no degradation upon bending to strain %.[43] Kim et al. also reported flexible IGZO TFT on PET substrate with PMMA as gate dielectrics.[155] However, they did not perform the bending test evaluation. Up to now, the reports of flexible transparent ZnO TFTs with pristine polymeric gate dielectrics have been still quite limited.

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. 10.

Fig. 10. (color online) The schematic descriptions of the stacked ZnO/polymeric dielectrics/polymer structure in flat and bent states, respectively. In the bent state, the stress is mainly located within the polymeric dielectrics layer.
2.3.3. Organic/inorganic hybrid dielectrics

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 ( –2.6 for PVP), which would exhibit smaller capacitance at a given thickness than inorganic dielectric materials.

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. 11.[68] Al2O3 nanoparticles (size nm) was added into PVP/poly(melamine-co-formaldehyde) precursor at a concentration in a range from 0.25 wt% to 1.00 wt%. After being stirred overnight, the solution was spin-coated on silver (Ag) gate. The nanocomposite dielectric was then post-treated with hot plate and ultraviolet illumination. The capacitance of the pristine PVP was 14 nF/cm2 corresponding to a dielectric constant of 3.9. After adding 0.25, 0.5-wt% and 1-wt% Al2O3 nanoparticles, the capacitance increased to 19, 20, and 22 nF/cm2, and the corresponding dielectric constants increased to 6.1, 7.1, and 8.1. After that, the lead oxide (PbO), which is a high-k material with ,[157] was introduced into PVP polymer, and the dielectric constant increased to 21.2.[158] Along with the improved capacitance, the TFT with nanocomposite dielectrics could sustain the same strain as the device with pristine PVP dielectrics.[43,68] This means that the nanocomposite dielectric inherits the merits from both inorganic high-k material and organic polymer material. In addition, the incorporation of high-k nanoparticles into polymeric dielectrics was believed to improve the robustness against the plasma damage in the following sputtering process.[68] In general, organic materials are more hydrophobic than inorganic materials, which is undesirable for the direct growing inorganic semiconductor materials on the polymeric dielectrics.[159,160] However, few researchers havefocused the effects of incorporated nanoparticles on the surface energy of nanocomposite dielectric.

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]
2.4. Electrode layers

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.[163166]

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. 12 as growing ITO film has some microdefects in the form of pin-holes (Fig. 12(a)). Upon reaching a strain of 1.28%, small cracks originating from the microdefects appeared (Fig. 12(b)). Further increasing the strain to 1.42%, the initial crack propagated from the sites to finite size (Fig. 12(c)) and the resistivity began to increase as shown in the onset region in Fig. 12(e). At higher strain levels, the finite cracks increased and the width spanned to the whole sample (Fig. 12(d)). As a result, the resistivity increased markedly as can be seen in Fig. 12(e). They found that the COS decreased with ITO thickness, which means that for a thicker ITO film the safe operating range could be even smaller.

Fig. 12. Progressive cracking of a 100-nm thick ITO film on 100- m thick polyester substrate during tensile loading (along the horizontal direction). Unloaded ITO (a); at 1.28% strain (b), the arrow indicates the failure initiation on a coating defect); at 1.42% strain (c); and at 3.42% strain (d). Density of tensile cracks and normalized resistance change during tensile loading of 50 nm (solid symbols) and 100 nm (empty symbols) thick ITO (e).[163]

To seek for flexible transparent electrodes which can sustain higher mechanical strains, oxide–metal–oxide (OMO) stacked structure (Fig. 13(a)) has been proposed. The most commonly used metal materials are silver (Ag) and copper (Cu) because of their good ductilities and high conductivities. What is more, by optimizing the metal layer thickness, the figure of merit (FOM), which can comprehensively characterize the property of transparent electrode, can be improved.[167] By insetting a thin Ag layer into the middle of double 30-nm-thick IZO layers,[168171] the IZO/Ag/IZO stacked electrode showed improved optical transmittance and conductive resistivity (Fig. 13(b)).[171] Maximum improvement of FOM was obtained with a 12-nm-thick Ag layer (Fig. 13(c)). More importantly, the mechanical robustness was improved compared with a single ITO layer (Fig. 13(d)). Other reported stacked electrodes with improved electrical conductivity, optical transmittance and mechanical robustness involve ITO/Ag/ITO,[172,173] IZTO/Ag/IZTO,[174] GZO/Ag/GZO,[175] IZO/Ag/IZO, ZTO/Ag/ZTO,[176,177] IZO/Ag/IZO/Ag,[170] and ITO/Cu/ITO.[173]

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)[178182] 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 14(a) shows the SEM image of random meshed Ag NWs on a flexible substrate[182] and Figure 14(b) shows the resistance change as a function of mechanical strains.[179] Compared with the small COS of ITO shown in Fig. 12, the Ag NW flexible transparent electrode processes much better mechanical robustness, with only 3.9 times increase of resistance under 15% tensile strain and almost no change under 15% compressive strain.

