A HIGH CONCENTRATION PHOTOVOLTAIC MODULE UTILIZING MICRO-TRANSFER PRINTING AND SURFACE MOUNT TECHNOLOGY

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1 A HIGH CONCENTRATION PHOTOVOLTAIC MODULE UTILIZING MICRO-TRANSFER PRINTING AND SURFACE MOUNT TECHNOLOGY Bruce Furman 1, Etienne Menard 1, Allen Gray 2, Matthew Meitl 1, Salvatore Bonafede 1, David Kneeburg 1, Kanchan Ghosal 1, Rudolf Bukovnik 1, Wolfgang Wagner 1, John Gabriel 1, Steven Seel 1 and Scott Burroughs 1 1 Semprius, Inc. 49 Prospectus Dr Suite C, Durham, NC 27713, USA 2 Phononic Devices, Cary, NC 27513, USA ABSTRACT We describe a high concentration photovoltaic (CPV) module utilizing micro-transfer printed (µ-tp) dual-junction GaInP/GaAs solar cells and an ELO (Epitaxial Lift-Off) process used to fabricate very small cells (<.5 mm 2 ) using 1 st use and reused GaAs substrates. The benefits of this technology include high efficiency, simple distributed heat transfer at high concentration ratios, and short optical paths. This approach enables the use of low cost, high reliability surface mount assembly of large backplanes for integration into CPV modules. To minimize compound semiconductor use and maximize cell efficiency, we combine plano-convex primary and spherical secondary optics to concentrate sunlight 1X over a +/-.8 degree angle of acceptance. Receiver efficiencies of ELO dualjunction GaInP/GaAs cells of >3% at 1, sun concentration are reported. Coupled with a >8% efficient optical train, module efficiencies greater than 24% have been achieved with dual-junction µ-tp solar cells. receivers attached to a backplane using mature Surface Mount Technology (SMT) [4]. The small cell size carries three benefits: first, because of the highly effective distributed heat transfer, no heat sinks are required to passively cool the solar cells. Second, reduction of the primary aperture enables use of planoconvex primary lenses, which enhance both transmission and acceptance angle over typical Fresnel designs. Spherical secondary lenses, placed just over the microtransfer printed cells, serve to increase the module acceptance angle to +/-.8 degrees, illuminate the cells with high uniformity, and correct for chromatic aberrations typically found in refractive optical systems. Finally, the small primary aperture size, coupled with an appropriate primary f-number, allows for a much thinner CPV module, enabling less material use and lower wind load crosssection. The unique overall design approach results in a very low cost, high performance, light weight and reliable module with a very manageable profile. A 3D schematic illustration of the CPV module design is shown in figure 1. INTRODUCTION Historic challenges for the CPV industry include demonstration of a low cost design, achieving high module efficiencies, and demonstrating long term reliability. Semprius has taken a fundamentally new design approach to address each of these challenges by designing a thin, high performance, low cost, CPV module [1] based upon a patented micro-transfer printing (µ-tp) technique [2]. Micro-transfer printing technology enables GaAs substrate reuse and highly parallel assembly of large arrays of very small cells (<.5 mm 2 ). Small cells have low series resistance, they enable the use of small inexpensive short path optics, and they allow for distributed thermal dissipation. The cells are based on a conventional lattice matched dual-junction GaInP/GaAs design [3], with the cell structure grown over a sacrificial epitaxial release layer suitable for post-release transfer-printing. The micro-cells are transfer-printed in a massively parallel fashion onto an engineered substrate. The substrates are processed and tested in wafer format, singulated, and the resulting Figure 1 CPV module design. MICRO-TRANSFER PRINTED RECEIVER ASSEMBLY /1/$ IEEE 475

