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Overview of EMCORE’s Multi-junction Solar Cell Technology and High Volume Manufacturing Capabilities

David Danzilio

EMCORE Photovoltaics, 10420 Research Road SE, Albuquerque, NM 87123

david_danzilio@emcore.com

Keywords: Multi-junction, Solar, Photovoltaic, Cell

Abstract Over the last decade, III-V multi-junction solar cells have effectively displaced silicon solar cells for generating power on the majority of commercial and military satellites. This technology shift was driven by the substantially higher conversion efficiency (28.5% for muti-junction vs. 17% for Si), superior radiation tolerance and the potential for continual performance advances offered by InGaP/InGaAs/Ge solar cells. These advantages enable satellite solar arrays with higher specific power (watts/kg of array weight), reduced launch costs and longer satellite service life (>15years in geosynchronous orbit). These are important factors that favorably influence the economics of satellite manufacturing and the satellite services industry. This technology is also finding broad application in utility-scale concentrating photovoltaic systems where multi-junction solar cells provide a substantial cost and performance advantage over silicon solar cells. Furthermore the worldwide multi-junction solar cell industry is extremely price competitive with annual price erosion of 7-10% being common. Key to EMCORE’s success in this highly competitive industry is its well-developed compound semiconductor solar cell technology and mature manufacturing capability. This multi-million dollar investment in technology and production capacity has enabled EMCORE Photovoltaics to become the largest manufacturer of high efficiency multi-junction compound semiconductor solar cells in the world. INTRODUCTION EMCORE’s Photovoltaics Division was founded in 1998 in response to increasing demand for high efficiency solar cells in support of a growing commercial satellite market. The 70,000-ft2 factory, located in Albuquerque NM, finished construction in October 1998 and first commercial shipment of the highest efficiency dual-junction solar cells ever produced (23%) occurred at the end of 1999. Since the commercial release of the dual junction product, EMCORE has continuously improved upon the multi-junction architecture to meet the ever increasing requirements of its customers, releasing a 26% efficient triple junction cell (TJ) in 2000, an advanced triple junction cell (ATJ) with 27.5% average efficiency in 2001, incorporation of an on-board monolithic bypass diode into the ATJ product family (referred to as the ATJM) in 2002, deployment of a triple junction solar cell for concentrator applications (CTJ, 35%

efficiency under concentrated illumination), and most recently the release in 2006 of the BTJ/BTJM product family that provides the highest commercially available conversion efficiency of 28.5%, and is offered both with and without a monolithic bypass diode. In addition to producing state of the art multi-junction solar cells for space and terrestrial concentrator applications, EMCORE Photovoltaics also produces Coverglass Interconnected Cells (CICs), terrestrial receiver modules for concentrator systems, and fully integrated solar panels for satellite applications. These products are built in our state of the art, highly automated solar cell Fab and solar panel manufacturing operation with the capacity to produce over 200,000 4-inch diameter wafers and more than 150kW of solar panels annually. MULTI-JUNCTION SOLAR CELLS As the name suggests, multi-junction solar cells are comprised of three junctions in series, with each junction optimized for a different segment of the solar spectrum. EMCORE’s solar cells are based on the InGaP/InGaAs/Ge triple junction architecture, a schematic of which is shown in figure 1. In this configuration, each junction is defined in a different semiconductor layer, possesses a different bandgap

Figure 1 Cross Section of a MJ Solar Cell

11CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA

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and is therefore tuned to a different wavelength segment of the solar spectrum. The materials are arranged such that the bandgap of the junctions become progressively narrower from the top junction to the bottom junction. Thus high-energy photons are absorbed in the top junction, generating electron-hole pairs, and less energetic photons pass through to the lower junctions where they are absorbed and generate additional electron hole pairs. The current generated in the junctions is then collected at ohmic contacts formed at the top and bottom of the solar cell. By employing compound semiconductor materials in this arrangement, the solar cell makes more efficient use of the available energy in the solar spectrum and yields substantially higher conversion efficiency than single junction solar cells formed in silicon. EMCORE’s latest generation triple junction solar cell, called the BTJ, exhibits a minimum average conversion efficiency of 28.5% under the air mass zero (AM0) conditions of space. This solar cell technology has been adapted for use in terrestrial concentrator systems designed for large-scale (1-100MW) solar power stations and exhibit peak conversion efficiency of over 36% under concentrated illumination. The application of triple junction solar cells in concentrating photovoltaic systems represents a substantial growth area, and to meet this market opportunity, EMCORE has made, and continues to make substantial investments in production capacity to satisfy present and projected demand. SOLAR CELL FABRICATION The process flow for EMCORE’s ATJM family of high efficiency multi-junction solar cells is shown in figure 2. The ATJM product incorporates a patented monolithic by-pass

