# Why are these multi-junction solar cells 40% efficient on earth but 30% in space?

This video about Spectrolab's spacecraft power systems mentions Juno several times and it looks like the Juno panels are shown at several points in the video. Multi-junction cells are discussed, they have higher efficiency than silicon and so were the obvious choice for Juno.

But I was confused when I saw this:

Spectrolab's solar cells are approximately 30% efficient in space and exceed 40% efficiency in terrestrial applications.

Why is there such a large difference in these values? Is photovoltaic efficiency defined differently for space and terrestrial applications?

Although I normally advise against taking a percent of a percent because it can be confusing:

$$1- \frac {\text{approximately 30%}} {\text{exceeding 40%}} \approx \text{25% less}$$

above: Screen shot from the video found here:

• Speculation: Additional layers of protection (e.g. coverglass) are needed in space to handle the U.V. and atomic oxygen, these absorb some light and reduce efficiency. Also, the wider operating temperature range may not be optimal for efficiency. Aug 15 '16 at 2:32
• The 30% may be an end-of-life value, radiation causes the efficiency to decrease over time. Aug 15 '16 at 2:36
• I've asked the related question Is photovoltaic efficiency defined and measured differently for space vs terrestrial applications? to keep the issues of ratings and measurement procedure separate.
– uhoh
Aug 15 '16 at 4:04
• This may be caused by temperature, IIRC solar cells get less efficient at high temperatures. Aug 15 '16 at 7:29
• @Hobbes Thanks - the need to allow for temperature variation within a ratings specification is noted by (at)pericynthion above. It's a good point though, and an important issue on Earth as well!. In at least one spacecraft they will be liquid cooled!
– uhoh
Aug 15 '16 at 7:43

The 40+ percent efficiencies on earth are using solar concentrators, often with concentration ratios as high as 1000x. Most spacecraft including Juno use planar solar arrays without concentration. Record solar cell efficiencies as a function of time are published periodically by NREL and are available as a nice plot here: http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.

• A very nice plot indeed. Yep, the highest point there for 3-junction, non concentrator is 37.9% in 2014. OK you nailed it, thanks!
– uhoh
Aug 15 '16 at 22:58
• Actually, you might include a reference to the comments about the additional protective layers (UV and energetic particles), efficiency losses at elevated (unregulated) temperatures in vacuum, and de-rating for end-of-life (15 years) performance loss to get that 37.9% record down to the >30% space rating.
– uhoh
Aug 16 '16 at 1:15
• If you wouldn't mind, I could edit your answer a bit, add those points explicitly, and then accept it.
– uhoh
Aug 20 '16 at 4:51
• I don't think it will happen ;-)
– uhoh
Mar 29 '19 at 14:58
• The link doesn't work, the NREL chart is now available at Best Research-Cell Efficiency Chart. Oct 23 '20 at 7:39