FEATURE SOLAR
patterning steps and leads to longer current conduction paths in the silicon wafer. TOPCon reduces these losses, is easier to manufacture, and closes the gap on the efficiency record for back- contacted solar cells.
A solar cell built from two
‘The last few years have seen a number of research groups break the long-standing silicon solar cell efficiency record of more than 25 per cent. One of these groups is near production at that efficiency level, which is a big breakthrough,’ said David Young, senior staff scientist at the US Department of Energy’s National Renewable Energy Laboratory (NREL) in the High Efficiency Crystalline PV group.
One way to boost silicon’s practical efficiency is by stacking individual films, each of which is optimised to collect a specific band of wavelengths. ‘Growing a material directly on top of silicon is one option, but efficiencies are not very high because the lattice constants need to be just right. The easier way is to grow the top cell separately, lift it off its parent wafer, and glue it onto the silicon wafer,’ explained Young. Young’s team recently set a record for mechanically stacked cells, achieving 29.8 per cent efficiency for a dual-junction device. The top cell, made of gallium indium phosphide (GaInP), was developed by NREL. The bottom cell was developed by the Swiss Centre for Electronics and Microtechnology using silicon heterojunction technology.
efficiency got attention because it’s a new way to boost silicon efficiency. If you
could...grow the top cell cheaply, you could slap that onto a silicon cell and increase efficiency relatively cheaply,’ added Young. When they recognised the value of their GaInP cell, it was a natural choice to connect it to silicon.
Solar simulators
allow developers to test a cell’s response to light in a controlled environment
‘If you look at the solar spectrum and work out the two best bandgaps, it’s 1.1 eV for the bottom and 1.7 eV for the top cell,’ said Young. With a ~1.8 eV bandgap, GaInP achieves 20.8 per cent efficiency by itself. ‘Gluing’ it on top of a 1.1 eV silicon cell optimises the tandem for the solar spectrum. Engineering the optical coupling of the two cells, such that none of the bottom cell was in the dark and most of the unused light in the top cell passed through to the bottom cell, optimised the power from the tandem. Work continues to raise the individual efficiencies of high- bandgap, single-junction cells,
from tried-and-true III-V materials to inexpensive but unstable perovskites. Finding a high-bandgap material that could easily attach to silicon offers a valuable way forward.
Simulating the sun Techniques of metrology play an important role in advancing solar cell efficiency, by providing a detailed understanding of cell and module performance.
Solar simulators allow developers to test a cell’s
Although multi-junction cells made of semiconductor materials similar to GaInP can have efficiencies of up to 40 per cent, they are very expensive and are used only in satellites or high concentration applications. ‘Our 29.8 per cent
response to light in a controlled environment. Xenon lamp-based simulators have been around for 40 years and have found their way into the commercial and industrial sectors for solar cell production; now, LEDs also allow simulation of response to selected wavelengths. Excelitas Technologies manufactures a broad array of lighting components and subassemblies for companies that design finished solar simulators. It’s Cermax product line includes components that deliver power, mainly from xenon lamps, as part of a fully integrated system. ‘We are not constrained to design solely around xenon. We have the capability to deliver a 100 per cent xenon-based cell, but we also use argon and krypton to shift the spectrum,’ noted Jim Clemens, product manager for Cermax.
Open view of the light path in an AvaSpec-ULSi spectrometer with integrated electronics
www.electrooptics.com | @electrooptics
Often working with distribution partner Atlas Specialty Lighting in Florida, Excelitas created a unique product that incorporates a reflector, lamp, and power supply in a single package. ‘The product is very transportable, and people tend to like the results. We see the continued use of xenon
Pentacene molecules convert a single photon into two molecular excitations via singlet fission
for small systems that could go into a lab, or for a large platform that tests hundreds of solar cells in an array,’ added Atlas general manager Ralph Felton.
Because of the maturity of the xenon-based line of simulators, lamps can be manufactured within a broad range of output levels and specifications. ‘Imagine xenon as the brute force method. It’s going to produce a spectrum of many wavelengths, and you get what you get. But with LEDs, you can dial in [specific wavelengths] and be more precise,’ said Felton. Moving away from xenon-based light sources to LEDs will be a natural progression, thanks to the many advantages this technology brings. ‘You can simulate situations by varying 13 to 18 different wavelengths, which offers advantages for testing and analysis. But what you really want is to look at specific wavelengths that let you improve efficiency grading of the solar panel itself,’ explained Mark Gaston, Excelitas product manager for solid state lighting. While LED solar simulators exist today, integrated systems are challenging to develop. ‘Since the LED side is new and complex, people need an integrated solution. We offer an approach where we select LEDs, design optics and the appropriate heat sink, add them to a circuit board and build the final solar product,’ said Gaston. WaveLabs Solar Metrology Systems, based in Germany, grew out of its founders’ desire to revolutionise solar metrology with LED technology. According to WaveLabs sales engineer Jason Nutter, ‘LEDs offer options that are simply not available with other light sources. With the
MARCH 2016 l ELECTRO OPTICS 17
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Avantes
Lawrence W Chin, David Turban and Alex W Chin/University of Cambridge
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