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Abstract
We demonstrated that a well-designed nanopatterned cover improves photovoltaic efficiency across a wide range of incident angles (θ). A nanopatterned cover was created using an integrated ray-wave optics simulation to maximize the light absorption of the surface-textured Si photovoltaic device. A hexagonally arranged nanocone array with a 300 nm pitch was formed into a polymer using nanoimprinting, and the nanostructured polymer was then attached to a glass cover with an index-matching adhesive. Angle-resolved current density-voltage measurements on Si photovoltaic devices showed that the nanopatterned glass cover yielded a 2–13% enhancement in power conversion efficiency at θ = 0–60°, which accounted for its broadband antireflective feature. We performed all-season-perspective simulations based on the results of the integrated ray-wave optics simulations and solar altitude database of South Korea, which validated the sustainability of the developed nanopatterned cover during significant seasonal fluctuations.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Photovoltaics (PVs) benefit greatly from nanophotonics, which can improve the absorption of sunlight at a wide range of wavelengths and incident angles, enabling a high power conversion efficiency (PCE) [1–19]. Because broadband sunlight absorption is critical for PV technologies, the rational design of nanophotonic structures, such as surface texturing [5–13] and multilayer coatings [14–19], has garnered immense attention. Overall, light-trapping strategies for PVs consider two optical effects: reduced surface reflection and increased optical path within the active material [1,4]. The former is required for bulk (i.e., optically thick) PVs, in which light entering the active material is almost entirely absorbed during a single round trip. In this scenario, only the minimization of Fresnel reflection losses for incoming sunlight is considered. In comparison, the latter is effective for thin-film (i.e., wavelength-thick) PVs, in which a periodic grating can convert sunlight in free space into laterally propagating waveguide modes [1,4,5]. Moreover, the periodic grating can concurrently serve an antireflective function.
Such nanophotonic structures have been primarily incorporated into the top or bottom surfaces of photovoltaic cells through top-down [6,7,13,17,18] or bottom-up [9–12,14–16,19] fabrication. In the absence of encapsulation, the device performance is evaluated at the cell level. However, encapsulation is required to protect the PV efficiency from weather-related (e.g., humidity, ultraviolet radiation, oxidation, and temperature) degradation, unexpected shocks, and continuous mechanical vibrations [16,20]. Ethylene vinyl acetate (EVA) is commonly employed as a PV module encapsulant owing to its glass-like transparency in the visible and near-infrared regions, good electrical insulation, and low polymerization temperature [21]. In addition, its strong adhesion strength prevents the formation of air gaps, which would otherwise result in significant reflection losses at the EVA/air/PV interfaces. Because PV panels are frequently exposed to elements such as rooftops, damage from dirt and other natural calamities is prevalent. As a result, EVA-coated PV cells must be sealed with a high-durability glass cover. However, experimental attempts to incorporate an antireflective glass cover into a surface-textured PV cell or design principles to optimize the glass cover in terms of broadband antireflection have rarely been reported. These studies are important because reflection loss at an air/glass interface is typically 4% for normal incidence and steadily increases for larger incident angles.
In this study, we report a nanopatterned glass cover that enhances the absorption of sunlight into surface-textured crystalline Si PV cells across a wide range of incident angles. The glass cover was designed using an integrated ray-wave optics simulation on account of its macroscopic scale (e.g., 1 mm thickness). The absorption efficiency of the surface-textured Si photovoltaic cell was calculated as a function of the incident angle when a nanopatterned glass cover was employed. We prepared an antireflective glass cover by bonding a nanostructured poly(methyl methacrylate) (PMMA) film to bare glass. Measurements of the transmittance of the fabricated glass covers and current density-voltage (J-V) characteristics of Si PV modules were performed at various incident angles. Finally, we conducted all-season-perspective simulations based on the results of the integrated ray-wave optics simulations and solar altitude database of South Korea to predict whether the developed PV module can sustain its antireflective capability during seasonal variations. There have already been a vast number of existing studies in the field of antireflective patterns (i.e., moth-eye structures). The following points differentiate our study from others. First, we developed an integrated ray-optics simulation to investigate the synergistic effect of a nanopatterned cover and micropatterned Si PV cell. Second, using the integrated ray-optics simulation, we designed a nanopatterned cover to maximize the absorption of Si PV devices. The nanopatterned cover induced marginal diffraction while it provided a broadband antireflective function. Therefore, the principal path of light incident upon a PV cell was closer to its normal line, which allowed for a high PCE. Finally, angle-resolved PV performance was evaluated for surface-textured Si PV cells with a fabricated nanopatterned cover.
