Transparent Conductive Oxide Films Research Articles

High-quality transparent conductive indium oxide film deposition by reactive pulsed magnetron sputtering: Determining the limits of substrate heating

A transparent conductive oxide (TCO) should have a combination of high electrical conductivity and optical transmission property to fulfill the challenging demands of industrial applications. So far, doped TCOs have been mostly considered to fulfill the technological challenges. In this study, we demonstrate that indium oxide (In2O3) thin films without intentional doping can be deposited with high crystallinity under specific film growth conditions leading to thin films with high mobility, high electrical conductivity, and high transmittance. In2O3 thin films have been deposited by reactive pulsed direct current magnetron sputtering (pulsed DCMS) from an indium target on substrates in a temperature range from room temperature (RT) to 600 °C. Detailed investigations on In2O3 thin films are performed and the film properties such as crystallinity, microstructure, chemical bonding states, electrical and optical properties are revealed. Enhanced plasma density and ionization degree provided by the employed reactive pulsed DCMS and the intentional substrate heating during the thin film deposition give possibility to deposit highly crystalline thin films which are preferentially oriented in (222) direction. The substrate heating enhances the crystallinity of the grown films up to a certain optimum temperature: 400 °C. When the substrate temperature is above 400 °C, the carrier concentration and mobility decrease due to the grain refinement effect caused by growth competition of grains in different orientations. The films grown at 400 °C indicate the presence of high oxygen vacancy concentrations, which can directly be associated with a high charge carrier concentration despite the lack of intentional doping. The undoped In2O3 films in this study grown at 400 °C show highly promising and competitive electrical properties such as low resistivity of 1.28 × 10-4 Ω⋅cm and the high carrier mobility of 69 cm2/Vs. The average transmittance of the In2O3 films with the highest conductivity is found to be greater than 80% in the visible to the near-infrared spectral region owing to an enhancement in the carrier mobility and an optical bandgap in the range from 3.61 eV to 3.77 eV.

Manganese-doped transparent conductive magnetic indium oxide films integrated on flexible mica substrates with high mechanical durability

Recent advances in flexible electronic devices have stimulated demands for the fabrication of multifunctional materials on highly flexible substrates. However, flexible diluted magnetic semiconductor films have been inadequately reported thus far. In this study, I grew polycrystalline manganese (Mn)-doped indium oxide (In2O3) films on flexible mica substrates at three deposition temperatures (Td) using the pulsed laser deposition method. All the investigated Mn-doped In2O3 films exhibited high optical transparency, high electrical conductivity, room-temperature ferromagnetism, and excellent mechanical durability. The present experimental results demonstrate that Td considerably influences the optical, electrical, and magnetic properties as well as the mechanical durability of the flexible Mn-doped In2O3 films. Based on the robustness of inorganic mica at high deposition temperatures, this detailed investigation provides a strategy for fabricating flexible diluted magnetic semiconductor films, the properties of which could be carefully optimized by widely varying the Td.

Lightwave irradiation-assisted low-temperature solution synthesis of indium-tin-oxide transparent conductive films

Transparent conductive oxide (TCO) films have important applications in many areas. Unfortunately, TCOs are usually fabricated using vacuum and high-temperature methods, preventing them from applications in low-cost flexible devices. In this paper, facile low-temperature sol-gel method is described that can be used to fabricate high-quality TCO films. This study uses lightwave (LW) irradiation (at ∼280 °C) with indium-tin-oxide (ITO) as a typical example. Both structure and key properties of ITO films are investigated for different LW irradiation conditions. ITO can be formed via LW irradiation after a period as short as 5 min. Furthermore, it is found that LW irradiation can promote the formation of M − O framework, effectively remove Cl impurities, and facilitate the elimination of hydroxyl oxygen defects - even at temperatures as low as ∼280 °C. The optimal ITO films show excellent electronic properties, including low sheet-resistance (14.5 Ω·sq−1) and high conductivity (1.7 × 103 S cm−1). Moreover, ITO films also show high transmittance (above 87%). Overall, our ITO films have a figure of merit (FOM) of 1.72 × 10−2 Ω−1, which is comparable to (or higher than) those of previous ITO films that were produced using conventional vacuum and high-temperature methods. Our LW irradiation method provides facile and effective approach to produce high-performance TCO films at remarkably low cost. This means these films could be used in affordable flexible large-area devices.

