Sol–Gel‐Derived High‐Performance Stacked Transparent Conductive Oxide Thin Films

Single‐ and multi‐layer transparent conductive oxide (TCO) thin films exhibiting high performance, good packing density and low surface/interface roughness are deposited on silica glass substrates by the sol–gel method. The crystal and microstructural properties of the TCO thin films are evaluated as an alternate to films prepared by ultra‐high vacuum deposition. Tin‐doped indium oxide (ITO) thin films produced using a two‐step drying process showed low surface roughness because of dense packing structure not only horizontal but also vertical directions. As a result, electrical conductivity, carrier concentration, carrier mobility, and optical transmittance of 2.3 × 10³ S/cm, 8 × 10²⁰ cm^?3, 18 cm²/Vs, and over 98% at 500 nm, respectively, were achieved. A multilayer ZnO/ITO stacked structure was also fabricated using the sol–gel process. Our findings suggest that solution‐based methods show promise as an alternative to existing ultra‐high vacuum methods to fabricate TCO thin films.     

Concentration dependence of physical properties of low temperature processed ZnO quantum dots thin films on polyethylene terephthalate as potential electron transport material for perovskite solar cell

Colloidal Zinc oxide quantum dots (ZnO QDs) prepared with varying concentrations through precipitation method were deposited on flexible ITO/PET substrates using spin-coating technique. Various characterization tools were utilized to investigate the morphological, structural, electrical and optical properties of the films. The crystallinity of the films was found to improve with increasing ZnO QD concentration (ZQ_C) as evident from the X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) studies. Crystallographic and optical parameters were evaluated and explained in depth. The average nanograin size and bandgap were increased and decreased respectively, from ～5 nm to ～8 nm and 3.29 eV–3.24 eV with an increase in ZQ_C from 10 mg/mL to 70 mg/mL. Columnar structure growth of the films is revealed by AFM results. The films showed decent optical transparency up to 81%. All the ZnO films exhibited n-type semiconducting property as indicated by the electrical measurements with carrier mobility and low resistivity of 12.21–26.63 cm²/Vs and 11.84 × 10^?3 to 13.16 × 10^?3 Ω cm respectively. Based on the experimental findings, ZnO QD nanostructure film grown at 50 mg/mL is envisaged to be a potential candidate for flexible perovskite photovoltaic application.     

Optical and electrical properties of ITO film on flexible fluorphlogopite substrate

Indium Tin Oxide (ITO) films were prepared, at room temperature, on a fluorphlogopite substrate using magnetron sputtering technology. At various temperatures of 500 °C, 600 °C, 700 °C, 800 °C, and 900 °C, the samples were (had) annealed for 2 h (a 2-h duration). The results showed improvement in the crystalline performance of ITO film at selected annealing temperatures, with a significant reduction in resistivity at 800 °C. The lowest resistivity is 4.08 × 10^?4 Ω-cm, which is nearly an order of magnitude lower than the unannealed sample. All samples have an average light transmittance above 85% in the visible light range (400–800 nm), and with increasing annealing temperature, the average light transmittance tends to decrease. Besides, at the sensitive wavelength of 550 nm, the light transmittance is as high as 93.74%. The sheet resistance testing of the sample was through the number of bending times, which revealed that with the increase of the number of bending, the sheet resistance increases. However, after 1200 bending times, the change rate of the sheet resistance remains below 5%. Thus, the ITO film prepared on the flexible fluorphlogopite substrate revealed excellent optical and electrical properties, good flexibility, and improved stability after high-temperature annealing, which guarantees successful application in flexible electronic devices.     

