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We investigated the hybrid antireflective (AR) structures with zinc oxide nanorods (ZnO NRs) grown on micro pyramidal silicon (MP-Si) structures. The MP-Si structures were fabricated by a simple and cost-effective anisotropic wet chemical etching technique under different ratios between potassium hydroxide (KOH) and isopropyl alcohol (IPA). For the MP-Si structures etched at 10:9 vol% of KOH and IPA, the relatively low average reflectance (Ravg) value of 14.5% was obtained in the wavelength range of 300-1100 nm. Its solar weighted reflectance (SWR) value was also estimated to be 12.6% in the wavelength range of 400-1100 nm. The ZnO NRs were hydrothermally grown on the MP-Si structures at various solution concentrations. For the ZnO NRs (25 mM)/MP-Si, the Ravg and SWR values were further decreased to 3.6% and 3.8%, respectively. The omnidirectional AR behavior was also observed in the wide incident angle range of 20-70°. The hybrid ZnO NRs/MP-Si AR structures revealed a superhydrophilic surface with water contact angles of < 5 o.
© 2016 Optical Society of America
1. Introduction
In semiconductor industries, silicon (Si) is one of the most versatile materials and it has been utilized to fabricate various optoelectronic devices such as photovoltaic cells, photodetectors, and image sensors because of its low cost, abundance in nature, nontoxicity, and long-term stability [1–4]. However, flat Si substrate which has the high refractive index (ƞSi > 3.4) in the visible and near-infrared (IR) wavelength regions indicates strong surface Fresnel reflections as well as optical losses, leading to the degradation of efficiency of Si-based optoelectronic devices [5–7]. To overcome this issue, various antireflective (AR) structures including thin-film layers and multiple-stacking layers have been investigated, but they have fundamental limitations such as thermal instability in outdoor environments, poor adhesion on certain substrates, and need of precise control over the process conditions [8–10]. Moreover, their AR properties work properly for the specific wavelength ranges [8,11]. Meanwhile, the hybrid AR structures which consist of the combination of nano/micro structures motivated from the compound eye have been recently considered and developed [12–18]. These structures exhibit the superior AR characteristics in the wide ranges of wavelengths and incident angles due to the increased probability of light trapping and the gradually varied refractive index distribution [17–19]. In addition, there are many efforts to prevent surface recombination on textured surfaces using various techniques such as (NH4)2S agents [20], InGaP shells [20], and deposition of layer with SiO2 [21], SiNx [21], SiNx:H [22], SiO2/SiNx [23], and Al2O3 [24].
The photolithography and dry etching techniques are generally used to fabricate the hybrid structures, but they require expensive and toxic equipments, complex processes, and long process time. To simplify the fabrication methods, chemical techniques which provide several advantages of simple and fast processes, cost-effectiveness, massive production, and low working temperature have been recently suggested [25–27]. The disordered micro pyramidal Si (MP-Si) structures can be formed by an anisotropic wet etching process. Meanwhile, zinc oxide nanorods (ZnO NRs) are considered as one of promising AR nanostructures due to their high transparency, easy structural tunability, and compatibility with other materials [16,28,29]. Furthermore, the ZnO NRs with ƞZnO ~1.9 in the visible and IR wavelength regions can be suitable to make the more continuously and gradually varied refractive index profiles between air and Si. It is also well known that the ZnO NRs can be easily synthesized using the chemical growth solutions [30,31]. Therefore, it is meaningful to analyze the optical properties of hybrid ZnO NRs/MP-Si AR structures. In this work, we fabricated the MP-Si structures using the anisotropic KOH wet etching process under various solution concentrations. The ZnO NRs were hydrothermally grown on the as-fabricated MP-Si structures at different solution concentrations. Their structural and optical characteristics as well as the surface wetting behaviours were investigated. Furthermore, their angle-dependent reflectance spectra were explored. For the theoretical analysis of light propagation on the surface of ZnO NRs/MP-Si structures, the finite-difference time-domain (FDTD) simulations were also performed.
