Virtually Bare Nanocrystal Surfaces: Significantly Enhanced Electrical Transport in CuInSe2 and CuIn1−xGaxSe2 Thin Films upon Ligand Exchange with Thermally Degradable 1‐Ethyl‐5‐Thiotetrazole

A facile and safe ligand exchange method for readily synthesized CuInSe₂ (CIS) and CuIn_1‐xGa_xSe₂ (CIGS) nanocrystals (NCs) from oleylamine to 1‐ethyl‐5‐thiotetrazole, preserving the colloidal stability of the chalcopyrite structure, is presented. 1‐Ethyl‐5‐thiotetrazole as thermally degradable ligand is adapted for the first time for trigonal pyramidal CIS (18 nm), elongated CIS (9 nm) and CIGS NCs (6 nm). Exchanged NC solutions are processed onto gold electrodes yielding ordered thin films. These films are thermally annealed at 260 °C to completely remove 1‐ethyl‐5‐thiotetrazol leaving individual closely assembled NCs with virtually bare surfaces. The current–voltage characteristics of the NC solids are measured prior to ligand thermolysis in the dark and under illumination and after ligand thermolysis in the same manner. The conductivity of trigonal pyramidal CIS increases by four orders of magnitude (1.4 × 10^?9 S cm^?1 (dark) to 1.4 × 10^?5 S cm^?1 (illuminated)) for ligand‐free NC films. Elongated CIS NC films show a three orders of magnitude conductivity increase and CIGS NC films exhibit improved conductivity by two orders of magnitude. Conductivity enhancement thereby depends on the NC size accentuating the role of trap‐states and internal grain boundaries in ligand‐free NC solids for electrical transport. This approach for the first time offers the possibility to address chalcopyrite materials’ electrical properties in a virtually ligand‐free state.     

Strongly Anisotropic Thermal Conductivity of Free‐Standing Reduced Graphene Oxide Films Annealed at High Temperature

Thermal conductivity of free‐standing reduced graphene oxide films subjected to a high‐temperature treatment of up to 1000 °C is investigated. It is found that the high‐temperature annealing dramatically increases the in‐plane thermal conductivity, K, of the films from ≈3 to ≈61 W m^?1 K^?1 at room temperature. The cross‐plane thermal conductivity, K_⊥, reveals an interesting opposite trend of decreasing to a very small value of ≈0.09 W m^?1 K^?1 in the reduced graphene oxide films annealed at 1000 °C. The obtained films demonstrate an exceptionally strong anisotropy of the thermal conductivity, K/K_⊥ ≈ 675, which is substantially larger even than in the high‐quality graphite. The electrical resistivity of the annealed films reduces to 1–19 Ω □^?1. The observed modifications of the in‐plane and cross‐plane thermal conductivity components resulting in an unusual K/K_⊥ anisotropy are explained theoretically. The theoretical analysis suggests that K can reach as high as ≈500 W m^?1 K^?1 with the increase in the sp² domain size and further reduction of the oxygen content. The strongly anisotropic heat conduction properties of these films can be useful for applications in thermal management.     

Enhanced Thermoelectric Properties of Antimony Telluride Thin Films with Preferred Orientation Prepared by Sputtering a Fan-Shaped Binary Composite Target

p-Type antimony telluride (Sb₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputter deposition using a fan-shaped binary composite target. The deposition temperature was varied from 100°C to 300°C in increments of 50°C. The influence of the deposition temperature on the microstructure, surface morphology, and thermoelectric properties of the thin films was systematically investigated. x-Ray diffraction results show that various alloy composition phases of the Sb₂Te₃ materials are grown when the deposition temperature is lower than 200°C. Preferred c-axis orientation of the Sb₂Te₃ thin film became obvious when the deposition temperature was above 200°C, and thin film with single-phase Sb₂Te₃ was obtained when the deposition temperature was 250°C. Scanning electron microscopy reveals that the average grain size of the films increases with increasing deposition temperature and that the thin film deposited at 250°C shows rhombohedral shape corresponding to the original Sb₂Te₃ structure. The room-temperature Seebeck coefficient and electrical conductivity range from 101 μV K^?1 to 161 μV K^?1 and 0.81 × 10³ S cm^?1 to 3.91 × 10³ S cm^?1, respectively, as the deposition temperature is increased from 100°C to 300°C. An optimal power factor of 6.12 × 10^?3 W m^?1 K^?2 is obtained for deposition temperature of 250°C. The thermoelectric properties of Sb₂Te₃ thin films have been found to be strongly enhanced when prepared using the fan-shaped binary composite target method with an appropriate substrate temperature.     