2. Results
2.1 Design of a nanopatterned cover using an integrated ray-wave optics simulation
For encapsulated Si PV modules, incident sunlight is partly reflected at two separate interfaces: the air/cover and cover/Si (Fig. 1(a)). Therefore, rationally designed patterns must be incorporated into both interfaces to reduce each reflection. In general, bulk Si PVs employ micropatterned Si surfaces through anisotropic wet chemical etching, which enables them to absorb sunlight over a wide range of wavelengths and incident angles [22]. Compared with the reflection at the cover/Si interface, the air-to-cover reflection is nominal (e.g., approximately 4% for normal incidence). However, it becomes critical when PVs primarily harness the off-normal incidence of sunlight (e.g., building-integrated PVs) [10,23].
Fig. 1. (a) Schematics describing three Si PV module cases: (i) planar Si with a planar glass cover, (ii) micropatterned Si with a planar glass cover, and (iii) micropatterned Si with a nanopatterned glass cover. (b) Simulated solar reflectance of a nanopatterned cover (n = 1.48) with a variety of Dt values. (Inset) Schematics of the simulated structures describing the parameters of H, S, Db, and Dt values. For all the simulations, H = 300 nm, S = 100 nm, and Db = 200 nm. (c) Surface plots of simulated spectra of rod- (left) and cone-shaped (right) nanopatterns (n = 1.48) with a variety of Dt values. For all the simulations, H = 300 nm and S = 100 nm.
We explored the optimal pattern morphology for the air/cover (refractive index, n = 1.48) interface using a rigorous coupled-wave analysis (RCWA) simulation (Fig. 1(b)). The solar reflectance was calculated as the top diameter (Dt) of the pattern varied (i.e., from rod to cone). The other pattern parameters were fixed: height (H) = 300 nm, space (S) = 100 nm, and bottom diameter (Db) = 200 nm, which were derived from the capacity of our nanoimprinting system. The solar reflectance was acquired by averaging each reflectance value at 400–800 nm for normal incidence, weighted by the AM 1.5G spectrum. The simulated solar reflectance steadily decreased as the pattern morphology became conical, which is consistent with the results of previous studies [5,6]. It was practically zero at Dt < 50 nm. Subsequently, we simulated the reflectance spectra for rod- and cone-shaped nanopatterns for various Db values (Fig. 1(c)). Overall, the cone exhibited a lower reflectance at the considered wavelengths. It performed almost perfectly as a broadband antireflective structure when Db = 100–200 nm.