2. Experimental details
Figure 1 shows the schematic diagram for the fabrication of the ZnO NRs grown on MP-Si AR structure. Initially, the flat Si substrates with the size of 20 × 20 mm2 were cleaned with acetone, methanol, and de-ionized (DI) water for 10 min, respectively, and then subsequently dried with a nitrogen (N2) gas flow. To eliminate the oxide and organic impurities on the surface of Si substrates, they were rinsed away using a buffered oxide etchant solution and 5 wt% hydrofluoric acid solution for 1 min, respectively. After each step, the samples were dipped into DI water for 2 min, followed by the N2 blow. In order to fabricate the MP-Si structures, the cleaned Si substrates were immersed into a wet chemical etchant solution at 80 °C for 40 min. Herein, the mixture of potassium hydroxide (KOH), isopropyl alcohol (IPA), and DI water was utilized as a wet chemical etchant solution. After the formation of MP-Si structures, the samples were dipped into a mixture of hydrogen chloride, hydrogen peroxide, and DI water (HCl:H2O2:H2O = 1:1:5) at 85 °C for 10 min to remove the residual potassium particles. Subsequently, the MP-Si structures were loaded into the radio-frequency (RF) magnetron sputtering chamber to deposit a thin aluminium-doped ZnO (AZO) seed layer. For the deposition of 40-nm-thick AZO film, the RF magnetron sputtering was carried out at 6 mTorr of working pressure and 100 W of RF power with 30 sccm of Ar flow. Next, the ZnO NRs were hydrothermally synthesized on the as-fabricated samples. Herein, the growth solution consisting of equimolar zinc nitrate hexahydrate and hexamethylenetetramine (HTMA) in 200 mL of DI water was mechanically stirred at 300 rpm for 20 min. The equimolar concentration of zinc nitrate hexahydrate and HTMA was determined to be 15, 25, and 50 mM. Subsequently, 1 mL of polyethylenimine was added into the growth solutions and their stirring was continued for 2 h for homogeneity. The growth of ZnO NRs was carried at the oven temperature of 90 °C for 12 h. After the growth process, the ZnO NRs/MP-Si samples were cleaned with DI water and then dried under the N2 gas flow. The morphological and phase properties of ZnO NRs/MP-Si structures were characterized by utilizing a field-emission scanning electron microscope (FE-SEM; LEO SUPRA 55, Carl Zeiss) and X-ray diffractometer (XRD; M18XHF-SRA, Mac Science) with Cu Kα (λ = 1.5402 Å) radiation. The crystallinity and elemental mappings of ZnO NRs were investigated by using a transmission electron microscope (TEM; JEM-2100F, JEOL). The optical reflectance spectra and omnidirectional characteristics of hybrid AR structures were measured by using an UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere at normal incidence and a spectroscopic ellipsometry (V-VASE, J. A. Woollam Co. Inc.), respectively. The wettability was evaluated from the average water contact angles at three different positions on the surface of samples by using a contact angle measurement system (Phoenix-300, SEO Co., Ltd) with ~5 μL droplets of DI water at room temperature. To deeply understand the light behaviours on the as-fabricated structures, the FDTD simulations were also performed using a commercial software (FullWave, Rsoft Design).
3. Results and discussion
Figure 2 shows the XRD pattern of hybrid ZnO NRs (25 mM)/MP-Si AR structure. The XRD peak of MP-Si fabricated by the anisotropic KOH wet etching process was observed at ~69° which corresponds to the (400) plane according to the JCPDS card no. 80-0018 [32]. The other XRD peaks observed at 36.2 and 47.5° could be readily indexed to the (101) and (102) planes of the hexagonal wurtzite ZnO crystal structure, respectively, and were well-matched with the JCPDS card no. 89-1397. Although the ZnO NRs were grown vertically to the surface, the (101) and (102) directions of ZnO NRs were dominantly found due to the inclined planes of MP-Si. Except the XRD peaks of MP-Si and ZnO NRs, no other peaks were observed. The inset of Fig. 2 shows the energy-dispersive X-ray (EDX) spectrum of the ZnO NRs (25 mM)/MP-Si structure. From the EDX spectrum, the atomic percentage of zinc (Zn) and oxygen (O) was counted to be ~46.9 and 29.1%, respectively.
Fig. 2 XRD pattern of hybrid ZnO NRs (25 mM)/MP-Si AR structure. The EDX spectrum of the ZnO NRs (25 mM)/MP-Si structure is shown in the inset.
