Spray-deposited Al-doped ZnO transparent contacts for CdTe solar cells
Highlights
► Al-doped ZnO films deposited by ultrasonic spray pyrolysis of alcoholic solutions. ► Resistivity of 3×10−3 Ω cm and transmittance of 74% obtained for 3 μm thick film. ► Films applied as transparent electrical contacts in CdTe solar cells. ► Efficiency of 12.1% without antireflection coating achieved.
Introduction
Doped zinc oxide is a popular transparent conducting oxide (TCO) widely used in photovoltaic, window and display technologies [1], [2]. Although the conductivity and chemical stability of ZnO are not as good as for the indium tin oxide (ITO) standard [3], it exhibits a relatively wide intrinsic direct band gap of ∼3.3 eV [2] ensuring high visible transparency, can be textured during or after thin film deposition, which is critical for the efficient light trapping in thin film solar cells [4], and finally, may offer cost reduction as compared to In-based TCOs. Intrinsic ZnO (i-ZnO) is highly resistive because native point defects such as Zn-on-O antisite (ZnO), the Zn interstitial (ZnI) and the O vacancy (VO) cannot deliver sufficiently high donor concentrations [2]. Substitutional doping of ZnO with Al, In, Ga, B or F does not only lead to high carrier concentrations of 1020–1021 cm−3 but also improves the film corrosion resistance [5], [6]. Al-doped ZnO (AZO) is one of the most conducting representatives, which is typically deposited by magnetron sputtering at laboratory and industrial scale, with resistivity as low as 1–2×10−4 Ω cm [7], [8]. Sputtered AZO/i-ZnO bilayer is commonly employed as the front transparent contact in high efficiency Cu(In,Ga)Se2 thin film solar cells [9], although recent studies demonstrate that similar bilayer structure can also be successfully applied to CdTe thin film solar cells with efficiency of up to 14.0% [10] or 15.6% [11].
The true low-cost potential of ZnO-based TCOs can be fully realized when combined with a low-cost and high throughput deposition method. Chemical spray pyrolysis is a simple non-vacuum technique that can be applied at ambient pressure and can be easily scaled up for large area deposition. There is a considerable amount of literature data on spray deposited ZnO thin films over the last 40 years, e.g. [12], [13], [14], [15], [16], [17], [18] and many others. In a typical deposition process, salts of zinc and appropriate dopant are dissolved in water or organic solvents such as methanol, ethanol or isopropyl alcohol, and the solution is sprayed with an inert carrier gas onto a substrate heated to 300–600 °C. A small amount of acetic acid added to the solution was found to be beneficial for the film properties because it increases the solubility of Zn salt, suppresses hydrolysis reactions causing insulating hydroxides Zn(OH)2, and promotes the formation of zinc acetate complexes [19]. The lowest resistivity was obtained for In-doped ZnO by Major et al. [20], when they reported the value of 8–9×10−4 Ω cm and an average visible transmittance of 85%, the record that still stands today. For AZO, the lowest value of 3×10−3 Ω cm was reported by Olvera et al. [17], although the same group reported later 1.0×10−2 Ω cm with a transmittance of about 80% [21]. Annealing in hydrogen [15] or in vacuum [22] helps to improve electrical and optical properties, resulting in resistivity on the order of 10-3 Ω cm. Despite the numerous investigations of sprayed AZO films, only a few groups did actually implement them as transparent contacts in thin film solar cells, e.g. [14], [23], [24], which could be partially accounted to non-optimum film properties or the need for the high-temperature deposition or post-annealing, both being energy- and cost-intensive processes when it comes to possible industrial implementation.
The purpose of this study is to optimize the spray deposition of AZO films without any post-annealing treatment and to apply them as transparent contacts in CdTe solar cells. Spraying parameters, composition and acidity of the sprayed solution, and doping level are systematically investigated in order to obtain AZO films with optical transparency of >80% in the 400–900 nm wavelength range corresponding to the spectral response of the CdTe solar cell, maintaining a smooth morphology and an acceptable sheet resistance of below 10 Ω/□, as it is recommended for current solar cell technologies [25]. The influence of deposition parameters on morphology, crystallinity, as well as optical and electrical properties of the AZO thin films is discussed. The solar cells’ results obtained with the sprayed AZO contact are finally compared with the AZO reference prepared by magnetron sputtering.
Section snippets
Experimental
The AZO thin films were prepared from a 0.1 M solution of zinc acetate Zn(CH3COO)2 (99.99%, Aldrich) in methanol and acetic acid (glacial, Carbo Erba). The proportion of acetic acid was varied between 0 ml and 6 ml per 100 ml of the final solution. Aluminum chloride AlCl3 (99.999%, Aldrich) was added as the dopant source in variable concentrations from 0 to 0.014 M corresponding to the Al/(Al+Zn) ratio from 0 to 0.14.
The layers were deposited on 1 mm thick borosilicate glass substrates of 5×5 cm²
Effect of solution acidity
Acetic acid, initially added to enhance the solubility of zinc acetate, appeared to have a strong influence on the layer morphology as shown on the SEM images (Fig. 1). Increasing quantities of acetic acid were added to chemical solutions containing nominally 0.03 Al/(Al+Zn). Increase of the acetic acid content leads to bigger grain size on SEM images and the formation of faceted AZO layer. This observation is supported by XRD data (not shown), where the average crystallite size increases from
Conclusion
AZO layers deposited by USP are successfully applied as the front electrical contact in CdTe solar cells. It is confirmed that the amount of aluminum chloride as the dopant and acetic acid as the stabilizing agent are decisive for AZO layer properties, and for both series there is an optimum at which the highest optical transparency and the lowest electrical resistivity can be achieved. In our case these are 3 ml of acetic acid per 100 ml of the final solution and the nominal AlCl3 concentration
Acknowledgments
The authors would like to acknowledge the help of their group members at the Laboratory for Thin Films and Photovoltaics at Empa. This work was partially supported by the Swiss National Science Foundation, Project no. PZ00P2_126435/1 under “Ambizione” program.
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2022, Materials Today: ProceedingsCitation Excerpt :Among these, there are numerous advantages of ultrasonic spray pyrolysis, such as wide area depositions, easy experimental setup and cost efficiency [17–19]. Many researchers/authors documented single element ZnO doped (Al/In/Ga) coated using spray pyrolysis technique [20–27]. Minimum resistivity was obtained for Aluminum doped ZnO (AZO) is 5 × 10−3, for Indium doped ZnO (IZO) is 3.4 × 10−3 and Ga-doped ZnO is 9.3 × 10−3 Ω cm.
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