Performing optical simulations on the full PV module (PV cell + glass cover) rather than on its components individually is essential to examine the synergistic effect between the nanopatterned cover and the surface-textured PV cell. An independently designed nanopatterned cover may diminish the absorption efficiency of PVs if the incident sunlight deviates far from the normal line with respect to the cover/Si interface after passing through the patterned cover. To address these concerns, we developed an integrated ray-wave optics simulation that performs the following procedures (Fig. 2(a)) [24,25]. First, RCWA wave-optics simulations were performed to obtain all orders of forward and backward diffraction efficiencies of the nanopatterned cover at every polar and azimuthal angle of incidence. Angle-resolved absorption spectra were obtained experimentally for a micropatterned Si surface. A typical KOH-etched Si surface exhibits near-unity absorptivity at broad wavelengths (400–800 nm) and incident angles (0–70°) (Fig. 2(b)). Subsequently, diffuse properties extracted from the RCWA-simulated nanopatterned cover and measured micropatterned Si surface were applied to the corresponding air/cover and cover/Si interfaces in a ray-tracing simulation model (Fig. 2(a)). Note that the ray-tracing model treats the practical scales (e.g., over a few hundred microns in thickness) of a cover and photovoltaic cell. Finally, we obtained the solar absorptivity (Aave) (averaged over 400–800 nm, weighted by AM 1.5G spectra) of surface-textured Si PV cells with nanopatterned covers (i.e., full PV modules) as a function of the incident angle using the integrated ray-wave optics simulation.
Fig. 2. (a) Schematics describing how an integrated ray-wave optics simulation is used to simulate the angle-resolved absorptivity spectra of a surface-textured Si PV cell with a nanopatterned cover. (b) Measured angle-resolved absorptivity spectra of a surface-textured Si PV cell. (Inset) cross-section SEM image of the surface-textured Si. (c) Simulated angle-resolved Aave of surface-textured Si PV cells with three cover cases: (i) planar cover, (ii) nanorod-patterned cover with Dt = 200 nm, and (iii) nanocone-patterned cover with Dt = 0 nm. (Inset) Increment of Aave to the cone- and rod-shaped nanopatterns compared with the planar cover. (d) Simulated transmission distribution of the nanopatterned cover at various incident angles. The magnified result shows the logarithmic plot of the same data. For the simulation, the parameters of the nanocone pattern (n = 1.48) are H = 300 nm, S = 100 nm, Dt = 0 nm, and Db = 200 nm.
We analyzed nanocone (Dt = 0 nm) and nanorod (Dt = 200 nm) arrays with H = 300 nm, S = 100 nm, and Db = 200 nm using an integrated ray-wave optics simulation (Fig. 2(c)). For simplicity, we regarded the cover as a homogeneous medium (n = 1.48) because the refractive index contrast between PMMA and glass is marginal (e.g., n = 1.48 for PMMA and 1.52 for glass in the visible). For all the considered incident angles (0–70°), the nanocone-patterned cover yielded greater Aave values compared with the nanorod-patterned cover (Fig. 2(c)). The increments in the Aave of the nanocone structure relative to the planar structure remained nearly constant at 2% below 50° and rapidly increased over 50° (inset, Fig. 2(c)). When averaged over 0–70°, the Aave of the nanocone structure was larger than those of the planar and nanorod structures by 2.9% and 1.5%, respectively. Note that the light passing through the air/nanopatterned cover interface faithfully followed its specular beam path (determined by Snell’s law) with marginal diffraction for all angles because the nanocone array had subwavelength scales (e.g., 300 nm pitch) (Fig. 2(d)) [1,5]. Thus, while a subwavelength nanocone array provides a broadband antireflective function, its principal path in the Si PV cell is closer to the normal line, which aids in augmenting Si absorption.
2.2 Fabrication of a nanopatterned cover using nanoimprinting
We prepared a nanocone-patterned cover by bonding nanoimprinting-patterned PMMA to bare glass with an index-matching adhesive. First, a hexagonally arranged, subwavelength (i.e., 300 nm pitch) periodic pattern was embossed on a Si master mold (Fig. 3(a)). A polyurethane-acrylate (PUA) layer conformally covered the patterned Si mold to create a replica, and PMMA was then spin-coated onto the PUA replica. Finally, the nanopatterned PMMA was transferred to bare glass. The nanopatterned PMMA-transferred glass was visibly transparent as bare glass owing to its subwavelength scales in pattern, which was evident from the camera images (Fig. 3(b)). Figure 3(c) shows the scanning electron microscopy (SEM) images of the fabricated nanopatterned PMMA film, indicating that a hexagonal array of nanocones was precisely imprinted into the PMMA film. The structural parameters were characterized by atomic force microscopy (AFM) imaging (H = 300 nm, S = 100 nm, and Dd = 200 nm) as shown in Fig. 3(d).