Figure 3 shows the (a) TEM images, (b) high-resolution (HR) TEM images, (c) selective area electron diffraction (SAED) pattern, and (d) elemental mapping images of the single ZnO NR. As shown in Fig. 3(a), the ZnO NR was taken apart from the surface of ZnO NRs (25 mM)/MP-Si structure using the ultra-sonication. From the HR-TEM image, the interplanar spacing between lattice fringes was calculated to be 0.267 nm, corresponding to the (002) plane as well as the polar c-axis of ZnO NR. The crystallinity of ZnO NR could be confirmed from its SAED pattern. As shown in Fig. 3(c), the aligned dot patterns of ZnO NR indicated the single crystallinity and their indices were matched with (200), (110), and (002) planes. According to the elemental mapping images as shown in Fig. 3(d), the uniform distributions of Zn (red) and O (green) atoms in ZnO NR were also demonstrated.
Fig. 3 (a) TEM images, (b) high-resolution (HR) TEM images, (c) selective area electron diffraction (SAED) pattern, and (d) elemental mapping images of the single ZnO NR.
Prior to the growth of ZnO NRs on MP-Si structures, the morphology-dependent optical characteristics of MP-Si structures were investigated. Figure 4(a) shows the Si etching rate at different IPA concentrations. It is well known that the addition of IPA to KOH solution leads to the degradation of etching rate, the improvement of smoothness, and no chemical reactions with other elements [33–38]. However, the exact role of IPA has not been defined yet. The most considerable phenomenon is the adsorption of IPA particles onto the Si surface [33,34]; For an anisotropic KOH etching, in general, an overall etching process can be expressed as below by the following equation:
A Si etching rate using KOH solution is approximately determined by the following equation [33]:When a Si substrate is placed into a solution, the agents such as OH- ions and IPA particles are diffused and adsorbed to Si surface. With increasing the IPA concentration, the access probability of OH- ions and water particles is more restricted, leading to the reduction of etching rate. Then, an oxidation rate reaches the equilibrium state with a desorption rate, resulting in minimized roughness. Over a certain IPA concentration, residues stay on Si surfaces due to the slow desorption rate rather than the oxidation rate and it makes surface roughness increased. As shown in Fig. 4(a), the etching rates were estimated to be ~3.57, 1.94, 0.97, and 1.50 μm/min for the solution concentrations of 10:1, 10:5, 10:9, and 10:13 vol%, respectively. The inset of Fig. 4(a) shows the SEM images of the corresponding MP-Si structures at different etchant conditions. The MP-Si structures were architected by an anisotropic wet-chemical etching process using KOH and IPA mixture solutions at different volumetric concentrations. As shown in the inset of Fig. 4(a), the size/height of MP-Si structures was roughly estimated to be 5/2.5, 10/7.8, 10/9.0, and 20 µm/23.0 µm for the solution concentrations of 10:1, 10:5, 10:9, and 10:13 vol%, respectively. The most uniform structures were observed at 10:9 vol%. Figure 4(b) shows the measured reflectance spectra of the corresponding MP-Si structures over a broad wavelength range of 300-1100 nm. As expected, the bare Si exhibited a very high average reflectance (Ravg) value of > 40.3% in the wavelength range of 300-1100 nm. On the contrary, the reflectance was reduced by incorporating the MP structures onto the Si surface, indicating the Ravg values of 21.5, 16.3, 14.5, and 17.9% for the solution concentrations of 10:1, 10:5, 10:9, and 10:13 vol%, respectively. Since the lower reflectance was observed at the 10:5 and 10:9 vol%, it was considerable that the refractive index distribution of MP-Si with a size of 10 µm could be more gradually and linearly varied in comparison with the others. At the 10:9 vol%, the lowest reflectance was observed due to the relatively more uniform MP-Si structure. This result was attributed to the gradient refractive index between air and Si via the MP structures as well as the superior light scattering effect on the MP structures [17,39,40]. The micro structures can generate the highly-ordered reflected lights with large angles. Some portion of diffused lights propagates to the neighbouring structures and is able to be scattered repeatedly, leading to the light trapping in the MP-Si structures as well as the reduction of surface reflection.Fig. 4 (a) Si etching rate at different IPA concentrations. (b) Measured reflectance spectra of the bare Si and the MP-Si structures at different etchant conditions over a broad wavelength range of 300-1100 nm. The inset of Fig. 4(a) shows the SEM images of the corresponding MP-Si structures at different etchant conditions.
