IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017
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Solar-Cell Metasurface-Integrated Circularly Polarized Antenna With 100% Insolation Son Xuat Ta, Jae Jin Lee, and Ikmo Park
Abstract—A circularly polarized (CP) patch antenna loaded with a metasurface, which is realized with a lattice of 4 × 4 square solar cells, is presented. The antenna is composed of a slotted circular patch sandwiched between the metasurface and the ground plane. This arrangement not only allows for total exposure of the solar cells to sunlight (100% insolation) but also improves antenna performance greatly. The final design, with an overall size of 40 mm × 40 mm × 3.5 mm (0.87λo × 0.87λo × 0.076λo at 6.5 GHz) yields a VSWR 85% across its 3 dB AR bandwidth. Index Terms—Circular polarization, circular patch, insolation, metasurface, solar-cell-integrated antenna.
I. INTRODUCTION LONG with rapid development of autonomous communication systems, antennas integrated with solar cells have received much attention from researchers because such a combination could save valuable “real estate” and reduce design costs [1]. The primary challenge is compatibility between antennas and solar cells: The antennas should not block solar cells from functioning properly, and the effectiveness of antennas should not be significantly reduced by the presence of solar cells. Many efforts have been undertaken to integrate antennas with solar cells for satellite and terrestrial applications [1]–[9]. To achieve maximum insolation of solar cells, several techniques for designing antennas have been presented, such as reduced-footprint antennas [2]–[4], mesh patch antennas [5]– [7], and transparent conductor antennas [8], [9]. However, most of these designs cannot achieve 100% insolation of the solar cells. Several studies on solar antennas [10]–[17] have utilized amorphous solar cells as the primary radiating elements; conse-
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Manuscript received July 6, 2017; revised July 31, 2017; accepted August 9, 2017. Date of publication August 17, 2017; date of current version September 25, 2017. This work was supported in part by the National Research Foundation of Korea funded by the Korea government under Grant 2016R1A2B100932 and in part by the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning funded by the Ministry of Trade, Industry & Energy, Republic of Korea, under Grant 20164030201380. (Corresponding author: Ikmo Park.) S. X. Ta is with the Division of Computational Physics, Institute for Computational Science and the Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam (e-mail: tasonxuat@tdt. edu.vn). J. J. Lee and I. Park are with the Department of Electrical and Computer Engineering, Ajou University, Suwon 16499, South Korea (e-mail: jaejin@ajou. ac.kr;
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2017.2740570
quently, they can achieve a 100% insolation of the solar cells. However, these antennas are linearly polarized. This letter presents a solar-cell metasurface circularly polarized (CP) patch antenna with total exposure of the solar cells to sunlight (100% insolation). The antenna is composed of a slotted circular patch sandwiched between a metasurface of 4 × 4 square solar cells and the ground plane. This arrangement not only allows for total exposure of the solar cells to sunlight, but also improves antenna performance significantly [18], [19]. The features of the proposed solar antenna, including a no-shading area, planar configuration, broad bandwidth, stable radiation profile, high gain, and high radiation efficiency, have been demonstrated numerically and experimentally. II. ANTENNA DESIGN Fig. 1 shows the geometry of the proposed antenna, which is composed of a driven patch sandwiched between the solar cells and the ground plane, a 50 Ω SMA connector, and two RT/Duroid 5880 substrates (εr = 2.2 and tanδ = 0.0009). The driven element is a circular patch with a slot that excites two orthogonal modes with a 90° phase difference, which consequently produces the CP radiation. An extended strip was added into the driven element to improve impedance matching. The driven patch was printed on the top side of substrate 1. The outer part of the SMA connector was connected to the ground plane, and the inner part extended through substrate 1 to connect with the extended strip. The metasurface was realized with a lattice of 4 × 4 square solar cells, which were mounted on the top side of substrate 2 where it was stacked above substrate 1 with no air gap. The antenna was optimized using an ANSYS High-Frequency Structure Simulator (HFSS) to achieve a planar configuration and broadband characteristics at a frequency of approximately 6.5 GHz. Its optimized design parameters are as follows: P = 8.5 mm, g = 1.0 mm, Wcell = 7.5 mm, h1 = h2 = 1.5748 mm, Rp = 5.7 mm, Ls = 9.5 mm, Ws = 1 mm, Wf = 2.4 mm, Lf = 4.3 mm, Fy = 8.0 mm, and α = 45◦ . III. SOLAR CELL In this letter, a square solar cell with dimensions of Wcell × Wcell was used instead of the metallic patch of the conventional metasurface structure. The basic design structure of a solar cell [20] was used for the HFSS calculations. As shown in Fig. 2, the solar cell consisted of metal grids (e.g., bus bar and finger) on top, Hs thick gallium arsenide (GaAs) layers, and a metallic layer for a direct current (dc) contact on the bottom. The grid had a width of gw , and the spacing between grids was gs . The single solar cell had the following design parameters: Wcell = 7.5 mm, gw = 0.07 mm, and gs = 0.5 mm. To
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017
Fig. 3. (a) Photograph of fabricated 4 × 4 solar-cells on a 50.8 mm GaAswafer and (b) J–V curve of the solar cell.
