Elsevier

Energy

Volume 66, 1 March 2014, Pages 20-24
Energy

Improvement of haze ratio of DC (direct current)-sputtered ZnO:Al thin films through HF (hydrofluoric acid) vapor texturing

https://doi.org/10.1016/j.energy.2013.09.001Get rights and content

Highlights

  • AZO films are textured with vaporized HF (hydrofluoric acid) formed by mixing HF with an H2SiO3 solution.

  • We achieved a haze value of 78% at λ = 540 nm by increasing the HF concentration.

  • A haze value of about 58% was achieved at λ = 800 nm when vapor texturing was used.

  • Maximum Jsc of 17.59 mA/cm2 is obtained due to sufficient light trapping in AZO/glass.

Abstract

The textured ZnO:Al films are used to enhance light trapping in thin film solar cells. The wet etch process is used to texture ZnO:Al films by dipping in diluted acidic solutions such as HCl (hydrogen chloride acid) or HF (hydrofluoric acid). During this process, the acidic solution can damage the glass substrate, which could then be difficult to apply for the inline mass production process since this process needs to be performed outside the chamber. In this paper we report a new technique to control the surface morphology of DC-sputtered ZnO:Al films. The ZnO:Al films are textured with vaporized HF formed by mixing the HF with an H2SiO3 solution. We achieved a high haze value of 74.6% at a 540 nm wavelength by increasing the etching time and HF concentration. A haze value of about 58% was achieved at a 800 nm wavelength when vapor texturing was used. The ZnO:Al film texture by HCl had a haze ratio of about 9.5% at 800 nm and less than 40% at 540 nm. In addition to a low haze ratio, in the texturing by HCl it was very difficult to control etching and to keep reproducibility due to its very fast etching speed.

Introduction

Al-doped ZnO (ZnO:Al) thin films are one of the promising TCO (transparent conductive oxide) films which can be applied to thin film Si solar cells. Moreover, as it is very strong against hydrogen plasma, it can be adopted to microcrystalline Si solar cells (μc-Si:H) [1], [2].

The sputtered ZnO:Al films deposited in the conditions of low pressure and high temperature are more compact and dense than those deposited at relative high pressure and low temperature. The sputtered ZnO:Al films are usually textured by diluted HCl (hydrogen chloride acid) solution (0.5–1 Vol%) despite the low etching efficiency [4]. It is well known that the surface morphology of the ZnO:Al films etched by HCl solution strongly depends on the film properties which are determined by the deposition conditions such as pressure and temperature [5]. Although these films can be etched efficiently in the HF (hydrofluoric acid) solution [3], the surface of the films textured by HF solution has sharp and small structures, leading to a low haze ratio. Furthermore, the surface of the glass substrate might be damaged by HF during the wet etching process.

In the present work, we investigated a new vapor texturing technique to control the surface morphology and improve the haze ratio of the sputtered ZnO:Al. The HF vapor generated by the reaction of HF and silica hydrous gel (H2SiO3) solutions was used to texture the sputtered ZnO:Al films. It effectively etched only one side where the ZnO:Al film was deposited. The surface of the ZnO:Al textured by the vapor had pillar-like irregular and sharp structures compared to that of the films textured typically and the haze ratio increased dramatically for both long and short wavelengths.

Section snippets

Experimental

ZnO:Al (Al2O3 2 wt.% doped) films were deposited on the corning glass by a DC (direct current) magnetron sputtering system. The deposition temperature, argon gas flow rate, working pressure, power density, substrate rotation, the inter electrode distance between the target and substrate, and the thickness of ZnO:Al films during sputtering were fixed at 267 °C, 15 sccm, 2 mtorr, 2.74 W/cm2, 5 rpm, and 13.6 cm and 1 μm, respectively. Prior to deposition, glass substrates were cleaned

Results and discussion

HF vapor texturing for ZnO:Al films was carried out with varying etching times and HF concentrations at room temperature using a solution of HF:H2SiO3 in the volume ratio of 1:1. The HF solution easily becomes vaporized at high temperature. For safety, HF is combined with the H2SiO3 solution and then vaporized. The reaction formula for the combined solution of HF and H2SiO3 is [7]:6HF+H2SiO3H2SiF6+3H2OH2SiF62HF+SiF4

For the formation of uniform vapor, small amounts of solution are mixed at

Conclusion

We investigated a new vapor texturing technique to control the surface morphology and improve the haze ratio of the sputtered ZnO:Al. Vapor texturing effectively etched only one side of the substrate where the ZnO:Al film was deposited. The surface of the ZnO:Al textured by the vapor had pillar-like irregular and sharp structures compared to that of the films textured typically and the haze ratio increased dramatically for both long and short wavelengths. Sample E, in which the etched surface

Acknowledgments

This work was supported by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20113020010010).

This work was supported by the Human Resources Development program (No. 20124010203280) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy.

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