Rapid and facile low-temperature solution production of ZrO2 films as high-k dielectrics for flexible low-voltage thin-film transistors

A rapid lightwave (LW) irradiation method was presented for the low-temperature solution production of ZrO₂ films as high-k dielectrics for flexible high-performance thin-film transistors (TFTs). The LW irradiation process markedly decreased the required processing temperature and processing time. Microstructure characterizations confirmed the successful formation of ZrO₂ films with an ultrasmooth surface, large band gap (>5 eV) and low defect level. The ZrO₂ film produced via LW irradiation at ∼200 °C in only 8 min presented excellent dielectric properties, including a small leakage current of 3.3 × 10⁻⁸ A/cm² and a large capacitance of 296 nF/cm², significantly outperforming the films by the conventional high-temperature annealing process at 400 °C for 60 min. Furthermore, LW irradiation was extended to the channel layer. The rapid low-temperature solution-processed InZrO_x TFTs exhibited superior electrical characteristics, such as a high carrier mobility of 41.3 cm²V⁻¹s⁻¹ and a high on-off current ratio of 10⁵∼10⁶ at a low operation voltage of 3 V due to the employment of high-quality ZrO₂ dielectric films. Moreover, the flexible TFT on a polyimide (PI) plastic substrate achieved a high mobility of nearly 30 cm²V⁻¹s⁻¹, indicating that LW irradiation is highly promising for the rapid and low-temperature solution production of high-quality and flexible oxide electronic devices.     

High-performance indium-free flexible transparent ATO/Au/ATO tri-layer films by magnetron sputtering

Due to the scarcity of indium (In) in the earth and its potential harm to individuals, the development of In-free transparent conductive film is considered crucial. In this work, In-free SnO₂:Sb/Au/SnO₂:Sb (ATO/Au/ATO, SAS) tri-layer films with high transparency and conductivity were successfully prepared on polycarbonate (PC) substrates by RF and DC magnetron sputtering at room temperature. The influence of the Au layer thickness on microstructure, electrical and optical performances was systematically studied after fixing the ATO thickness to 50 nm. It was indicated by X-ray diffraction patterns that ATO is amorphous and Au is oriented along (111). The trend of increasing and then decreasing light transmission with Au layer thickness was observed in both experimental and simulation results. The improved figure of merit (FoM, 1.89 × 10^?2 Ω^?1) was achieved in SAS tri-layer film, the resistivity and average transmittance of which was lowered to 7.50 × 10^?5 Ω cm and 81.4%, respectively, when Au layer thickness is 11 nm. Moreover, the mechanism of the variation of optical and electrical properties at different Au layer thickness was proposed. Particularly, the SAS tri-layer films also exhibit superior flexibility, durability and adhesion. These results demonstrate SAS tri-layer films are promising alternative to ITO in flexible electronics applications.     

Highly ordered columnar ITO thin film with enhanced thermoelectric and mechanical performance over wide temperature range

Indium tin oxide (ITO) based thin films offer the possibility to improve the performance of high-temperature thermocouples by providing good sensitivity and reliability over a wide temperature range. In this study, a thin but robust ITO-based thin-film thermocouple, with a low-crystallised highly ordered columnar structure, was fabricated. The electrical conductivity exhibited a high temperature-dependent sensitivity owing to the increasing density with increasing temperature. The nano-hardness and interfacial robustness were evaluated and found to exhibit excellent service reliability at high temperatures because of the low thermal stress. Furthermore, the similar mechanical and electrical performances of the thin films, after annealing at 600 and 800 °C, demonstrated that the enhanced performance was mainly determined by the orientation of the ITO thin films. An enhanced Seebeck coefficient (～100 μV K^?1) was obtained for the ITO thin film after annealing at 1000 °C, resulting in a special structure with profuse nanoholes. These results highlight the good mechanical performance and stability of the thermoelectric properties of highly ordered columnar thin films over a wide temperature range, and can serve as a guide for the preparation of thin but robust functional ceramic-based materials.     

Influence of a Ni interlayer on the optical and electrical properties of trilayer GZO/Ni/GZO films

Trilayer GZO/Ni/GZO films were deposited onto polycarbonate (PC) substrates with RF and DC magnetron sputtering, and then the influence of a Ni interlayer on the optical and electrical properties of the films was investigated. A 2-nm-thick Ni interlayer decreased the resistivity to 6.4×10⁻⁴ Ω cm and influenced the optical transmittance.Although optical transmittance deteriorated with Ni insertion, the films showed a relatively high optical transmittance of 74.5% in the visible wavelength region. The figure of merit (FOM) of a GZO single layer film was 1.2×10⁻⁴ Ω⁻¹, while that of the GZO/Ni/GZO films reached a maximum of 8.2×10⁻⁴ Ω⁻¹.Since a higher FOM results in higher quality transparent-conductive oxide (TCO) films, it is concluded that GZO films with a 2 nm Ni interlayer have better optoelectrical performance than single-layer GZO films.     