Fig. 14. Random meshed Ag NW electrode. (a) SEM image of Ag NWs on flexible transparent substrate.[182] (b) Surface resistance ratio ( ) of the Ag NW/polymer electrode comparing tensile and compressive strains.[179]
3. Bending effect on flexible TFT

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.

3.1. Bending test systems

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. 15(a)) and 2) arched up under side extrusion (Fig. 15(b)). In type 1, the probe tips contact well with the electrodes. However, the variation of bending radius is not convenient in this setup. On the contrary, in type 2, the bending radius can be tuned by varying the distance between the two splints. But the probe tips might contact poorly with the electrodes especially when the substrate is thin or soft.[88] Thus, attaching flexible polymer substrate onto a flexible metal sheet might be a good choice to combine good contact and flexibility.

Fig. 15. Two types of bending test systems. (a) Substrate wrapped around a rigid rod and (b) arched up under side extrusion.
3.2. Strain in film

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. (1), under the premise that the substrate is much thicker than the film, or Eq. (2), under the assumption of neglecting the difference in Young’s modulus between substrate ( and film ( . Both the two mechanical models simplified the calculation process and were suited well for many other cases.[43,68,186]

where is the thickness of substrate, the thickness of film, and R the bending radius.

For the case in which neither the premise nor the assumption holds, one could go to Eq. (3)[187189]

where and .

The relationships between ε and , R, η, χ in Eq. (1) and Eq. (3), are visualized in Figs. 16(a) and 16(b) (setting and mm). As can be seen in Fig. 16(a), at a fixed radius, the strain can be reduced by utilizing a thin substrate. At a fixed , the strain increases markedly with radius decreasing, and the critical failure radius increases with increasing. Taking and the difference between and into account, the strain could become smaller with η and χ increasing as can be seen in Fig. 16(b).

Fig. 16. (color online) Visualizations of relationships (a) between R and ( ) and (b) between χ and η.

For a given film (fixed and , besides using thinner and more elastic substrates described above, there are other ways to reduce the strains in functional films. By encapsulating the film and forming an encapsulation/film/substrate sandwiched structure, the strains in film can be further reduced.[189] If the configuration meets

where and are the Young’s modulus and thickness of the encapsulation layer, respectively, the film without any strain is right in the neutral surface. Consequently, the functional film will not fail to work until the substrate and encapsulation are ineffective. In this approach, Sekitani et al. fabricated ultra-flexible organic TFT with a sandwiched poly-chloro-paraxylylene/pentacene-TFT/PI structure[190] and Kinkeldei et al. reported sandwiched PI/IGZO TFT/PI structure with a small bending radius of 125 m.[40] Park et al. proposed that inserting a buffer layer between film and substrate can also reduce the film strain.[191]

3.3. Bending direction

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. 17(a)) and perpendicular (Fig. 17(b)) to the current flow have different effects on electrical performance.[42] As shown in Fig. 17(c), saturation mobility ( is slightly affected by the strain parallel to the channel up to %, while begins to be degraded when under a strain perpendicular to channel. The degradations of both on- and off- currents of flexible TFT under the perpendicular strain are explained as being due to the crack of brittle chromium (Cr) gate. The cracks disconnect the parts of gate, leaving some of the channel areas uncontrolled, thus making the off-current increased and the on-current reduced. Whereas, no crack occurs under the parallel strain when . Once ε exceeds 0.72%, the cracks become unstable and destroy the device permanently.

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.[192194] The influence of strain on the electron transport mobility can be described as follows:

where μ0 and are the charge carrier mobilities respectively in flat and bent conditions, and m is an empirical proportionality constant which depends on the channel material.[195] For IGZO, Petti et al.[72] and Münzenrieder et al.[194] found that the tensile and compressive strains could induce TFT parameters shifting towards opposite directions as shown in Fig. 18. Under a tensile strain, the field mobility and subthreshold swing are increased, and the threshold voltage is reduced; while under a compressive strain, the field mobility and subthreshold swing are reduced, and the threshold voltage is increased. The dependence of electrical performance on the strain types can be explained with the electrical structure change associated with crystal structure deformation.[85,196]

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]
3.4. Reports on strain test

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 3 summarized is the state-of-art results about bending test of flexible transparent ZnO TFTs. Most of the polymer substrates listed in Table 3 are PI, PET, and PEN with thickness values ranging from 1 to several hundred microns. For either wrapped around rigid rods or arched up with two parallel plates, majority of the devices are bent with tensile strain parallel to the current flow (channel length direction). However, compressive strain or strain perpendicular to current flow should also been considered since they could have different influences on the flexible ZnO TFT performance.[42,194] Generally, smaller bending radii are desirable, because it means more flexibilities and serviceabilities. Bending radius can vary in a large range from 25 m to 10 cm depending mainly on the thickness of the substrate. Strain is in inverse proportion to bending radius as shown in Eqs. (1)–(3), and is the normalized parameter to describe the mechanical state in functional layer whatever the substrate thickness is. Fatigue test concerns the stability and reliability of flexible device. Typically, the fatigue tests conduct 102 to 106 times with respect to the stressing times.

