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Journal of the Korean Physical Society, Vol. 60, No. 11, June 2012, pp. 1944∼1948

Multi-layer Coating of SiO2 Nanoparticles To Enhance Light Absorption by Si Solar Cells Yoon-Ho Nam, Han-Don Um, Kwang-Tae Park, Sun-Mi Shin, Jong-Wook Baek and Min-Joon Park Department of Bio-Nano Technology, Hanyang University, Ansan 426-791, Korea

Jin-Young Jung, Keya Zhou, Sang-Won Jee, Zhongyi Guo∗ and Jung-Ho Lee† Department of Material and Chemical Engineering, Hanyang University, Ansan 426-791, Korea (Received 4 April 2012, in final form 19 April 2012) We found that multi-layer coating of a Si substrate with SiO2 dielectric nanoparticles (NPs) was an effective method to suppress light reflection by silicon solar cells. To suppress light reflection, two conditions are required for the coating: 1) The difference of refractive indexes between air and Si should be alleviated, and 2) the quarter-wavelength antireflection condition should be satisfied while avoiding intrinsic absorption loss. Light reflection was reduced due to destructive interference at certain wavelengths that depended on the layer thickness. For the same thickness dielectric layer, smaller NPs enhanced antireflectance more than larger NPs due to a decrease in scattering loss by the smaller NPs. PACS numbers: 78.20.-e, 78.20.Bh, 78.20.Ci, 78.35.+c, 78.40.-q Keywords: Silicon solar cell, Antireflection coating, SiO2 nanoparticles, Multi-layer coating, Quarter-wavelength theory, Rayleigh scattering DOI: 10.3938/jkps.60.1944

I. INTRODUCTION

Crystalline silicon (Si) materials are currently the dominant materials used in the production of solar cells because of their earth abundance and well-established processing techniques. However, the high refractive index of Si decreases its efficiency in solar cells because more than 40% of the incident light is reflected back. Therefore, antireflection coatings (ARCs) are critically important for increasing the conversion efficiency of Si solar cells. Several ARC techniques have been developed, such as surface texturing, deposition of silicon nitride (SiNx) films, and nanoparticle (NP) coating. Texturing the surface is known to be useful for light trapping because the light reflected from the textured surfaces can be redirected onto other surfaces in a repetitive manner, thus reducing the overall reflectance [1–4]. Because an increase in surface area also causes an increase in surface recombination velocity, which degrades the conversion efficiency [5], commercially available SiNx films deposited via plasma enhanced chemical vapor deposition (PECVD) are generally used to reduce reflectance, despite their high cost-of-ownership [6,7]. ∗ E-mail: † E-mail:

[email protected] [email protected]; Fax: +82-31-400-4723

Low cost metal or dielectric NPs such as Ag, Au, SiO2 , and TiO2 have recently been considered as alternative ARCs; however, metal NPs were shown to have a negative impact [8,9] on light absorption by Si because they revealed their own light absorption. In contrast, dielectric NPs, especially SiO2 NPs, have low extinction coefficients at wavelengths ranging from 400 to 1200 nm, so there is effectively no intrinsic absorption [10,11]. Dielectric NPs have, therefore, been researched as alternative ARCs [12–15]. Dielectric NPs have been coated as monolayers or multi-layers and reduce light reflection if two conditions are met. The first condition is that the refractive index of the ARCs should match those of air and Si, which is called optical impedance matching; the refractive indices of air and Si are about 1 and 3.5 ∼ 4 (wavelength dependent), respectively. The second condition is that the optical thickness (the thickness times the refractive index) of the ARCs should be equal to λ/4. This is called the quarter-wavelength antireflection (AR) effect and is shown below in Eq. (1): λmin = n · d, 4

(1)

where n is the refractive index of the ARC, d is the thickness of ARC, and λmin is the wavelength of maximum suppression of light reflection caused by destructive interference. According to Eq. (1), λmin varies with the

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Multi-layer Coating of SiO2 Nanoparticles To Enhance Light Absorption by Si Solar Cells – Yoon-Ho Nam et al.