Figure 5 shows the (a) top-view and cross-sectional SEM images of the MP-Si structures (KOH:IPA = 10:9) covered by the ZnO NRs grown at 0, 15, 25, and 50 mM and (b) surface wettability of the bare Si and the ZnO NRs (0 and 25 mM)/MP-Si structures. From the SEM images, it was confirmed that the ZnO NRs were grown vertically to the surface of MP-Si structures. Their average diameter/height were approximately obtained to be 65/2600, 70/2300, and 95 nm/1900 nm, indicating the aspect ratio of 40, 33, and 20, for the growth solution concentrations of 15, 25, and 50 mM, respectively. With increasing the growth solution concentration, herein, the diameter of ZnO was increased while the height and aspect ratio were decreased. This may be attributed to the rapid isotropic growth of ZnO NRs which enhances the growth rate on overall facets of ZnO NRs by the excessive Zn2+ ions [30,41]. Moreover, the ZnO NRs grown at 25 and 50 mM were densely and fully covered over all the surfaces of MP-Si structures while the ZnO NRs grown at 15 mM were sparsely observed. Since the relatively high aspect ratio indicates more gradually and linearly varied refractive index distribution, it could be expected that the ZnO NRs grown at 25 mM sufficiently reduce the surface reflection. Additionally, the surface wettability is closely related to the surface roughness. From the photographs in Fig. 5(b), both the bare Si and MP-Si structure had a hydrophilic surface with the water contact angle (θC) values of 47 and 10°, respectively, while the surface of ZnO NRs (25 mM)/MP-Si structure exhibited a very low θC value of < 5°, i.e., superhydrophilicity. The highly hydrophilic surface of MP-Si structure was explained by the well-known Wenzel’s equation [42–44] because the roughness ratio could be small due to the lowest etching rate as well as the improvement of smoothness. Furthermore, the ZnO NRs/MP-Si structure exhibited a superhydrophilicity much lower than the vertically grown ZnO NR array films [42,45,46] and the flat surface of single-crystalline ZnO [47]. This may be attributed to the hydrophilic property of ZnO itself and ZnO grown on the inclined surfaces which allow water to fill the grooves easier. Therefore, the ZnO NRs/MP-Si with the superhydrophilic surface can be useful to improve the self-cleaning effect for photovoltaic applications.
Fig. 5 (a) Top-view and cross-sectional SEM images of the MP-Si structures (KOH:IPA = 10:9) covered by the ZnO NRs grown at 0, 15, 25, and 50 mM and (b) surface wettability of the bare Si and the ZnO NRs (0 and 25 mM)/MP-Si structures.
Figure 6 shows the (a) measured reflectance spectra of the ZnO NRs (0, 15, 25, and 50 mM)/MP-Si structures and (b) calculated electric (E)-field intensity distributions of the bare Si and the ZnO NRs (0 and 25 mM)/MP-Si structures at λ = 550 nm using the FDTD simulations. As shown in Fig. 6(a), the reflectance of MP-Si structures was further suppressed over the wide wavelength range by introducing the ZnO NRs on their surfaces. The reflectance below 390 nm was significantly dropped due to the high absorption of ZnO NRs. The Ravg values of hybrid AR structures were calculated to be 8.0, 3.6, and 4.6% for the ZnO NRs grown at 15, 25, and 50 mM, respectively. This may be ascribed to the combination of the scattering effect and gradient refractive index effect as discussed in Fig. 4 and Fig. 5(a). To investigate the AR properties of the as-fabricated ZnO NRs/MP-Si structures for photovoltaic applications, their solar-weighted reflectance (SWR) values were estimated. Herein, the SWR is defined as the ratio of the usable photons reflected to the total useable photons. The SWR values in the wavelength range of 400-1100 nm can be calculated following the equation [48]:
where Is(λ) is the spectral irradiance under the condition of AM 1.5G and R(λ) is the total reflectance according to Fig. 4 and Fig. 5(a). Obviously, the much lower SWR value of 3.8% was found from the ZnO NRs (25 mM)/MP-Si structure compared to the bare Si (SWR = 37.7%) and the ZnO NRs (0, 15, and 50 mM)/MP-Si structures (SWR = 12.6, 8.2 and 5.1%, respectively). Thus, the ZnO NRs/MP-Si structures could be utilized as hybrid AR layers to enhance the efficiency of Si-based solar cells. The photographic images of Fig. 6(a) show the bare Si and the ZnO NRs (0, 15, 25, and 50 mM)/MP-Si structures under the fluorescence light. From the image, the reflected white light was