ZnO/Al₂O₃ core/shell nanorods array as excellent anti-reflection layers on silicon solar cells

Highlights

• ZnO nanotip arrays were synthesized by hydrothermal methods as antireflection layer.

• The total reflectance is low around 7.8% from 400 nm to 1000 nm.

• The total reflectance can reduce to 5.5% after coating of alumina thin film.

• The power conversion efficiency can be enhanced to 17.79%.

Abstract

A simple, low-temperature hydrothermal method and atomic layer deposition (ALD) were used to fabricate ZnO nanostructures as subwavelength-structure antireflection layers (SWS ARLs) on Si solar cells. ZnO seed layers with wafer-scale uniformity were prepared, and ALD was used to reproduce two types of ZnO-based structures, nanorod arrays (NRAs) and nanotip arrays (NTAs). The study examined diammonium phosphate concentrations during growth, conducted simulations based on three-dimensional finite-difference time-domain and reflection analyses, performed X-ray diffractometer, field-emission scanning electron microscope, and high-resolution transmission electron microscope characterizations, measured total reflectance spectra by using a spectrophotometer with integrated spheres, and ran solar simulations to determine the efficiency of the Si solar cells. Coating the ZnO NTAs on the Si solar cells yielded a low total reflectance over a broad band range and produced omnidirectional light scattering on the cells, causing incident light to have a shallow penetration depth near the p–n junction and leading to an increase in short current density (_Jsc). Coating the ZnO NTAs with an Al₂O₃ shell induced continuous variation in the refractive index, further decreasing the total reflectance to approximately 5.5%, and protected the ZnO NTAs from the harmful acidic environment. Significantly increasing the J_sc and η levels of the Si solar cells, the Al₂O₃@ZnO-NTA antireflection structure produced a high efficiency of 17.79%. Its superior performance, including low and wideband reflectance, a low process temperature, and a significant increase in efficiency, indicates the potential of this antireflective structure for enhancing solar cell efficiency in photovoltaic devices.

Introduction

Commercial silicon (Si) solar cells have been developed to convert sunlight into electricity with efficiencies of approximately 20% [1]. To improve the conversion efficiency, many studies have focused on antireflection layers (ARLs) to effectively suppress the Fresnel reflection over a broad band in the visible region of the solar spectrum. In general, ARLs consist of one or more dielectric layers in the form of either a quarter-wave thickness film that exhibits a wavelength-sensitive reduction in the reflection because of interference [2], [3], [4] or a mesoporous film that has the advantage of light trapping, resulting in a more broadband response [5], [6], [7]. Xi et al. used oblique-angle deposition at various angles to coat five layers of titanium dioxide (TiO₂) and silicon dioxide (SiO₂) nanorods with an optimized overall refractive index gradient, achieving a low reflectance [8]. Striemer and Fauchet applied a thin, porous Si layer as an ARL on the Si surface, a large index discontinuity was broken, resulting in a lower broadband reflectivity [9]. However, a multilayered dielectric film has limitations in the coating of materials, as well as various physical and chemical properties that affect the adhesion, thermal mismatch, and stability of the thin-film stack [10]. In addition, a gradient refractive index of ARLs could be achieved by gradually changing the porosity of discrete layers to obtain an excellent broadband reflectivity (<3%) in the visible spectrum. The overall thickness of these porous layers was too thick to degrade the electrical characteristics of the Si solar cells.

Currently, developing ARLs with a subwavelength structure (SWS), which is a surface-relief grating with a period shorter than the wavelength of light, has gained considerable attention [11], [12], [13], [14], [15]. According to the experimental results of some studies investigating SWS surfaces, a two-dimensional SWS surface does not depend on the polarization direction and shows promise for use in many optical components [16]. Light strikes the SWS as if it encounters a thin layer with an effective refractive index (n_e) between the refractive indices of air and Si, thus avoiding the abrupt transition from air to device and enhancing the light trapping. However, if the wavelength of incident light is considerably shorter than the structure dimension, the multiple reflection process could become dominant for light trapping [17].