Dispersion and microwave processing of nano-sized ITO powder for the fabrication of transparent conductive oxides

Aqueous dispersions of tin-doped indium oxide (ITO) nanopowder were prepared and the effect of the addition of PEG 400, Tween 80 and β-alanine as dispersants was investigated using zeta potential and particle size distribution measurements. Both PEG 400 and β-alanine were found to produce stable dispersions that were used to deposit ITO thin films on glass substrates by dip and spin coating methods. The ITO thin films were heat-treated using both conventional and microwave heat treatment in order to improve the inter-particle connections and hence the resistivity and transparency of the films. All the films exhibited an average transmittance of >80% over the visible spectrum after being subjected to the heat treatment process. ITO films prepared with no dispersant showed very high resistivity values for both heating methods, however addition of 2 wt% PEG 400 to the dispersion yielded a reduction in the resistivity values to 1.4×10⁻¹ Ω cm and 3.8×10⁻² Ω cm for conventionally and microwave treated films, respectively. The surface morphological studies confirmed that addition of dispersants improved the film uniformity and inter-particle connections of the ITO films considerably.     

Al3+ doping induced changes of structural,morphology, photoluminescence,optical and electrical properties of SnO2 thin films as alternative TCO for optoelectronic applications

Highly transparent and conductive pure (SnO₂) and aluminum doped tin oxide (Al:SnO₂) thin films were deposited on glass substrates by the sol-gel spin-coating method. The structural, morphological, optical and electrical properties of the prepared thin films at different doping rates have been studied. X-ray diffraction results revealed that all the films were polycrystalline in nature with a tetragonal rutile structure. SEM images of the analyzed films showed a homogeneous surface morphology, composed of nanocrystalline grains. The EDS results confirmed the presence of Sn and O elements in pure SnO₂ and Sn, O, Al in doped SnO₂ thin films. The optical results revealed a high transmittance greater than 85% in the visible and near infrared and a band gap varying between 3.82 and 3.89 eV. PL spectra at room temperature showed that the most dominant defects correspond to oxygen vacancies. A low resistivity of order varying between 10^?3 and 10^?4 Ω cm and a high figure of merits ranging between 10^?3 and 10^?2 Ω^?1 in the visible range were obtained. The best performances were obtained for samples containing 2 at. % Al, which could be used as an alternative TCO layer for future optoelectronic devices.     

The Sintering Trajectory and Electrical Properties of Niobium‐Doped Titania Sputtering Targets

Nb‐doped TiO₂ (TNO) films, which are highly conductive and transparent, can be used as transparent conductive oxide (TCO) films. A predominant manufacturing method for TCO film is magnetron sputtering, and the material of the sputtering target affects the performance of the film. The objective of this study was to investigate the sintering densification, microstructure, and electrical properties of TNO and TiO₂ sputtering targets. The results showed that the segregation of Nb at the grain boundary in TNO helps to facilitate densification and inhibit grain growth. After 1200°C sintering, the sintered density of TNO target achieves almost 100% of the theoretical density. Moreover, the Nb₂O₅ additive greatly improves the electrical properties, decreasing the resistivity of TiO₂ from >10⁸ Ωcm to 4.6 × 10¹ Ωcm. Correlations between TNO sputtering target investigated in this study and TNO sputtered film reported in the literature are also preliminarily established. The resistivity of TNO film with an anatase structure is obviously lower than that of TNO target with a rutile structure.     

