Synthesis and Characterization of System In(O,OH)S/i-ZnO/n+-ZnO
In this work, we fabricated system In(O,OH)S/i-ZnO/n+-ZnO to be used as potential optical window in thin films solar cells. i-ZnO/n+-ZnO thin films were synthesized by reactive evaporation (RE) method and In(O,OH)S thin films were synthesized by chemical bath deposition (CBD) method; all thin films were deposited on soda lime glass substrates. Thin films were characterized through X-ray diffraction (XRD), atomic force microscopy (AFM), and spectral transmittance measurements. Structural results indicated that both thin films were polycrystalline; furthermore, morphological results indicated that both thin films coated uniformly soda lime glass substrate; besides, optical characterization indicated that system had more than 80% of visible radiation transmittance.
Since their discovery, thin film solar cells have been considered a real option to compete with conventional silicon based solar cells; fewer amounts of chemical reagents and lower costs of manufacture than costs in silicon solar cells construction are main features of these devices. In last decades, research in this area was directed to study new ways to improve the efficiency of solar cells; currently, highest efficiency conversion reported to solar cells based on chalcopyrite absorbent layers (CIGS cell) is near to 21.0% ±0.6 (laboratory scale) .
General chalcopyrite-based solar cell structure is shown in Figure 1; in typical structure (known as substrate configuration) layers are deposited in sequence, molybdenum back contact, p-type CIGS absorber layer, n-type buffer layer, transparent conductor oxide (TCO), and top contact grid . Inside absorbent layer, pairs of electron-hole are produced and light path and electron extraction from device rely on optical window (composed of buffer layer and TCO); buffer layer decreases the mechanical tension stress between absorbent layer and TCO electrode; besides, TCO is required for electron extraction from solar cells . Usually, these devices use a CdS thin film as buffer layer in their structure; this layer improves mechanical heterojunction between absorbent layer and TCO; however CdS depicts environmental hazards during manufacture of the layers; besides, this material has a small forbidden energy band gap ( = 2.4 eV); this low-energy band gap reduces photocurrent at wavelengths near the ultraviolet region, and this incomplete absorption of the visible spectrum for CdS buffer layers is considered to be responsible for the drop in quantum efficiency of chalcopyrite-based solar cells [4–6]. Currently, deposition of Cd-free buffer layers without deteriorating stability of solar cell is an important research topic in the area. Best results of conversion efficiency are reported when buffer layers deposited by chemical bath deposition (CBD) are incorporated inside solar cells; this method is suitable for depositing uniform and adherent buffer layer in a large area; furthermore a variety of binary and ternary compounds can be sensitized by this way [7, 8]. Typical TCO as In2O3, SnO2 F:SnO2 and In:SnO2, and ZnO are used in solar cell as electrical contact to extract electric power from solar cells [9–13]. Among these TCO’s materials, Zinc Oxide (ZnO) has reported optimal results; it is a potential alternative to conventional indium-tin-oxide (ITO), which has been the most commonly TCO used in thin films solar cells [14, 15]. Different methods have been used to synthesize thin films of ZnO, such as spray pyrolysis , sputtering , MOCVD , and reactive evaporation (RE) . Most of these methods reported transmittances near to 80% and low resistivity values at large range; however in most cases it is necessary to add dopants such as aluminum or indium to achieve suitable resistivity values. Different structures like ZnO/ and ZnO/ZnSe have been used as optical window in the fabrication of solar cells with structures CuInSe2//ZnO and ZnO/ZnSe/CdTe, although best results were reported to CdS/In:SnO3 system [20, 21].
In this paper we deposited In(O,OH)S thin films by the CBD method and i-ZnO/n+-ZnO thin films by RE method. Results showed that system In(O,OH)S/i-ZnO/n+-ZnO could be used as optical window to thin films solar cells.
In this work we sensitized highly conductive and transparent thin films (n+-ZnO, resistivity of 8 × 10−4 Ωcm) to be used as TCO in solar cells; furthermore we also deposited highly transparent and resistive thin films (i-ZnO, resistivity of 106 Ωcm) to be used as a diffusion barrier in solar cells; these thin films were deposited by RE in two-step process. The ionization of the reagent species was achieved through a low power glow discharge (GD) for applying a voltage of about 500 V-DC. Condition operations were as follows: glow discharge (2–20 mA), Zn temperature evaporation (470–510°C), partial pressure during GD (2 × 10−2–5 × 10−2 mBar), and flow (15–20 mLmin−1) .
