Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10^− 4 and 4 × 10^− 4Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.     

Half wave rectification of inorganic/organic heterojunction diode at the frequency of 1 kHz

An inorganic/organic vertical heterojunction diode has been demonstrated with p-type Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) deposited by spin coating on n-type Ga-doped ZnO (GZO) thin films. Transparent conducting GZO thin films are deposited on glass substrate by rf-magnetron sputtering. Electrical properties of GZO thin films are investigated depending on the processing temperatures. The resistivity, mobility and carrier concentration of the GZO thin films deposited at processing temperatures of 500 °C are measured to be about 3.6 × 10⁻⁴ Ω cm, 23.8 cm²/Vs and 7.1 × 10²⁰ cm³, respectively. The root mean square surface roughness of the GZO thin films is calculated to be ~ 0.9 nm using atomic force microscopy. Current-voltage characteristics of the n-GZO/p-PEDOT:PSS heterojunction diode present rectifying operation. Half wave rectification is observed with the maximum output voltage of 1.85 V at 1 kHz. Low turn-on voltage of about 1.3 V is obtained and the ideality factor of the n-GZO/p-PEDOT:PSS diode is derived to be about 1.8.     

Polyaniline micropattern onto flexible substrate by vapor deposition polymerization-mediated inkjet printing

An approach to pattern a conducting polymer on various flexible substrates using vapor deposition polymerization-mediated inkjet printing method was demonstrated. Complex patterns of doped emeraldine salt polyaniline were obtained via chemical oxidation polymerization of vaporized aniline monomer on inkjet-printed oxidant patterns. The features of pattern were precisely controlled by inkjet printing with a micrometer-scale resolution. Fourier transformed infrared attenuated total reflection analysis was conducted in order to confirm the polymerization of aniline monomer and UV-visible spectroscopy analysis was used to investigate the oxidation state of obtained polyaniline. The minimum width of patterned line was ca. 80 μm. The sheet resistance of patterned polyaniline films was 3.8 × 10³ Ω/□ for an average patterned film thickness of ca. 450 nm.     

Highly conducting and transparent tin-doped CdO thin films for optoelectronic applications

Highly conducting and transparent thin films of tin-doped cadmium oxide were deposited on quartz substrate using pulsed laser deposition technique. The effect of growth temperature on structural, optical and electrical properties was studied. These films are highly transparent (78-89%) in visible region, and transmittance of the films depends on growth temperature. It is observed that resistivity increases with growth temperature after attaining minimum at 150 °C, while carrier concentration continuously decreases with temperature. The lowest resistivity of 1.96 × 10^− 5 Ω cm and carrier concentration of 5.52 × 10²¹ cm³ is observed for the film grown at 150 °C. These highly conducting and transparent tin-doped CdO thin films grown via pulsed laser deposition could be an excellent candidate for future optoelectronic applications.     

Enhanced conductivity in iodine doped polyaniline thin film formed by thermal evaporation

Thermal evaporation technique was employed to deposit pristine and iodine doped polyaniline (PANI) thin films on glass substrates. PANI was synthesized by the chemical oxidation method. Iodine doping was carried out by evaporation. The polymer synthesized was characterized by Thermo Gravimetric Analysis (TGA), Fourier Transform Infra Red (FTIR) and Ultraviolet-Visible (UV-VIS) spectroscopy. The evaporation temperature was optimized from TGA measurements. The thin film was deposited in vacuum at 1.33 × 10^− 4 Pa by thermal evaporation of PANI. The polymer film was characterized by FTIR and UV-VIS spectroscopy. The surface morphology of the films was studied by field emission scanning electron microscopy. The resistivity was measured by van der Pauw technique. The conductivity of the doped films was seen to increase with the iodine concentration and many fold increase in conductivity was observed in comparison to the pristine films. The increase in conductivity is due to the generation of polaron band in the band gap upon iodine doping.     