refractive index and the layer thickness. In this work, we varied the layer thickness to determine λmin while the refractive index was kept the same for various coating thicknesses. To minimize light reflection using SiO2 NPs on a Si substrate in the visible and the NIR regions, the thickness of the multilayer needs to be determined. Additionally, the diameter of the NPs, which affects light scattering, is another important factor to consider when developing an antireflection coating. As the diameter of NPs increases, the influence of light scattering becomes dominant so that incident light is largely scattered [12,16]. In other words, scattering loss can be enhanced by large-diameter NPs. For the same layer thickness, a monolayer should have NPs with a larger diameter than a multi-layer because the thickness of a monolayer depends only on the diameter of NPs; consequently the possibility of light scattering is larger. We synthesized multilayered SiO2 NPs to improve light absorption by Si solar cells. Light reflection was reduced because of destructive interference at certain wavelengths that depended on the layer thickness. When the diameter of NPs was varied for the same thickness of the dielectric layer, smaller NPs enhanced the antireflection effect to a greater extent than larger NPs because scattering loss was decreased.

II. EXPERIMENTAL DETAILS Single-crystal p-type Si (100) with a thickness of 525 µm was used as a substrate (2.5 × 2.5 cm2 ). The Si substrate was cleaned in H2 SO4 :H2 O2 (4:1) for 20 min, rinsed with de-ionized (DI) water, and then dried using N2 gas. Multi-layered SiO2 NPs were made using colloidal SiO2 NPs of 25 ± 5 nm (30 wt%) and 104 ± 10 nm (30 wt%) in diameter. Colloidal solutions were used without any further purification and were diluted to 10 wt%, and 5 wt% with DI (18.2 MΩ·cm−1 ) water. The SiO2 NPs were dropped on the Si substrate by drop-casting and were then spin-coated to create multilayered SiO2 NPs of the desired thicknesses by using various spinning speeds and times. The morphologes and the thicknesses of the layers were characterized by using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). To compare the optical properties of the multilayered SiO2 NPs quantitatively, we measured the total reflection spectra by using an UV-VIS-NIR spectrophotometer (Lambda 750, Perkin Elmer) with an integrating sphere attachment over the 400- to 1200-nm wavelength range.

III. RESULTS AND DISCUSSION SEM images in Fig. 1 show a uniform coating of multilayered SiO2 NPs on the Si substrate. We were able to

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Fig. 1. Scanning electron microscopy (SEM) images of SiO2 NPs layers on Si substrates. Top-view and crosssectional images for SiO2 NPs: (a) average diameter of NPs: 25 nm and (b) average diameter of NPs: 104 nm.

Fig. 2. (Color online) Total reflection spectra of multilayered SiO2 NPs on Si substrates for various layer thicknesses: (a) average diameter of NPs: 25 nm and (b) average diameter of NPs: 104 nm.

control the thickness by adjusting the spin-coating conditions based on the RPM speed and the concentration of the SiO2 solution. The layer thickness decreased as the RPM speed was increased and as the concentration of the SiO2 solution was decreased. Figures 2(a) and (b) show the total reflection spectra of multilayered SiO2 NPs on a Si substrate compared with a bare Si substrate. The multi-layered SiO2 NPs reduced light reflection over wavelengths from 400 – 1200 nm. The wavelength of maximum suppression, λmin , of the light reflection caused by destructive interference is displayed with arrows in Fig. 2. For the substrate with a coating of 67 nm in thickness (25-nm diameter NPs), the destructive interference occurred in a rather high energy region. This thickness is not suitable for ARCs in Si solar cells because Si solar cells are sensitive to light in the visible and the near-infrared (NIR) regions rather than to light in higher energy region. Thus, the coating needs to be thicker to shift the destructive interference positions into the visible and the NIR regions. For coatings of 196 nm (25-nm NP diameter) and 180 nm (104-nm NP diameter) in thickness, the wavelength of maximum suppression shifted to a longer wavelength because the layer thickness increased. However, if the layer becomes too thick, an oscillating behavior in spectra was observed because the layer thickness matched not only λ/4, but also 3λ/4, 5λ/4, etc. Oscillation spectra were observed for layers of 490 nm in thickness (25-nm NP diameter) and 510 nm in thickness (104-nm NP diameter), indicating that these thicknesses are not suitable for AR coatings. Next, we determined the optimal layer thickness for