2.1. Buffer Layer Deposition
In this work, thin films of In(O;OH)S were deposited by CBD process for using reagents indium(III) chloride 2.5 × 10−2 M, thioacetamide 3.5 × 10−1 M, acetic acid 3.0 × 10−1 M, sodium citrate dihydrate 3.0 × 10−2 M, temperature at 70°C, and pH of 2.5. Thin films of In(O;OH)S were grown on soda lime (SL) substrates; chemical reaction can be written as follows:After reaction synthesis, thin films were dried at ambient temperature under stream.
2.2. Thin Films Characterization
The film thickness was measured through a Veeco Dektak 150 profilometer. The surface morphology of thin films was studied through PSI Microscope and atomic force microscopy (AFM). The optical properties of thin films were studied through transmittance measurements carried out with a Perkin Elmer Lambda 2S spectrophotometer. Finally, X-ray diffraction patterns of the samples were recorded in Shimadzu 6000 diffractometer with a source of Cu-Kα radiation (λ = 0.15418 nm) in a range diffraction angle 2θ between 20° and 80°. The electrical properties of TCO thin films were measured for using the four-probe method.
3. Results and Discussions
3.1. Optical Characterization
Optical transmittance of the thin films was studied from 300 nm to 900 nm of electromagnetic spectrum. Figure 2 shows the transmission spectra of In(O;OH)S (90 nm); these spectra show that buffer layer has optical transmission higher than 80% in the visible range and a sharp absorption edge at visible region; this result is typical to buffer layers and it is higher than typical transmittance of CdS thin films layers. Furthermore, Figure 2 also shows transmission spectra of i-ZnO (40 nm and resistivity of 106 Ωcm) and n+-ZnO (900 nm and resistivity of 8 × 10−4 Ωcm) thin films; these spectra show that both i-ZnO and n+-ZnO have optical transmission between 80% and 90%, and the absorption coefficient was determined as follows:where α is the absorption coefficient and is the film thickness (obtained by profilometry). The optical energy band gap () of the films was determined using the relation:where is a constant depending on the transition probability; the optical band gap of the films was determined by extrapolating the linear portion of the ()2 versus plot on the -axis [23–25]. Figure 3 shows value to fitting equation (3). The band gap to buffer layer was In(O,OH),S (2.71 eV); this high value is due to CBD process; although CBD is a common fabrication process of thin films, most of reports did not explain mechanism of reaction; in literature two different processes are proposed to explain thin films formation: (a) homogeneous precipitation (HP): this mechanism is known as ion-by-ion precipitation; in this process thin films are generated by direct reaction of the ions under substrate surface; in first step free ions diffuse until substrate surface; after that, first nucleation centers of semiconductor are generated on substrate surface; after, nucleation center grows by addition of ions from solution; finally crystals grow and join other ones (coalescence process) and thin film of semiconductor is generated. (b) Heterogeneous precipitation (HeP): this mechanism is known as cluster-by-cluster precipitation; in this process thin films are generated by direct reaction of species into the reaction solution; in first step, colloidal particles are generated into solution (chalcogenide of metal or hydroxide of metal); after that, particles diffuse until substrate surface and first nucleation centers of semiconductor are generated on surface substrate; after that, nucleation center grows by addition of ions from solution. Finally crystals grow and join other ones (coalescence process) and thin film of semiconductor is generated [26, 27]. In CBD process both mechanisms can be present during thin films generation; the predominance of one mechanism to the other one will be determined by experimental condition (temperature, reagent concentration, pH, rate agitation, and complexing agents) . The value of In(O,OH)S is 2.71 eV; this value is greater than reported to In2S3; during synthesis process coprecipitation of other species and intermediates (In(OH)3, In2O3) is common; in our case CBD process deposits In(O,OH)S; this result is confirmed by XRD results. Furthermore, band gap of In(O,OH)S is greater than CdS (2.4 eV); this result is very positive due to the fact that blue response of device could be improved by the incorporation of this buffer layer .
Figure 3 also shows to TCO; the main difference between the two types of films is observed in the interference patterns; i-ZnO thin film did not show pattern because of its reduced thickness (40 nm) and n+-ZnO thin film shows typical interference patterns of this kind of layers.
