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Abstract
In this article, anti-reflection coatings (ARCs) with double-layer metal nanoparticles for Cu(In, Ga)Se2 (CIGS) solar cells were proposed. A study of three ARCs nanostructures (Ag-SiO2-Ag, Au-SiO2-Au and Au-SiO2-Ag structures) on CIGS solar cells was presented using the finite-difference time-domain method. Various ARC nanostructures with different metals (Au and Ag), metal nanoparticle radius, spacing layer (SiO2) thickness, and period (corresponding to the particle density) on CIGS solar cells were optimized in detail. The Au-SiO2-Ag structure was demonstrated to have a better anti-reflection ability when the thickness of the SiO2 layer is 45 nm, the radius of the metal particle is 14 nm, and the side length of the period is 41 nm. The results demonstrated that the reflectivity of the CIGS solar cells with the optimal ARCs structure was reduce by 83% compared with the case of no ARCs. And compared with the MgF2 anti-reflection film, the reflectivity was reduced by 37%.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fresnel reflection occurs when light enters the interface between two media with different refractive indices [1]. This reflection reduces the performance of many optical devices, such as photodetectors, solar cells and light-emitting diodes. The energy loss for bare monocrystalline silicon solar cells is high up to 30% caused by the serious reflection at long-wavelength [2]. In light-emitting diodes, some of the light emitted by the semiconductor is reflected back into the semiconductor with nearly 35% of the energy loss [3]. Therefore, reducing reflectivity is critical for improving device performance. For solar cells, the traditional method of reducing reflectivity is generally to deposit monolayer or multilayer films on the surface. For example, Pakhuruddin et al. designed ZnO anti-reflective film on the thin film silicon solar cell [4] and Xiao et al. prepared a TiO2/SiO2 double-layer anti-reflection coating for GaAs solar cells [5], both of which obviously reduced the reflectivity in a certain wavelength region. However, it is difficult to achieve low reflection in a broadband for anti-reflection coatings (ARCs) [6]. Therefore, effectively minimizing the energy loss caused by reflectively within a broad wavelength region is crucial for high performance devices, especially for thin film devices.
Several typical light trapping methods have been proposed to reduce the reflection for the thin film solar cells, such as plasmonic metallic structures [7,8] and ordered dielectric light trapping structures (e.g. photonic crystals) [9]. Metal nanoparticles (NPs) supporting local surface plasmons can provide an advanced light trapping mechanism to significantly reduce the light reflection of solar cells [10–12]. There are two aspects to have to be addressed about lowering the reflection of devices via using NPs, the first involves the light scattering mechanism, the second relates to the local field enhancement induced by NPs. The local field enhancement means that when metal NPs are irradiated by light, the free electrons on the surface collectively oscillate to generate surface plasmon polaritons, which enhances the local electromagnetic field of the metal surface. Moreover, local field on metal surface enhancement significantly enhances the light absorption of the material. The light scattering mechanism means that the strong scattering of NPs enables the energy of the plasmon to couple into the optoelectronic device, thereby reducing the reflectance and enhancing the absorption of light by the device. In addition, since the incident light at a single angle is scattered by the NPs and enters the optoelectronic device at different angles, the distance traveled by the light in the device increases, thereby thinning the device [13] and reducing the material consumption.
Metal NPs act as a light trapping structure to help solar cells captures more incident photons [14], which is a new way to reduce the reflectivity of materials. The effectiveness of light trapping depends largely on the polarizability of the metal NPs, which is affected by the shape, size, distribution and media environment of the NPs [15,16]. In recent years, the research and application of metal NPs have been systematized. Jheng et al. studied the effect of different sizes deceahedral Au NPs on the performance of polymer solar cells. When 43 nm sized Au NPs were added to the device, the power conversion efficiency is significantly enhanced from 3.67% to 4.22% [17]. Bai et al. obtained good results in related fields. Firstly, the spherical metal NPs were studied, and high-quality light trapping structures were obtained by eliminating small NPs. The short-circuit current density of silicon solar cells increased by 10.73% [18]. Next, decahedron-shaped Au NPs were prepared and used as a light trapping structure on double-junction and triple-junction solar cells, the photovoltaic power efficiency reaches up to 28.63% and 31.90% [19]. These efforts have strongly demonstrated that metal NPs can effectively reduce the reflectivity of solar cells. However, these studies are limited to monolayer metal NPs, making the device still weak for harvesting light scattering. Nowadays, the preparation of NPs has been rapidly promoted, and double-layer NPs can be prepared, enabling more light capture in solar cells. Double-layered metal NPs have a high degree of flexibility (such as changing particle size and spacing layer thickness) for the regulation of plasmon polaritons. Moreover, the coupling effect was found to play an important role in the optical properties of the double-layer metal nanoarrays [20], which broadens the plasmon resonance spectrum. Thus, using double-layer structures of metal NPs may achieve better performance for solar cells.