Comparative study on IWO and ICO transparent conductive oxide films prepared by reactive plasma deposition for copper electroplated silicon heterojunction solar cell

Comparative study on IWO and ICO transparent conductive oxide films prepared by reactive plasma deposition for copper electroplated silicon heterojunction solar cell

Investigation of In2O3:SnO2 films with different doping ratio and application as transparent conducting electrode in silicon heterojunction solar cell

Transparent conducting oxide films were performed from SnO 2 -doped In 2 O 3 with different doping ratio of 97:3 and 95:5 (ITO) by pulsed DC magnetron sputtering in argon and oxygen atmosphere. The effect of oxygen content on the structural, electrical and optical properties of films was investigated. Proper oxygen content improved the properties of ITO films. All films showed the polycrystalline structures. The optimal Hall mobility and resistivity reached 32.2 cm 2 /V·S and 4.5 × 10 −4 Ω cm, respectively. The average transmittance in the 400–1100 nm wavelength range was above 88 %. The optimized ITO films with Lightly and heavily doped ratio are appropriately chosen as front and back electrode of silicon heterojunction solar cells respectively, and the conversion efficiency of more than 24 % were obtained. • Different Sn-doped In 2 O 3 transparent conductive films (ITO) were investigated by magnetron sputtering. • Lightly and heavily doped ITO films present good transmittance and electrical properties respectively. • Lightly and heavily doped ITO films were used appropriately as the front and back electrodes respectively. • Efficiency of the copper electroplating silicon heterojunction solar cell can reach 24.34 %.

Embedded Nanopattern for Selectively Suppressed Thermal Conductivity and Enhanced Transparency in a Transparent Conducting Oxide Film

Embedded Nanopattern for Selectively Suppressed Thermal Conductivity and Enhanced Transparency in a Transparent Conducting Oxide Film

Sputtering processes to deposit various transparent conductive oxide films

Sputtering processes to deposit various transparent conductive oxide films

Thermal Stability of the Copper and the AZO Layer on Textured Silicon

Transparent conductive oxide (TCO) film is the most widely used front electrode in silicon heterojunction (SHJ) solar cells. A copper metallization scheme can be applied to the SHJ process. The abundance of zinc in the earth’s crust makes aluminum-doped zinc oxide (AZO) an attractive low-cost substitute for indium-based TCOs. No work has focused on the properties of the copper and AZO layers on the textured silicon for solar cells. This work deposited an aluminum-doped zinc oxide layer and copper metal layer on textured (001) silicon by a sputtering to form Cu/AZO/Si stacks. The structures of Cu/AZO/Si are characterized by scanning electron microscope (SEM), scanning transmission electron microscope (STEM), and energy-dispersive X-ray spectrometer (EDS). The results show that the copper thin film detached from AZO in the valley of the textured silicon substrate at a temperature of 400 °C. Additionally, the gap between the copper and AZO layers increases as temperature increases, and the 65 nm thickness AZO layer was found to be preserved up to 800 °C.

The ZnO-In2O3 Oxide System as a Material for Low-Temperature Deposition of Transparent Electrodes.

The development of optoelectronic devices based on flexible organic substrates substantially decreases the possible process temperatures during all stages of device manufacturing. This makes it urgent to search for new transparent conducting oxide (TCO) materials, cheaper than traditional indium-tin oxide (ITO), for the low-temperature deposition of transparent electrodes, a necessary component of most optoelectronic devices. The article presents the results of a vertically integrated study aimed at the low-temperature production of TCO thin films based on a zinc-indium oxide (ZIO) system with acceptable functional characteristics. First, dense and conducting ceramic targets based on the (100-x) mol% (ZnO) + x mol% (In2O3) system (x = 0.5, 1.5, 2.5, 5.0, and 10.0) were synthesized by the spark plasma sintering method. The dependences of the microstructure and phase composition of the ZIO ceramic targets on the In2O3 content have been studied by powder X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy methods. Then, a set of ZIO thin films with different Zn/In ratios were obtained on unheated glass substrates by direct current (dc) magnetron sputtering of the sintered targets. Complex studies of microstructure, electrical and optical properties of the deposited films have revealed the presence of an optimal doping level (5 mol% In2O3) of the ZIO target at which the deposited TCO films, in terms of the combination of their electrical and optical properties, become comparable to the widely used expensive ITO.

Determining femtosecond laser fluence for surface engineering of transparent conductive thin films by single shot irradiation.