Microstructures of YSZ and CGO Thin Films Deposited by Spray Pyrolysis: Influence of Processing Parameters on the Porosity

Microstructures of yttria‐stabilized zirconia (YSZ) thin films deposited by spray pyrolysis at 370 °C on sapphire are investigated. The as‐deposited films are predominantly amorphous and crystallize upon heating at temperatures above 370 °C, developing grains in the range of 5 nm to several 100 nm. During post‐deposition heat treatment up to 800 °C, ～ 50 vol% porosity develops in the center of the films with gradients towards almost dense interfaces to the air and substrate. The reason for this porosity is the decomposition of residues from the precursor and the free volume liberated due to crystallization. Dense YSZ thin films consisting of one monolayer of grains are obtained with annealing temperatures exceeding 1200 °C. In gadolinium‐doped‐ceria (CGO) thin films similar microstructures and porosity are found after low‐temperature heat treatments indicating that the precursor residues due to the deposition method are the main cause of the porosity. Grain growth stagnation in annealed thin films is observed in both the YSZ and in CGO thin films. Stagnating grain growth in the thin films is rather caused by reduced grain boundary mobility, here predominately due to a “secondary phase”, i.e., pores, than to other effects. The stagnation ceases at higher annealing temperatures after densification has taken place.     

Fabrication and Electrochemical Properties of Epitaxial Samarium‐Doped Ceria Films on SrTiO3‐Buffered MgO Substrates

Thin films of samarium‐oxide‐doped (20 mol%) ceria (SDC) are grown by pulsed‐laser deposition (PLD) on (001) MgO single‐crystal substrates. SrTiO₃ (STO) prepared by PLD is used as a buffer layer on the MgO substrates to enable epitaxial growth of the fluorite‐structured SDC film; the STO layer provides a proper crystalline match between SDC and MgO, resulting in highly crystalline, epitaxial SDC films grown in the (001) orientation. Film conductivity is evaluated by electrochemical impedance spectroscopy measurements, which are performed at various temperatures (400–775 °C) in a wide range of oxygen partial pressure (pO₂) values (10^?25?1 atm) in order to separate ionic and electronic conductivity contributions. At 700 °C, SDC/STO films on (100) MgO exhibit a dominant ionic conductivity of about 7 × 10^?2 S cm^?1, down to pO₂ values of about 10^?15 atm. The absence of grain boundaries make the SDC/STO/MgO heterostructures stable to oxidation‐reduction cycles at high temperatures, in contrast to that observed for the more disordered SDC/STO films, which degraded after hydrogen exposure.     

Microstructures and electrical properties of SrRuO3 thin films on LaAIO3 substrates

Conductive SrRuO₃ thin films have been deposited using pulsed laser deposition on LaA10₃ substrates at different substrate temperatures. Structural and microstructural properties of the SrRuO₃/LaAlO₃ system have been studied using x-ray diffraction, scanning electron microscopy, and scanning tunneling microscopy. Electrical properties of SrRuO₃ thin films have been measured. It was found that the film deposited at 250°C is amorphous, showing semiconductor-like temperature dependence of electrical conductivity. The film deposited at 425°C is crystalline with very fine grain size (100∼200?), showing both metallic and semiconductor-like temperature dependence of electrical conductivity in different temperature regions. The film deposited at 775°C shows a resistivity of 280 μΩ.cm at room temperature and a residual resistivity ratio of 8.4. Optimized deposition conditions to grow SrRuO₃ thin films on LaA10₃ substrates have been found. Possible engineering applications of SrRuO₃ thin films deposited at different temperatures are discussed. Bulk and surface electronic structures of SrRuO₃ are calculated using a semi-empirical valence electron linear combination of atomic orbitals approach. The theoretical calculation results are employed to understand the electrical properties of SrRuO₃ thin films.     