The Cu(In, Ga)Se2 (CIGS) thin film solar cell has unique properties such as high power conversion efficiency, low manufacturing cost and long-term stability, which is regarded as one of the most promising photovoltaic technologies for future energy [21]. However, the solar incidence of light suffers from the Fresnel reflection on the surface of window layer of CIGS solar cells, resulting in the deficiency of the photo generated current [22]. It is hoped that the question will be resolved (fall away) with our proposed novel anti-reflection structures. Herein, the finite-difference time-domain (FDTD) method is applied to design three ARCs with the structures of Ag-SiO2-Ag, Au-SiO2-Au, and Au-SiO2-Ag. Our results confirm that the reflectivity of CIGS solar cells is 16% when wavelength ranges from 420 nm to 730 nm, but it decreases to less than 3% upon introduction the anti-reflective structure of Au-SiO2-Ag. Namely, there is a 83% relative reduction in reflectivity in comparison with that of bare CIGS, and a 37% reduction in contrast to that of the MgF2 anti-reflection film.
2. Device structure and simulation method
Two metals, Au and Ag, were selected as the front located nanoparticle materials for light trapping, owing to surface plasmon resonance phenomenon. The combination of SiO2 and metallic plasmonic nanostructures has the advantage of enhancing light-matter interactions, resulting in a large field enhancement at the gap between the particles and the metal surface. In addition, the introduction of SiO2 protects the underlying metal, especially easily oxidized metals (such as Ag). Thus, we designed three structures of ARCs for CIGS solar cell: double-layer Ag NPs (Ag-SiO2-Ag), double-layer Au NPs (Au-SiO2-Au), and the structure with lower-layer Ag NPs and upper-layer Au NPs (Au-SiO2-Ag). Since the stability of Ag is worse than that of Au (Ag is easily corrosive in the environment), the structure with lower-layer Au NPs and upper-layer Ag NPs is not considered here.
A 0.1 µm-thick MgF2 film is used as the commonly traditional anti-reflection film for the CIGS solar cell, as shown in Fig. 1(a)
Fig. 1 CIGS solar cell and its ARC structure. (a) MgF2 coating (b) The solar cell structure consisting of ARCs. (c) Double-layer metal NPs coatings.
In recent years, the manufacturing process of the CIGS solar cells with a structure of glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al/ARCs is very universal. Huang et al. had sequentially deposited the back contacts of Mo, CIGS, buffer CdS, intrinsic ZnO (i-ZnO), ZnO:Al layers by sputtering, two-step or co-evaporation, chemical bath deposition, and sputtering processes, respectively [24]. And the process of preparing metal NPs has matured. Bai et al. prepared small-sized Ag NPs by magnetron sputtering and thermal annealing [18,25]. Therefore, the fabrication of our structure can be realized recently.
The ARCs structure is above the window layer, which is both the anti-reflection layer and the passivating layer. The high-doped cap layer of the CIGS solar cell is below the electrod, which has excellent electrical conductivity. In our modeling, the simple p region of device is considered just absorbed layer CIGS, but the complex n region consists of i-ZnO and CdS. The photogenerated carriers are transported to the top window layer and then spread laterally to the cap layer which is then collected by the electrodes.
