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High-performing solar cells are a dime a dozen nowadays, but almost all of them have one thing in common. They can’t break through the theoretical limit for solar conversion efficiency, established back in the mid-20th century. Well, that was then. The photovoltaic researchers of the 21st century are ready to move on, and exhibit A is a theory called the “bulk photovoltaic effect.”
The Bulk Photovoltaic Effect Is Just A Theory …
If the bulk photovoltaic effect rings a bell or two, you may be thinking back to August 9 of 2016, when CleanTechnica first caught wind of the theory.
“A team based at Drexel University has demonstrated how to break the Shockley-Queisser limit, which dictates the maximum amount of sunlight that a solar cell can convert to electricity,” CleanTechnica reported.
That description jumped the gun just a bit. As of 2016, research into the bulk photovoltaic effect was still in the theory formulation phase. A real-world demonstration was still years away. Still, if the bulk photovoltaic theory pans out, it could open the door to a new generation solar cells with a simplified structure, leading to lower costs and more rapid, widespread adoption.
“The new solar cell breakthrough is a significant step forward in solar cell R&D, as teams around the world vie to squeeze the maximum amount of energy from solar-friendly material whilst juggling conversion efficiency with size, weight, flexibility, durability and cost,” CleanTechnica noted in 2016.
… But This “Peculiar Phenomenon” Could Blow The Photovoltaic Field Wide Open
Many other researchers have contributed to the bulk photovoltaic field in the eight years since 2016. Last year, Zhenbang Dai of the University of Texas at Austin and Andrew M. Rappe of the University of Pennsylvania recapped the state of affairs in an article published by the journal Chemical Physics Reviews, under the title, “Recent progress in the theory of bulk photovoltaic effect.”
Dai and Rappe trace early investigations of the bulk photovoltaic effect to the late 1960’s. It was just few years after the Shockley-Queisser limit was established, and researchers were already beginning to develop a radically new approach to solar cell design.
The bulk photovoltaic effect involves deploying just one type of semiconductor in a solar cell. That’s a foundational contrast with conventional solar cells. A typical solar cell falls under the Shockley-Queisser limit because it deploys two kinds of semiconductors. The familiar silicon solar cell, for example, is composed of two different iterations of silicon. The junction or interface between the two materials is both the deciding and the limiting factor.
Getting rid of the junction may seem like a simple way to circumvent the Shockley-Queisser limit, but it opens up a whole new can of worms.
“… intense theoretical investigations have been made to understand the physical origin of BPVE [bulk photovoltaic effect], which seems to be a rather peculiar phenomenon,” Dai and Rappe explain, adding that “one would naively expect that in the absence of built-in field (as exists in a p–n junction), the oscillating light field would only drive the charge carriers to oscillate periodically without inducing a net current.”
“Thus, the breaking of such intuition implies that we must go beyond the linear response regime to fully account for the BPVE,” they emphasize.
Hold Onto Your Bulk Photovoltaic Effect Hats
Having tracked the latest developments in bulk photovoltaic theory, Dai and Rappe advised that the theory has been advanced to the point of direct experimentation. “We hope that our review can provide a perspective on the current stage of the BPVE theory and encourage the community to extend the exploration of this fruitful field,” they concluded.
As if in answer, last week Shinshu University in Japan announced the results of a new study confirming the Dai/Rappe guidance about going “beyond the linear response regime.”
Billed as a first-of-its-kind real world demonstration of the bulk photovoltaic effect, the study deployed alpha-phase indium selenide (α-In2Se3) as the semiconductor, sandwiched between two layers of graphite. The details are available under the title, “Bulk photovoltaic effect of an alpha-phase indium selenide (α-In2Se3) crystal along the out-of-plane direction,” in the journal Applied Physics Letters.
Shinshu University also kindly summarized the study in a plain language press release.
“Today, most solar cells employ p–n junctions, leveraging the photovoltaic effect that occurs at the interface of different materials,” the school notes. “However, certain crystalline materials exhibit an intriguing phenomenon known as the bulk photovoltaic (BPV) effect.”
“In materials lacking internal symmetry, electrons excited by light can move coherently in a specific direction instead of returning to their original positions,” Shinshu elaborates. “This results in what is known as ‘shift currents,’ leading to the generation of the BPV effect.”
Next Steps To The Low Cost Solar Cell Of The Future
As noted by Shinshu University, researchers who study the bulk photovoltaic effect previously identified α-In2Se3 as a good candidate for experimentation. However, the Shinshu team was the first to transition from theory to a real-life demonstration.
“After testing with different external voltages and incident light of various frequencies, the researchers verified the existence of shift currents in the out-of-plane direction, confirming the above mentioned predictions,” the school noted.
The research team was also the first to determine that α-In2Se3 is not just a good candidate. It is a great candidate. The team compared the performance of α-In2Se3 to other proposed materials and found a quantum efficiency several orders of magnitude higher.
For the record, quantum efficiency is not the same as solar cell conversion efficiency. Think of quantum efficiency as the number of marbles that can fit into a one-gallon bucket. Conversion efficiency would be the number of marbles you can take from the bucket with one hand.
Still, a high level of quantum efficiency indicates the potential for a respectably high level of solar conversion efficiency. It may take many more years of R&D before the bulk photovoltaic effect makes it onto the affordably priced rooftop solar array of the future, but the Shinshu study should help accelerate the research trajectory.
In the meantime, other new solar cell formulas are already testing the boundaries of Shockley-Queisser. Another approach to keep an eye on thin film solar technology, which deploys inexpensive materials and an economical manufacturing process.
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Image (cropped): The field of bulk effect photovoltaic theory has finally leaped off the chalkboard and onto the laboratory bench as researchers unlock the secrets behind the solar conversion efficiency of a “peculiar phenomenon” (artist rendition by Ella Marushchenko for Drexel University, 2016).
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