Table 3.

Results of bending test of flexible transparent ZnO TFTs in the period from 2010 to 2016.

.
4. Conclusions and perspectives

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 2. The operation and evaluation of strain test are highlighted in Section 3.

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)[28,46,93,95,197200] and zinc chloride (ZnCl2) into ethylene glycol (C2H6O2).[62] Thus, a high-temperature ( ) post-annealing is essential to fully decompose the organic components and produce a pure-phase metal oxide from the solution phase, which is not compatible with most of the flexible substrates.[49] To solve this problem, some low-temperature post-treatment is conducted on solution processed ZnO TFTs, including microwave annealing[37,197199,201] and ultraviolet photo annealing.[46,202,203] However, most of the researches are based on rigid silicon or glass substrates, very limited reports on flexible substrates.[63] So, we suggest that low-temperature post-treatment can be a promising direction for solution processing, especially printing, techniques. The flexibility also involves mechanical stability issue, which never presents in traditional rigid devices. The strain in the film will induce mechanical stress field which can change the crystal, and thus electrical, structure and even crack the oxide films.[204] Critical parameters such as energy band, mobility, dielectricity, and thermal conductivity can also be affected. Thus, coupling of multi-physics involving mechanical stress field needs to be rigorously studied. Finally, the commercialized application still requires the long-term stable, repeatable, reliable, large-area uniform, and t low cost techniques.

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.