In recent years, there has been increasing interest in optoelectronic applications of transparent conductive oxide (TCO) thin-film-based materials and devices fabricated using patterning techniques. Meanwhile, femtosecond laser processing is a convenient method that further improves the performance of TCO-based functional devices and expands their application prospects. In this study, we proposed a simple and effective strategy to determine the fluences required for laser processing TCOs. We investigated the modification of an indium tin oxide (ITO) film induced by a femtosecond laser (45/150 fs, 800 nm) at different pulse fluences. The results reveal that the laser modification of ITO films is highly dependent on the irradiated pulse fluences. Several distinct types of final micro/nanostructures were observed and may be attributed to superficial amorphization, spallation ablation, stress-assisted delamination, boiling evaporation, and phase explosion. The final micro/nanostructures were studied in detail using optical microscopy, scanning electron microscopy, transmission electron microscopy and a surface profiler. At a lower fluence above the melting but below the ablation threshold, a laterally parabolic amorphous layer profiled with maximum thicknesses of several tens of nanometers was quantitatively attained. At a higher fluence, stress-assisted delamination and superheated liquid-induced micro-honeycomb structures emerged. Furthermore, the electron and lattice temperature evolutions were also obtained using a two-temperature model to prove the ablation mechanism and ascertain the micro/nanostructure formation principle. The predicted surface temperatures confirmed film amorphization without ablation below 0.23 J/cm2. These results reveal the interaction mechanism between femtosecond laser pulse and ITO film including the competition between the free electron heating of intraband transition and the multiphoton absorption of the interband transition, which promotes the potential applications for femtosecond laser processing TCO films and other wide-band-gap semiconductors such as photodetectors, solar cells, UV-light-emitting diodes, and flat-panel displays.

High stability of flexible perovskite transparent conductive oxide film via van der Waals heteroepitaxy

Transparent conducting oxides (TCOs), such as indium-tin oxide (ITO) and indium gallium zinc oxide (IGZO), are essential components in the design of flexible optoelectronic devices. However, it is still a challenge to prepare low-cost flexible transparent conductive film for achieving the flexible functionality together with high optical transparency and electrical conductivity. Herein, fluorphlogopite (mica) is selected as the bendable and transparent substrate for the fabrication of low-cost perovskite CaVO3 TCO epitaxial film. Benefitting from thin mica substrate with high Young’s modulus of 200 GPa, flexible CaVO3 TCO epitaxial film exhibits high mechanical stability in terms of electrical conductivity and optical transparency after bending up to 105 cycles. The change of resistance can reflect the bending curvature and strain state. Our results provide a new pathway to design superior mechanical flexibility and stability CaVO3 TCO films for application in flexible, light-weight, and portable smart optoelectronic devices.

ZnO/Ag/graphene transparent conductive oxide film with ultrathin Ag layer

We insert a silver (Ag) layer between zinc oxide (ZnO) and graphene films of a bilayer structure to create trilayer transparent conductive oxide films with improved conductivities due to the bridge provided by the Ag layer to transport free electrons. To construct trilayer ZnO/Ag/graphene transparent conductive films, the Ag and ZnO layers are deposited successively on a graphene/glass substrate by magnetron sputtering from solid Ag and powder ZnO targets. The results show that the electron concentration in the trilayer films increases one order of magnitude to 1021 cm−3 upon inserting dispersed Ag dots and three orders of magnitude to 1023 cm−3 upon adding a relatively continuous 10.5-nm-thick Ag layer. However, the electron mobility drops dramatically from 10−1 to 10−2 cm2 V−1 s−1 because the dispersed Ag dots of a thin Ag layer form narrow bridges, which limit electron transport. The continuous 10.5-nm-thick Ag layer not only acts as a wide bridge but also provides electrons; therefore, the resistivity of the ZnO/Ag(continuous)/graphene trilayer decreases significantly, while the mobility of the trilayer film remains of the same order of magnitude as that of the continuous Ag layer. Of course, the transparency of the trilayer film decreases slightly upon inserting the Ag layer. To create frontier electrodes, the ZnO/Ag/graphene multilayer structure must be built up to form transparent conductive oxide films.