Dielectric and electrical conductivity properties of multi-stage spark plasma sintered HA–CaTiO3 composites and comparison with conventionally sintered materials

One of the different issues limiting the wider application of monolithic hydroxyapatite (HA) as an ideal bone replacement material is the lack of reasonably good electrical transport properties. The comprehensive electrical property characterization to evaluate the efficacy of processing parameters in achieving the desired combination of electroactive properties is considered as an important aspect in the development of HA-based bioactive material. In this perspective, the present work reports the temperature (RT-200 °C) and frequency (100 Hz–1 MHz) dependent dielectric properties and AC conductivity for a range of HA–CaTiO₃ (HA–CT) composites, densified using both conventional pressureless sintering in air as well as spark plasma sintering in vacuum. Importantly, the AC conductivity of spark plasma sintered ceramics [～upto 10^?5 (Ω cm)^?1] are found to be considerably higher than the corresponding pressureless sintered ceramics [～upto 10^?8 (Ω cm)^?1]. Overall, the results indicate the processing route dependent functional properties of HA–CaTiO₃ composites as well as related advantages of spark plasma sintering route.     

Hydrogen-assisted pulsed KrF-laser irradiation for the in situ photoreduction of graphene oxide films

We report on the use of pulsed KrF-laser irradiation for the in situ reduction of graphene oxide (GO) films under both vacuum and partial hydrogen pressure. By exposing GO films to 500 pulses of a KrF-laser, at a fluence of 10 mJ/cm², their sheet resistance (R_s) is dramatically reduced from highly insulating (∼10¹⁰ Ω/sq) to conductive values of ∼3 kΩ/sq. By increasing the laser fluence, from 10 to 75 mJ/cm², we were able to identify an optimal fluence around 35 mJ/cm² that leads to highly conductive films with R_s values as low as 250 Ω/sq and 190 Ω/sq, under vacuum (10⁻⁵ Torr) and 50 mTorr of H₂, respectively. Raman spectroscopy analyses confirmed the effective reduction of the KrF-laser irradiated GO films through the progressive recovery of the characteristic 2D band of graphene. Furthermore, systematic Fourier-transform infrared spectroscopy analysis has revealed that KrF-laser induced reduction of GO preferentially occurs through photodissociation and removal of carboxyl (COOH) and alcohol (OH) groups. A direct correlation is established between the electrical resistance of photoreduced GO films and their COOH and OH bond densities. The KrF-laser induced reduction of GO films is found to be more efficient under H₂ background than under vacuum. It is concluded that our KrF-laser reduced GO films mainly consist of turbostratic graphite built from randomly organized few-layers-graphene building blocks, which contains some residual oxygen atoms and defects. Finally, by monitoring the KrF-laser fluence, it is shown that reduced GO films combining optical transmission as high as ∼80% along with sheet resistance as low as ∼500 Ω/sq can be achieved with this room-temperature and on-substrate process. This makes the laser-based reduction process developed here particularly attractive for photovoltaic hybrid devices using silicon substrates.     

Realization of high transparent conductive vanadium-doped zinc oxide thin films onto flexible PEN substrates by RF-magnetron sputtering using nanopowders targets

RF-magnetron sputtering has been carried out at room temperature to deposit vanadium-doped zinc oxide (VZO) nanostructured thin films onto flexible PEN substrates. The sputtering targets of compacted VZO nanopowder have been prepared using a rapid and inexpensive Sol-Gel synthesis followed by a supercritical drying process. Structural and morphological study of VZO particles in the targets has been carried out via X-ray diffraction and Transmission Electron Microscopy (TEM). The nanostructured thin films have been characterized to analyze the structural, morphological, electrical and optical properties as a function of vanadium content from 0 to 4 at.%. Structural characterization of VZO thin films revealed that the deposited thin films have been grown preferentially along (002) and exhibit the hexagonal wurtzite structure. The cross-sectional and microstructural analysis performed by Scanning Electron Microscopy (SEM) confirms the columnar growth of nanostructures. The deposited thin films exhibit transparent behavior with transmission >70% in the visible region. It has been observed that nanostructured thin films with vanadium content of 2% have demonstrated the lowest resistivity (6.71 × 10^?4 Ω cm) with Hall mobility of 10.62 cm² V^?1 s^?1. The deposited vanadium doped nanostructured thin films would have potential applications in electronic and optoelectronic devices.     