The typical intrinsic defects in ZnO are the following: O vacancy (), Zn vacancy (), Zn interstitial (), O interstitial (), and antisite Zn (). Zn interstitials and oxygen vacancies are known to be the predominant ionic defect types [26, 27, 30, 31]. This kind of defects can be controlled and we can obtain two types of ZnO conductivities; synthesis under excess of oxygen reduces and synthesis under defect of oxygen increases .
Figure 3 shows that n+-ZnO thin film had an value of 3.35 eV; besides, i-ZnO thin film had an value of 3.40 eV; this value is according to other authors [32–36]. Optical results also show that i-ZnO antidiffusion layer does not affect transmittance of TCO; this is very important because this layer reduces species diffusion between heterojunctions to give chemical stability to solar cells on time. Finally, if we take into account only visible radiation (400 nm–750 nm) absorption effect, the optical window constructed in this work permits near to 83% of visible radiation reach to absorbent layer; results indicated that system In(O;OH)S/i-ZnO/n+-ZnO has suitable optical properties to be used as optical window in thin film solar cells.
3.2. Structural Characterization
ZnO films were characterized through XRD measurements; Figure 4 shows experimental XRD patterns to i-ZnO and n+-ZnO. Results show that the films of i-ZnO and n+-ZnO have the same crystal structure corresponding to a hexagonal crystallographic structure (JCPDS # 36-1451); thin films had a preferential orientation in the plane of growth (002); this result is according to other reports [32, 33]. The signal intensity in n+-ZnO thin film pattern is higher than to signal i-ZnO due to differences in the thickness of thin films. Furthermore, Figure 4 shows experimental XRD patterns to buffer layers. XRD pattern shows two signals located at 2θ = 33.7° and at 2θ = 34.1°; these signals can be assigned to (001) and (200) planes from orthorhombic indium oxide hydroxide (InOOH) (JCPDS # 17-0549); third reflection is located at 2θ = 48.37°; this can be assigned to (2212) plane from tetragonal β-In2S3 phase (JCPDS #25-0390) [34, 35].
3.3. Morphological Characterization
Figure 5 shows AFM images of substrate and thin films deposited in this work; in Table 1 are listed the corresponding grain size average and roughness (Rms) values, which were determined through the ProScan image analysis software.
Figure 5(a) shows typical AFM image of soda lime glass (SLG) substrate; this figure shows a smooth surface and a reduced grain size (Table 1), typical values to SLG substrate. Figure 5 shows morphological changes after thin films deposition process; Figure 5(b) shows AFM image of In(O;OH)S thin film deposited by CBD on SLG substrate; Figure 5(b) shows crystalline grains that are well connected and they spread uniformly without any cracks; the image also shows crystallites are packed together; the average grain size of the films was near to 90 nm and roughness was 7 nm. Figures 5(c) and 5(d) show AFM images of n+-ZnO and i-ZnO thin films deposited by RE on SLG. Results show that i-ZnO fill up uniformly SLG substrate; particles are distributed randomly throughout the SLG substrate without any crack. Furthermore, Figure 5(d) shows that the grain size and roughness of the films of n+-ZnO are bigger than i-ZnO; this could be due to time used during synthesis process; time used in RE method is bigger to n+-ZnO than i-ZnO. Finally, Figures 5(c) and 5(d) show that thin films are constituted by grains composed of crystallites with reduced size; these grains form a compact and uniform layer; when synthesis time increases are clear, those crystallites produced during first instants tend to form aggregates.
In this work we synthesized both thin films of ZnO and i-ZnO by RE method and thin films of In(O;OH)S by CBD method; optical results indicated that system ZnO/i-ZnO/In(O;OH)S had transmittance about 80% in visible region of electromagnetic spectrum. Furthermore, structural characterization indicated that TCO and buffer layer were polycrystalline; besides, morphological characterization indicated all thin films covered uniformly materials used as substrates. Finally, the antidiffusion layer could avoid migration of different chemical species through the heterojunction. Our results suggest that the system we proposed could be used as alternative optical windows in thin films solar cells.
The authors declare that they have no competing interests.
This research was supported by Universidad del Atlántico (Project code CB20-FGI2016) and Universidad Nacional de Colombia.
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