Uniformity of gallium doped zinc oxide thin film prepared by pulsed laser deposition

Recently, transparent conducting oxide thin films have attracted attention for the application to transparent conducting electrodes. In this work, we evaluated the uniformity of electrical, optical and structural properties for gallium doped zinc oxide thin films prepared on the 10 × 10 cm² silica glass substrate by pulsed laser deposition. The resistivity, carrier concentration, mobility, bonding state and atomic composition of the film were uniform along in-plane and depth direction over the 10 × 10 cm² area of the substrate. The film showed the average transmittance of 81-87%, resistivity of 1.4 × 10^− 3 Ω cm, carrier concentration of 9.7 × 10²⁰/cm³ and mobility of 5 cm²/Vs in spite of the amorphous X-ray diffraction pattern. The gradual thickness distribution was found, however, the potential for large-area and low temperature deposition of transparent conducting oxide thin film using pulsed laser deposition method was confirmed.     

Influence of Ag thickness of aluminum-doped ZnO/Ag/aluminum-doped ZnO thin films

Highly conducting aluminum-doped ZnO (30 nm)/Ag (5-15 nm)/aluminum-doped ZnO (30 nm) multilayer thin films were deposited on glass substrate by rf magnetron sputtering (for top/bottom aluminum-doped ZnO films) and e-beam evaporation (for Ag film). The transmittance is more than 70% for wavelengths above 400 nm with the Ag layer thickness of 10 nm. The resistivity is 3.71 × 10^− 4 Ω-cm, which can be decreased to 3.8 × 10^− 5 Ω-cm with the increase of the Ag layer thickness to 15 nm. The Haacke figure of merit has been calculated for the films with the best value being 8 × 10^− 3 Ω^− 1. It was shown that the multilayer thin films have potential for applications in optoelectronics.     

The characteristics of chemical and heat stability properties of chromium-vanadium-aluminum co-doped zinc oxide films for dye-sensitized solar cells

Transparent conducting oxide thin films are used as front contact material for dye-sensitized solar cells. This study investigated the effects of chromium (Cr) and vanadium (V) contents on the chemical and heat stability properties of aluminum-doped zinc oxide (AZO) thin films using pulsed direct current magnetic sputtering on Corning 1737F glass substrates. The experimental results show that Cr and V doping is useful for improving the chemical and thermal stability of AZO films. The energy gap for AZO thin films is between 3.65 and 3.69 eV. The resistivity of the AZO:Cr:V thin film was 4.23 × 10^-4 Ω cm at a Cr/V ratio of 0.30/0.23 wt.%, deposition power of 150 W, working distance of 5.5 cm, substrate temperature of 473 K, working pressure of 0.4 Pa, and frequency of 10 kHz. This value is lesser than (and therefore superior to) the resistivity of SnO₂:F (FTO) films (6.5 × 10^-4 Ω cm), but greater than that of SnO₂:In (ITO) thin films (1.2 × 10^-4 Ω cm). The resistivity increased by about 0.27% after electrolyte etching, which is similar to the 0.16% increase observed for the ITO thin film. After a thermal cycle test at 673 K, the resistivity of the AZO:Cr:V film increased to 5.42 × 10^-4 Ω cm, which is better than the resistivity of the ITO and FTO films after the same thermal cycle.     

Improvements of spatial resistivity distribution in transparent conducting Al-doped ZnO thin films deposited by DC magnetron sputtering

The relationship between two techniques developed for improving the resistivity distribution on the substrate surface in transparent conducting Al-doped ZnO (AZO) thin films prepared at a temperature of 200 °C by dc magnetron sputtering depositions (dc-MSD) using various sintered AZO targets has been investigated. One improvement method superimposes an rf component onto the dc-MSD (rf + dc-MSD). The other improvement method uses conventional dc-MSD with a low resistivity AZO target prepared under optimized conditions. An improvement of resistivity distribution resulted from a decrease in the resistivity of targets used in the preparation of AZO thin films by dc-MSD either with or without superimposing rf power. However, the resistivity distribution of AZO thin films resulting from depositions using rf-superimposed dc-MSD with lower-resistivity targets was not significantly improved over that of AZO thin films prepared by conventional dc-MSD using targets with the same low resistivities. The use of rf superimposition only resulted in improved resistivity distribution in thin films when the AZO targets had a resistivity higher than around 1 × 10^− 3 Ω cm. It should be noted that sintered AZO targets optimized for the preparation of AZO thin films with lower resistivity as well as more uniform resistivity distribution on the substrate surface tended to exhibit a lower resistivity.     