To investigate the optimization of ARCs, the integrated light reflection was calculated by integrating the light reflection over the spectrum and weighting to the AM1.5G standard solar photon spectrum [26] in the 300-1200 nm spectral range. The integral formula is as follows [27]:
where I is the photon number of reflection meaning the reflectance spectra integrated over a wavelength range of 300-1200 nm, which is consistent with the meaning of reference [27], IAM1.5G is the photon flux density of the AM1.5G solar spectrum, λ is the wavelength and R(λ) is the reflectivity at wavelength λ. When the value of I decreases, the structure will have stronger anti-reflection ability, which leads to better coating effect. This is because the more photons a solar cell absorbs, the more electron-hole pairs it forms. For the CIGS solar cell, the light reflectivity is calculated by simulation.In order to obtain light reflectivity: the value of R(λ), the FDTD method was used to calculate the light reflection in the solar cell. The plane waves used by light source are from 300 nm to 1200 nm, which are emitted vertically. Perfectly matched layers (PML) were used in the vertical direction to avoid the interference of the light reflected from the simulation boundaries and periodic boundary conditions were used in the lateral direction to simulate two layers ordered arrays of NPs. We set maximum mesh step as 1 nm in x, y and z direction in the simulation region. The simulation time was 1000 fs. Refractive index data for metals (Au, Ag and Mo) and SiO2 were obtained from Palik [28], CIGS and ZnO from Li [29,30], and the rest of the material data were measured from Filmetrics company [31]. The thickness of SiO2 layer, the radius of metal NPs and the side length of the period (c shown in Fig. 2
Fig. 2 The square (c*c) structure unit of numerical calculation, where c is the space between two adjacent NPs.
For the CIGS solar cell, the light transmission is zero on the bottom surface. The relationship between reflection and absorption is defined as A = 1 - R, where A and R are the absorption and reflection coefficients, respectively.
3. Results and discussions
In order to optimize the ARC structure of CIGS solar cells, we studied the relationship between reflectivity and wavelength with different SiO2 thicknesses for different structures when the side length of the period and radius of metal NPs were fixed at 49 nm and 15 nm. Figures 3(a-c)
Fig. 3 Reflectivity depends on wavelength with different SiO2 layer thickness, when the side length of the period is 49 nm and the metal particle radius is 15 nm. (a) Ag-SiO2-Ag structure. (b) Au-SiO2-Au structure. (c) Au-SiO2-Ag structure. (d) The photon number of reflection with different SiO2 layer thickness for the three ARC structures.
The plasmonics on the surface of the metal NPs significantly reduce the reflectivity of the solar cell. There are two aspects to explain why metal NPs reduce the reflection of the devices [15,33]. On the one hand, when metal NPs are irradiated by light, the free electrons on the surface collectively oscillate to generate surface plasmon polaritons, which enhances the local electrical field intensity. The local field on metal surface enhancement significantly increases the light absorption of the solar cell. On the other hand, the strong scattering of the NPs enables the energy of the plasmon to couple into the solar cell, thereby reducing the reflectance and enhancing the absorption of the solar cell. The local electrical field intensity is greatly enhanced for the double-layer NPs as a result of adding more sharp corners or singularities [34] which play an important role in scattering light.
The photon number of reflection calculated from Eq. (1) is shown in Fig. 3(d) for the three ARC structures. The results show that the three ARC structures exhibit a minimum photon number in each curve when the SiO2 layer thickness is 45 nm. It is noted that, for the three ARCs, the Au-SiO2-Ag structure has the strongest absorption capacity for CIGS solar cells.
When the thickness of SiO2 layer is fixed at 45 nm, Fig. 4
Fig. 4 The results of optical simulation calculation according to Eq. (1). The photon number of reflection varies with the side length of the period and the radius of metal particle when the thickness of SiO2 layer is 45 nm. (a) Ag-SiO2-Ag structure. (b) Au-SiO2-Au structure. (c) Au-SiO2-Ag structure.
Fig. 5 Comparison of the five structures. The side length of the period for three structures (Ag-SiO2-Ag structure, Au-SiO2-Au and Au-SiO2-Ag structure) is 45 nm, 37 nm and 41 nm, respectively. And the radius of metal particle is 14 nm. (a) Dependence of reflectivity on the wavelength. (b) The photon number of reflection with different SiO2 layer thickness.
In order to further verify that the parameters derived from Fig. 4 are optimal, Fig. 5(b) shows the relationship between the photon number of reflection which is calculated by Eq. (1) and the thickness of the SiO2 layer. The results show that when the SiO2 layer thickness is 45 nm, the photon number of reflection is really minimal. For ARCs of CIGS solar cells, compared with the case of no ARCs, the reflectivity of Ag-SiO2-Ag, Au-SiO2-Au and Au-SiO2-Ag structures is reduced by 77%, 82% and 83%, respectively. Meanwhile, compared with the MgF2 anti-reflection film, the reflectivity is reduced by 15%, 34% and 37%, respectively. In addition, the optimal structure of Au-SiO2-Ag reduces the reflectivity by 4% compared with the optimal structure of Au-SiO2-Au. This discrepancy is mainly attributed to the metal particle differences between the two structures. It can be clearly seen that the reflectivity is significantly mitigated. Hence, we may conclude that double-layer metal NPs significantly reduce the reflectivity of the CIGS solar cell, especially for the Au-SiO2-Ag structure.