Surface free energy and wettability properties of transparent conducting oxide-based films with Ag interlayer

ITO, ZTO, AZO and Ag are commonly used in transparent conducting oxide (TCO)/metal/TCO electrodes to form multilayered thin films on a suitable substrate. A detailed surface free energy (SFE) knowledge of these films is critical to design desirable TCO-based sandwich structures. In this study, TCO/Ag/TCO multilayer thin films were coated onto glass substrates using ITO, ZTO, AZO and Ag targets by magnetron sputtering. The wettability properties of TCO, Ag interlayer and TCO/Ag/TCO were evaluated by contact angle measurements of seven different liquids having various surface tension values. The dispersive and polar components of SFE were calculated using geometric and harmonic mean approaches. The acidic and basic components of SFE were calculated using van Oss-Chaudhury-Good method. Following this, the work of adhesion values between TCO films and Ag interlayer were estimated using SFE values of the films. The results show that the SFE components of the surfaces differ depending on the TCO type, the total SFE values of TCO/Ag/TCO films were lower than that of TCO films, and AZO/Ag adhesion is stronger than the other TCO/Ag structures. The reasons behind these differences were discussed by evaluating the SFE, XRD, AFM and SEM analysis simultaneously.

Improvement of the Optoelectrical Properties of a Transparent Conductive Polymer via the Introduction of ITO Nanoparticles and Its Application in Crystalline Silicon/Organic Heterojunction Solar Cells.

Compared with conventional transparent conductive indium tin oxide (ITO) films, poly(3,4-ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) as a conductive polymer material has been diffusely applied in organic optoelectronic devices. However, its optoelectrical properties need to be further improved. Therefore, a simple and universal approach with introducing ITO nanoparticles (NPs) was proposed to improve the optoelectrical properties of PEDOT:PSS thin films. The results show that the vertical conductivity (σDC⊥) and average transmittance (from 300 to 1200 nm) of PEDOT:PSS films were enhanced about 26.8 and 6.3%, respectively. Crystalline silicon (c-Si)/organic heterojunction solar cells (HSCs) with PEDOT:PSS/ITO NP hybrid films were fabricated and performances led to further improvement. The spatial distributions of relative electrical field intensity and the carrier generation rate of the HSCs under the standard AM 1.5 G condition were simulated, which were in good agreement with the experimental conclusions.

Plasma-assisted deposition of indium tin oxide thin films by sublimation using an anodic vacuum arc discharge

With the anodic vacuum arc discharge as a method of physical vapor deposition (PVD) excellent properties of thin transparent conductive oxide films can be achieved, but the relations between the process parameters and the coating properties have been insufficiently described in the literature. This paper is intended to narrow this gap by investigations on indium tin oxide (ITO) thin films deposited on borosilicate glass sheets by sublimation of indium tin oxide (90:10) using an anodic vacuum arc. The arc discharge is based on a hot hollow cathode electron source and a special evaporator connected as anode. The concentration of the discharge onto the sublimating material is achieved by combining the magnetic fields of an axially symmetrical permanent magnet system and a solenoid coil. Coating rates of about 15 nm/s were achieved with an arc current of 60 A. Measurements with a planar Langmuir probe have shown that the vapor is highly ionized (about 95%). The discharge voltages are relatively low (< 60 V), so it can be assumed that the particle energies are correspondingly low. Substrate temperature (30–300 °C), oxygen gas flow (0–100 sccm) and gas pressure (0.04–0.4 Pa) were varied to study their influence on the thin film properties. Specific electrical conductivity, density and mobility of charge carriers, as well as optical properties were determined and compared with those of other PVD processes. Low specific electrical resistance of 1.8–2.0 x 10–4 Ω cm was reached at a substrate temperature of 200–300 °C independently of the pressure. The mean optical transmission (in the spectral range between 400 nm and 1100 nm) of 100 nm thick ITO films deposited at a temperature of 200 °C and at an optimized oxygen flow was determined to 84%. The roughness of the layers determined by atomic force microscopy is significantly lower than typically for magnetron sputtered films. At a pressure of 0.04 Pa, the crystalline bixbyite phase can already be detected at a coating temperature of approx. 30 °C, whereby the layer growth begins with the formation of an amorphous structure and changes to the crystalline phase with increasing layer thickness. At a temperature above 100 °C and low coating pressure the layers are completely crystalline. At a higher pressure the transition from amorphous to crystalline phase is shifted towards higher temperature. For lower oxygen flows the crystallites exhibit mainly a preferred (211) orientation. For higher oxygen flows the texture of crystallites is changed and the intensity of (440) and (622) is increased. The formation of these special crystallographic textures can probably explain the very low roughness values of the ITO layers measured by atomic force microscopy. The investigations have shown how the film properties can be adjusted by the process parameters and that the ITO sublimation using an anodic vacuum arc is suited very well for plasma assisted ITO thin film deposition.