Fig. 3. (a) Schematics describing fabrication of a nanopatterned PMMA-transferred glass cover. (b) Camera images of planar- (left) and nanopatterned PMMA-transferred (right) glass cover. (c) Top-view (left) and cross-section (right) SEM images of a fabricated nanopatterned PMMA with S = 100 nm and Db = 200 nm. (d) AFM image (left) and height profile (right) of the fabricated nanopatterned sample in (c).
2.3 Angle-resolved measurements
We obtained angle-resolved transmittance spectra of the nanopatterned PMMA-transferred (Fig. 4(a)) and planar (Fig. 4(b)) glass covers to verify the broadband antireflective function. The nanopatterned sample exhibited significantly enhanced transmittance compared with the planar sample at the considered wavelengths (400–800 nm) and incident angles (θ = 5–70°). At θ < 50°, the solar transmittance (Tave) (average over 400–800 nm, weighted by AM 1.5 G) was >0.91, outperforming the planar sample (Tave > 0.89) (see the bottom panels in Figs. 4(a) and 4(b)). This feature is more apparent at larger angles. At θ > 50°, the nanopatterned sample had a Tave of >0.79, which is in stark contrast with the Tave (>0.59) of the planar sample (see the top panels in Figs. 4(a) and 4(b)). As a result, the increase in transmittance was approximately 2.1% at θ < 50° (inset, Fig. 4(c)), and substantially increased to >10% at θ > 60° (Fig. 4(c)). These experimental results support the simulated data in Figs. 1(c) and 2(d); a cone-shaped nanopattern, which is optically analogous to a graded-index film, functions as a broadband antireflective structure [5,6,26]. Note that these measured data included the backside reflection from the glass/air interface such that they exhibited slightly reduced transmittance values, relative to the simulated results of Fig. 1.
Fig. 4. (a,b) Measured angle-resolved transmittance spectra of nanopatterned PMMA-transferred (a) and planar (b) glass covers. (c) Increment in Tave of the nanopatterned sample in (b) compared with the planar sample in (a). (Inset) Plot with incident angles of 0–60°.
2.4 Photovoltaic experiments
We integrated a fabricated nanopatterned (H = 300 nm, S = 100 nm, Dd = 200 nm, and Dt = 0 nm) glass cover on top of a Si PV cell using an index-matching adhesive (Fig. 5(a)). We obtained J-V characteristic curves at various incidence angles for Si PV modules with nanopatterned and planar glass covers. Figure 5(b) shows the results at three discrete angles (θ = 0°, 30°, and 60°); the enhancements in the short-circuit current (Jsc) were 1.6%, 2.6%, and 13.7% for 0°, 30°, and 60°, respectively. In addition, we plotted the PCE values of the same Si PV modules for θ = -60°–60° [Fig. 5(c)]. The nanopatterned cover sample outperformed its counterpart planar cover sample at the considered incident angles. The increase in PCE was 6.6% on average over θ < ±50° (upper panel, Fig. 5(c)), but it was sharply pronounced above ±>50°. This observation is quantitatively consistent with the results of the integrated ray-wave optics simulations shown in Fig. 2(d) and the angle-resolved transmittance measurements shown in Fig. 4(c). As discussed in Fig. 4, the nanocone-patterned cover was designed to obtain broadband (400–800 nm), omnidirectional (0°–70°) antireflective performance. The reference planar sample relatively underperformed at θ > 50°, which led to the dramatic enhancements in the average transmittance and power conversion efficiency at the large off-angles (Figs. 4(c) and 5(c)).