Reference
[1] Hoffman R L Norris B J Wager J F 2003 Appl. Phys. Lett. 82 733
[2] Nomura K Ohta H Ueda K Kamiya T Hirano M Hosono H 2003 Science 300 1269
[3] Wager J F 2003 Science 300 1245
[4] Nomura K Ohta H Takagi A Kamiya T Hirano M Hosono H 2004 Nature 432 488
[5] Carcia P F McLean R S Reilly M H Nunes G Jr 2003 Appl. Phys. Lett. 82 1117
[6] Park J S Kim T W Stryakhilev D Lee J S An S G Pyo Y S Lee D B Mo Y G Jin D U Chung H K 2009 Appl. Phys. Lett. 95 13503
[7] Tripathi A K Smits E C P van der Putten J B P H van Neer M Myny K Nag M Steudel S Vicca P O’Neill K van Veenendaal E Genoe J Heremans P Gelinck G H 2011 Appl. Phys. Lett. 98 162102
[8] Reyes P I Ku C J Duan Z Lu Y Solanki A Lee K B 2011 Appl. Phys. Lett. 98 173702
[9] Kamiya T Hosono H 2010 Npg Asia Mater. 2 15
[10] Dagdeviren C Hwang S W Su Y Kim S Cheng H Gur O Haney R Omenetto F G Huang Y Rogers J A 2013 Small 9 3398
[11] Fortunato E Barquinha P Martins R 2012 Adv. Mater. 24 2945
[12] Petti L Münzenrieder N Vogt C Faber H Büthe L Cantarella G Bottacchi F Anthopoulos T D Tröster G 2016 Appl. Phys. Rev. 3 21303
[13] Cherenack K Zysset C Kinkeldei T Münzenrieder N Tröster G 2010 Adv. Mater. 22 5178
[14] Lee S Jeon S Chaji R Nathan A 2015 Proc. IEEE 103 644
[15] Makarov D Melzer M Karnaushenko D Schmidt O G 2016 Appl. Phys. Rev. 3 11101
[16] Karnaushenko D Münzenrieder N Karnaushenko D D Koch B Meyer A K Baunack S Petti L Tröster G Makarov D Schmidt O G 2015 Adv Mater. 27 6797
[17] Münzenrieder N Cantarella G Vogt C Petti L Büthe L Salvatore G A Fang Y Andri R Lam Y Libanori R Widner D Studart A R Tröster G 2015 Adv. Electron. Mater. 1 1400038
[18] Kwon J Y Lee D J Kim K B 2011 Electron. Mater. Lett. 7 1
[19] Ahn B D Jeon H J Sheng J Park J Park J S 2015 Semicond. Sci. Technol. 30 64001
[20] Mativenga M Geng D Kim B Jang J 2015 ACS Appl. Mater. Interfaces 7 1578
[21] Kim S J Yoon S Kim H J 2014 Jpn. J. Appl. Phys. 53 02BA02
[22] Romeo A Lacour S P 2015 37th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC) August 25–29 2015 Milano, Italy
[23] Salvatore G A Münzenrieder N Kinkeldei T Petti L Zysset C Strebel I Büthe L Tröster G 2014 Nat. Commun. 5 2982
[24] Kagan C R Andry P 2003 Thin-Film Transistors 2 New York CRC Press
[25] Fleischhaker F Wloka V Hennig I 2010 J. Mater. Chem. 20 6622
[26] HCherenack K Munzenrieder N S Troster G 2010 IEEE Electron Dev. Lett. 31 1254
[27] Park K Lee D K Kim B S Jeon H Lee N E Whang D Lee H J Kim Y J Ahn J H 2010 Adv. Funct. Mater. 20 3577
[28] Lee C Y Lin M Y Wu W H Wang J Y Chou Y Su W F Chen Y F Lin C F 2010 Semicond. Sci. Technol. 25 105008
[29] Song K Noh J Jun T Jung Y Kang H Y Moon J 2010 Adv. Mater. 22 4308
[30] Zhao D Mourey D A Jackson T N 2010 IEEE Electron Dev. Lett. 31 323
[31] Kim D H Cho N G Kim H G Kim I D 2010 Electrochem. Solid State Lett. 13 H370
[32] Nomura K Aoki T Nakamura K Kamiya T Nakanishi T Hasegawa T Kimura M Kawase T Hirano M Hosono H 2010 Appl. Phys. Lett. 96 263509
[33] Su N C Wang S J Huang C C Chen Y H Huang H Y Chiang C K Chin A 2010 IEEE Electron Dev. Lett. 31 680
[34] Liu J Buchholz D B Hennek J W Chang R P H Facchetti A Marks T J 2010 J. Am. Chem. Soc. 132 11934
[35] Liu J Buchholz D B Chang R P H Facchetti A Marks T J 2010 Adv. Mater. 22 2333
[36] Cheong W S Bak J Y Kim H S 2010 Jpn. J. Appl. Phys. 49 05EB10
[37] Jun T Song K Jeong Y Woo K Kim D Bae C Moon J 2011 J. Mater. Chem. 21 1102
[38] Jung Y Jun T Kim A Song K Yeo T H Moon J 2011 J. Mater. Chem. 21 11879
[39] Mativenga M Choi M H Choi J W Jang J 2011 IEEE Electron Dev. Lett. 32 170
[40] Kinkeldei T Munzenrieder N Zysset C Cherenack K Tröster G 2011 IEEE Electron Dev. Lett. 32 1743
