Antireflective silicon nanostructures with hydrophobicity by metal-assisted chemical etching for solar cell applications
© Yeo et al.; licensee Springer. 2013
Received: 4 March 2013
Accepted: 21 March 2013
Published: 8 April 2013
We present broadband antireflective silicon (Si) nanostructures with hydrophobicity using a spin-coated Ag ink and by subsequent metal-assisted chemical etching (MaCE). Improved understanding of MaCE, by conducting parametric studies on optical properties, reveals a design guideline to achieve considerably low solar-weighted reflectance (SWR) in the desired wavelength ranges. The resulting Si nanostructures show extremely low SWR (1.96%) and angle-dependent SWR (<4.0% in the range of 0° to 60°) compared to that of bulk Si (SWR, 35.91%; angle-dependent SWR, 37.11%) in the wavelength range of 300 to 1,100 nm. Relatively large contact angle (approximately 102°) provides a self-cleaning capability on the solar cell surface.
Over the past decades, a great deal of efforts has been carried out to improve the conversion efficiency of crystalline silicon (c-Si) solar cells, which occupy most of the solar cell market [1, 2]. To achieve a high-efficiency c-Si solar cell, antireflective layers/structures are inevitably necessary for enhancing the transmission of the sunlight into the solar cells by suppressing surface reflection, which is caused by the refractive index difference at the air/c-Si interface. Recently, subwavelength-scale nanostructures have attracted considerable attention as a promising antireflective structure to minimize unwanted reflection losses, due to their long-term stability, and broadband and omnidirectional antireflection properties [3–10]. To produce subwavelength-scale Si nanostructures, a dry etching method using nanoscale mask patterns has been commonly employed [7–10]. However, this method is complex, expensive, and inadequate for mass production and may cause damage to the crystal structure and surface morphology due to high-energy ions . In recent years, metal-assisted chemical etching (MaCE), based on the strong catalytic activity of metal in an aqueous solution composed of HF and an oxidant, has attracted great interest as a method for fabricating Si nanostructures for electronic and optoelectronic devices [2, 6, 12–18]. This is a simple, fast, cost-effective, and high-throughput method for fabricating various Si nanostructures without any sophisticated equipment or ion-induced surface damages. The antireflection properties of nanostructures are strongly correlated with their dimensions and etching profiles [4–8], which can be controlled by adjusting the pattern of the metal catalyst  and etching conditions, such as etching time, etchant concentration, and etching temperature for MaCE [6, 12–16]. However, the antireflection characteristics of Si nanostructures, which take into account the etchant concentration and etching temperature of MaCE, have been less explored. Therefore, it is meaningful to investigate the optimum Si MaCE condition to achieve desirable antireflective Si nanostructures for practical solar cell applications. Another aspect of this parametric study is that we could confirm the self-cleaning effects of the fabricated structures as well as the optical properties .
In this paper, we investigated the influence of Si MaCE conditions including the concentration of HNO3 (i.e., oxidant), HF, deionized (DI) water, and etching temperature on the morphologies and optical properties of Si nanostructures for obtaining the most appropriate antireflective Si nanostructures with self-cleaning function for solar cell applications. Antireflection properties of the fabricated Si nanostructures were systematically investigated by hemispherical reflectance spectrum measurement and calculating solar-weighted reflectance (SWR). The surface wetting behavior of the Si nanostructures was also analyzed by the water contact angle measurement.
Results and discussion
To investigate the effects of HF concentration on the period and height of Si nanostructures produced by MaCE, a number of structures within a unit area and average height were roughly estimated from SEM images. With increasing HF concentration, the counted number of structures decreased, which means that the period of the fabricated Si nanostructures increased. This is primarily due to the enhancement of lateral etching of Si MaCE because the lateral etching of Si can be enhanced by increasing HF concentration, when the oxidant is sufficient for providing extra positive holes (h+) from the etching front (i.e., metal/silicon interface) to the side of the already formed Si nanostructures [11, 15]. Hence, the nanostructures can disappear without distinguishable structure formation, leading to the period increases, if the lateral etching is larger than the radius of the nanostructures . The average height of the Si nanostructures increased from 308 ± 22 to 1,085 ± 147 nm as the HF concentration increased. This is due to the fact that the overall etching rate was influenced by the removal of oxidized Si by HF when the oxidant was sufficient for generating oxidized Si . For this reason, the measured hemispherical reflectance decreases as the HF concentration increases. It is worth noting that the calculated SWR increased from 5.20% to 7.62% as the HF concentration increased from 8% to 14% even though the height of the Si nanostructures much increased. This is mainly because the main energy density region of the solar energy spectrum is located in the short-wavelength region (around 500 nm). This indicates that the HF concentration is crucial for obtaining Si nanostructures with desirable distribution for practical solar cell applications.
We investigated the influence of Si MaCE conditions, including the concentration of HNO3, HF, and DI water as well as etching temperature, on the morphologies and optical properties of the fabricated Si nanostructures to achieve the optimum Si MaCE condition, resulting in desirable antireflective Si nanostructures with self-cleaning function, for practical solar cell applications. The optical properties of the fabricated Si nanostructures were strongly correlated with Si MaCE conditions. The Si nanostructures fabricated by an optimum MaCE condition demonstrated the extremely low SWR of 1.96% and an angle-dependent SWR of <4% up to an AOI of 60°, compared to that of bulk Si (SWR, 35.91%; angle-dependent SWR, 37.11%) in the wavelength range of 300 to 1,100 nm, as well as a hydrophobic characteristic with a water contact angle of approximately 102°. These results provide improved understanding of Si MaCE and guidelines to achieve desirable antireflective Si nanostructures with self-cleaning capability for high-efficiency c-Si solar cells.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. 2011–0017606).
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