Low‐Temperature Superionic Conductivity in Strained Yttria‐Stabilized Zirconia

Very high lateral ionic conductivities in epitaxial cubic yttria‐stabilized zirconia (YSZ) synthesized on single‐crystal SrTiO₃ and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order ～1 Ω^?1cm^?1) are observed at 500 °C for 58‐nm‐thick films on MgO. The results indicate a superposition of two parallel contributions – one due to bulk conductivity and one attributable to conduction along the film–substrate interface. Interfacial effects dominate the conductivity at low temperatures (<350 °C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk‐like conductivity is observed. The films have a negligible grain‐boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150–500 °C are of great fundamental importance but may also be technologically relevant for low‐temperature applications.     

Vertically Aligned Nanocomposite Thin Films as a Cathode/Electrolyte Interface Layer for Thin‐Film Solid Oxide Fuel Cells

A thin layer of a vertically aligned nanocomposite (VAN) structure is deposited between the electrolyte, Ce_0.9Gd_0.1O_1.95 (CGO), and the thin‐film cathode layer, La_0.5Sr_0.5CoO₃ (LSCO), of a thin‐film solid‐oxide fuel cell (TFSOFC). The self‐assembled VAN nanostructure contains highly ordered alternating vertical columns of CGO and LSCO formed through a one‐step thin‐film deposition process that uses pulsed laser deposition. The VAN structure significantly improves the overall performance of the TFSOFC by increasing the interfacial area between the electrolyte and cathode. Low cathode polarization resistances of 9 × 10^?4 and 2.39 Ω were measured for the cells with the VAN interlayer at 600 and 400 °C, respectively. Furthermore, anode‐supported single cells with LSCO/CGO VAN interlayer demonstrate maximum power densities of 329, 546, 718, and 812 mW cm^?2 at 550, 600, 650, and 700 °C, respectively, with an open‐circuit voltage (OCV) of 1.13 V at 550 °C. The cells with the interlayer triple the overall power output at 650 °C compared to that achieved with the cells without an interlayer. The binary VAN interlayer could also act as a transition layer that improves adhesion and relieves both thermal stress and lattice strain between the cathode and the electrolyte.     

Crystallization and Microstructure of Yttria‐Stabilized‐Zirconia Thin Films Deposited by Spray Pyrolysis

The crystallization and microstuctural evolution upon thermal treatment of yttria‐stabilized zirconia (YSZ, Zr_0.85Y_0.15O_1‐δ) thin films deposited by spray pyrolysis at 370 °C are investigated. The as‐deposited YSZ films are mainly amorphous with a few crystallites of 3 nm in diameter and crystallize in the temperature range from 400 °C to 900 °C. Fully crystalline YSZ thin films are obtained after heating to 900 °C or by isothermal dwells for at least 17 h at a temperature as low as 600 °C. Three exothermic heat releasing processes with activation energies are assigned to the crystallization and the oxidation of residuals from the precursor. Microporosity develops during crystallization and mass loss. During crystallization the microstrain decreases from 4% to less than 1%. Simultaneously, the average grain size increases from 3 nm to 10 nm. The tetragonal phase content of the YSZ thin film increases with increasing temperature and isothermal dwell time. Based on these data, gentle processing conditions can be designed for zirconia based thin films, which meet the requirements for Si‐based microfabrication of miniaturized electrochemical devices such as micro‐solid oxide fuel cells or sensors.     

Thermoelectric Properties of Al-Doped ZnO Thin Films

We have prepared 2 % Al-doped ZnO (AZO) thin films on SrTiO₃ substrates by a pulsed laser deposition technique at various deposition temperatures (T _dep = 300–600 °C). The thermoelectric properties of AZO thin films were studied in a low temperature range (300–600 K). Thin film deposited at 300 °C is fully c-axis-oriented and presents electrical conductivity 310 S/cm with Seebeck coefficient ?65 μV/K and power factor 0.13 × 10^?3 Wm^?1 K^?2 at 300 K. The performance of thin films increases with temperature. For instance, the power factor is enhanced up to 0.55 × 10^?3 Wm^?1 K^?2 at 600 K, surpassing the best AZO film previously reported in the literature.     

