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The NREL solar cell efficiency chart tracks the efficiency of research-level photovoltaics since 1976, and the most up-to-date version is presented below (it was updated in April 2020).

3-J (3-junction) solar cells have seen a steady improvement in efficiency from about 33% in 1999 to 44.4% in 2013, at which point no further research seems to have been done on 3-J solar cells (according to the NREL chart at least). At around the same time, the company Soitec put 4-J cells on the map, the fist one having an efficiency of 44%, followed by a steady increase in efficiency up to 47.1% in 2015. When I say "steady" increase, I mean that the slope we see in the curve for 4-J cells is roughly the same as the slope we see for the 3-J cells from 1999 to 2013, which was excellent progress in increasing solar cell efficiency.

But suddenly it stopped.

  • No new reports of progress with 4-J solar cells have been put on the map since 2015.
  • No 5-J concentrator solar cells have been put on the map at all.
  • One 6-J concentrator solar cell was reported by NREL in the summer of 2019, which was the first 4+ junction concentrator cell reported in 4 years.

What happened?


enter image description here Source: https://www.nrel.gov/pv/cell-efficiency.html

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    $\begingroup$ I think due to the fact that the maximum theoretical efficiency of an infinite junctions SC (a hypothetical SC with an infinite number of layers) is around 68% and already the highest SC has 47% efficiency and these multi-junctions SCs become more and more expensive when you add a layer, there is not much motivation for researchers/industry to improve it. In other word, there should be a financial motivation to improve the efficiency of SCs, which I think is absent here. $\endgroup$
    – Mithridates the Great
    May 4, 2020 at 1:10
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    $\begingroup$ @AloneProgrammer thanks for your insights! I do wonder why they stopped working on 4-J though, because the efficiency was steadily improving just like with 3-J, and the improvement didn't really stop between 2013 and 2015, but then suddenly they skipped 5-J concentrators and went straight to 6-J ... there seems to be something more to the story! $\endgroup$
    – Nike Dattani
    May 4, 2020 at 1:13
  • $\begingroup$ @AloneProgrammer, the theoretical efficiency limit of 68% is for non-concentrated sunlight. With concentrated sunlight it is 86.8%. My question mentioned that I was referring only to concentrator solar cells. $\endgroup$
    – Nike Dattani
    May 8, 2020 at 4:27
  • $\begingroup$ This question has a long term bounty posted on Meta for 200-400 rep. Read the description for how to earn this reward. $\endgroup$
    – Tyberius ♦
    Jul 14, 2020 at 18:21

1 Answer 1

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I suspect it's a combination of three factors:

First, there's a fundamental limit of the efficiency of solar cells. A solar cell is effectively a heat engine that uses the 6000K Sun as the heat source and the Earth's ~300K ambient temperature as the heat sink. Given that, the efficiency of a "perfect" solar cell tops out at around 95% for an infinite-sized Sun, or 69% for the Sun as it actually appears in the sky.

Second, increasing the junction count produces diminishing returns. You mention that the slope of four-junction cells follows the trend of three-junction cells -- in contrast, changing from single-junction to two-junction, and two-junction to three-junction cells both produced an immediate jump in efficiency. Yes, you get more efficiency by adding more junctions, but each additional junction requires more effort for less improvement.

And third, solar panels are a reasonably mature commercialized technology. Efficiency is "good enough", and research effort is directed more towards making cheaper cells than making efficient cells. Most users, if they need more power, can just put up a bigger array, they don't need a more-efficient one.

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  • $\begingroup$ Just a quick response to: "You mention that the slope of four-junction cells follows the trend of three-junction cells -- in contrast, changing from single-junction to two-junction, and two-junction to three-junction cells both produced an immediate jump in efficiency." I was looking at concentrators since those are what give the state-of-the-art efficiency: Does the slope not look roughly linear from the 1983 purple triangle, all the way up to the latest 6-J from 2019? There is a small jump from 2-J to 3-J but it's no surprise to me considering that it's 1994 to 2000 [cot'd] $\endgroup$
    – Nike Dattani
    May 5, 2020 at 3:36
  • $\begingroup$ whereas 3J & 4J points are plotted for the same year. I accept that adding more junctions may be more effort than it's worth, but this graph makes me curious why there's no 5J concentrator on this map, & why no new 4J or 3J concentrators were added to the map since 2015 and 2013 respectively. For 3J it's somewhat understandable that after 14yrs researchers ran out of ideas/motivation to improve, but 3yrs of results for 4J seems weak, & why no 5J on the map before 6J? Maybe they started working on perovskites, but it doesn't explain why there's no 5J & why they did 6J rather than improving 4J. $\endgroup$
    – Nike Dattani
    May 5, 2020 at 3:42
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    $\begingroup$ @NikeDattani, look at who's setting the records for multijunction cells: it's nearly all National Renewable Energy Laboratory (pure research) or Boeing Spectrolab (solar panels for satellites). There's not much interest in ultra-high-efficiency cells -- compare to things like single-junction GaAs, silicon, and emerging technologies, where you see a huge variety of names, including ones like "Panasonic", "LG", and "General Electric". The major research push right now is not for efficient cells, it's for cheap-to-manufacture cells. $\endgroup$
    – Mark
    May 5, 2020 at 20:52

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