The NASA Artemis program will establish a long-term presence on the Moon and set the stage for the first human mission to Mars. But before creating a sustainable foundation on the lunar surface, NASA must launch the Power and Propulsion Element (PPE) and other modules to construct Gateway, a Moon-orbiting space station.

To meet NASA’s power, mass and radiation resistance requirements, the PPE is designed with highly efficient, multi-junction solar cells. Testing these advanced cells is challenging for legacy lamp-based solar simulators, but powerful LED-based solar simulators can ensure the PPE’s solar cells perform to NASA’s standards.

Multi- Versus Single-Junction Cells

A multi-junction solar cell is essentially a stack of multiple solar cells that uses multiple bandgaps, or junctions, to capture a broader range of solar energy than standard single-junction cells allow — boosting a solar cell’s efficiency. Most space applications require more power per area than single-junction cells can provide, so three-junction (3J) solar cells have been the space-industry standard for decades.

Programmable LED solar simulator (pLEDss) technology utilizes hundreds of small LEDs and advanced controls to non-invasively deliver crucial metrics for solar cell health. The LEDs are housed in pLEDss “heads” that are placed close to the cells. Thanks to advanced controls, each of the hundreds or thousands of LED channels in the system is independently adjustable — enabling superior spectral adjustability, spatial uniformity and temporal stability.

Less Is More

To reduce design, construction and launch costs, it is imperative for NASA to utilize available space as efficiently as possible. Multi-junction cells are optimized for increased efficiency, making them excellent components to support lightweight, compact spacecraft designs. With area and mass at a premium, NASA selected advanced solar cells with more than three junctions for PPE. This allows for lighter spacecraft designs that feature more compact solar arrays without sacrificing energy output. This weight reduction directly benefits rocket payloads — where every kilogram saved counts.

In the past, traditional solar simulation methods that used lamps were sufficient for simple solar cell designs. But multi-junction cell designs are more complex, and traditional methods perform progressively worse when applied to cells with three junctions and more. The pLEDss technology can deliver on the shortcomings of lamp-based simulators while also saving time and costs.

How pLEDss Ensures Sophisticated Cell Performance

A key feature of the patented pLEDss technology is the ability to individually control each LED light, allowing engineers to apply enhanced tests using spectral and spatial non-uniformities — a flexibility that traditional methods lack. This degree of control also enables pLEDss to gain deep insight into a cell’s health, providing information on potential hotspots, manufacturing defects, efficiency and more.

For example, manufacturing defects in a solar cell can generate different currents at each junction. With traditional solar simulator systems, it is impossible to measure these current differences between cells after the cells have been assembled into strings. However, pLEDss technology can measure the current at each junction in every cell to find the defects — even in an assembled string.

This innovative simulator technology also takes up less space than traditional methods. The pLEDss heads can be placed much closer to the cells, as opposed to lamps which are placed away from the test article. Considering that PPE’s solar arrays are about the size of a football field’s end zone, utilizing lamp-based simulators would introduce a great amount of complexity. Additionally, pLEDss technology is automated, saving NASA significant time and resources, as well as ensuring accuracy, repeatability and speed.

Programmable LED solar simulator technology was designed to address the challenges of multi-junction cells — regardless of the number of junctions. In fact, pLEDss has already measured cells with three, four and five junctions. Six-junction capable pLEDss systems have been built and are awaiting the next generation of advanced solar cells. These capabilities ensure that pLEDss will meet the needs of spacecraft using the advanced solar cells of the future — for the Artemis missions and beyond.

Learn more in our white paper.