Enhancing optical and electrical properties of Al-doped ZnO coated polyethylene terephthalate substrates by laser annealing using overlap rate controlling strategy

Aluminum-doped zinc oxide (AZO) layers were deposited on polyethylene terephthalate (PET) flexible substrates and optimized by laser annealing using a 532 nm nanosecond pulsed laser. Effects of overlap rates, i.e. laser spot overlap rate (SO_R) and laser scan line overlap rate (LO_R), on AZO/PET films were investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), UV–visible transmittance spectra and digital four-point probe instrument, respectively. Laser annealing could greatly enhance grain crystallinity, increase crystallite size and avoid damage to the PET flexible substrates, thus effectively enhance transmittance and conductivity of the films. The results showed that the AZO/PET film annealed by using 85% SO_R and 60% LO_R presented the highest average visible transmittance of 76.2% and the lowest resistivity of 1.95×10⁻³ Ω cm, which respectively improved by approximately 23% and 75% compared to those of the as-deposited AZO/PET film. This work may be of great importance from the viewpoint of performance optimization of transparent conductive oxide (TCO) flexible films.     

Superior optoelectrical properties of magnetron sputter-deposited cerium-doped indium oxide thin films for solar cell applications

Indium tin oxide (ITO) is the most commonly used front contact material for a variety of photovoltaic technologies. However, the presence of a high free carrier concentration in ITO thin films results in the well-known phenomenon of free carrier absorption in the near-infrared (NIR) region of the solar spectrum. This causes optical losses especially in those solar cells where the active layer is designed to preferentially absorb NIR photons. Therefore, a combination of high carrier mobility and high NIR transparency is desired for advanced transparent conductive oxides for substituting ITO in solar cells. In this work, cerium-doped indium oxide (ICeO) thin films are deposited by pulsed DC magnetron sputtering, giving a remarkable 137% improvement of the mobility (71 cm² V^?1 s^?1) compared to the previous record value of 30 cm² V^?1 s^?1 for DC magnetron sputtered cerium-doped ITO films on glass. When compared to conventional ITO films prepared in this work, the highest mobility of ICeO is found to be almost four times higher and also the NIR transmission is substantially enhanced. Theoretical modelling of the experimental results indicates that neutral impurity scattering limits the carrier mobility in our films. With the recent advancements in single and multi-junction organic and perovskite solar cells, the development of ICeO/glass substrates (as possible replacements for the commonly used ITO/glass substrates) demonstrates significant potential in minimizing optical losses in the NIR region.     

Effect of silver (Ag) ions irradiation on the structural,optical and photovoltaic properties of Mn doped TiO2 thin films based dye sensitized solar cells

Dye sensitized solar cell (DSSC) is an emerging energy harvesting tool which converts direct sunlight into electrical energy. These cells have much better properties in contrast with silicon based solar cells because of their flexible nature, light weight, low cost, environment friendly nature, and involvement of a simple manufacturing process. Since, a photoanode is the backbone of DSSC, we synthesized a pure and 1% manganese (Mn) doped titanium dioxide (TiO₂) films by sol-gel method which are irradiated with silver (Ag) ions at two different concentrations (2 × 10¹⁴ and 4 × 10¹⁴) ions-cm^?2. X-ray diffraction revealed that Mn doping followed by Ag irradiation transformed TiO₂ from pure anatase to rutile phase. Ultraviolet–visible spectroscopy exposed the reduction in band gap of TiO₂ film during this doping and irradiation process. Therefore, absorption is enhanced with red shift in UV-range. When these films are used as a photoanode in DSSC, 1% Mn doped TiO₂ film exposed with Ag at the concentration of (2 × 10¹⁴) ions-cm^?2 exhibited maximum efficiency of 2.40%.     

Improving the electrical properties of niobium-doped titania sputtering targets by sintering in oxygen-deficient atmospheres

Niobium-doped titania (TNO) film can be used as a transparent conductive oxide (TCO) film due to its excellent conductivity and visible transparency. The performances of TNO sputtering targets are thus critical issues in optimizing sputtered films. This study clarifies the influences of inert and reducing atmospheres on the microstructure, densification, crystal structure, and electrical properties of TNO sputtering targets. The results indicate that a sintering atmosphere of 90% Ar–10% H₂ can result in a lower sintered density, larger grain size, and lower resistivity than can an atmosphere of Ar, followed by one of air. Sintering in 90% Ar–10% H₂ or Ar obviously decreases the resistivity of TiO₂, from >10⁸ Ω cm to <10⁻¹ Ω cm, and the TNO target, from >10¹ Ω cm to <10⁻¹ Ω cm. The resistivity of TNO target sintered at 1200 °C in 90% Ar–10% H₂ is as low as 1.8×10⁻² Ω cm.     