Hydrogen roles approaching ideal electrical and optical properties for undoped and Al doped ZnO thin films

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Investigation of the Chemical Reaction between Silver Electrodes and Transparent Conductive Oxide Films for the Improvement of Fill Factor of Silicon Heterojunction Solar Cells

We evaluated the fill factor (FF) degradation mechanism in silicon heterojunction (SHJ) solar cells with high mobility In2O3 film as a high carrier mobility transparent conductive oxide (TCO) film. In particular, we focused on the electrode formation using a high productive screen-printing technique. We found the In2O3 film is easier to be reduced than the traditional TCO such as tin-doped indium oxide (ITO). Thus, the Ag atom inside the electrode is easily oxidized during the cure annealing process and results in higher contact resistance at electrode/TCO interface and deteriorate FF characteristic. We introduced novel cation catalyst additive for paste polymerization which is less reactive with In2O3 and improve the contact resistance by suppressing the silver oxidation. We also demonstrated SHJ cell fabrication and prove the effect of the developed silver paste.

Evaluation of correlation between fill factor and high mobility transparent conductive oxide film deposition temperature in the silicon heterojunction solar cells

We investigated the degradation factor of the conversion efficiency of the silicon heterojunction solar cells. In particular, we clarified the effect of the transparent conductive oxide film deposition conditions on the film quality and interface states, and the relationship between these factors and the conversion efficiency. Heat process causes a decrease in carrier mobility due to a decrease in the grain size of the TCO film, but has an effect of improving the TCO/a-Si:H(p) interface states. In particular, the silicon oxide layer between the TCO film and a-Si:H(p) increases the series resistance and moderates the band bending at the TCO/a-Si:H(p) interface, leading to FF degradation. Since In2O3:H film easily forms a silicon oxide layer at the TCO/a-Si:H(p) interface, the interface treatment is important factor in the cell manufacturing process.

Wide Band Gap Al and In Co-doped ZnO Films for Near-Infrared Plasmonic Application

In transparent conducting oxide films, tuning of plasmonic resonance is directly controlled by free electron concentration and thus by activated dopants. In this study, large area AlxInyZn1-x-yO thin films at various concentrations were prepared by spray coating using water as a solvent. The effect of Al/In dopant ratio on the structural, electrical, optical, and plasmonic properties was investigated. Tuning of optical response to a well-defined plasmon resonance is correlated to the above properties of AlxInyZn1-x-yO films. Theoretical fitting based on the Drude-Lorentz (D-L) theory was utilized for extracting the dielectric spectra and cross-over wavelength (ωc). The studies revealed plasmonic properties in NIR for the films with Al/In ratios of A5I5, A2.5I7.5, and A0I10, indicating In as the most activated dopant. Surface plasmon mode simulated using the extracted permittivity values showed the influence of mobility of these films on the broadening of the dip. The minimum plasmonic loss suggests the suitability as an alternative plasmonic material in the near infrared.

Numerical evaluation of one-dimensional transparent photonic crystal heat mirror coatings for parabolic dish concentrator receivers

High temperature solar receivers can convert concentrated solar power to heat to drive chemical production or mechanical cycles to produce electricity. Transparent windows with selective coatings that reflect thermal radiation can be used to cover receivers to improve their efficiency. However, in parabolic dish concentrators (PDC) these windows can reflect or absorb incoming solar radiation, and may actually reduce the efficiency and power output from PDC receivers. In this work numerical analysis shows that one-dimensional transparent photonic crystal heat mirrors (TPCHMs), which have the form of a modified dielectric mirror, can be designed to be highly transmissive to solar radiation but highly reflective towards thermal radiation. Results show that TPCHMs can be designed to provide significant enhancements to PDC receiver efficiencies operating at lower solar concentration ratios. Specifically, numerical analysis shows efficiency improvements of at least 62%, and 193% compared to either open-face receivers or the case when the receiver window is coated with a transparent conducting oxide film at operating temperatures of 1000 K and 1500 K, respectively. These results suggest that TPCHM covers have potential for enhancing the performance of smaller PDC systems operating at relatively low solar concentration ratios.