clearly observed on the surface of bare Si, indicating a high surface reflection. For the MP-Si structure without ZnO NRs, there was no reflected light but its surface still looked bright due to the scattering effect. For the hybrid AR structures, the black color without reflected light could be found on their surfaces and especially, among them, the darkest black color was achieved from the ZnO NRs (25 mM)/MP-Si structure, indicating the significantly reduced surface reflection. The AR characteristics of the ZnO NRs/MP-Si structure were also represented by the calculated E-field intensity distributions as shown in Fig. 6(b). The insets of Fig. 6(b) show the high-magnification images of E-field distributions at the corresponding interfaces. For the bare Si, the strong plane E-field in the air was observed because its flat surface provided the high surface reflection. In contrast, the diffracted E-fields were observed in the MP-Si and the ZnO NRs/MP-Si structures due to the scattering effect. For the ZnO NRs/MP-Si structure, the strong E-field was held on the interfaces and even more some portion of E-field could be penetrated into the MP-Si structures due to the gradually varied refractive index profiles. As a result, the hybrid ZnO NRs/MP-Si AR structures can play a key role in guiding and trapping the light into their structures, probably leading to the efficiency improvement of Si-based photovoltaic devices.Fig. 6 (a) Measured reflectance spectra of the ZnO NRs (0, 15, 25, and 50 mM)/MP-Si structures and (b) calculated E-field intensity distributions of the bare Si and the ZnO NRs (0 and 25 mM)/MP-Si structures at λ = 550 nm using the FDTD simulations. The photographic images of the bare Si and the ZnO NRs (0, 15, 25, and 50 mM)/MP-Si structures under the fluorescence light are shown in the inset of (a). The high-magnification images of E-field distributions at the corresponding interfaces are shown in the inset of (b).
Since the solar irradiance arrives to the surface of photovoltaic devices with various incident angles (θinc) by the scattering in the atmosphere and the positional variation of sun along the time, the investigation on angle-dependent optical characteristics of ZnO NRs/MP-Si structures is necessary. Figure 7 shows the contour plots of variations of the measured reflectance spectra as a function of θinc for the (a) bare Si, (b) MP-Si, and (c) ZnO NRs (25 mM)/MP-Si structures in the wavelength range of 300-1100 nm for un-polarized light. As expected, the incident light was highly reflected (R > 30%) on the flat surface of the bare Si in overall regions. On the other hand, the low reflectance region of < 5% was observed at the θinc = 20-60° and 20-70° from the MP-Si and ZnO NRs/MP-Si structures, respectively. The Ravg values of the corresponding structures at the wavelengths of 300-1,100 nm and θinc = 20-80° were estimated to be 33.6, 5.9, and 1.8%, respectively. Thus, the ZnO NRs/MP-Si structure with the omnidirectional AR properties is applicable for Si-based photovoltaic devices.
Fig. 7 Contour plots of variations of measured reflectance spectra as a function of θinc for the (a) bare Si, (b) MP-Si, and (c) ZnO NRs (25 mM)/MP-Si structures in the wavelength range of 300-1100 nm for un-polarized light.
4. Conclusion
We fabricated the hybrid ZnO NRs/MP-Si AR structures by using simple and cost-effective anisotropic wet chemical etching and hydrothermal processes. To achieve the minimum reflectance, various ratios of KOH to IPA and concentrations of ZnO growth solutions were used. The optimized experimental conditions for MP-Si and ZnO NRs were determined as 10:9 vol% (KOH:IPA) and 25 mM, respectively. The Ravg and SWR values of ZnO NRs (25 mM)/MP-Si were 3.6 and 3.8%, respectively, over the broad wavelength range. Its omnidirectional AR characteristics were also confirmed from the incident angle-dependent reflectance spectra. From the theoretically calculated E-field intensity distributions, the reduced surface reflection at the rough interfaces was also confirmed in good agreement with the experimentally measured data. From these results, the ZnO NRs/MP-Si structures with broadband and omnidirectional AR characteristics can be a promising candidate to enhance the device efficiency in Si-based photovoltaic applications.
Funding
National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A2A01068407).
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