In the past decade, one-dimensional structures have attracted exceptional attention in both optical enhancements and electric improvements for photovoltaic applications because of their unique architecture [18]. From the optical viewpoint, nanowire arrays (NWAs) have significantly suppressed the reflection in wide ranges of wavelength and the angle of incidence [11], [19], [20], [21], [22]. Superior antireflection characteristics of NWAs, including polarization insensitivity, omnidirectionality, and broadband working range, have been demonstrated [20], [23], [24]. In addition, this decreasing reflectance was due to light trapping structures, demonstrating that NWAs can efficiently absorb solar light and can be potentially applied to photovoltaics [25], [26]. Yang et al. used silica bead arrays, which prepared by dipcoating process, as etching masks on Si substrate. Then a deep reactive ion etch (d-RIE) process with SF6 and C4F8 gas was employed to generate a Si-NWAs-SWS of various heights [27]. Although the etching process was used to fabricate Si NWA-SWSs, the manufacturing costs of etching processes, such as RIE, were expensive. He et al. developed a Si hierarchical structure (Si pyramidal/Si microgroove) as ARL by wet etching process [15]. The Si hierarchical structure exhibited an average total reflectance as low as 13.2% from 300 to 1000 nm and the power conversion efficiency of 15.2% with a high short-circuit current density (J_sc) of 36.4 mA/cm². However, the surface recombination loss resulting from dry or wet etching would further hindered the applications of SWSs in commercial Si solar cells [28].

Recently, nanowires synthesized using various bottom-up approaches based on chemical vapour deposition have demonstrated antireflection coating consisting of single materials such as indium tin oxide, TiO₂, SiO₂, zinc oxide (ZnO), and gallium phosphate [6], [8], [11], [12], [13], [29], [30], [31]. Among these materials, ZnO is attractive as an SWS material because of its favorable transparency, appropriate refractive index (n = 1.931), and the ability to form textured coating through anisotropic growth. Estrich et al. deposited Ga-doped ZnO films on a Si substrate through pulsed laser deposition to demonstrate superior ARL performance compared with a SiN_x single-layer ARL [32]. Therefore, ZnO nanostructures possess high potential for trapping light radiation in ARL applications. Many methods can be used to fabricate ZnO NRAs or NWAs [33], [34], [35]. In contrast to a gas-phase process, the solution synthesis was a simple, low-temperature, and low-cost method for fabricating ZnO NRAs. According to Lee et al., a low reflectance of approximately 7% was achieved using ZnO NRAs grown on Si wafer through a low-temperature hydrothermal method involving optimal 1,3-diaminopropane (DAP) [11]. However, although previous studies have presented favorable antireflection properties with optimal ZnO NRAs, they have focused little on the evaluation of the properties of Si p–n junction solar cells. In addition, the as-grown ZnO NRAs showed a near-band emission (NBE) at approximately 380 nm and a broad defect band emission (DBE) between 550 and 700 nm [36], [37], indicating that the incident light was absorbed by the ZnO NRAs and was not caught by the Si solar cells to generate photocurrent.

In this study, ZnO nanorods arrays (NRAs), ZnO nanotip arrays (NTAs), and Al₂O₃-coated ZnO NTAs (Al₂O₃@ZnO-NTAs) were used as SWS-ARLs on Si solar cells through a modified hydrothermal process and atomic layer deposition (ALD). The morphologies, crystallinity, phonon vibrations, and optical properties of the ZnO-based SWS-ARLs were studied. In addition, the light trapping performance was demonstrated by simulations based on finite-difference time-domain (FDTD) analysis and reflection analysis at first. The performance of various types of ZnO-based SWS-ARLs that covered the Si solar cells was compared with that of the bare Si solar cells. The results demonstrated that Al₂O₃@ZnO-NTAs effectively improved the antireflection and increased the conversion efficiency of the Si solar cells by up to 17.79%.