diode for shadow protection. The manufacturing process begins with procurement and qualification of 100-mm diameter, 145µm (5.5 mil) thick Ge wafers. Substrate qualification is required as the bottom junction is formed at the substrate/epi interface, and wafer quality has a strong influence on the performance of the bottom subcell. Once qualified for use, the multi-junction epitaxial layer structure is grown in EMCORE designed multi-wafer (13-14 wafer/run) MOCVD systems. The epitaxial structure is comprised of more than 50 distinct layers and is on the order of 10µm thick. The epitaxial structure is then characterized using a suite of characterization tools (X-ray, PL, electrochemical CV etc.). Although this characterization data provides feedback on the growth process, historically the only method of determining the quality of the epitaxial structure (and hence whether a reactor should be turned over to production) was to fully process and test solar cells. Recently, EMCORE has developed a measurement technique that enables the quantitative assessment of the full epitaxial structure immediately after the first metal step. This technique enables rapid feedback to the growth engineers enabling faster reactor tuning, reduced downtime and higher reactor availability. The fabrication process proceeds with a lithography/etch process to define the bypass diode and employs a selective etch to define the area of the diode which has been grown on top of the triple junction epitaxial structure. The diode etch is followed by the Mesa lithography/etch process and etches through the entire epitaxial structure, into the Ge substrate and defines the active area of the solar cell. TEL MarkV coat/develop tracks along with Canon 1x projection aligners are utilized for the photolithographic operations in the above steps (as well as all others). As MJ space solar cells are large area devices (27-31 cm2 each – only 2 per wafer!), the exposure field required (i.e., the entire wafer) dictates the use of 1x projection systems as the field size of reduction steppers is not adequate to pattern even one space cell. Once the diode and Mesa are defined, the front side metal grid is formed using industry standard photo patterning, metal deposition and lift-off techniques. With the grids defined on top of a highly doped n+ cap (to ensure ohmic behavior), the cap layer is patterned and selectively etched to remove this material from between the grid fingers and reveal the window for the top junction. This etch is quite critical as it has a large influence on the absorption of incoming photons by the multi-junction structure. The process continues with the reactive deposition of an anti-reflection coating of a TiOx/Al2O3 dielectric stack whose spectral characteristics are optimized to minimize reflections as well as maximizing the end-of-life performance of the solar cells. The solar cell manufacturing process concludes with the deposition of the backside metal to form the p-contact and is followed by an anneal step to ensure ohmic behavior. The Figure 2 ATJM Process Flow

12 CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA

Figure 3 Completed ATJM CIC

space solar cells are then on-wafer tested under AM0 conditions using a multi source solar simulator, and terrestrial cells are tested under concentrated illumination (500x-1000x). During this test step, full I-V curves are taken from which relevant figures of merit are calculated (i.e. conversion efficiency, output voltage, output current etc.). Wafers are then marked with both human readable characters as well as 2-D barcodes to enable easy identification and traceability. Once tested and marked, the solar cells are mounted onto film frames and diced using industry standard methods. After dicing, the solar cells are picked from the wafer and visually inspected to rigorous criteria as these products are destined for use in space where they must survive for >15 years in a harsh environment, without failure. A completed BTJM solar cell with silver plated kovar interconnects attached and coverglass applied is shown below in figure 3.

Although the manufacturing of multi-junction solar cells appear to be relatively simple in comparison to FET/HEMT or HBT based III-V devices/circuits, one should not conclude that these products are easy to produce. While many requirements of the solar cell manufacturing are relaxed in comparison to mainstream FET/HEMT/HBT processing (e.g., 5-10µm minimum geometries, one front side metal layer, minimal process steps) there are numerous unique requirements that make these products extraordinarily difficult to manufacture in volume. Driving the degree of manufacturing difficulty is the fact that, for space use, many of these solar cells have total areas in the range of 27-31cm2 with nearly all the area being electrically active semiconductor (try making an HBT of that size!). As one 100-mm wafer generates only two space solar cells, large emphasis is placed on defect control and yield management within the manufacturing process. This is particularly important in the area of epitaxial growth, as a single large epi defect will result in the loss of an entire cell (and half of a wafer). Furthermore, small epitaxial defects can generate recombination centers that reduce electrical performance and