[41] Marrs M A Moyer C D Bawolek E J Cordova R J Trujillo J Raupp G B Vogt B D 2011 IEEE Trans. Electron Dev. 58 3428
[42] Munzenrieder N Zysset C Kinkeldei T Troster G 2012 IEEE Trans. Electron Dev. 59 2153
[43] Lai H C Tzeng B J Pei Z Chen C M Huang C J 2012 SID Symp. Dig. Tech. Pap. 43 764
[44] Erb R M Cherenack K H Stahel R E Libanori R Kinkeldei T Münzenrieder N Tröster G Studart A R 2012 ACS Appl. Mater. Interfaces 4 2860
[45] Kim D I Hwang B U Park J S Jeon H S Bae B S Lee H J Lee N E 2012 Org. Electron. 13 2401
[46] Kim Y H Heo J S Kim T H Park S Yoon M H Kim J Oh M S Yi G R Noh Y Y Park S K 2012 Nature 489 128
[47] Ji L W Wu C Z Fang T H Hsiao Y J Meen T H Water W Chiu Z W Lam K T 2013 IEEE Sens. J. 13 4940
[48] Kim S H Yoon J Yun S O Hwang Y Jang H S Ko H C 2013 Adv. Funct. Mater. 23 1375
[49] Hong K Kim S H Lee K H Frisbie C D 2013 Adv. Mater. 25 3413
[50] Lin Y H Faber H Zhao K Wang Q Amassian A McLachlan M Anthopoulos T D 2013 Adv. Mater. 25 4340
[51] Yang W Song K Jung Y Jeong S Moon J 2013 J. Mater. Chem. 1 4275
[52] Zhou J Wu G Guo L Zhu L Wan Q 2013 IEEE Electron Dev. Lett. 34 888
[53] Seo J S Jeon J H Hwang Y H Park H Ryu M Park S H K Bae B S 2013 Sci. Rep. 3 2085
[54] Hyung G W Park J Wang J X Lee H W Li Z H Koo J R Kwon S J Cho E S Kim W Y Kim Y K 2013 Jpn. J. Appl. Phys. 52 71102
[55] Hsu H H Chang C Y Cheng C H 2013 Phys. Status Solidi-Rapid Res. Lett. 7 285
[56] Hsu H H Chang C Y Cheng C H Yu S H Su C Y Su C Y 2013 Solid-State Electron. 89 194
[57] Hsu H H Chang C Y Cheng C H 2013 IEEE Electron Dev. Lett. 34 768
[58] Münzenrieder N Petti L Zysset C Kinkeldei T Salvatore G A Tröster G 2013 IEEE Trans. Electron Dev. 60 2815
[59] Münzenrieder N Zysset C Petti L Kinkeldei T Salvatore G A Tröster G 2013 Solid-State Electron. 84 198
[60] Zysset C Münzenrieder N Petti L Büthe L Salvatore G A Tröster G 2013 IEEE Electron Dev. Lett. 34 1394
[61] Sharma B K Jang B Lee J E Bae S H Kim T W Lee H J Kim J H Ahn J H 2013 Adv. Funct. Mater. 23 2024
[62] Dai M K Lian J T Lin T Y Chen Y F 2013 J. Mater. Chem. 1 5064
[63] Park S Cho K Yang K Kim S 2014 J. Vac. Sci. Technol. 32 62203
[64] Wee D Yoo S Kang Y H Kim Y H Ka J W Cho S Y Lee C Ryu J Yi M H Jang K S 2014 J. Mater. Chem. 2 6395
[65] Chen H Cao Y Zhang J Zhou C 2014 Nat. Commun. 5 4097
[66] Xu H Pang J Xu M Li M Guo Y Chen Z Wang L Zou J Tao H Wang L Peng J 2014 ECS J. Solid State Sci. Technol. 3 Q3035
[67] Xu H Luo D Li M Xu M Zou J Tao H Lan L Wang L Peng J Cao Y 2014 J. Mater. Chem. 2 1255
[68] Lai H C Pei Z Jian J R Tzeng B J 2014 Appl. Phys. Lett. 105 33510
[69] Ok K C Park S H K Hwang C S Kim H Shin H S Bae J Park J S 2014 Appl. Phys. Lett. 104 63508
[70] Nakajima Y Nakata M Takei T Fukagawa H Motomura G Tsuji H Shimizu T Fujisaki Y Kurita T Yamamoto T 2014 J. Soc. Inf. Disp. 22 137
[71] Rim Y S Chen H Liu Y Bae S H Kim H J Yang Y 2014 ACS Nano 8 9680
[72] Petti L Münzenrieder N Salvatore G A Zysset C Kinkeldei T Büthe L Tröster G 2014 IEEE Trans. Electron. Dev. 61 1085
[73] Münzenrieder N Voser P Petti L Zysset C Büthe L Vogt C Salvatore G A Tröster G 2014 IEEE Electron Dev. Lett. 35 69
[74] Lee G J Kim J Kim J H Jeong S M Jang J E Jeong J 2014 Semicond. Sci. Technol. 29 35003
[75] Li H U Jackson T N 2015 IEEE Electron Dev. Lett. 36 35
[76] Liu N Zhu L Q Feng P Wan C J Liu Y H Shi Y Wan Q 2015 Sci. Rep. 5 18082
[77] Kim J Jeong S M Jeong J 2015 Jpn. J. Appl. Phys. 54 114102
[78] Honda W Harada S Ishida S Arie T Akita S Takei K 2015 Adv. Mater. 27 4674
[79] Jo J W Kim J Kim K T Kang J G Kim M G Kim K H Ko H Kim Y H Park S K 2015 Adv. Mater. 27 1182
[80] Motomura G Nakajima Y Takei T Tsuzuki T Fukagawa H Nakata M Tsuji H Shimizu T Morii K Hasegawa M Fujisaki Y Yamamoto T 2015 ITE Trans. Media Technol. Appl. 3 121
[81] Jung S W Koo J B Park C W Na B S Oh J Y Lee S S Koo K W 2015 J. Vac. Sci. Technol. 33 51201
[82] Jin S H Kang S K Cho I T Han S Y Chung H U Lee D J Shin J Baek G W Kim T Lee J H Rogers J A 2015 ACS Appl. Mater. Interfaces 7 8268
[83] Park M J Yun D J Ryu M K Yang J H Pi J E Kwon O S Kim G H Hwang C S Bak J Y Yoon S M 2015 J. Mater. Chem. 3 4779
[84] Petti L Frutiger A Münzenrieder N Salvatore G A Büthe L Vogt C Cantarella G Tröster G 2015 IEEE Electron Dev. Lett. 36 475
[85] Tripathi A K Myny K Hou B Wezenberg K Gelinck G H 2015 IEEE Trans. Electron Dev. 62 4063
[86] Hsu H H Chiu Y C Chiou P Cheng C H 2015 J. Alloys Compd. 643 S133
[87] Li Y S He J C Hsu S M Lee C C Su D Y Tsai F Y Cheng I C 2016 IEEE Electron Dev. Lett. 37 46
[88] Zhang Y Mei Z Cui S Liang H Liu Y Du X 2016 Adv. Electron. Mater. 2 1500486
[89] Zhang L R Huang C Y Li G M Zhou L Wu W J Xu M Wang L Ning H L Yao R H Peng J B 2016 IEEE Trans. Electron Dev. 63 1779
[90] Wang B Yu X Guo P Huang W Zeng L Zhou N Chi L Bedzyk M J Chang R P H Marks T J Facchetti A 2016 Adv. Electron. Mater. 2 1500427
[91] Park C B Na H I Yoo S S Park K S 2016 Appl. Phys. Express 9 31101
[92] Jung S W Choi J S Park J H Koo J B Park C W Na B S Oh J Y Lim S C Lee S S Chu H Y 2016 J. Nanosci. Nanotechnol. 16 2752
[93] Kim J Kim J Jo S Kang J Jo J W Lee M Moon J Yang L Kim M G Kim Y H Park S K 2016 Adv. Mater. 28 3078
[94] Oh H Cho K Park S Kim S 2016 Microelectron. Eng. 159 179
[95] Zeumault A Ma S Holbery J 2016 Phys. Status Solidi 213 2189
[96] Nakata M Motomura G Nakajima Y Takei T Tsuji H Fukagawa H Shimizu T Tsuzuki T Fujisaki Y Yamamoto T 2016 J. Soc. Inf. Disp. 24 3
[97] Narushima S Mizoguchi H Shimizu K Ueda K Ohta H Hirano M Kamiya T Hosono H 2003 Adv. Mater. 15 1409
[98] Schein F L vonWenckstern H Grundmann M 2013 Appl. Phys. Lett. 102 92109
[99] Schein F L Winter M Böntgen T vonWenckstern H Grundmann M 2014 Appl. Phys. Lett. 104 22104
[100] Schlupp P Schein F L vonWenckstern H Grundmann M 2015 Adv. Electron. Mater. 1 1400023
[101] Chen W C Hsu P C Chien C W Chang K M Hsu C J Chang C H Lee W K Chou W F Hsieh H H Wu C C 2014 J. Phys. Appl. Phys. 47 365101
[102] Pal B N Sun J Jung B J Choi E Andreou A G Katz H E 2008 Adv. Mater. 20 1023
[103] Brillson L J Dong Y Tuomisto F Svensson B G Kuznetsov A Y Doutt D Mosbacker H L Cantwell G Zhang J Song J J Fang Z Q Look D C 2012 J. Vac. Sci. Technol. 30 50801
[104] Brillson L J Lu Y 2011 J. Appl. Phys. 109 121301
[105] Chasin A Nag M Bhoolokam A Myny K Steudel S Schols S Genoe J Gielen G Heremans P 2013 IEEE Trans. Electron Dev. 60 3407
[106] Zhang J Li Y Zhang B Wang H Xin Q Song A 2015 Nat. Commun. 6 7561
[107] Sugimura T Tsuzuku T Kasai Y Iiyama K Takamiya S 2000 Jpn. J. Appl. Phys. 39 4521
[108] Hemour S Wu K 2014 Proc. IEEE 102 1667
[109] Grover S Moddel G 2011 IEEE J. Photovolt. 1 78
[110] Kimura Y Sun Y Maemoto T Sasa S Kasai S Inoue M 2013 Jpn. J. Appl. Phys. 52 06GE09
[111] Lee D, 2016 CES 2016: Hands-on with LG’s roll-up flexible screen, January 5, 2016, BBC News
[112] Cervant E 2015 Report: flexible displays will dominate the future with foldable, rollable and even stretchable panels, September 8, 2015, Android Auth.
[113] Park J S Maeng W J Kim H S Park J S 2012 Thin Solid Films 520 1679
[114] Choi C H Lin L Y Cheng C C Chang C 2015 ECS J. Solid State Sci. Technol. 4 P3044
[115] Rim Y S Bae S H Chen H DeMarco N Yang Y 2016 Adv. Mater. 28 4415
[116] Harris K D Elias A L Chung H J 2015 J. Mater. Sci. 51 2771
[117] Ni H Liu J Wang Z Yang S 2015 J. Ind. Eng. Chem. 28 16
[118] Chien C W Wu C H Tsai Y T Kung Y C Lin C Y Hsu P C Hsieh H H Wu C C Yeh Y H Leu C M Lee T M 2011 IEEE Trans. Electron Dev. 58 1440
[119] Cantarella G Münzenrieder N Petti L Vogt C Büthe L Salvatore G A Daus A Tröster G 2015 IEEE Electron Dev. Lett. 36 781
[120] Rogers J A Someya T Huang Y 2010 Science 327 1603
[121] Wagner S Lacour S P Jones J Hsu P I Sturm J C Li T Suo Z 2004 Phys. E Low-Dimens. Syst. Nanostructures 25 326
[122] Sekitani T Someya T 2012 MRS Bull. 37 236
[123] http://www.corning.com/in/en/products/display-glass/products/corning-willow-glass.html
[124] Leterrier Y 2003 Prog. Mater. Sci. 48 1
[125] Knez M Nielsch K Niinistö L 2007 Adv. Mater. 19 3425
[126] Kim H Lee H B R Maeng W J 2009 Thin Solid Films 517 2563
[127] Münzenrieder N Salvatore G A Petti L Zysset C Büthe L Vogt C Cantarella G Tröster G 2014 Appl. Phys. Lett. 105 263504
[128] Kamiya T Nomura K Hosono H 2009 J. Disp. Technol. 5 273
[129] Dehuff N L Kettenring E S Hong D Chiang H Q Wager J F Hoffman R L Park C H Keszler D A 2005 J. Appl. Phys. 97 64505
[130] Chiang H Q Wager J F Hoffman R L Jeong J Keszler D A 2005 Appl. Phys. Lett. 86 13503
[131] Jackson W B Hoffman R L Herman G S 2005 Appl. Phys. Lett. 87 193503
[132] Janotti A Van de Walle C G 2009 Rep. Prog. Phys. 72 126501
[133] Janotti A deWalle C G V 2005 Appl. Phys. Lett. 87 122102
[134] Liu L Mei Z Tang A Azarov A Kuznetsov A Xue Q K Du X 2016 Phys. Rev. 93 235305
[135] Suresh A Wellenius P Dhawan A Muth J 2007 Appl. Phys. Lett. 90 123512
[136] Saji K J Jayaraj M K Nomura K Kamiya T Hosono H 2008 J. Electrochem. Soc. 155 H390
[137] Jeon I Y Lee J Y Yoon D H 2013 J. Nanosci. Nanotechnol. 13 1741
[138] Kim K A Bak J Y Choi J S Yoon S M 2014 Ceram. Int. 40 7829
[139] Lee Y G Choi W S 2013 Electron. Mater. Lett. 9 719
[140] Chong E Jo K C Lee S Y 2010 Appl. Phys. Lett. 96 152102
[141] Kim C J Kim S Lee J H Park J S Kim S Park J Lee E Lee J Park Y Kim J H Shin S T Chung U I 2009 Appl. Phys. Lett. 95 252103
[142] Park J S Kim K Park Y G Mo Y G Kim H D Jeong J K 2009 Adv. Mater. 21 329
[143] Chong E Kim S H Lee S Y 2010 Appl. Phys. Lett. 97 252112
[144] Chong E Chun Y S Lee S Y 2010 Appl. Phys. Lett. 97 102102
[145] Noh J Jung M Jung Y Yeom C Pyo M Cho G 2015 Proc. IEEE 103 554
[146] Subramanian V Cen J de la F Vornbrock F Grau G Kang H Kitsomboonloha R Soltman D Tseng H Y 2015 Proc. IEEE 103 567
[147] Grau G Subramanian V 2016 Adv. Electron. Mater. 2 1500328
[148] Lim N Kim J Lee S Kim N Cho G 2009 IEEE Trans. Adv. Packag. 32 72
[149] Sekitani T Nakajima H Maeda H Fukushima T Aida T Hata K Someya T 2009 Nat. Mater. 8 494
[150] Chang J B Liu V Subramanian V Sivula K Luscombe C Murphy A Liu J Fréchet J M J 2006 J. Appl. Phys. 100 14506
[151] Niinistö L Nieminen M Päiväsaari J Niinistö J Putkonen M Nieminen M 2004 Phys. Status Solidi 201 1443
[152] Jen S H Bertrand J A George S M 2011 J. Appl. Phys. 109 84305
[153] Pecunia V Banger K Sirringhaus H 2015 Adv. Electron. Mater. 1 1400024
[154] Facchetti A Yoon M H Marks T J 2005 Adv. Mater. 17 1705
[155] Kim D H Choi S H Cho N G Chang Y Kim H G Hong J M Kim I D 2009 Electrochem. Solid-State Lett. 12 H296
[156] Hwang B U Kim D I Cho S W Yun M G Kim H J Kim Y J Cho H K Lee N E 2014 Org. Electron. 15 1458
[157] Shannon R D 1993 J. Appl. Phys. 73 348
[158] Han W Lee H S Bangi U K H Yoo B Park H H 2016 Polym. Adv. Technol. 27 245
[159] Chen R Kim H McIntyre P C Bent S F 2005 Chem. Mater. 17 536
[160] Lee J P Jang Y J Sung M M 2003 Adv. Funct. Mater. 13 873