2 The core of the Semprius CPV module is comprised of an array of multi-junction III-V cells which are distributed over an engineered substrate using Semprius proprietary micro-transfer printing process. This process is schematically illustrated in Figure 2. The process starts with the fabrication of dual-junction GaInP/GaAs cells on a source wafer. After the cells are suspended from the GaAs substrate by sacrificial etching of a release layer, a microfabricated elastomeric stamp with posts at the pitch of the target substrate is brought into intimate contact with the surface of the wafer to selectively bond the array of solar cells through a van der Waals interaction (Figure 2A). The stamp is then peeled away (Figure 2B) and laminated against the target substrate to complete the array transfer of the ultra-thin solar cells (Fig. 2B-D). Figure 2E shows an optical micrograph of the source wafer after pick-up of an array of cells. Figure 2F, shows a printed target substrate exhibiting the same cell pitch. Figure 3 Target substrate with partial assembly of lenses (a), side view of aligned ball lenses over cells (b), top view with no optic on bad cell (c), top view of singulated SMT receiver (d), and bottom view of SMT receiver with solder pads (e). a a X c b d e A B C D RECEIVER CHARACTERIZATION Figure 4 shows the quantum efficiency (QE) of two test cells grown both on 1 st use GaAs (a) and on a reused substrate (b), both with well current matched top and bottom junctions. These results were obtained on large (1cm) test site areas on the same wafer as the ELO cells used in receiver and module testing. Figure 5 shows examples of cell efficiencies as a function of concentration ratio for: (a) prototype receivers grown on 1 st use GaAs, and (b) prototype receivers, grown in the same run as (a) but on reused GaAs substrates. Both the 1st use and reused wafer cells show an efficiency of >3% at 1, Sun concentration. E F Figure 2 Schematic representation of the microtransfer printing process (a-d), micrograph of source (e) and target (f) substrates. After transfer is complete, the engineered substrate can then be processed using conventional electronic device processing techniques to complete the receiver fabrication. Small spherical secondary lenses are then integrated with the engineered substrate wafer to comprise an array of µ-tp receivers as shown in figure 3. These receiver wafers can then be tested under concentrated light prior to the receiver assembly into a full module. Figure 3e shows the bottom surface of a singulated receiver with solderable pads. External Quantum Efficiency (a.u.) 8% 7% 6% 5% 4% 3% 2% 1% % Re-used Substrate Re-used Substrate Virgin Substrate Virgin Substrate Top Junction A B wavelength (nm) Bottom Junction FIGURE 4 External quantum efficiency of dualjunction cells showing structure grown on top of an epitaxial release layer with well current-matched top and bottom junctions using (a) 1 st use substrate and (b) reused substrate Figure 5 shows an example of P-I-V curves for (a) 1 st use cell and (b) reused cell at 1X solar concentration for a single µ-tp receiver. Figure 6 shows typical performance /1/$ IEEE 476

3 of the dual junction µ-tp cells fabricated from epi grown on 1 st use or reused GaAs substrates. Figure 5 Cell efficiencies as a function of concentration ratio using (a) 1 st use and (b) reused substrate. Current (A) Efficiency (%) I-V 1st use I-V re-use P-V 1st use P-V re-use 48 1st use epi 48 reuse epi concentration (# suns) 1st use Reuse VOC (V) ISC (A) VMP (V) IMP (A) PMAX.81 (W).8 FF (%) Voltage (V) Figure 6 P-I-V curves of µ-tp cells on 1 st use and reused GaAs substrates assembly process. The Semprius process involves four principle steps: First, solder is applied using a screening mask onto a CPV backplane. Second, functional components such as binned receivers, as shown in figure 8, are placed onto the CPV backplane using a conventional high speed pick and place tool. Third, the populated backplane is then reflowed in nitrogen to melt the solder and interconnect all the components. The last operation is to test the CPV backplane prior to module assembly. Receiver Backplane Lens Array Frame Figure 7 Module assembly process and photograph of assembled CPV backplane. Frequency Receiver Test / Bin SMT Backplane Assembly Backplane Test Final Module Assembly 1st use lot 24 reuse lot 24 1st use lot reuse lot More P max (W) Module Testing CPV MODULE ASSEMBLY AND CHARACTERIZATION The Semprius CPV module design requires the assembly of a large array of receivers onto a monolithic backplane. In order to reduce manufacturing costs, Semprius is leveraging standard low cost and