Fig. 5. (a) Schematic describing how to integrate a nanopatterned cover into a surface-textured Si PV cell. The index-matching adhesive (n = 1.46) was used to integrate with the nanopatterned PMMA and a Si PV cell. (b) Measured J-V curves of planar- and nanopatterned cover-integrated Si PV modules at θ of 0°, 30°, and 60°. (Inset) Camera images of the nanopatterned (top) and planar cover-integrated Si PV modules (bottom). (c) Measured PCE values of the planar and nanopatterned devices in (a) as a function of θ. (Upper panel) Increment in PCE values as a function of θ. (d) Simulated average monthly Jsc values of the planar and nanopatterned cover-integrated Si PV modules. (Upper panel) Average monthly peak θs values of South Korea.
The PV modules are mounted outdoors, where the amount of solar irradiation varies throughout the year, depending on the solar altitude. Therefore, we simulated the performance variation of the nanopatterned cover-integrated Si PV module over one year (lower panel, Fig. 5(d)). We also plotted the peak solar altitude angles (θs = 90° – θ) in South Korea (37°N, 127°E) [27], which had maximum and minimum θs values of 28.8° and 73.8°, respectively (top panel, Fig. 5(d)). Based on the solar altitude database of South Korea and simulated angle-resolved PV absorptivity in Fig. 2(c), we calculated the monthly average Jsc for the nanopatterned and planar samples (bottom panel, Fig. 5(d)). We assumed that the external quantum efficiency spectra of the Si PV cell used in the model was constant, irrespective of θ [28]. The prediction model demonstrated that the nanopatterned glass cover sustained its antireflective function throughout the year, yielding improved Jsc values. The enhancement in Jsc was relatively significant in the winter. It is worth noting that PMMA, which we used for the uppermost patterned layer, is vulnerable to weathering by ultraviolet light and temperature variations in a real word setting. Therefore, high-durability polymers should be employed for nanoimprinting, thereby retaining the pristine PV efficiency from weather-related degradation. For example, ethylene chlorotrifluoroethylene can provide significant weight reduction, excellent ultraviolet resistance, and self-cleaning function. Hence, if a highly durable, antireflective polymer matrix is used for encapsulation without a glass cover, lightweight Si photovoltaic modules can be achieved with an improved PCE. This strategy will facilitate the use of building-integrated PVs. In this case, the integrated ray-optics simulation incorporating the new polymer matrix should be updated.
3. Conclusions
We have theoretically and experimentally demonstrated the synergistic effect of a nanopatterned cover and micropatterned Si PV cells. The nanopatterned-cover-integrated Si PV module was optimized to provide broadband, omnidirectional antireflective function using integrated ray-wave optics simulations. The designed structure was fabricated using a nanoimprinting process and integrated into a Si PV cell using an index-matching adhesive. The nanopattern-transferred glass cover enhanced the PCE of the Si PV cell by 8.4% when averaged over 0°–60°, compared with a planar glass cover sample. Finally, we verified that the developed Si PV module could sustain its enhanced Jsc (more than 5%) in all seasons, even when various solar altitude angles were considered. The nanopatterned glass cover exhibited a more significant performance at large off-angles (e.g., θ > 50°), which highlights that it can be a standard design strategy for building-integrated PV modules [23]. Furthermore, the integrated ray-wave optics simulation developed herein can be extensively utilized to design other macroscopic optoelectronic devices containing highly diffractive elements [29]. Finally, we highlight that a broadband, omnidirectional antireflective pattern is extensively used for different spectra and applications. If appropriately designed, it can work in the mid-infrared (3–14 µm) [30,31], which is crucial for shortwave (3–5 µm) and longwave (7.5–14 µm) infrared thermography.
Funding
Korea Institute of Energy Technology Evaluation and Planning (20203030010200); National Research Foundation of Korea (2020R1A2B5B01002261, 2021M3D1A2049865).
Acknowledgments
S.-K. Kim thanks the National Research Foundation of Korea for supporting this study. J. H. Kim thanks the Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning for supporting this study.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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