Precursors and physical-chemical conditions to deposit the ZnO, CZTS, and CZT films by spray pyrolysis method.
Abstract
The paper presents the investigation on the influence of substrate temperature Ts and the sprayed initial solution volume Vs on structural, substructural, optical properties, and elemental composition of ZnO and Cu2ZnSnS4 (CZTS) films as well as state-of-the-art of studying the Cd1−xZnxTe (CZT) films obtained by spray pyrolysis technique. The single-phase nanocrystalline ZnO films with average crystallite size of DC = 25–270 nm and thickness of d = 0.8–1.2 μm can be deposited at substrate temperatures of Ts > 473 K. The continuous CZTS films with optimal thickness (d = 1.3 μm) for application as absorber layers in solar cells were deposited at the sprayed initial precursor volume of Vs = 5 ml. The increase of the substrate temperature up to 673 K caused the significant improvements in the stoichiometry of ZnO films. The optimal stoichiometry ratio of CZTS films for application in solar cells was obtained at Vs = 3–4 ml. Optical study of ZnO films showed that these films have a high-transmission coefficient values of T = 60–80%. To the best of our knowledge, there is the lack of works devoted to the study of CZT films obtained by spray pyrolysis technique.
Keywords
- ZnO
- Cu2ZnSnS4
- Cd1−xZnxTe
- thin films
- pulsed spray pyrolysis
1. Introduction
ZnO is an
Cd1–xZnxTe (CZT) solid solutions are perspective alternative absorption materials to Сu(In,Ga)(S,Se)2 in the tandem solar cells having the band gap value of
Among the methods to deposit the ZnO, CZTS, and CZT films, special attention is paid to the spray pyrolysis technique having unique advantages: simplicity, efficiency, and cheapness. This technique provides the non-vacuum deposition of a large-area thin film with well-controlled properties.
It was shown [8, 9] that the greatest influence on physical properties and elemental composition of ZnO film has a substrate temperature
Thus, the investigation of the influence of deposition conditions on structural, substructural, and optical properties of ZnO, CZTS, and CZT films deposited by spray pyrolysis technique is the perspective in terms of its application in highly efficient optoelectronic devices.
2. ZnO, CZTS, and CZT thin films deposition methods. Peculiarities of the spray pyrolysis technique
The wide range of methods is well developed to deposit ZnO, CZT, and CZTS films which split into physical (for example, magnetron sputtering [12, 13, 14]) and chemical (for example, spray pyrolysis [5, 8, 9, 10]) techniques. Typically, the physical methods allow to obtain more perfect films with a higher structural quality, and these methods provide a precise control of the films thickness and low content of defects in deposited material compare to chemical methods, but physical deposition techniques require the usage of more complicated equipment and presence of high level of vacuum, thus they are energy-consuming. In contrary, chemical techniques to deposit ZnO, CZTS, and CZT films are low-cost and energy savers. Among them, spray pyrolysis method is considered as the most promising technique. This technique is simple and non-vacuum providing the deposition of the continuous, porous, nanostructured films, and multilayered structures [15].
Taking into account the increased interest to the nanosized materials with properties, significantly different to bulk materials (caused by quantum-size effect), several scientific groups have obtained the nanocrystalline ZnO and CZTS films [16, 17]. It is important to note that the works dedicated to the study of the nanosized structures used chemical techniques for films deposition. ZnO, CZTS, and CZT films deposited by spray pyrolysis technique are not yet well-studied; this fact conditioned the aim of our study.
The image of laboratory setup developed for the deposition of ZnO, CZTS, and CZT films by pulsed spray pyrolysis is showed in Figure 1. It consists of a spraying gun with initial precursor volume reservoir (1), spraying nozzle (2), and microcontroller block (3), allowing the control of the number of spraying cycles, time, and pauses between cycles. To the spraying gun, the compressor with pressure regulator (4) is connected with the aim of producing the air flow for transportation of the dispersed precursor onto heated substrate surface. Between the spraying gun and the compressor, an electromagnetic valve (5) is installed, where the “open” and “closed” regimes are controlled by the microcontroller block (3). The heating of substrate (6) is provided by the heating plate (7). During the deposition of films by spray pyrolysis technique, the properties of ZnO, CZTS, and CZT condensates are dependent on the precursor choice and physical, chemical deposition conditions. Table 1 presents the overview of deposition conditions and precursors typically used to deposit the ZnO, CZTS, and CZT films by spray pyrolysis technique.