Post-annealing Effect on Microstructures and Thermoelectric Properties of Bi0.45Sb1.55Te3 Thin Films Deposited by Co-sputtering

p-Type Bi_0.45Sb_1.55Te₃ thermoelectric (TE) thin films have been prepared at room temperature by a magnetron cosputtering process. The effect of postannealing on the microstructure and TE properties of Bi_0.45Sb_1.55Te₃ films has been investigated in the temperature range from room temperature to 350°C. x-Ray diffraction analysis shows that the annealed films have polycrystalline rhombohedral crystal structure, and the average grain size increases from 36?nm to 64?nm with increasing annealing temperature from room temperature to 350°C. Electron probe microanalysis shows that annealing above 250°C can cause Te reevaporation, which induces porous thin films and dramatically affects electrical transport properties of the thin films. TE properties of the films have been investigated at room temperature. The hole concentration shows a trend from descent to ascent and has a minimum value at the annealing temperature of 200°C, while the Seebeck coefficient shows an opposite trend and a maximum value of 245?μV?K^?1. The electrical resistivity monotonically decreases from 19.8?mΩ?cm to 1.4?mΩ?cm with increasing annealing temperature. Correspondingly, a maximum value of power factor, 27.4?μW?K^?2?cm^?1, was obtained at the annealing temperature of 250°C.     

Optimized La0.6Sr0.4CoO3–δ Thin‐Film Electrodes with Extremely Fast Oxygen‐Reduction Kinetics

La_0.6Sr_0.4CoO_3–δ (LSC) thin‐film electrodes are prepared on yttria‐stabilized zirconia (YSZ) substrates by pulsed laser deposition at different deposition temperatures. The decrease of the film crystallinity, occurring when the deposition temperature is lowered, is accompanied by a strong increase of the electrochemical oxygen exchange rate of LSC. For more or less X‐ray diffraction (XRD)‐amorphous electrodes deposited between ca. 340 and 510 °C polarization resistances as low as 0.1 Ω cm² can be obtained at 600 °C. Such films also exhibit the best stability of the polarization resistance while electrodes deposited at higher temperatures show a strong and fast degradation of the electrochemical kinetics (thermal deactivation). Possible reasons for this behavior and consequences with respect to the preparation of high‐performance solid oxide fuel cell (SOFC) cathodes are discussed.     

Chlorine-doped CdS thin films from CdCl<Subscript>2</Subscript>-mixed CdS powder

The CdS:Cl thin films have been prepared using thermally evaporated, CdCl₂-mixed CdS powder at 200°C substrate temperature. The percentage of CdCl₂ in the mixture varied from 0% to 0.20%. The electrical properties and the grain size of the deposited films were investigated. The results show that light doping, resistivity, carrier concentration, and mobility follow Seto’s model for polycrystalline material. However, with heavy doping, these properties undergo a saturation trend. The saturation behavior can be understood in terms of the rapid formation of the A-center complexes in the films. The deposited films were annealed at 250°C and 300°C. The resistivity of pure and lightly doped CdS films increased with annealing temperature, whereas carrier concentration and mobility in these films decreased. However, for the higher doping concentrations, the resistivity decreased, whereas carrier concentration and mobility showed improvement. These changes in electrical properties of the deposited films with annealing and doping concentration are attributed to a reduction in the lattice defect sites in CdS upon annealing. The experimental results are interpreted in terms of a modified version of Seto’s model for polycrystalline materials.     