Transparent conducting Al and Y codoped ZnO thin film deposited by DC sputtering

Transparent conducting Al and Y codoped zinc oxide (AZOY) thin films with high transparency and low resistivity were deposited by DC magnetron sputtering. The effects of substrate temperature on the structural, electrical and optical properties of AZOY thin films deposited on glass substrates have been investigated. X-ray diffraction spectra indicate that no diffraction peak of Al₂O₃ or Y₂O₃ except that of ZnO (0 0 2) is observed. The AZOY thin film prepared at substrate temperature of 250 °C has the optimal crystal quality inferring from FWHM of ZnO (0 0 2) diffraction peak, but the AZOY thin film deposited at 300 °C has the lowest resistivity of 3.6 × 10⁻⁴ Ω-cm, the highest mobility of 30.7 cm² V⁻¹ s⁻¹ and the highest carrier concentration of 5.6 × 10²⁰ cm⁻³. The films obtained have disorderly polyhedral surface morphology indicating possible application in thin film solar cell with good quality and high haze factor without the need of post-deposition etching.     

Synthesis of c-axis preferred orientation ZnO:Al transparent conductive thin films using a novel solvent method

Transparent aluminum doped zinc oxide (ZnO:Al, AZO) conducting thin films with a high-preferential c-axis orientation were synthesized using a new sol-gel formula. The films were deposited using a spin-coating route onto borosilicate glass substrates. We used propylene glycol methyl ether (PGME) as the solvent in place of ethylene glycol monomethyl ether (EGME), which is commonly used because it is easier to deposit onto the substrates. PGME is also superior in terms of health and safety. PGME solvent does not need to settle for several days before use and can be spin-coated as soon as the raw material and solvent are mixed. The effects of this novel solvent on the structural, morphological, electrical and optical properties are discussed using XRD, SEM, a four-point probe and UV-VIS spectrophotometry. It was found that the films produced with PGME showed a high-preferential c-axis orientation and compact microstructure in comparison films produced using EGME. The electrical resistivity of AZO thin films produced with PGME solvent was lowered to 3.474 × 10^− 3Ω cm after annealing in 95 N₂/5H₂ atmosphere. In addition, the optical transmittances of AZO thin films on glass plates were higher than 90% in the visible wavelength region.     

Chemical synthesis of highly stable PVA/PANI films for supercapacitor application

Polyvinyl alcohol (PVA)/polyaniline (PANI) thin films were chemically synthesized by adopting two step process: initially a thin layer (200 nm) of PVA was spin coated by using an aqueous PVA solution onto fluorine doped tin oxide (FTO) coated glass substrate, afterwards PANI was chemically polymerized from aniline monomer and dip coated onto the precoated substrate. The thickness of PANI layer was varied from 293 nm to 2367 nm by varying deposition cycles onto the precoated PVA thin film. The resultant PVA/PANI films were characterized for their optical, morphological and electrochemical properties. The FT-IR and Raman spectra revealed characteristic features of the PANI phase. The SEM study showed porous spongy structure. Electrochemical properties were studied by electrochemical impedance measurement and cyclic voltammetry. The electrochemical performance of PVA/PANI thin films was investigated in 1 M H₂SO₄ aqueous electrolyte. The highest specific capacitance of 571 Fg⁻¹ was observed for the optimized thickness of 880 nm. The film was found to be stable for more than 20,000 cycles. The samples degraded slightly (25% decrement in specific capacitance) for the first 10,000 cycles. The degradation becomes much slower (10.8% decrement in specific capacitance) beyond 10,000 cycles. This dramatic improvement in the electrochemical stability of the PANI samples, without sacrificing specific capacitance was attributed to the optimized PVA layer.     