[161] Minami T 2005 Semicond. Sci. Technol. 20 S35
[162] Pasquarelli R M Ginley D S O’Hayre R 2011 Chem. Soc. Rev. 40 5406
[163] Leterrier Y Médico L Demarco F Månson J A E Betz U Escolà M F Kharrazi Olsson M Atamny F 2004 Thin. Solid Films 460 156
[164] Kim Y S Hwang W J Eun K T Choa S H 2011 Appl. Surf. Sci. 257 8134
[165] Park Y S Kim H K Jeong S W Cho W J 2010 Thin Solid Films 518 3071
[166] Ko Y D Lee C H Moon D K Kim Y S 2013 Thin Solid Films 547 32
[167] Bender M Seelig W Daube C Frankenberger H Ocker B Stollenwerk J 1998 Thin Solid Films 326 67
[168] Park Y S Choi K H Kim H K Kang J W 2010 Electrochem. Solid-State Lett. 13 J39
[169] Park Y S Kim H K 2010 J. Vac. Sci. Technol. 28 41
[170] Kim H K Lim J W 2012 Nanoscale Res. Lett. 7 67
[171] Cho S W Jeong J A Bae J H Moon J M Choi K H Jeong S W Park N J Kim J J Lee S H Kang J W Yi M S Kim H K 2008 Thin Solid Films 516 7881
[172] Park Y S Choi K H Kim H K 2009 J. Phys. Appl. Phys. 42 235109
[173] Park Y S Park H K Jeong J A Kim H K Choi K H Na S I Kim D Y 2009 J. Electrochem. Soc. 156 H588
[174] Choi K H Nam H J Jeong J A Cho S W Kim H K Kang J W Kim D G Cho W J 2008 Appl. Phys. Lett. 92 223302
[175] Park H K Jeong J A Park Y S Na S I Kim D Y Kim H K 2009 Electrochem. Solid-State Lett. 12 H309
[176] Choi Y Y Kim H K Koo H W Kim T W Lee S N 2011 J. Vac. Sci. Technol. 29 61502
[177] Lim J W Oh S I Eun K Choa S H Koo H W Kim T W Kim H K 2012 Jpn. J. Appl. Phys. 51 115801
[178] Lee J Lee P Lee H Lee D Lee S S Ko S H 2012 Nanoscale 4 6408
[179] Yu Z Zhang Q Li L Chen Q Niu X Liu J Pei Q 2011 Adv. Mater. 23 664
[180] Lim J W Cho D Y Eun K Choa S H Na S I Kim J Kim H K 2012 Sol. Energy Mater. Sol. Cells 105 69
[181] Lee J Y Connor S T Cui Y Peumans P 2008 Nano Lett. 8 689
[182] Hu L Kim H S Lee J Y Peumans P Cui Y 2010 ACS Nano 4 2955
[183] Zhang K Han K Shi S Bahl G Tawfick S 2016 Adv. Electron. Mater. 2 1600003
[184] Kim H J Kim Y J 2014 IOP Conf. Ser. Mater. Sci. Eng. 62 12022
[185] Lee M H Hsu S M Shen J D Liu C 2015 Microelectron. Eng. 138 77
[186] Dauzou F Bouten P C P Dabirian A Leterrier Y Ballif C Morales-Masis M 2016 Org. Electron. 35 136
[187] Gleskovas H Wagner S Suo Z 1999 MRS Online Proceedings Library Archive 557 653
[188] Gleskova H Wagner S Suo Z 1999 Appl. Phys. Lett. 75 3011
[189] Suo Z Ma E Y Gleskova H Wagner S 1999 Appl. Phys. Lett. 74 1177
[190] Sekitani T Iba S Kato Y Noguchi Y Someya T Sakurai T 2005 Appl. Phys. Lett. 87 173502
[191] Park S K Han J I Moon D G Kim W K 2003 Jpn. J. Appl. Phys. 42 623
[192] Gleskova H Wagner S Suo Z 2000 J. Non-Cryst. Solids 266�?69 1320
[193] Chen B W Chang T C Hung Y J Hsieh T Y Tsai M Y Liao P Y Chen B Y Tu Y H Lin Y Y Tsai W W Yan J Y 2015 Appl. Phys. Lett. 106 183503
[194] Munzenrieder N Cherenack K H Troster G 2011 IEEE Trans. Electron Dev. 58 2041
[195] Heremans P Tripathi A K de Jamblinne de Meux A Smits E C P Hou B Pourtois G Gelinck G H 2016 Adv. Mater. 28 4266
[196] Rockett A 2008 The Materials Science of Semiconductors Springer US
[197] Song K Young Koo C Jun T Lee D Jeong Y Moon J 2011 J. Cryst. Growth 326 23
[198] Yoo Y B Park J H Lee S J Song K M Baik H K 2012 Jpn. J. Appl. Phys. 51 40201
[199] Hwang Y H Kim K S Cho W J 2014 Jpn. J. Appl. Phys. 53 04EF12
[200] Moon S W Cho W J 2015 J. Semicond. Technol. Sci. 15 249
[201] Oh S M Jo K W Cho W J 2015 Curr. Appl. Phys. 15
[202] Hwang Y H Seo S J Jeon J H Bae B S 2012 Electrochem. Solid-State Lett. 15 H91
[203] Yang Y H Yang S S Chou K S 2010 IEEE Electron Dev. Lett. 31 969
[204] Park J Kim C S Ahn B D Ryu H Kim H S 2015 J. Electroceramics 35 106