Improving optical and electrical performances of aluminum-doped zinc oxide thin films with laser-etched grating structures

A laser etching method was performed to achieve the dual purpose of fabricating grating structures and laser annealing on aluminum-doped zinc oxide (AZO) thin films, and thus improve the film photoelectric performances. Different line spacings and laser fluences were adopted to systematically explore the optimal laser etching condition. Too narrow line spacings or too high laser fluences led to light reflections at the grating external surface to cause more light dissipation, and too wide line spacings or too low laser fluences resulted in relatively small total grating lateral areas being detrimental to multiple internal light reflections. Moreover, too narrow line spacings brought about laser-caused lattice disorder and too high laser fluences produced laser-ablated spots or overburned traces. Therefore, using the medium line spacing and laser fluence, e.g. 40 μm and 0.6 J/cm² in the present work, was more suitable for synchronously realizing grating structure fabrication and laser annealing. The corresponding AZO film exhibited the maximum figure of merit of 2.89 × 10^?2 Ω^?1, which was 1.6 times that of the untreated AZO film. This study is expected to expand performance improvement methods of TCO films and promote the application of laser-etched grating structures.     

Structural and optoelectronic properties of transparent conductive c-axis-oriented ZnO based multilayer thin films with Ru interlayer

ZnO and Ru multilayer thin films are deposited using the sputtering deposition technique at room temperature. The effects of the Ru interlayer thickness and annealing temperature on the properties of multilayer thin films have been studied. An X-ray diffraction study reveals that ZnO layers are highly c-axis-oriented. The use of an Ru interlayer improves the crystalline quality of the subsequently deposited ZnO layers. Moreover, the crystalline quality of the entire structure is further enhanced through thermal annealing in a vacuum. Atomic force microscopy images show that the surface roughness of the multilayer thin films increases with a Ru interlayer thickness greater than 6 nm. The roughness of the film surface increases in correlation with annealing temperatures. This accounts for the decreased optical transmittance of the multilayer thin films annealed at temperatures higher than 450 °C. The electrical resistivity of multilayer thin films decreases with an increase in the metallic interlayer thickness. Thermal annealing at 450 °C causes low resistivity in multilayer thin films. The lowest resistivity reached ～5.4 × 10^?4 Ω cm for multilayer films with a 10-nm-thick Ru interlayer annealed at 450 °C.     

Oxidation behavior and electrical properties of metal interconnects with Ce-doped Ni–Mn spinel coatings

To improve the high-temperature oxidation resistance and electrical conductivity of ferritic stainless steels, protective Ce-doped NiMn₂O₄ spinel coatings were fabricated on the surface of SUS430 steel by electrophoretic deposition (EPD). The phase structure and microstructure of Ce-doped NiMn₂O₄ in both powder and coating forms were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The high-temperature oxidation of the NiMn₂O₄ spinel coating before and after Ce doping in the air at 800 °C for 168 h was studied by weight gain experiments. The area-specific resistance (ASR) of coatings was measured by a standard four-probe method. It was found that the Ce-doped NiMn₂O₄ spinel powder displayed a stable structure, high crystallinity, fine grain size, and decreased agglomeration when the Ce content was fixed at 0.05 mol?L^?1. The oxidation kinetics of NiMn₂O₄-coated SUS430 steel before and after Ce doping obeyed a parabolic law with parabolic rate constants of 4.58 × 10^?15 g² cm^?4 s^?1 and 1.83 × 10^?15 g² cm^?4 s^?1, respectively. When oxidized at 800 °C for 50 h, the ASR value of the coated samples before and after Ce doping stabilized at about 15.2 mΩ?cm² and 14.5 mΩ?cm², respectively. This work demonstrated that the Ce-doped NiMn₂O₄ spinel coating improved the high-temperature oxidation resistance and the electrical conductivity of metal interconnects.