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Origami Solar-Tracking Concentrator Array for Planar Photovoltaics Kyusang Lee, Chih Wei Chien, Byungjun Lee, Aaron Lamoureux, Matt Shlian, Max Shtein, P.C. Ku, and Stephen Forrest ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00592 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 22, 2016

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Origami Solar-Tracking Concentrator Array for Planar Photovoltaics Kyusang Lee1†, Chih-Wei Chien1†, Byungjun Lee1, Aaron Lamoureux3, Matt Shlian2, Max Shtein3, P.C. Ku1 and Stephen Forrest1,3,4*, 1

Department of Electrical Engineering and Computer Science, University of Michigan 2 School of Art and Design, University of Michigan 3 Department of Materials Science and Engineering, University of Michigan 4 Department of Physics, University of Michigan †

These authors contributed equally to this work. Corresponding author. E-mail: [email protected]

*

Keywords : Solar concentrator, tracker, origami, photovoltaic, GaAs, low-profile Abstract Solar tracking concentrators can potentially lead to low-cost photovoltaic modules that minimize the use of costly semiconductor materials by improving optical collection and coupling. However, solar concentrators and accompanying trackers have proven to be expensive, bulky and heavy, thereby resulting in increased balance-of-system costs. Here we demonstrate a lightweight and low-profile, and potentially low cost planar solar tracking concentrator based on the ancient Japanese art of origami. The tightly packed hexagonal concentrator and tracker arrays are fabricated by cutting and folding thin reflecting sheets that capture and direct concentrated light onto a small, high efficiency GaAs solar cell. The tracker enables single-axis solar tracking via a simple one-dimensional translational motion of an actuator with minimal energy expense (~2.9J/m2/day). Further, we demonstrate stable operation over 10,000 cycles. The solar concentrated cell achieves a 450% increase in diurnal energy output compared with an equivalent, unconcentrated cell. The potentially low cost and low profile of the origami concentrators may lead to their wide deployment on rooftops and other building integrated applications. 1

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Concentrated photovoltaics (CPV) integrated with solar trackers are distinct from conventional photovoltaics in that an auxiliary optical system and tracking structure is used to focus the solar flux onto a small active device area while mechanically following the sun as it journeys through the ecliptic.1,2 In this way, the required area of expensive solar cell material required to generate a desired power output is significantly reduced while the performance of the solar cell is enhanced by the increased incident light intensity compared to unconcentrated cells.3 Conventional CPV systems utilize Fresnel lenses4,5, Fresnel reflectors6, parabolic troughs7,8, or parabolic dishes9,10,11. To maintain a maximum concentration factor, solar concentrators require a precise solar tracking system to accommodate their narrow light acceptance angle.12,13 As a result, most tracking CPV systems are bulky and heavy, requiring costly support structures that resist wind loading. Moreover, these structures lead to an increase in the amount of energy needed to actuate the module positioning equipment. As a result, the applications of concentrated PVs have been limited to utility-scale solar power plants. For widespread local power generation applications, new approaches that minimize volume and weight without a loss in efficiency are essential. Here we demonstrate a shallow-profile, mini-solar tracking concentrator whose design and construction are inspired by the ancient Japanese art of origami. Origami uses only folding and cutting to transform a two-dimensional sheet into a variety of complex three-dimensional structures.14,15 Origami has been widely adapted in various industrial applications such as in packaging technology due to its potential low cost.16 Here, both concentrator and tracker are fabricated from a thin plastic sheet by origami process, then combined that results in a low cost, relatively low technical barrier for fabrication (and hence a significant advantage in time to deployment) and extremely light-weight module. This construction can lead to a potentially

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dramatic reduction of the balance-of-system cost compared to conventional tracking systems based on unwieldy integrated lenses and/or metal reflectors. The origami concentrator provides a concentration factor of up to 6 suns (relative), while the integrated tracker achieves a wide light incident acceptance angle (over ±70°). Finally, its low weight leads to a minimal expense of actuation energy (~2.9 J/m2/day, or on the order of 0.001% of harvested energy which is almost three order of magnitude lower than that of the conventional tracker). The simulated output power from the origami module suggests a 450% enhancement over an analogous flat PV system. The low-profile of the entire system distinguishes the flat panel concentrator from conventional bulky concentrator technology. The luminescent solar concentrators (LSCs) are the alternative low-profile concentrators; however, LSCs suffer from low optical coupling efficiency (~20%) and limited spectral coverage.17 Furthermore, very small scale of the origami solar tracking concentrator provides flat plate type packaging, improved thermal dissipation18 and interconnection degrees-of-freedom19. With this combination of attributes, the CPV system can lead to application to a wide variety of market segments including roof-top panels, buildingintegrated PVs (BIPVs), and solar farms.