№ | Initial precursor | Solvent | Concentration (М) | Substrate type | Substrate temperature, | Ref. |
---|---|---|---|---|---|---|
ZnO films deposition | ||||||
1 | Zinc chloride (ZnCl2) | H2O | 0.10 | Silicon | 623–823 | [21] |
H2O | 0.10 | Glass | 773 | [22] | ||
H2O | 0.10 | Glass | 523–723 | [23] | ||
2 | Zinc acetate (Zn(CH3COO)2∙2H2O) | H2O | 0.04 | Glass | 573 | [24] |
3 | Zinc acetate Zn(CH3COO)2 | H2O + CH3OH | 0.20 | Glass | 693 | [25] |
H2O | 0.50 | Glass | 453–723 | [26] | ||
H2O | 0.10 | Glass | 623 | [27] | ||
CZT films deposition | ||||||
4 | Cadmium chloride (CdCl2) Zinc chloride (ZnCl2) Tellurium chloride (TeCl4) | H2O | 0.02 (1:1:3) | Glass | 250–325 | [10, 11] |
CZTS films deposition | ||||||
5 | Copper chloride (CuCl2) Zinc chloride (ZnCl2) Tin chloride (SnCl2) Тhiourea (CS(NH2)2) | (CH3)2SO | 0.010 0.005 0.005 0.040 | Soda-lime glass | 623 | [28] |
6 | Copper chloride (CuCl2) Zinc chloride (ZnCl2) Tin chloride (SnCl2) Тhiourea (CS(NH2)2) | H2O + C2H5OH | 0.020 0.010 0.010 0.080 | Soda-lime glass | 553–633 | [29] |
It should be noted that in order to obtain initial molecular solution for the deposition, the typical materials are metal salts dissolved in polar solvents, particularly in water, ethanol, etc. The most common substrates used are the non-oriented glass and silicon slides. The average substrate temperature is in the range of 250–823 K. It should be noted that these values are lower in comparison to the substrate temperatures used in physical methods.
3. Morphological, structural, and substructural properties of ZnO and CZTS films obtained by spray pyrolysis technique
The surface morphology, structural, substructural, optical properties, and chemical composition of ZnO and CZTS films deposited by spray pyrolysis method are determined by its physical, chemical, and technological deposition conditions.
3.1. The morphological properties
SEM images of ZnO films deposited at different substrate temperatures are presented in Figure 2a–d [19, 20]. It has been shown that at substrate temperatures higher than 473 K, crack-free and continuous nanocrystalline ZnO films with a good adhesion to substrate were formed.
The average grain size in the condensates was in the range of
One of the main film parameters of CZTS films is its thickness, which is typically controlled by the dispersed precursor volume
Thus, we have studied the CZTS films deposited by spray pyrolysis technique at different sprayed initial precursor volumes which had the higher thickness than studied in Ref. [9].
In Figure 2e–h, the SEM images of CZTS films and its cross-section deposited at different
3.2. Structural and substructural properties
Structural and substructural properties of ZnO and CZTS films have a significant influence on functional characteristics of devices [18, 19, 20]. Thus, its study is an important scientific objective. For example, band gap of zinc oxide films can be significantly increased by means of nanostructuring due to the quantum-size effects. At the same time, CZTS films as the absorber layers is SC should have the crystallites with sizes larger that diffusion length of minority charge carriers [6]. However, the films obtained by spray pyrolysis are usually characterized by high levels of microdeformations, microstresses, and density of dislocations in comparison to the values observed in the condensates deposited by physical vacuum methods, e.g., thermal evaporation, magnetron sputtering, etc.
The detailed description of methods applied to study structural, substructural, and optical properties of films is described elsewhere [18, 19, 20].
In Figure 3a, the XRD patterns of ZnO films deposited at different substrate temperatures are presented. On the diffraction patterns of the low-temperature films is dominated the diffraction line at angles 35.60–36.10° that corresponds to the reflection from (101) plane of ZnO hexagonal phase. On the diffraction patterns of the films deposited at
In Figure 3b, the X-ray patterns of CZTS films deposited at different dispersed solution volumes are shown. As can be seen from Figure 3b, on X-ray patterns is dominated the line on angles 28.05–28.50° which corresponds to the reflection from (112) CZTS tetragonal phase crystallographic plane. There are also presented lines at angles 47.15–47.50° and 55.55–56.45° which correspond to the reflection from (220) and (312) CZTS planes, respectively. It should be noted that during the increasing of precursor volume the intensity of peaks is increased and its half-width is decreased. It may be caused by the increasing of film thickness and by improvement of the films’ crystalline quality. It is well-known that intensities ratio between the number of diffraction reflections from kesterite and stannite crystallographic planes is different [30]. Taking into account this fact, determination of these ratios gives an opportunity to estimate precisely the materials dominate phase. The measured intensity ratio
Lattice parameter of the materials is a characteristic which is very sensitive to stoichiometry varying, impurities introduction, oxidation, etc. Thus, the precise determination of these values allows us to study the corresponding processes.
In Figure 4, the dependencies of ZnO and CZTS films lattice parameters
InFigure 5, the results of measurements
As can be seen from Figure 5c, in CZTS films during the increasing of the dispersion solution volume from 2 to 5 ml, CDS values are almost were not changed:
In CZTS films, during the increase of the dispersion solution volume microstresses level is decreased, wherein the smallest
In Figure 6, the results of measurements of the dislocations concentration on the boundaries (
The smallest values of
3.3. The study of the stoichiometry
Energy dispersed analysis of the X-ray spectra (EDAX) allows us to determine the elemental composition of ZnO and CZTS films obtained in present work. Results determined for films deposited at different physical-chemical and technological conditions are presented in Table 2. As it can be seen, ZnO films have some oxygen surplus on zinc. Besides, films stoichiometry is increased during the increasing of the substrate temperature. This fact is confirmed by the concentration ratios
ZnO | |||||||
---|---|---|---|---|---|---|---|
473 | 41.8 | 58.2 | 1.4 | ||||
523 | 42.3 | 57.7 | 1.4 | ||||
573 | 42.6 | 57.4 | 1.3 | ||||
623 | 44.3 | 55.7 | 1.2 | ||||
673 | 44.0 | 56.0 | 1.2 | ||||
Stoichiometry | 50.0 | 50.0 | 1.0 | ||||
CZTS | |||||||
2 | 28.6 | 21.4 | 14.3 | 35.8 | 0.8 | 1.5 | 0.8 |
3 | 27.0 | 17.3 | 14.7 | 40.8 | 0.8 | 1.2 | 0.7 |
4 | 27.7 | 16.3 | 15.1 | 40.9 | 0.9 | 1.0 | 0.6 |
5 | 26.4 | 15.2 | 15.4 | 43.0 | 0.9 | 1.0 | 0.6 |
Stoichiometry | 25.0 | 12.5 | 12.5 | 50.0 | 1.0 | 1.0 | 0.5 |
The control of CZTS films elemental composition is a complex and important task because of its probable determination of the phase conditions, crystal structure, optical, and electrical properties of investigated layers. It was estimated that in CZTS films some copper, zinc, and tin are present in surplus and has some sulfur deficiency. Sulfur losses in films during the pyrolytic reaction of the initial precursor near the surface of the heated substrate may be caused by its high volatility [41]. It should be noted that stoichiometry of studied films is some improved during the increasing of dispersed precursor volume. Also, the obtained ratio
4. Optical properties of ZnO and CZTS films obtained by spray pyrolysis technique
4.1. Optical properties
The study and control of the optical properties of ZnO, CZTS, and CZT films is an important task with the aim of their usage in optoelectronic devices, especially for SCs development. It is well-known that optical characteristics of these films heavily dependent on morphological, structural, substructural properties, chemical composition, and physical (chemical) and technological deposition conditions.
In present work, the transmission light coefficient of ZnO films was in the range of
As can be seen from Figure 7a, band gap
In Figure 7b, dependence of the materials
4.2. Raman and Fourier transform IR (FTIR) spectra
Raman spectroscopy is an additional to X-ray diffraction analysis method of studying the phase composition and quality of ZnO, CZTS, and CZT thin films.
