# 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. – pericynthion Aug 15 '16 at 2:32
• The 30% may be an end-of-life value, radiation causes the efficiency to decrease over time. – 2012rcampion 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. – Hobbes 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