Results and Discussion Origami concentrator An array of origami solar concentrators is folded from a 127 µm thick polyethylene terephthlate glycol-modified (PETG) sheet. The material choice for the concentrator and tracker is not limited to PETG, but most of the flexible thin sheet can be employed for this process. Figure 1A shows a schematic illustration of the procedure used to fabricate the origami concentrator and a photograph of a space-filling 2 X 2 concentrator array. The origami concentrator is designed

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using SolidWorks software (SolidWorks Corporation) to have a 2 mm wide hexagonal base and six, 6.9 mm tall parabolic sidewalls (Fig. 1B). Prior to folding, the origami concentrator was cut using a razor, and then the surface was coated by e-beam deposition with a 100 nm thick Ag film to achieve a high reflectivity across a broad spectrum. To enhance the reliability of system by combining with packaging process, the dielectric layer overcoating of Ag surface or Al reflector can be employed. The concentrator performance was simulated assuming collimated blackbody (5778 ℃) light is incident normal to the surface of the solar cell (assuming a perfect absorber) along with a hexagonal concentrator with a perfectly reflecting surface. Ray tracing (Tracepro, Lambda Research Corporation) shows 88.5% light concentration for the design in Fig. 1A. This is equivalent to ~8 sun concentration from the geometric ratio of 9.0 between the area of the solar cell and concentrator aperture. To determine the concentration factor, the current density-voltage (J-V) characteristics of GaAs solar cells of the same active material area, with and without an origami concentrator, were measured for a single cell plus concentrator, and then for arrays under a simulated AM1.5G solar spectrum, with results shown in Figure 1C (see details in Supplementary Fig. 1). Without concentrators, a short-circuit current density of Jsc = 23.1 ± 0.1 mA/cm2, open-circuit voltage of VOC = 0.91 V, and a fill factor FF =73.3 ± 1.3 % were obtained. With the origami concentrator, Jsc increased by a factor of 6.1 ± 0.1 to 140.9 ± 0.2 mA/cm2 while FF slightly decreased to 69.3 ± 1.2 % due to the non-optimized contact grid design. The discrepancy between geometrical simulation and measured concentration factor is mainly due to the less than unity reflection and absorption of concentrators and photovoltaic cells, respectively. The maximum output power increased by a factor of 6.1 ± 0.1 due in part to increases in Voc (to 0.97V) at the increased light

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intensity. The power conversion efficiency of concentrated GaAs cell is PCE = 15.2 ± 0.1 %. The optimized GaAs module fabricated by epitaxial lift-off technology is expected to provide PCE over 24%.20

Origami tracker Figure 2A illustrates the fabrication process of the origami tracker fabricated from a 127 μm thick PETG sheet by cutting and bending. The hexagonal GaAs solar cell is attached to each tracker to form a tangential plane at the top extremum of a 7 mm high sinusoidally bent film. The solar cell is tilted by horizontally translating a 3D-printed polylactic acid actuation plate whose edges are pushed on the shoulders of the bent film that support the concentrator. This results in tilting by shifting the solar cell away from the center of the curvature, as shown in Fig. 2B. Figure 2C shows three different tracking angles accessed by the lateral movement. Figure 2D shows the dependence of solar cell angle on the linear position of the actuation plate. The horizontal translation of the actuator transforms linearly into the rotation angle of solar cell, greatly simplifying the mechanical design and control mechanism required for tracking. The tracker efficiency is compared with a non-concentrated GaAs solar cell at various light incident angles in Fig. 2E. In contrast to the non-concentrated cell, the single tracking cell shows nearly a constant energy output for solar angles ranging from 0 o to 70o. The robustness of our design to multiple actuation cycles is shown in Fig. 2F. Here, we continuously tilted the parabolic concentrator via automated motorized actuation for over 10,000 cycles, corresponding to over 27 yr of operation. This is longer than the guaranteed operational lifetime of commercial flat panel PV modules. The period of each cycle was 2.8 second. Over this duration, there is no observed change in performance, indicating reliable operation of this

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simple and lightweight mechanism. Moreover, the external contacts patterned by photolithography avoid wiring that may lead to decreased module reliability.