Raman spectra of ZnO films measured in the range of frequencies 90–800 cm−1 are presented in Figure 8a. In spectra, a number of different intensity lines on the next frequencies: 95–98, 333–336, 415, 439–442, 572, and 578–587 cm−1 are observed. Using the reference data, these lines were interpreted by us as the next phonon modes:
FTIR spectroscopy is an addition to X-ray diffraction analysis and Raman spectroscopy technique, which allows to obtain an information about the elemental composition of the studied material and its contamination by the precursor impurities. The number of frequencies, where the light absorption and transmission in films are performed, allows us to determine the functional links between chemical elements which are part of the studied materials.
In Figure 8b, FTIR reflection spectra of ZnO films deposited at different substrate temperatures are presented. Although that thin films were deposited in air by chemical technique obtained spectra were comparatively pure.
At low frequencies (460–475 cm−1), there is observed minima, which due to the reference data [48], correspond to Zn-O vibrational mode. It should be noted that FTIR spectra obtained on films deposited in all range of substrate temperatures have a C-Cl vibrational mode [50]. The presence of this connection may be caused by the usage of HCl acid, which was added as a precursor during its preparation. The acid paths are also observed in films. In FTIR spectra of ZnO films deposited at
It is well known that in CZTS films, the presence of secondary phases, such as CuxSy, ZnxSy, SnxSy, CuxSnSy, ZnO, and ZnxSnOy, is available [39, 51, 52, 53]. They are characterized by affiliated lattices, and they indicate on XRD patterns refractions on similar angles. It complicates the phase analysis by XRD technique. Thus, for precise identification of the secondary phases in CZTS compound, the researchers often use Raman spectroscopy in addition to XRD analysis [54]. It allows to identify not only secondary phases, but also kesterite and stannite. In Table 3, the results of study the Raman spectra of CZTS films using as an excitation source the radiation of several lasers are presented. At all spectra regardless on the precursor volume and excitation laser type, the main peak on frequencies of (339–340) cm−1 is presented. It is well correlated to the results of previous studies [52, 54, 55]. In Raman spectra obtained using the green laser, lines on the next frequencies: 142, 340, and 664 cm−1 are also observed, which correspond to
Experimental data | Literature data | ||||||
---|---|---|---|---|---|---|---|
Raman shift, cm−1 | Symmetry | Mode | Reference | ||||
2 | 3 | 4 | 5 | ||||
Raman shift (cm−1) | |||||||
Green-laser ( | |||||||
142 | 143–144 | E | [56] | ||||
340 | 338–339 | A | [54] | ||||
664 | 672 | A | [56] | ||||
Red laser ( | |||||||
339 | 338–339 | A | [52] | ||||
663 | 672 | A | [57] | ||||
UV-laser ( | |||||||
340 | 341 | A | [56] | ||||
— | 560 | — | — | 541 | — | [45] | |
664 | 672 | A | [57] |
Usage of the red- and UV-lasers as phonons excitation source allows us to increase the method’s sensitivity onto the revealing of compounds with optical band gap close to
5. Conclusions
As a results of the complex study of structure, substructure, optical properties, and elemental composition of ZnO, CZTS, and CZT films obtained by pulsed spray pyrolysis technique dependent on the physical (chemical) and technological deposition conditions, it was determined that ZnO nanocrystalline films have an average grain size of
It has been shown that in ZnO during the increasing of substrate temperature there is a tendency to the increasing of the CDS; however, in CZTS films, their CSD values were weakly depended on the dispersed solution volume.
Lattice parameters values in ZnO and CZTS films deposited at
It has been estimated that during the increase of
It has been determined that during the increasing of substrate temperature to 623 K stoichiometry of ZnO layers was improved (
Study of the optical characteristics of ZnO films allow to estimate the high values of transmission coefficient
CZT film spectra (х = 0.32) had a mode LO2(ZnTe). In these spectra, intensive peaks corresponded to A1(Te) and ETO(Te) tellure modes were also determined. CZT film spectra (х = 0.75) have a weak mode A1(Te), peaks of LO1(CdTe), TO1(CdTe), TO2(ZnTe), and LO2(ZnTe) modes, and also LO2(ZnTe) mode resonant replica.
The results of a research study of the ZnO, CZTS, and CZT thin films will be used for the development of the devices, primarily, in third generation high-efficiency thin-film solar cells.
Acknowledgments
This work was supported by the Ministry of the Education and Science of Ukraine (Grants numbers: 0116U002619, 0115U000665c, 0116U006813, and 0117U003929).
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