Published on November 28th, 2016 | by Tina Casey0
New Solar Cell Efficiency Research Breaks 30-Year Logjam
November 28th, 2016 by Tina Casey
A research team from Technion — the Israel Institute of Technology — is on to a new solar cell approach that could put the Shockley-Queisser conversion efficiency limit to rest once and for all.
That’s great news for clean energy fans who are concerned that the incoming Donald J. Trump administration will cut US funding for foundational solar energy research that leads to improved efficiency and falling costs. After all, if the US drops the ball, Israel, or for that matter, China, Switzerland, Japan, South Korea, and other top players around the globe have already picked it up.
30 Years Of Solar Cell Efficiency Research
For those of you new to the topic, solar cell efficiency refers to the ability of a material to convert sunlight to an electric current.
There is plenty of room in the marketplace for low-efficiency solar cell materials. One example is the emerging class of thin film solar cells, which can be integrated into windows and other surfaces.
However, for utility scale installations, small scale rooftop solar arrays, and many other applications, a high efficiency solution is more compact and economical.
For space travel and other specialty uses, you can just throw economy out the window and go for the highest efficiency possible, regardless of cost.
Either way, the hunt has been on for new materials — and combinations of materials — that fit the high efficiency bill.
The problem is that solar cell efficiency is limited by the fact that when solar cells absorb sunlight, they also collect a lot of heat, which dissipates in the form of lost energy.
This dissipation is what the Shockley-Queisser efficiency limit refers to. The theory is that under optimal sunlight, the maximum conversion efficiency for a single solar cell material is 41%. More generally the Shockley-Queisser limit is a little over 30%.
It is possible to reclaim some of that heat energy before it dissipates, but engineering a device that can tolerate the high level of heat is a problem.
According to the Technion team, research in the solar thermalphotovoltaics (that is, sunlight plus heat plus light-induced electricity) field has been ongoing for more than 30 years, and so far researchers have little to show for their efforts:
After over thirty years of research, the record conversion efficiency for STPV [solar thermalphotovoltaics] stands at 3.2% for an absorber operating temperature of 1,285 K.
The STPV Efficiency Breakthrough
The Technion team claims a conversion efficiency of 50% for their new solar cell, or an improvement of about 70% over the “conventional value” of 30%.
So how’d they do that?
Here’s the explainer from the study, published last month in the journal Nature:
Here heat is harvested by a low bandgap photoluminescent absorber that emits thermally enhanced photoluminescence towards a higher bandgap photovoltaic cell, resulting in a maximum theoretical efficiency of 70% at a temperature of 1,140 K.
Did you get all that?
The Technion Foundation offers a more descriptive take:
“Solar radiation, on its way to the photovoltaic cells, hits a dedicated material that we developed for this purpose, and the material is heated by the unused part of the spectrum,” says graduate student Assaf Manor, who led the study as part of his PhD work. “In addition, the solar radiation in the optimal spectrum is absorbed and re-emitted at a blue-shifted spectrum…”
“Blue-shifted” refers to shorter wavelengths, which can then be harvested by the solar cell. The end effect is that the cell can convert heat as well as light to an electrical current.
Don’t run right out to Home Depot for your advanced STPV solar cell. The research team expects to work on the new device for another five years ago before it’s up to speed.
Meanwhile, if you’d like to read up on solar thermalphotovoltaics, CleanTechnica also covered the topic back in 2014 under the title, “Solar Thermophotovoltaics — Getting To 80% Efficiency.”
For another approach to breaking the Shockley-Queisser barrier, check out this new research from Drexel University with an assist from the inventor of the photocopier.