Origami concentrator module design The PETG-based CPV structure is constructed from a thin sheet; hence the overall system weight is extremely small (~1.4 kg/m2, based on a 1 mm thick ABS plastic actuator frame and excluding the actuating motors whose additional weight does not scale simply with area). Moreover, both the volume and area occupied by the CPV module remains unchanged during tracking, making the system suitable for area-limited applications such as rooftop installations. Due to the shallow profile of the miniature concentrator (total height: 13.7 mm), negligible shadowing between two adjacent systems is expected, eliminating the need for gaps between two panels. However, we must consider the light coupling loss due to the shadowing by adjacent concentrators (Fig. 3A). An optimal design of the entire tracking array must consider the packing density, shadowing, and concentrator shape. The energy harvesting efficiency of each mini-concentrator consists of two parts: the packing density, which is the ratio between the usable concentrator area and the total occupied area, and the light-collection efficiency, defined as the fraction of light reflected onto the solar cell within each concentrator. Figure 3B shows a 2D projection of tightly packed polygons with various numbers of facets as viewed from normal and oblique angles. At normal incidence, the void between the polygons leads to optical losses. However, at an oblique incident angle shadowing is introduced. Thus, the angle-dependent packing density as a function of the number of facets is calculated by considering a rhombus unit cell connecting the center-of-mass of adjacent polygons in projection of a packed array, as shown in Fig. 3C. The number of facets

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considered is from 4 to 7 since there is little optical efficiency advantage of having a larger number of facets. The light collection efficiency is simulated by ray-tracing considering a perfectly reflecting surface, with results shown in the inset of Fig. 3C (see Supplementary Figure 2S for details). A pentagonal concentrator shows the highest theoretical light collection efficiency of 93%, which is 4.5% higher than a hexagonal concentrator. However, its relatively low packing density (92%) results in a reduced output power. Multiplying the packing density by the lightcollection efficiency, the simulated energy harvesting efficiency is found to depend on the number of concentrator facets, as shown in Fig. 3D. An array of hexagonal parabolic concentrators achieves the highest light harvesting efficiency of 88.5%.

Energy harvesting using origami solar tracking concentrator The origami solar tracking concentrator array was integrated with GaAs solar cells and single plate actuator as shown in Fig. 4A. The performance of a 2 × 2 array of solar tracking concentrators is characterized vs. source angle, with results in Fig. 4B. The concentrator exhibits greater than 1 sun concentration over ±25°, consistent with the simulation normalized to the measured peak concentration factor (dotted line). The energy output from a fixed array rolls off sharply at large incidence angles since the concentrator is primarily effective when the incident beam is nearly parallel to the concentrator optical axis. However, the relatively wide acceptance angle of the concentrator compared to high concentration PV, such as for a parabolic dish or Fresnel lens, results in a larger tolerance for collection of diffuse sunlight.8 As the incidence angle is increased the array shows additional losses compared to an individual concentrator due

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to shadowing from the overlapping from neighboring concentrators, as predicted in Figs. 3A and B. Based on the measured angle-dependent concentration factors, the energy harvested in Phoenix, AZ (33.45° N, 112.07° W) versus the time of day was calculated during the summer solstice (Fig. 4C). The energy harvested is considerably higher throughout the day for the concentrator and tracker, than for a non-tracking concentrated cell, which in turn is higher than for a non-tracking, non-concentrated cell. The energy output is found by integrating the area under the curves in Fig. 3C. We find nearly the identical output for the stationary planar cell and the cell with only a concentrator. The cell integrated with an origami concentrator and tracker shows peak energy output at 9:00 hr and 15:00 hr when the module is directly facing the sun; at other times the sun is not perfectly aligned with the concentrator axis due to constraints of the actuation and tilting design. The tracking cell shows a ~450% increase in energy harvested per unit area of semiconductor surface used compared to a non-tracking conventional cell (Fig. 4D). Finally, the elasticity of the thin PETG film minimizes the energy required to actuate the system for solar tracking.21 Stress-strain measurements using a texture analyzer indicate that an energy of only ~2.9 J is needed to actuate a 1 m2 module over the course of a day (Fig. 4E). After cycling the actuator 10,000 times, the strain energy decreases only marginally by ~2.8% possibly due to plastic deformation of tracker, as shown in Fig. 4E. The inset shows the energy required during cycle 1 and cycle 10,000. This change corresponds to 20% power conversion efficiency Si or GaAs solar cell. Furthermore, the concentrator PV array can be packaged into a low-profile flat plate configuration by simple encapsulation under a transparent “window” layer (see Supplementary Fig. 3) In such a package, the array can exhibit both structural robustness and isolation from

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wind-loading and other environmental factors that reduce performance (see Supplementary Figs. 4S and 5S).