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Journal of the Korean Physical Society, Vol. 60, No. 11, June 2012

Table 1. Average absorptance and relative enhancement βabs as functions of the ARC layer thickness. Diameter

Thickness (nm)

25 nm (±5 nm)

104 nm (±10 nm)

67 139 154 196 100 134 154 180

Si solar cell applications. Figure 3 shows the average reflectance over wavelengths of 400 – 1200 nm for eight samples with multi-layered SiO2 NPs as a function of layer thickness. As shown in the 25-nm group of Fig. 3, the average reflectance decreased as the thickness decreased from 196 to 139 nm and rose rapidly at a thickness of 67 nm, which explains why layers that are too thin are not suitable for obtaining the AR effect for visible and NIR light. A 139-nm-thick layer showed the lowest reflectance out of the four samples, so it could be considered for an optimized antireflection layer. All samples showed similar average reflectances in the 104-nm group of Fig. 3 although the 154-nm-thick layer had the lowest average reflectance. Meanwhile, when we evaluated layers with thicknesses of 154 nm and 134 nm (104-nm NP diameter), the average reflectance was somewhat higher than that for the 154-nm- and the 139-nm-thick layers (25-nm NP diameter). These phenomena are discussed later. Based on the reflectance, we calculated the average absorptance using the formula A = 1 − R. The relative absorption enhancement, βabs , is extracted with
1200 nm AARC (λ)dλ nm βabs =
400 , (2) 1200 nm Abare (λ)dλ 400 nm

where Abare (λ) and AARC (λ) are the average absorptances of bare Si and of the Si with multilayered SiO2 NPs over wavelengths of 400 – 1200nm, respectively. The βabs listed in Table 1 were calculated by normalizing the AARC (λ) value to the value for bare Si. As expected, samples with lower reflectance had greater light absorption. Note that the Si solar cells are more sensitive to visible light because the spectral irradiance of visible light is much higher than that of other light in the AM 1.5G solar spectrum. Thus, light reflectance should be decreased in the visible region in particular. Figures 4(a) and (b) show the optical absorption intensity, opt(λ), for various layer thicknesses with respect to the AM 1.5G solar spectrum. An ideal sample, which realizes 100% absorption, is shown with bare Si for comparison. The

Average absorptance (%) (400 – 1200 nm) 68.90 76.85 76.08 73.04 72.31 73.62 74.33 73.58

Relative enhancement, βabs 1.161 1.288 1.273 1.222 1.219 1.232 1.245 1.231

Fig. 3. (Color online) Average reflectance of eight samples of multi-layered SiO2 NPs as a function of the layer thickness. The thicknesses of the 25-nm diameter group (black squares) were 196 nm, 154 nm, 139 nm, and 67 nm. The thicknesses of the 104-nm diameter group (red circles) were 180 nm, 154 nm, 134 nm, and 100 nm.

optical absorption intensity was obtained using opt(λ) = φ(sun) · A(λ),

(3)

where opt(λ) is the optical absorption intensity, (φ(sun) is the AM 1.5G solar spectrum intensity, and A(λ) is the optical absorptance. As the layer thickness changed, the optical absorption intensity changed as a function of wavelength. To specifically confirm the optical enhancement according to the wavelength zone of light, we calculated the relative enhancement in the optical absorption intensity γabs via normalization to the value for bare Si. We divided the wavelengths into three zones with their own γabs , which are given by Eq. (4), which only shows Zone I representing the entire wavelength range (400 – 1200 nm) used in this work:


1200 nm

nm γabs,I =
400 1200 nm 400 nm

optARC (λ)dλ , optbare (λ)dλ

(4)

Multi-layer Coating of SiO2 Nanoparticles To Enhance Light Absorption by Si Solar Cells – Yoon-Ho Nam et al.