Low‐Temperature,Nontoxic Water‐Induced Metal‐Oxide Thin Films and Their Application in Thin‐Film Transistors

Here, a simple, nontoxic, and inexpensive “water‐inducement” technique for the fabrication of oxide thin films at low annealing temperatures is reported. For water‐induced (WI) precursor solution, the solvent is composed of water without additional organic additives and catalysts. The thermogravimetric analysis indicates that the annealing temperature can be lowered by prolonging the annealing time. A systematic study is carried out to reveal the annealing condition dependence on the performance of the thin‐film transistors (TFTs). The WI indium‐zinc oxide (IZO) TFT integrated on SiO₂ dielectric, annealed at 300 °C for 2 h, exhibits a saturation mobility of 3.35 cm² V^?1 s^?1 and an on‐to‐off current ratio of ≈10⁸. Interestingly, through prolonging the annealing time to 4 h, the electrical parameters of IZO TFTs annealed at 230 °C are comparable with the TFTs annealed at 300 °C. Finally, fully WI IZO TFT based on YO_x dielectric is integrated and investigated. This TFT device can be regarded as “green electronics” in a true sense, because no organic‐related additives are used during the whole device fabrication process. The as‐fabricated IZO/YO_x TFT exhibits excellent electron transport characteristics with low operating voltage (≈1.5 V), small subthreshold swing voltage of 65 mV dec^?1 and the mobility in excess of 25 cm² V^?1 s^?1.     

Highly transparent and conducting zinc oxide films deposited by activated reactive evaporation

Highly transparent and conducting undoped zinc oxide films have been obtained with a best resistivity of ～1.1 × 10^-3 Ω cm, a carrier density of ～1.5 × 10²⁰ cm^?3 and a mobility of ～38 cm²V^{?1s ?1}. These were produced by activated reactive evaporation at a deposition rate of 2 to 8Å/s with a substrate temperature ≤200° C. The films deposited by this process were found to have resistivities that were thickness independent and also were relatively insensitive to deposition parameters. In terms of conductivity, it was found that films deposited at higher temperatures (T > 300°+ C) were always inferior to the films deposited below 200° C. High temperature vacuum annealing (350° C) significantly degraded the resistivity of the undoped films deposited at low temperature; this was attributable to a drop in both the electron concentration and the mobility. Aluminum doping was found to be able to stabilize the electron concentration while the drop in mobility was found to be related to the choice of substrate.     

Optical and electrical properties of nickel oxide thin films synthesized by sol–gel spin coating

Nickel oxide thin films were prepared by the sol–gel technique combined with spin coating onto glass substrates. The as-deposited films were pre-heated at 275 °C for 15 min and then annealed in air at different temperatures. The effects of the annealing temperature on the structural and optical properties of the films are studied. The results show that 600 °C is the optimum annealing temperature for preparation of NiO films with p-type conductivity and high optical transparency. Then, by using these optimized deposition parameters, NiO thin films of various thicknesses were deposited at the same experimental conditions and annealed under different atmospheres. Surface morphology of the films was investigated by atomic force microscopy. The surface morphology of the films varies with the annealing atmosphere. Optical transmission was studied by UV–vis spectrophotometer. The transmittance of films decreased as the thickness of films increased. The electrical resistivity, obtained by four-point probe measurements, was improved when NiO layers were annealed in N₂ atmosphere at 600 °C.     

High rate (~7 nm/s), atmospheric pressure deposition of ZnO front electrode for Cu(In,Ga)Se2 thin‐film solar cells with efficiency beyond 15%

Undoped zinc oxide (ZnO) films have been grown on a moving glass substrate by plasma‐enhanced chemical vapor deposition at atmospheric pressure. High deposition rates of ~7 nm/s are achieved at low temperature (200 °C) for a substrate speed from 20 to 60 mm/min. ZnO films are highly transparent in the visible range (90%). By a short (~minute) post‐deposition exposure to near‐ultraviolet light, a very low resistivity value of 1.6·10⁻³ Ω cm for undoped ZnO is achieved, which is independent on the film thickness in the range from 180 to 1200 nm. The photo‐enhanced conductivity is stable in time at room temperature when ZnO is coated by an Al₂O₃ barrier film, deposited by the industrially scalable spatial atomic layer deposition technique. ZnO and Al₂O₃ films have been used as front electrode and barrier, respectively, in Cu(In,Ga)Se2 (CIGS) solar cells. An average efficiency of 15.4 ± 0.2% (15 cells) is obtained that is similar to the efficiency of CIGS reference cells in which sputtered ZnO:Al is used as electrode. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of Annealing Ambient on the Electrical and Optical Properties of Aluminum-Doped ZnO Films Produced via a Sol–Gel Process

In this study, aluminum-doped ZnO (AZO) thin films were prepared by a sol–gel with spin coating process. The AZO films were annealed by a two-step process. The films were first annealed in air or nitrogen at 500°C for 3 h, followed by annealing in three types of ambient, i.e., vacuum (10^?3 Torr or 10^?6 Torr) or forming gas (10% H₂/90% N₂), at 500°C for 4 h. The effect of the annealing ambient on the microstructure, electrical and optical properties of the AZO films was explored by x-ray diffraction, field-emission scanning electron microscopy, four-point probe sheet resistivity measurements, Hall voltage measurements, and ultraviolet–visible spectroscopy. The results showed that the size of AZO particulates in the films was determined mainly by the first annealing step. The films annealed in air in the first step were composed of larger AZO particulates than those annealed in nitrogen. The conductivities of the AZO films were significantly increased by the second annealing step. Second annealing in a high-vacuum system (10^?6 Torr) led to the highest AZO film conductivity among the three ambients. Regardless of the various annealing processes, the films remained transparent under visible light and exhibited a sharp absorption edge in the ultraviolet region. The highest conductivity, i.e., 168 S cm^?1, was obtained from films annealed first in air and then in vacuum of 10^?6 Torr.     

Structural and Electrical Characterization of ZnO Films Grown by Spray Pyrolysis and Their Application in Thin‐Film Transistors

The role of the substrate temperature on the structural, optical, and electronic properties of ZnO thin films deposited by spray pyrolysis using a zinc acetate precursor solution is reported. Analysis of the precursor compound using thermogravimentry and differential scanning calorimetry indicates complete decomposition of the precursor at around 350 °C. Film characterization using Fourier Transform Infrared Spectroscopy (FTIR), photoluminescence spectroscopy (PL), and ultraviolet–visible (UV–Vis) optical transmission spectroscopy suggests the onset of ZnO growth at temperatures as low as 100 °C as well as the transformation to a polycrystalline phase at deposition temperatures >200 °C. Atomic force microscopy (AFM) and X‐ray diffraction (XRD) reveal that as‐deposited films exhibit low surface roughness (rms ≈ 2.9 nm at 500 °C) and a crystal size that is monotonously increasing from 8 to 32 nm for deposition temperatures in the range of 200–500 °C. The latter appears to have a direct impact on the field‐effect electron mobility, which is found to increase with increasing ZnO crystal size. The maximum mobility and current on/off ratio is obtained from thin‐film transistors fabricated using ZnO films deposited at >400 °C yielding values on the order of 25 cm² V^?1s^?1 and 10⁶, respectively.     

SnS?Sno2 conversion of chemically deposited SnS thin films

The thermal decomposition of chemically deposited SnS thin films to SnO₂ films by air annealing at temperatures up to 400°C is discussed. The conversion of a 0.7 μm thick SnS thin film to an SnO₂ film involves the creation of non-stoichiometric SnS, SnS + SnS₂ mixed phase and non-stoichiometric SnO₂ (i.e. SnO_{2 ? x}), as concluded from X-ray diffraction patterns, optical transmission spectra and electrical characteristics. The SnO₂ thin films obtained in this manner are photoconductive, with a lowest sheet resistance (in the dark) of about 10⁵ Ω/□ and an activation energy (E_a) of 0.1 eV for the electrical conductivity observed for the SnS films annealed at 325°C. This was found as the onset temperature for conversion of the SnS + SnS₂ phase to the non-stoichiometric SnO_{2 – x} film. Elevation of the annealing temperature to 400°C results in an elevation of the sheet resistance to about 10⁹ Ω/□ with the value of E_a at 1.3 eV, indicating an improvement in the degree of stoichiometry.