Fluorine-doped ZnO transparent conducting thin films prepared by radio frequency magnetron sputtering

Fluorine-doped ZnO transparent conducting thin films were prepared by radio frequency magnetron sputtering at 150 °C on glass substrate. Thermal annealing in vacuum was used to improve the optical and electrical properties of the films. X-ray patterns indicated that (002) preferential growth was observed. The grain size of F-doped ZnO thin films calculated from the full-width at half-maximum of the (002) diffraction lines is in the range of 18-24 nm. The average transmittance in visible region is over 90% for all specimens. The specimen annealed at 400 °C has the lowest resistivity of 1.86 × 10^− 3 Ω cm, the highest mobility of 8.9 cm² V^− 1 s^− 1, the highest carrier concentration of 3.78 × 10²⁰ cm^− 3, and the highest energy band gap of 3.40 eV. The resistivity of F-doped ZnO thin films increases gradually to 4.58 × 10^− 3 Ω cm after annealed at 400 °C for 4 h. The variation of the resistivity is slight.     

Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering

Highly conducting and transparent thin films of tungsten-doped ZnO (ZnO:W) were prepared on glass substrates by direct current (DC) magnetron sputtering at low temperature. The effect of film thickness on the structural, electrical and optical properties of ZnO:W films was investigated. All the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity first decreases with film thickness, and then increases with further increase in film thickness. The lowest resistivity achieved was 6.97 × 10⁻⁴ Ω cm for a thickness of 332 nm with a Hall mobility of 6.7 cm² V⁻¹ s⁻¹ and a carrier concentration of 1.35 × 10²¹ cm⁻³. However, the average transmittance of the films does not change much with an increase in film thickness, and all the deposited films show a high transmittance of approximately 90% in the visible range.     

Deposition of transparent conductive mesoporous indium tin oxide thin films by a dip coating process

Cetyltrimethyl ammonium bromide (CTAB) templated mesoporous indium tin oxide (ITO) thin films were deposited on quartz plates by an evaporation-induced self-assembly (EISA) process using a dip coating method. The starting solution was prepared by mixing indium chloride, tin chloride, and CTAB dissolved in ethanol. Five to fifty mole percent Sn-doped ITO films were prepared by heat-treatment at 400 °C for 5 h. The structural, adsorptive, electrical, and optical properties of mesoporous ITO thin films were investigated. Results indicate that the mesoporous ITO thin films have an ordered two-dimensional hexagonal (p6mm) structure, with nanocrystalline domains in the inorganic oxide framework. The continuous thin films have highly ordered pore sizes (>20 Å), high Brunauer-Emmett-Teller (BET) surface area up to 340 m²/g, large pore volume (>0.21 cm³/g), outstanding transparency in the visible range (>80%), and show a minimum resistivity of ρ = 1.2 × 10⁻² Ω cm.     

Structural and optical characterization of Al-doped ZnO films prepared by thermal oxidation of evaporated Zn/Al multilayered films

Aluminum-doped zinc oxide (ZnO:Al) thin films (t = 68–138 nm) were prepared by thermal oxidation in air flow, at 720 K, of the multilayered metallic Zn/Al thin stacks deposited in vacuum onto glass substrates by physical vapor deposition. The effect of Al content (3.7–8.2 at.%) on the structural (crystallinity, texture, stress, surface morphology) and optical (transmittance, absorbance, energy band gap) characteristics of doped ZnO thin films was investigated. The X-ray diffraction spectra revealed that the Al-doped ZnO films have a hexagonal (wurtzite) structure with preferential orientation with c-axis perpendicular to the substrate surface. A tensile residual stress increasing with Al content was observed. The films showed a high transmittance (about 90%) in the visible and NIR regions. The optical band gap value was found to decrease with Al content from 3.22 eV to 3.18 eV. The results are discussed in correlation with structural characteristics and Al content in the films.     