From Fig. 5(a), two resonant absorption peaks can be found in the curve for the Au-SiO2-Ag structure, and they are located at 395 nm and 665 nm. To further understand the enhancement of optical absorption for the Au-SiO2-Ag structure, the mode distribution of λ = 395 nm and λ = 665 nm is shown Fig. 6 and 7
Fig. 6 The field distribution on Au NPs of X-Y plane for the Au-SiO2-Ag structure: (a) λ = 395 nm; (b) λ = 665 nm. Other parameters are unchanged (the thickness of the SiO2 layer is 45 nm, the radius of the metal particle is 14 nm, and the side length of the period is 41 nm).
Fig. 7 The mode distribution of X-Z plane for the Au-SiO2-Ag structure: (a) E: λ = 395 nm; (b) H: λ = 395 nm; (c) E: λ = 665 nm; (d) H: λ = 665 nm. Other parameters are unchanged (the thickness of the SiO2 layer is 45 nm, the radius of the metal particle is 14 nm, and the side length of the period is 41 nm).
Figure 6 shows the mode distribution of X-Y plane, which located on the Au NPs of the structure. As shown in Fig. 6(a), for the resonant absorption peak at 395 nm, the field distribution displays the existence of quadrupole plasmon modes. As shown in Fig. 6(b), when the resonant absorption peak is at 665 nm, the field distributions indicate the presence of dipole plasmon mode at this resonance peak [36].
For a better comparison, we also have chosen the X-Z plane. Figures 7(a) and (b) are the electric field distribution (E) and magnetic field distribution (H) at a wavelength of 395 nm, respectively. As shown in Fig. 7(a) and (b), two field relative maximum points appear on the upper surface of the Ag NPs. Local field enhancement at the SiO2/Ag interface, indicates the excitation of surface plasmons resonance. Figures 7(c) and (d) are the electric field distribution and magnetic field distribution at a wavelength of 665 nm, respectively. The electric field in the ZnO layer is significantly enhanced. The scattering effect of the metal nanoparticles increases the transmission optical path of the incident light, which is beneficial to increase the absorption of the incident light by the solar cell.
Compared with other published anti-reflective structures, our structure has its unique advantages. With the ZnO monolayer anti-reflection structure on top of thin film silicon solar cell, the surface reflectance is suppressed to only 2% at the wavelength of 600 nm [4]. However, the reflectivity of the Au-SiO2-Ag structure we designed is approximately close to zero at the wavelength of 400 nm. And the reflectivity is less than 5% in the 400-1000nm wavelength range. Compared with the reflectivity of the anodic aluminum oxide (AAO)/TiO2 double-layer anti-reflection structure of GaAs solar cells [35], our Au-SiO2-Ag structure also has a wider spectrum with the low reflectivity, which has better anti-reflective effect.
4. Conclusion
We investigated the effects of plasma scattering on reflection of CIGS solar cells decorated with double-layer metal NPs. The FDTD method was used to optimize the structural parameters of three kinds of the ARCs. And we find that Au-SiO2-Ag structure is the optimal ARC structure and when the thickness of the SiO2 layer is 45 nm, the radius of the metal particle is 14 nm, and the side length of the period is 41 nm, the CIGS solar cells have the lowest reflectivity. The reflectivity of ARC structure is reduced by 83% and 37% compared with the case of no ARCs and the MgF2 anti-reflection film. Therefore, the ARC structure greatly reduces the reflection and improves the photoelectric efficiency. This ARC structure with double-layer metal NPs can be used as a scheme for investigating plasma enhanced absorption in solar cells and enables us to develop guiding principles for solar cell design.
Funding
Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications) (IPOC2016ZT01); National Natural Science Foundation of China (61674020, 61574019, 61474008); the International Science & Technology Cooperation Program of China (2011DFR11010); the 111Project of China (B07005).
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