adversely affect product yield. Near perfection is required in the epitaxial growth process to result in electrically conforming material on a consistent basis. These factors, along with numerous others, result in a very challenging technology that requires the highest degree of engineering expertise to produce these products in high volume. PACKAGING – CICs and SOLAR PANELS To be made useful for the intended application, whether it be a solar panel for a geosynchronous satellite or a concentrating PV system operating at 1000x, the solar cells are packaged in a manner that will ensure they perform reliably for the required operating life of the system they are powering. In the case of a GEO satellite, the solar panels must provide adequate power for a minimum of 15 years while enduring large temperature swings (-180°C to +150°C) and in a harsh radiation environment with little more than a thin piece of glass for protection. Unlike a mobile phone, the implications of failure are loss of a $100M-$200M asset (not exactly a throw away item) and adverse financial implications to the service provider, or in the case of a military satellite, the loss of critical battlefield capability. No satellites powered with EMCORE products have exhibited any on-orbit anomaly, and EMCORE’s unique record is due to robust cell design and rigorous control of the packaging process. The process of integrating the solar cells into a solar panel begins with the attachment of interconnects to the cell. As shown in figure 3, silver-plated interconnects are attached to multiple bond pads on the front side using a parallel gap welding process. This is followed by the application of a thin (4-6 mil) coverglass, and is bonded to the solar cell using a transparent adhesive. The adhesive is then cured in a convection oven and the CIC is electrically tested using a solar simulator to ensure the addition of the cover glass (which is covered with an anti-reflection coating) has not shifted the performance of the solar cell outside of normal limits. It is well understood that interconnect to cell welding is perhaps the most critical process in determining the on orbit reliability of solar panels. This process must be robust and well controlled, as these connections must survive tens of thousands of thermal cycles (-180°C to +150°C for GEO satellites, -110°C to +100°C for LEO) without failure. To exceed the reliability requirements of our customers, EMCORE has invested substantial resources to procure the most advanced welding toolset available and to identify and control the critical input variables to the welding process. This effort has resulted in a process that exhibits pull strengths of 1 to 2 kg (depending on the type of weld) and process capability indices ranging from 2.0 to over 15. This advanced toolset and process control methodology has been built into an advanced pick and place tool that fully

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13CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA

automates the placement of interconnects, executes the welding step while exerting real time control of the input variables, store all relevant process information, applies the adhesive and places the coverglass onto the solar cell. The automated pick and place tool, shown in figure 4, eliminates variability and hence reliability risks posed by competing approaches as well as reducing labor cost to a minimum.

Solar panel manufacturing proceeds by welding completed CICs in series to form strings. The number of CICs in a particular string is determined by the voltage requirement of the mission, and all strings on the panel are typically of the same length (voltage). Additionally, as each series connected solar cell must pass nominally the same current, all the CICs in a particular string are chosen from the same current bin as determined in prior electrical test operations. The strings are then arranged into circuits such that the required number of strings is configured in parallel in order to meet the current requirement of the satellite. The circuits are then bonded to lightweight solar panel substrates using an industry standard adhesive and cured under vacuum to ensure that no air pockets are formed. This bonding process is repeated several times until the entire panel has been populated with solar cells in a geometric configuration that maximizes the packing density. The circuits are then wired together on both the frontside and backside of the solar panel using NASA certified wiring techniques.

Once assembled, the solar panels undergo extensive environmental and electrical testing, which typically includes temperature cycling under vacuum, electrical evaluation using a large area pulsed solar simulator (LAPSS), ambient thermal cycling and additional electrical testing using the LAPSS. Only upon successful completion of the environmental testing and a thorough review of all test data with the customer can the hardware be shipped for integration into the solar array. Due to space constraints, the above represents an abbreviated description of the solar panel assembly and test process. Many important details have been omitted and a more comprehensive description will be provided during the oral presentation. CONCLUSIONS EMCORE Photovoltaics is the world’s largest manufacturer of high efficiency multi-junction solar cells for space and terrestrial solar power applications. Through continuous investment in production capacity and engineering discipline, EMCORE has driven MJ junction solar cells to commercial viability. This investment has resulted in wide availability of high performance, ultra high reliability solar power products for new markets. EMCORE’s multi-junction technologies will also continue to offer our customers the benefits of well-defined product/technology roadmap to future improvement realized in performance, cost and extended application. ACKNOWLEDGEMENTS The author would like to thank a number of people who contributed to this article, particularly Jody Wood, Roger Esra, Robert Gallegos, Navid Fatemi, Patrick Park, Brad Clevenger, Paul Sharps and Viven Bercier. ACRONYMS AM0: Air Mass Zero ATJ: Advanced Triple Junction ATJM: Advanced Triple Junction with Monolithic Diode BTJ: Best Triple Junction BTJM: Best Triple Junction with Monolithic Diode CIC: Coverglass Interconnected Cell GEO: Geosynchronous Orbit LAPSS: Large Area Pulsed Solar Simulator LEO: Low Earth Orbit MJ: Multi Junction TJ: Triple Junction

Figure 4 Automated Tool for CIC Manufacture

14 CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA