The aim of this study is to examine the technological challenge of the electrochemical formation of zinc oxide and Al-doped ZnO films (ZnO:Al, AZO) as transparent conductive oxide coatings with complex architectures for solar cell photoanode materials. A cathodic electrodeposition of AZO was performed using aqueous nitrate electrolytes at 25°C. A significant positive deviation in aluminum percentage in the films was demonstrated by the LAES, EDX, and XPS methods, which originates from aluminum hydroxide sedimentation. The photoluminescent characteristics of the ZnO films reveal low band intensities related to intrinsic defects, while the samples with 1 at.% of aluminum show a strong and wide PL band at
Zinc oxide (ZnO) is a multifunctional material, mostly due to its transparent and conducting characteristics and its advantages in a wide range of technological applications such as electrodes in solar cells and in flat-panel displays, touch control panels, shields with electromagnetic protection, light-emitting diodes (LEDs), and smart windows. In order to improve the electrical and optical properties of transparent semiconductive ZnO films, oxide was doped with a number of metal or nonmetal elements. Aluminum-doped ZnO films (AZO) are considered the most promising alternative to expensive indium-based ITO for transparent conducting oxide materials for solar cells.
AZO thin films can be prepared by various techniques such as sol-gel synthesis [
It should be noted that only very few articles describe the electrochemical deposition of zinc and aluminum oxide. Most of these electrochemical approaches reported for ZnO film preparation use an aqueous bath of zinc salt such as zinc chloride and zinc nitrate, with the commonly used zinc precursors in zinc electrolytic baths [
Most publications on the deposition of ZnO from zinc nitrate baths describe a uniform film growth for heating up to 80°C only when individual ZnO particles are round-shaped or hexagon-like, depending on deposition conditions. According to Refs. [
In Ref. [
In this article, electrocrystallization of zinc oxide at ITO substrates was carried out using nitrate electrolytic baths with Zn(NO3)2 as a zinc precursor and aluminum nitrate as a source of Al3+ ions. The chemical composition of zinc and aluminum coprecipitation products obtained in an electrical field at room temperature (RT) was analyzed precisely to reveal any admixture phases. The effect of the aluminum percentage on the optical and electrical properties of AZO was assessed through UV–vis optical transmittance, photoluminescent spectroscopy, and two-fold electrical measurements.
Al-doped ZnO films have been prepared by mutual electrochemical deposition of zinc and zinc oxide. The process has been carried out in a three-electrode cell at room temperature from nitrate aqueous baths. The zinc bath composition included just nitrates, namely, 0.2 М Zn(NO3)2, 0.5 М KNO3, and 0.0032 М Al(NO3)3. The working and counter electrodes have been placed parallel to each other and separated by 2 cm. The reference electrode is a saturated aqueous Ag/AgCl/KCl(s) electrode connected to the cell via a Luggin capillary. A platinum wire of 0.5 mm diameter has been applied as a counter electrode. The samples with a different theoretical thickness of
The sample morphologies of ZnO films are examined using a Leo Supra 50 VP scanning electron microscope or a Carl Zeiss NVision 40 scanning electron microscope (SEM). Both the instruments are equipped with EDX Oxford Instruments attachments for the local chemical analysis.
TG-DTA analysis has been performed with preliminary delaminated pristine coatings of complex composition. All measurements have been carried out using STA 409 PC Luxx coupled with a quadrupole mass spectrometer QMS 403С Aëolos (NETZSCH). Annealing has been done in argon as a gas carrier with a flow rate of 30 ml/min. The samples have been annealed up to 800°C with a heating rate of 5°/min and an isotherm at 800°C for 30 min.
Mass spectrometry with inductively coupled plasma (ICP MS) analyses of electrolyte contents have been carried out using an ICP spectrometer Perkin-Elmer ELAN DRC-II. Standard solutions have been prepared to correspond to a range of concentrations of the components to be determined at 0-10
Laser atomic emission spectroscopy (LAES) has been carried out for semiquantitative analysis of aluminum percentage in the films deposited. The spectral recording system is implemented on a monochromator-spectrograph with a 4-position turret of replaceable diffraction gratings and a CCD detector from Toshiba or on several Paschen-Runge-type polychromators with five Toshiba CCD linear detectors with the following characteristics: range of wavelengths 177-800 nm, reproducibility ±0.01 nm, spectral
The XPS spectra have been recorded on a laboratory spectrometer PHI5500VersaProbeII. The excitation source is monochromatized Al K
The dark DC electrical resistivity of the films studied has been measured at room temperature by a routine 2-terminal technique in a capacitor configuration. The contacts to the films and to the substrate were prepared using Ag paint.
Experiments on the deposition of Al-doped zinc oxide were carried out using conductive transparent ITO substrates to examine the microstructure and composition for further application as photoanodes. The corresponding cyclic voltammetry (CV) curves and preferred electroplating modes are presented in Figure
Cyclic voltammograms of ITO electrode in zinc nitrate electrolytes with different concentrations of aluminum nitrate: (a) 1—Al-free zinc nitrate electrolyte and 2—Zn-free aluminum nitrate electrolyte. (b) 1—Al-free electrolyte, 2—1 at.% of aluminum, and 3—2 at.% of aluminum. Scan rate—20 mV/s.
Zinc deposition electrolytes and applied deposition methods.
Electrolytes | Electrolyte composition | C(Zn) (ICP MS), mol/l | C(Al) (ICP MS), mol/l | Al/Zn (ICP MS) | Deposition potential, deposition rate |
---|---|---|---|---|---|
Al-0 | 0.2 М Zn(NO3)2 | -1.1 V (-1.3 ÷ -1.1 V) vs. Ag/AgCl | |||
0.5 М KNO3 | |||||
Al-1 | 0.2 М Zn(NO3)2 | 99.02 | 0.98 | 0.0099 | -1.1 V (-1.3 ÷ -1.1 V) vs. Ag/AgCl |
0.5 М KNO3 | |||||
0.0032 М Al(NO3)3 | |||||
Al-2 | 0.2 М Zn(NO3)2 | 98.05 | 1.95 | 0.0199 | -1.1 V (-1.3 ÷ -1.1 V) vs. Ag/AgCl |
0.5 М KNO3 | |||||
0.0064 М Al(NO3)3 | |||||
Al-4 | 0.2 М Zn(NO3)2 | 96.03 | 3.97 | 0.0413 | -1.1 V (-1.3 ÷ -1.1 V) vs. Ag/AgCl |
0.5 М KNO3 | |||||
0.0128 М Al(NO3)3 | |||||
Al-6 | 0.2 М Zn(NO3)2 | 94.00 | 6.00 | 0.0638 | -1.1 V (-1.3 ÷ -1.1 V) vs. Ag/AgCl |
0.5 М KNO3 | |||||
0.0192 М Al(NO3)3 | |||||
Al-10 | 0.2 М Zn(NO3)2 | 90.07 | 9.93 | 0.1102 | -1.1 V (-1.3 ÷ -1.1 V) vs. Ag/AgCl |
0.5 М KNO3 | |||||
0.0256 М Al(NO3)3 |
According to CV data and other studies [
The reduction of nitrate ions (
Reduction of Zn2+ ions to metallic zinc occurs below −1.0 V:
This deposition mechanism indicates that zinc hydroxide Zn(OH)2 is an appropriate precursor for ZnO electrochemical growth because of its slower deposition rate and easy zincite formation on heating. The formation of zinc hydroxide is optimal for controlling the morphology of the product, including nucleation processes and growth and phase boundary formation between the ZnO grains. On the other hand, slow electrocrystallization of metal zinc with the subsequent oxidation is an alternative way to obtaining a uniform transparent coating. The formation of zincite-like ZnO occurs right at the cathode if the concentration of hydroxyl ions is low. The growth kinetics and also habitus of the crystallites is mostly determined by the concentration of Zn2+ ions in the near-cathode region.
In order to identify the electrodeposition processes and to verify the electrochemical behavior of the electrodes in nitrate baths, cyclic voltammetry measurements were carried out. Figure
The XPS data confirms the formation of the aluminum hydroxide phase at the working electrode in the process of electrocrystallization at -1.1 V. An analysis of the Al2p and O1s line profiles has shown the presence of a single Al3+ compound (Figure
Characteristic XPS bands for Al 2p and O 1s for the sample with theoretical percentage of Al 2 at.%.
An XRD analysis was carried out to identify the formation of Zn(OH)2 and Al(OH)3 at the electrodes. It was found that most reflections in diffractograms correspond to Zn(OH)2 (file JCPDS 36-1451) while no reflections of the individual Al(OH)3 phase were found.