Fig. 1. Geometry of the proposed antenna: (a) cross-sectional view, (b) driven patch, and (c) fabricated sample.
Fig. 2.
Single-junction GaAs thin-film solar cell.
reduce the calculation time, one layer of GaAs (εr = 12.9, μr = 1, tanδ = 0, and Hs = 0.385 mm) was used for all simulations. Fig. 3(a) shows a photograph of the fabricated 4 × 4 solar cells on a 2 in GaAs wafer. The single-junction GaAs solarcell structure was grown by the metal organic chemical vapor deposition system on p-type GaAs (100) substrates. Trimethylgallium and trimethylindium were used as group III precursors, while arsine 740 (AsH3 ) and phosphine (PH3 ) were used as As and P sources, respectively. Silane (SiH4 ) and diethylzinc (DEZn) were used as n- and p-doping sources, respectively. Ultrahigh-purity hydrogen gas (H2 ) was used as a carrier gas. The reactor pressure and temperature were kept at 50 mbar
and 680 °C, respectively. The grown GaAs solar cell was fabricated by photolithography, metal evaporation, rapid thermal annealing, wet chemical etching, and back-end processes. The AuGe/Ni/Au and the Ti/Pt/Au structures were used as n- and p-type contact metals, respectively. The n-type GaAs ohmic layer was selectively etched in an NH4 OH:H2 O2 :DI (2:1:10) solution after the contact formation. The GaAs solar cell was isolated by wet chemical etching and sawing processes. The MgF2 /ZnS double layers were deposited on the top surface for antireflection coating. Fig. 3(b) shows the current density– voltage (J–V) characteristics of a complete single-junction GaAs thin-film solar cell measured at room temperature (25 °C). The contact resistivity measured by a transmission line model method is 4 × 10−4 Ω · cm2 . The highest power conversion efficiency (PCE) of the fabricated GaAs solar cells was investigated as 22.4% (Vo c = 1.04 V, Jsc = 25.44 mA/cm−2 , FF = 85%), and the average PCE of 16 cells was investigated as 22.1% under AM1.5 G illumination by a class AAA solar simulator (Wacom: WXS-220S-L2). IV. SIMULATION AND MEASUREMENT RESULTS To investigate antenna performance in the presence of solar cells, the proposed antenna was characterized for two different configurations (i.e., the metasurface was implemented with copper patches and solar cells), and these results are shown in Fig. 4. With the metasurface comprising a lattice of copper patches, the antenna yielded a VSWR 90%, across its 3 dB AR bandwidth. V. DC CONNECTIONS To extract the photovoltaic-generated dc current from the individual solar cells and to mitigate the effects of connections on the antenna performances, 56 nH chip inductors [21] were used to connect the grid and the bottom contacts of the solar cells, as shown in Fig. 6(a). The inductor has a maximum dc resistance of 0.9 Ω and an impedance of ∼2.3 kΩ at 6.5 GHz. The solar-cell antenna with chip inductors was characterized via the HFSS. In the simulations, each inductor with a size of 1 mm × 0.4 mm was modeled as a lumped RLC element with an inductance of 56 nH. The inductors are classified into two groups; one is used to connect the grid contacts, and the other is used to connect the bottom contacts. Moreover, eight bias lines were inserted to deliver the dc power. A performance comparison of the proposed antenna without/with dc connec-
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017
(0.87λo × 0.87λo × 0.076λo at 6.5 GHz), yielded a VSWR < 2 bandwidth of 6.05–7.33 GHz, 3 dB AR bandwidth of 6.0–7.05 GHz, broadside gain of 6.9–9.0 dB within the 3 dB AR bandwidth, and radiation efficiency >85%. With the promising features of no shading, a planar structure, broad impedance and CP radiation bandwidths, high gain, and high radiation efficiency, the proposed antenna is a good candidate for autonomous communication systems. REFERENCES
Fig. 6. (a) Top view of metasurface solar cells with chip-inductor connections; simulated (b) |S 1 1 | and (c) AR and broadside-gain values of the solar-cell antenna with and without connections.
tions is illustrated in Fig. 6(b) and (c). The presence of the dc connections slightly degraded the CP radiation bandwidth of the antenna near 7.0 GHz. Nonetheless, the antenna with dc connections yielded the broadband characteristic; its VSWR < 2 bandwidth was 6.0–8.2 GHz, and its AR < 3 dB bandwidth was 5.83–6.72 GHz. Its broadside gain was also better at 7.5 dBic across the operational bandwidth. These results indicate that the proposed antenna works well with the presence of the dc connections between the individual solar cells. VI. CONCLUSION A solar-cell metasurface-based CP antenna with 100% insolation was presented. The antenna comprised a driven, circular patch with a diagonal slot sandwiched between the metasurface of 4 × 4 square solar cells and the ground plane; this arrangement allowed 100% exposure of the solar cells to sunlight. The final prototype, with an overall size of 40 mm × 40 mm × 3.5 mm
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