Conclusion Origami-based solar concentrators and trackers have been demonstrated to substantially increase the energy harvesting capability of solar cells while greatly decreasing the weight, cost and complexity of non-tracking flat solar panels. A reduction of costly semiconductor material usage by up to a factor of 450% is possible by integrating the solar cell, the concentrator with a concentration factor of 6 suns and the tracker (±70° tracking). This approach should significantly reduce the balance of system costs due to the extreme light weight of the module (~1.4 kg/m2). Further, solar tracking only requires a one-dimensional translational actuator motion that simplifies the system while incurring minimal energy expense (~2.9 J/m2/day). The enhanced energy harvesting of our unique design can be directly translated into a solar-to-electrical energy conversion cost reduction. Finally, the low-profile CPV module enables solar tracking without occupying excessive volume or area. Therefore, integrated origami solar tracking CPVs provide a means to dramatically accelerate the deployment of inexpensive solar energy for a wide variety of applications, including rooftops, building facades, and even utility-scale energy generating plants. Furthermore, the proposed technology can potentially inspire countless applications across any disciplines including microelectronics, optoelectronics, biomedical technology, and mechanical devices.

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Methods Epitaxial growth of the GaAs solar cells The epitaxial layers of the GaAs solar cells are grown by gas-source molecular beam epitaxy (GSMBE) on a 2-inch diameter Zn-doped (100) p-GaAs substrates. The layering scheme is as follows: 5×1018 cm−3 Be-doped GaAs (0.2 µm) buffer layer, 2×1018 cm−3 Be-doped Al0.8Ga0.2As (0.050 µm) back surface field layer, 2×1017 cm−3 Be-doped p-GaAs (4 µm) base layer, 1×1018 cm−3 Si-doped n-GaAs (0.15 µm) emitter layer, 6×1017 cm−3 Si-doped Al0.8Ga0.2As (0.03 µm) window layer and 5×1018 cm−3 Si-doped n-GaAs (0.1 µm) contact layer. The GaAs and Al0.8Ga0.2As layers are grown at 600 °C.

Solar cell fabrication Following epitaxial growth, the wafer is separated into 6mm × 4mm rectangles using a dicing saw. Then the front surface contact gird is photolithographically patterned using the LOR 3A and S-1827 (Microchem) bi-layer photoresist. A Ni(5 nm)/Ge(20 nm)/Au(30 nm)/Ti(20 nm)/Au(700 nm) metal contact is deposited by e-beam evaporation. After the metal layer is lifted-off, the exposed, highly doped 100 nm thick GaAs contact layer is selectively removed by plasma etching. The device mesas are defined using photolithograpy and subsequent wet etching using H3PO4:H2O2:deionized H2O (3:1:25). A Pd(5 nm)/Zn(20 nm)/Pd(20 nm)/Au(500 nm) metal contact is deposited by e-beam evaporation on the substrate. After annealing the sample for 1 min at 400 ℃ to form ohmic contact, the sidewalls are passivated with 1 μm thick polyimide applied by spin coating. After curing the sample at 300 ℃ for 30 min, the polyimide is selectively removed by photolithography and plasma etching. The external contact pad is

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patterned with Ti (10 nm)/Au (500 nm). Finally, a bilayer anti-reflection coating consisting of TiO2 (49 nm) and MgF2 (81 nm) is deposited by e-beam evaporation.

Mechanical characterization of tracker The trackers were systematically tilted using a Thorlabs 25mm motorized actuator (Z825B). Images were taken at the desired displacement using a digital camera, and orientation angles were subsequently measured using ImageJ software (W.S. Rasband, US National Institutes of Health, Bethesda, Maryland, USA). The force – displacement characteristics of the origami tracker were measured using a TA.XTPlus Texture Analyzer (Texture Technologies, Hamilton, Massachusetts, USA) software package. Each sample was cycled from -7 mm (-60°) to +7 mm (+60°), for a total of 10,000 cycles. The strain energy was calculated as the amount of energy required to actuate a single cycle. The strain fade was calculated as the percentage difference in strain energy between cycle 1 and cycle 1,000 (that is, 100% x

,

).

Supporting Information Further details related to the characterization, measurement, and simulation. The supporting information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.XXXXXXXX.