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Table 2. Relative enhancement in optical absorption intensity γabs for different wavelength regions as functions of the layer thickness. Diameter

25 nm (±5 nm)

104 nm (±10 nm)

Thickness (nm) 67 139 154 196 100 134 154 180

400 – 1200 nm 0.306 0.341 0.337 0.323 0.322 0.328 0.327 0.325

Fig. 4. (Color online) Optical absorption intensity and relative enhancement γabs for layers of various thicknesses with respect to the AM 1.5G solar spectrum: (a) average diameter of NPs: 25 nm and (b) average diameter of NPs: 104 nm.

Likewise, Zone II for the visible region (400 – 800 nm) and Zone III for the NIR region (800 – 1200 nm) were separately extracted by only varying the wavelength ranges. These γabs values over different wavelength regions are listed in Table 2. In the 25-nm-diameter group, the 139-nm-thick layer showed superior optical absorption intensity because the average reflectance of this layer was the lowest and the highest enhancement was achieved in the visible region without much degradation

Relative Enhancement, γabs 400 – 800 nm 0.363 0.384 0.374 0.354 0.384 0.372 0.353 0.354

800 – 1200 nm 0.249 0.297 0.299 0.249 0.260 0.283 0.301 0.295

Fig. 5. (Color online) Relative absorption enhancement βabs of NPs with different diameters for similar layer thicknesses. (a) The layer thickness is 139 nm for the 25-nm NP diameter and 134 nm for the 104-nm NP diameter. (b) The layer thickness is 154 nm for both diameters.

in the NIR region. In the 104-nm-diameter group, all samples showed similar γabs values due to a counterbalancing effect between enhancement in the visible region and degradation in the NIR region although the average reflectance was the lowest when using a 154-nmthick layer. As a result, both the AR ability of ARCs and their light wavelength sensitivity need to be considered simultaneously when designing ARCs. Until now, we focused on the AR effect as a function of layer thickness. Because λmin influences the average

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Journal of the Korean Physical Society, Vol. 60, No. 11, June 2012

reflectance in a certain wavelength region based on Eq. (1), the layer thickness d and the n values are important factors for light reflection. In this work, the samples had similar d and n values and should, therefore, have had similar AR properties. The AR effect, however, could also be influenced by the diameter of the SiO2 NPs. Figure 5 shows βabs values for discriminating different diameter samples of similar thicknesses. The 25-nm-diameter sample showed greater enhancement than the 104-nmdiameter sample in all wavelength regions. The different AR effects can be understood by using Rayleigh scattering, which explains the scattering behavior of light by NPs with dimensions much smaller than the wavelength of light. The scattering cross-section of NPs was calculated using Eqs. (5) and (6):  4 1 2π α2 , (5) σsca = 6π λ   ε−1 , (6) α = 4πR3 ε+2 where σsca is the scattering cross-section, λ is the wavelength, α is the polarizability, R is the diameter of NPs, and ε is the dielectric constant. The scattering crosssection describes the possibility of light being scattered by NPs and is proportional to the square of the polarizability of NPs. Additionally, larger diameter NPs have a higher polarizability than smaller diameter NPs. Thus, larger diameter NPs have greater scattering crosssections than smaller diameter NPs, which causes an increase in the scattering loss. This explains why the absorption enhancement is lower for the 104-nm-diameter samples than for the other samples, as shown in Fig. 5. Due to scattering loss, therefore, the layers with larger diameter NPs had worse AR effects than the layers with smaller diameter NPs. IV. CONCLUSION We synthesized multi-layered SiO2 NPs on Si substrates as ARCs and investigated their AR characteristics based on the quarter-wavelength AR effect. ARCs of various thicknesses enhanced light absorption in the visible and the NIR regions. In addition, absorption enhancement in the visible region associated with the AM 1.5G solar spectrum was crucial for the Si substrate. Layers with larger diameter NPs revealed the worse AR effects due to scattering loss than layers with smaller

diameter NPs. Thus, the AR effect could be controlled by using the diameter of the SiO2 NPs as well as by using the ARC layer thickness. ACKNOWLEDGMENTS This work was supported by the research fund of Hanyang University (HY-2010-N).

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