All concentrations of metals in the baths were measured experimentally by mass spectrometry (ICP MS). The determined values of the concentrations are fairly close to the corresponding theoretical values, with a margin of error not exceeding 1-5%. The data obtained is given in Table
LAES data for the deposited films show a systematic increase in the aluminum percentage in the bulk of the films which contain much more aluminum than required (Table
Characteristics of ZnO:Al coatings after annealing at 530°C for 1 hours. 1
Electrolytes | Zn (LAES), at% | Al (LAES), at% | Al/Zn (LAES) | Zn (EDX), at% | Al (EDX), at% | Al/Zn (EDX) | Zn (XPS), at% | Al (XPS), at% | Al/Zn (XPS) |
---|---|---|---|---|---|---|---|---|---|
Al-0 | 0 | 34.5 | 0 | 0 | |||||
Al-1 | 90.9 | 9.07 | 0.10 | 3.03 | 1.60 | 0.53 | 21.0 | 5.0 | 0.24 |
Al-2 | 86.0 | 14.0 | 0.16 | 2.97 | 5.59 | 1.88 | 7.5 | 15.0 | 2.00 |
Al-4 | 86.3 | 13.7 | 0.16 | 0.88 | 1.58 | 1.80 | 9.7 | 7.2 | 0.74 |
Al-6 | 86.0 | 14.0 | 0.16 | 1.63 | 9.26 | 5.68 | 1.3 | 18.1 | 13.90 |
Al-10 | 84.3 | 15.7 | 0.19 | 3.82 | 16.13 | 4.22 | 23.6 | 18.9 | 23.60 |
Graphical representation of the Al/Zn ratio in electrolytes and the samples before annealing according to (a) ICP MS data for the electrolytes (1), (b) LAES data (2), and (c) XPS data (3) for 2
Table
The EDX data showed a uniform distribution of zinc and aluminum over the surface of the film, but found a complete discrepancy between the ratios of metals in the volume of the films, which is due to the low accuracy of the method (Table
A semiquantitative analysis of the surface composition of the films was achieved by XPS spectroscopy. The same effect of the positive deviation of the surface concentration of aluminum was found, and the mole fraction of surface aluminum exceeds its volume percentage. The Al/Zn ratios obtained from the XPS survey spectra are higher than the values found for the bulk of the film by the LAES and EDX methods, which show an integral result for the coating (Figure
According to experimental data, the aluminum content on the surface of the film is much higher than the bulk content, as can be seen in Figure
ITO is an appropriate substrate for ZnO crystal growth both in aqueous and alcohol media. As reported by Lebedev et al. [
SEM micrographs of ZnO coatings deposited in different aqueous nitrate baths with various Al percentage, namely, (a) ZnO electrolyte, (b) 1 at.% of aluminum, (c) 2 at.% of aluminum, (d) 4 at.% of aluminum, (e) 6 at.% of aluminum, and (f) 10 at.% of aluminum.
On the other hand, it is interesting that there are still not many techniques for porous ZnO template synthesis, as porous semiconductive coatings have great potential for different electronic applications including photovoltaics [
In the micrographs, it is clear that the grains are thin and flake-like with an average diameter of about 1
In accordance with TG-DTA data (Figure
TG-DTA data for pristine ZnO films deposited from nitrate bath at RT before their annealing at 500°C in air. 1—weight loss in w.%, 2—DSC effect, and 3—ICP MS data for releasing gas products with mass numbers of 44 and 18, respectively.
The electron diffraction (ED) data of individual deposit crystallites also correspond well to the hexagonal ZnO lattice but not to zinc hydroxide Zn(OH)2. A diffractogram of an individual flat plate-like crystallite is presented in Figure
TEM and electron diffraction data for a single particle of ZnO produced via the annealing process in the Al-0 film.
The transport characteristics of the samples were studied to reveal the formation of an inert Al2O3 phase at the surface or ZnO doping by aluminum. The change in the phase composition of the samples during annealing is indicated by the change in the electrical resistivity of the samples, which decreases from (2-4)x106 Ohm·cm approximately by factor 2 for Al-1 - Al-4 samples but not for the Al-6 and Al-10 ones. Generally, the Al doping resulted in the fairly high conductivity of samples. However, insulator admixtures such Al2O3 segregate under the annealing conditions onto the ZnO grain boundaries increasing the resistance of the films. Such effect is the most significant for the samples with higher aluminum content.
Also, assuming that the Al/Zn ratio is in an appropriate range, it is expected that during annealing Al3+ ions diffuse into the ZnO lattice and some of the Zn2+ sites are replaced by Al3+. According to the defect theory, Al3+ ion substituted in the crystal lattice acts as an interstitial impurity. At the same time, the electrical conductivity of AZO films depends on the oxygen vacancy and the contribution of
Al-doped ZnO films were studied by optical spectroscopy in the UV-vis range to characterize their optical transparency and reflectance inputs. Figure
Optical transmittance spectra of ZnO coatings of 1—Al-0, 2—Al-1, and 3—Al-2 with different thicknesses of 1000 nm and 2000 nm.
Both undoped and Al-doped films have strong absorption in the UV region and good transmittance of 75% in the visible region. The absorption intensity of Al-doped films increases slowly in the visible range, and the absorption peaks are not obvious in comparison with undoped ZnO where the absorption intensities increase rapidly at the absorption edge. This means that Al doping degrades the film crystal quality. Otherwise, Al doping leads to a slight blue shift of the film absorption edge and the shift increases monotonically with an increase in Al concentration as reported by [
The ion Al3+ radius is smaller than that of Zn2+. The substitution of lattice Zn2+ with Al3+ would widen the ZnO bandgap. The blue shift of the absorption edge of the doped films indicates that the doped Al3+ ions are located in the lattice site forming
Photoluminescence data is shown in Figure
Photoluminescence spectra of ZnO:Al coatings deposited from zinc-aluminum nitrate electrolytes. (a) PL spectra for Al-0 (Al-free) films at 1—77 K and 2—293 K. (b) The PL spectra of 1—Al-0 and 3—Al-1 (1 at.% of Al (theoretical)) collected at 77 K. The excitation wavelength of 337 nm.
The near-band-edge luminescence (NBE) of crystalline ZnO was collected at 77 K and 293 K, and there are only strong lines in the spectra at 370 nm (3.35 eV) and 395 nm (3.14 eV) correspondingly. In the PL spectrum collected at 77 K both green and yellow lines of intrinsic defects,
In the PL spectra of the samples with the lowest theoretical percentage of aluminum of 1 at.%, a strong maximum at
For the comparison, all samples with higher theoretical percentage of aluminum (with Al-reached surfaces) showed a different character of their PL spectra. These spectra included the blue shoulder right near the NBE bands at
The samples obtained in the zinc-aluminum nitrate aqueous electrolyte are, most likely, doped ZnO:Al or ZnO:Al with some Al2O3 admixture at the surface. An important advantage of this electrolyte composition is the intensive oxidation of the electrolytic precipitate. No metal phase has been detected in the samples which means ZnO films can be obtained from nitrate electrolyte in a single step, without any additional oxidants. Also, a positive deviation in the aluminum percentage in the sample could be reduced if electrolytes are used with a smaller concentration of the dopant. The doping of ZnO with Al has produced a new photoluminescence maximum originated from different kinds of point defects as oxygen vacancies are included both as a substitute for Zn by Al3+ ions in the ZnO lattice with coordinated interstitial oxygen.
On the other hand, the microstructure of the films is porous, which is appropriate for semiconductive scaffolds for solar cells but less suitable for most TCO layers in microelectronics. Subsequent studies should focus on improving this deposition technique.
The data used to support the findings of this study are obtained by all authors at Lomonosov Moscow State University (MSU), Lebedev Physical Institute (LPI) of the Russian Academy of Sciences, Prokhorov General Physics Institute of the Russian Academy of Sciences (GPI), and ZAO “SPECS”. The composition characterization data (ED, XRD, XPS, and TG-DTA MS data) used to support the findings of this study are included within the article. The SEM data used to support the findings of this study are included within the scope of the article. The diffuse spectroscopy data are performed in MSU and used to support the findings of this study which are included within the article. The photoluminescence spectroscopy data carried out in LPI and used to support the findings of this study are included within the article. The twofold resistance measurements are carried out in GPI using an original setup. LAES measurements are obtained using an original setup developed by ZAO “SPECS”.
The project was performed using the equipment and setups of the collective resource center of Moscow State University “Technologies for synthesis of new nanostructured materials and their complex characterization.”
The authors declare that they have no conflicts of interest.
Authors are grateful to colleagues from Moscow State University for their assistance in experiments, namely, Prof. Andrei V. Shevelkov, Prof. Alexander V. Knotko, Dr. Olga V. Boytsova, Dr. Tatyana Shatalova, Dr. Sergey S. Abramchuk, and Prof. Eugene Goodilin. This work is supported by the Russian Foundation for Basic Research (grant 19-03-00849_a).