Acknowledgements This work was performed in part at the Laurie Nanofabrication Facility, a member of the National Nanotechnology Infrastructure Network supported in part by the National Science Foundation (NSF). M. Shlian, M. Shtein acknowledge the partial financial support of the

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National Science Foundation (NSF), grant NSF 1240264 under the Emerging Frontiers in Research and Innovation (EFRI) program, Department of Air Force grant: FA9550-12-1-0435 (Shtein), and NanoFlex Power Corp. (K.L. and S.F.) All data used to obtain the conclusions in this paper are presented in the paper and/or the Supplementary Materials. Other data may be requested from the authors.

Contributions K.L. and C-W.C designed, fabricated and characterized the origami solar tracking concentrator module and analyzed the data, B.L. contributed to the concentrator optical simulation. A.L performed mechanical characterization. M. Shilian contributed to origami design. M.S., S.F. and P-C.K supervised the work and helped in data analysis. All authors discussed the results and commented on the manuscript.

Competing financial interests The authors declare no competing financial interests.

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Figure captions Figure 1: A. (Left) Schematic illustration of the fabrication procedure of an origami solar concentrator array. The folding/cutting pattern was facilitated by using SolidWorks (SolidWorks Corporation). (right) Image of fabricated 2 × 2 concentrator array. B. Schematic illustration of a 6.9 mm tall origami concentrator with 2 mm and 6 mm wide base and concentrator aperture length, respectively. C. The current density-voltage characteristics of a GaAs solar cell with and without the origami concentrator.

Figure 2: A. Design of the actuation structure and the attachment of the solar cell. B. Operation of the tracker using a 3D printed actuator that is linearly translated in the horizontal direction. C. Directing angle of the tracker depending on the lateral movement of the actuator at several different angles indicated. D. Orientation angle of the vs. linear displacement of the actuation plate. E. Normalized short circuit current of a GaAs solar cell with and without the integrated tracker. A non-tracking cell shows cosθ coupling loss, while the cell integrated with the tracker exhibits almost no loss until θ >70o. F. Panel orientation (θ) versus cycle number for -60° (at x = -7 mm, where x is lateral displacement of actuator), 0° (at x = 0 mm), and +60° (at x = +7 mm). As shown, there is negligible change in orientation due to cycling.

Figure 3: A. Shadowing in a tightly packed concentrator array at an oblique light incident angle. B. Projection view of packed concentrator arrays with various shapes as viewed from normal and oblique angle. Red dotted line shows the connection of nearest polygons center used for packing density calculation. Solid, dashed and dotted outline of polygons indicate the first, second and third row of polygons from projection view, respectively. C. Packing density vs. tracking angle of square, pentagonal, hexagonal and heptagonal concentrator arrays. Inset: Light collection efficiency as a function of the number of concentrator facets. D. The maximum light harvesting efficiency of various polygonal shaped concentrators. Hexagonal concentrators show the highest energy harvesting efficiency.

Figure 4: A. Schematic illustration of the origami integrated photovoltaic system comprised of an array of concentrators, trackers and actuator. B. Measured energy output normalized to solar cell area at different light incident angles. The sample consists of a 2 × 2 array of solar cells and 15


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origami concentrators. In the tracking array, solar cells are aligned perpendicular to the incident light at all angles. C. Hourly energy generation using origami solar tracking concentrator array compared with a flat cell on July 1st at Phoenix, AZ (33.45° N, 112.07° W). D. Comparison of integrated energy harvesting over the course of a day in Fig. 4C. E. Strain energy versus cycle number for actuation between -60o and +60o for >10,000 cycles. The strain energy decreases by ~2.8%. Inset: Stress versus actuator displacement (x) at cycle 1 and cycle 10,000, showing strain fade by the slight change in the curves at x>0.

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ACS Photonics

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Origami Solar-Tracking Concentrator Array for Planar Photovoltaics Kyusang Lee1†, Chih-Wei Chien1†, Byungjun Lee1, Aaron Lamoureux3, Matt Shlian2, Max Shtein3, P.C. Ku1 and Stephen Forrest1,3,4*, 1

Department of Electrical Engineering and Computer Science, University of Michigan 2 School of Art and Design, University of Michigan 3 Department of Materials Science and Engineering, University of Michigan 4 Department of Physics, University of Michigan

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ACS Photonics

Figure 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

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Origami Solar-Tracking Concentrator Array for ... - ACS Publications

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