Pulsed laser deposition of indium tin oxide films on flexible polyethylene naphthalate display substrates at room temperature

Pulsed laser deposition was used to deposit high-quality indium tin oxide (ITO) thin solid films on polyethylene napthalate (PEN) flexible display substrates. The electrical, optical, microstructural, mechanical and adhesive properties of the functional thin layer were investigated as a function of a narrow range of background oxygen gas pressure at room temperature, which is the most desirable thermal condition for growing transparent conducting oxides on flexible display polymer substrates. ITO films (240 ± 35 nm thick) deposited on PEN at room temperature in the range of 0.33 to 2.66 Pa background oxygen pressure are observed to exhibit low electrical resistivity (~ 10^− 4 Ω cm) and high optical transmission (~ 90%). Electromechanical uniaxial tensile testing, of the hybrid thin structures, results in crack onset nominal strains of around 2%. The ITO surface adhesion reaches a maximum at 1.33 Pa deposition pressure.     

ITO thin films prepared by a microwave heating

ITO thin films were prepared by irradiating 2.45 GHz of microwave with an output power of 700 W using a commercial kitchen microwave oven. A substrate temperature went up and down rapidly between 100 and 650 °C in a minute by a dielectric loss of SnO₂ layer pre-deposited on a glass substrate. We found that the electrical and optical properties of films were affected by the atmosphere in a microwave irradiation, while the sintering was completed within a few minutes. Although the electrical resistivity was not reduced below 5.0 × 10^− 4 Ω·cm in this study, the results lead to the possibility of a practical rapid synthesis of ITO transparent conducting oxide films.     

Visible light enhanced TiO2 thin film bilayer dye sensitized solar cells

Visible light enhanced nitrogen-sulfur (N-S) doped titanium dioxide (TiO₂) thin films were prepared by the sol-gel method using thiourea as a dopant. The physical and chemical properties of the TiO₂ thin films were greatly influenced by the amount of thiourea added to the sol-gel solution. The greatest shift to longer wavelengths for visible light absorption was observed with 0.6 g of thiourea in the precursor solution, while 0.4 g yielded the largest particle sizes. These single-cycle dip-deposited N-S doped TiO₂ thin films were used as visible light harvesters as well as blocking layers in dye sensitized solar cells. When deposited directly on conducting fluorine doped tin oxide electrodes, photo-conversion efficiencies were reduced. However, the opposite configuration, with N-S doped thin films on top of nanoporous TiO₂, yielded an increased open-circuit voltage of 0.84 V, a short-circuit current density of 9.86 mA cm⁻², and an overall conversion efficiency of 5.88% greater than that of a standard cell. The effectiveness of the blocking layer on the cell efficiencies was analyzed by electrochemical impedance spectroscopy.     

Synthesis and optical characterization of amorphous carbon nitride thin films by hot filament assisted RF plasma CVD

Carbon nitride thin films were synthesized by hot filament assisted radio frequency plasma chemical vapour deposition using methane and nitrogen gas mixture on silicon and glass substrates. The films were deposited at different substrate bias and at different substrate temperatures. At higher substrate bias (>−120 V) there was no deposition on the substrate, but complete etching of the deposited layer was observed. X-ray diffraction studies indicated the films were amorphous. The Fourier transform infrared spectra showed that the films produced exhibited high transmittance with the presence of the C-N stretching band at 1260 cm⁻¹. For the films deposited at a lower substrate temperature C=N peaks were also present. Raman spectroscopic study indicated the presence of D and G peaks whose relative height varied with substrate temperature. The transmittance versus wavelength measurement in the UV-VIS-NIR region showed the high transmittance in the NIR region. The optical band gap of the films was calculated to be 2.0 eV and the refractive index varied within 1.6-1.7 for the wavelength range 800-1800 nm.