highly reliable SMT which has been in use in the electronics industry for 4+ years. Figure 7 shows a schematic representation of the process and a photo of a populated backplane. This process allows parallel fabrication of all module components including receivers, backplanes, frames, and lens arrays. The core of this assembly process is based on SMT similar to a conventional printed circuit board Figure 8 Distribution of SMT receivers prior to backplane assembly. A total of 5 receivers were tested. Figure 9 shows a comparison of two backplanes built from a distribution of receivers as shown in figure 8 and tested under similar conditions. The P-I-V curves indicate no differences in performance at the module level. These results confirm that the fabrication of the µ-tp receivers using reused GaAs substrates showed no negative effects in either cell or module performance. After test, the backplane is assembled into a frame, the primary lens aligned and the module sealed /1/$ IEEE 477

4 Current (A) I-V 1st use substrate I-V reuse substrate P -V 1st use substrate P-V reuse substrate Voltage (V) Current (A) voltage (V) I pcx I fresnel P pcx P fresnel Figure 9 P-I-V curves for 2 modules built with receivers from 1 st use (red) and reused GaAs substrates (green). Figure 1 shows an array of CPV engineering modules mounted on a dual axis tracker in Durham, NC. Currently 31 modules are mounted on a FIENA SF-2 2-axis tracker along with moitoring and control instrumentation. Figure 11 P-I-V curves showing differences in performance between plano-convex and Fresnel primary lens arrays. Figure 11 shows a comparison of two lens arrays on the same test module under similar conditions. The planoconvex lens results in approximately a 5% increase in P MAX on this module relative to the Fresnel lens array. C urren t (I) Voltage (V) I-V P-V Figure 12 P-I-V curve of CPV module P ow er (W ) Figure 1 Semprius CPV engineering modules on a 2 axis tracker in Durham NC. Figure 12 shows a representative module P-I-V curve measured under the following conditions: DNI = 995 W/m2, T AMBIENT = 9 C. The module produced the following output results: V OC = 49.4 V, I SC =.7 A and P MAX = 28.3 W, with an optical aperture of.117 m 2 resulting in a module efficiency of 24.3%. A fill factor of 81.6% was also calculated. An Angle of Acceptance (AOA) curve on a comparable module is shown in Figure 13. An angular acceptance angle of ±.8 degrees was measured in both elevation and azimuth /1/$ IEEE 478

5 second group was built from a lower distribution of receivers AOA +/ Figure 13 AOA data measured on a Semprius CPV engineering module..2 Azimuth Error (º) Elevation Error (º) DNI (W/m 2 ) V o c (V ) DNI (W/m2) Ambient T Backplane 1 T 6 Backplane 2 T 55 A :24 1:48 13:12 :36 18: 2:24 Time 55.7 B.6 5 slope =.1 (A/ o C).5 slope = -.62 (V/ o C) 45 Voc.4 4 Isc Backplane T ( o C) 45 4 Temperature o C C u rre n t (A ) Figure A) Module backplane temperature as a function of the ambient air temperature and DNI. B) Module open circuit voltage (Voc) and short circuit current (Isc) as a function of the module backplane temperature :55 1:19 12:43 :7 17:31 Time Module 2-9 Module 2-1 Module 2-12 Module 1-4-glass Module 2-13 Module 2-14 DNI Figure 14 Full day on-sun data collected on 3/5/21 in Durham, NC on a 2 axis tracker. Figure 14 shows the performance over an entire day for several modules. Small disruptions in the curve are a result of tree shadings in the morning and clouds in the afternoon. Two groups of modules can be observed; one group with high power (-28 W) during most of the day and a second group with expected power (23- W). Performance differences are a result of intentional µ-tp receiver performance binning from test results. The high power is built with top tier receivers from the test, while the DNI (W/m2) Under worse case thermal operating conditions (no wind, high DNI conditions) the module backplanes typically operate at 2- º C above the temperature of the ambient air (Fig. A). We have observed backplane temperatures 2-ºC above ambient at higher ambient conditions with no wind. As previously mentioned, the small µ-tp cells enable distributed heat dissipation resulting in excellent thermal performance. At standard operating condition (85 W/m 2 DNI) the individual concentrator cells operate at approximately C above the backplane temperature [1]. The temperature response of a well engineered CPV module should exhibit temperature coefficients proportional to an individual concentrator cell. Figure B presents the open circuit voltage and the short circuit current of a module as a function of the module backplane temperature. Division of the module voltage and current temperature coefficients by the number of concentrator cells wired in series and parallel respectively are in good agreement with cell temperature coefficients extracted indoors using a concentrated solar simulator (β Voc = -3.6 mv/ C) and α Isc =.7 ma / C [5]. These results confirm /1/$ IEEE 479

6 that the thermal expansion of the primary lens and backplane plates are well managed by the secondary ball lenses which effectively relax alignment tolerances. Relative Distribution Rev 2. Modules Rev 2.1 Modules Rev 2.2 Modules Efficiency (%) Figure 16 Module optical efficiency for 3 groups of 2- junction modules: Rev 2., 2.1 and 2.2. During the past, several groups of modules have been assembled and tested. During rev. 2., modules were built with a wide variety of design options and materials. These include selection of epi material, variation in receiver fabrication procedures, backplane material, lens design and materials, and module alignment options. A distribution of module optical efficiencies is shown in figure Jul-9 Aug-9 Oct-9 Dec-9 Jan-1 Mar-1 May-1 Jun-1 Date Module 1/1 Module 2-1 Module 2-2 Module 2-7 Module 2-9 Module 2-11 Figure 17 Long term module power over a 1 year period for several modules. The efficiency of the rev. 2. modules varies significantly (16-24%) as expected. These results were used to optimize our process for a group of rev 2.1 modules. The goal was to produce a very stable and reproducible module utilizing the best design features and a large percentage of functional receivers. A small increase in optical efficiency observed in the rev 2.2 modules resulted from an improvement in our PCX lens array efficiency. Currently a number of modules are being tested in Durham, NC as well as a number of other sites. Figure 17 shows a sampling of the power produced at 85 W/m 2 DNI for six development modules over the past 6-12 months. CONCLUSIONS The Semprius CPV module design approach addresses each of the major challenges of the CPV industry. This approach enables significant benefits in cost, performance, and reliability. Micro-transfer printing eliminates one of the limitations in using small cells by enabling massively parallel bonding of a large array of small cells as well as reuse of the GaAs substrate. Small cells allow for distributed thermal dissipation eliminating the need for expensive heat sinks, small short focal length optics for efficient optics and a thin profile, and low series resistance. Combined with wafer level SMT receiver and module assembly the design approach is scalable to high volume manufacture resulting in a very low installed cost ($/kwh). ACKNOWLEGMENTS The authors would like to thank Tanya Moore, Mark Perkins, and Miroslav Samarskiy for assembly of receivers, backplanes, and modules, and the DOE for their support in part through NAT The authors would also like to thank the involvement of the DuPont electronic materials group and Brian Cary at Advanced Technical Ceramics for their support and test materials in this project and Sandia national labs and NREL for early engineering module evaluation. REFERENCES [1] S. Burroughs et al., A new approach for a low cost CPV module design utilizing micro-transfer printing technology, 6 th International Conference on Concentrated Photovoltaic Systems, 21 [2] E. Menard, R. G. Nuzzo and J. A. Rogers, Bendable single crystal silicon thin film transistors formed by printing on plastic substrates, Applied Physics Letters 86(9),, [3] C. Algora, V. Díaz, The influence of series resistance on the guidelines for the manufacture of concentrator p-on-n GaAs solar cells, Prog. Photovoltaics 8, 2, pp [4] R. Prasad, Surface mount technology: principles and practice, Chapman & Hall,1997 [5] D. King, W. Boyson and J. Kratochvil, Photovoltaic Array Performance Model, 24, SAND /1/$ IEEE 48