Science

Breakthrough allows inexpensive solar cells to be fabricated from any semiconductor

Breakthrough allows inexpensiv...
A new technique allows photovoltaic solar cells to be produced using any semiconductor (Photo: Shutterstock)
A new technique allows photovoltaic solar cells to be produced using any semiconductor (Photo: Shutterstock)
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Two approaches to engineering the screening properties of SFPV electrodes - fingered electrodes on the left, graphene electrodes on the right
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Two approaches to engineering the screening properties of SFPV electrodes - fingered electrodes on the left, graphene electrodes on the right
Where are all the solar panels we were promised? (Photo: Shutterstock)
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Where are all the solar panels we were promised? (Photo: Shutterstock)
How a solar cell works (Image: Brian Dodson)
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How a solar cell works (Image: Brian Dodson)
A new technique allows photovoltaic solar cells to be produced using any semiconductor (Photo: Shutterstock)
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A new technique allows photovoltaic solar cells to be produced using any semiconductor (Photo: Shutterstock)
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Despite their ability to generate clean, green electricity, solar panels aren't as commonplace as the could be. The main sticking point, of course, is price. Due to their need for relatively expensive semiconductor materials, conventional solar cells don't yet have a price-efficiency combination that can compete with other sources of electricity. Now Profs. Alex Zettl and Feng Wang of Lawrence Berkeley National Laboratory and the University of California at Berkeley have developed seriously unconventional solar cell technology that allows virtually any semiconductor material to be used to create photovoltaic cells.

A solar cell works according to these steps. First, sunlight hits the solar cell and is absorbed by the semiconductor of which the solar cell is made. In the absorption process, electrons are freed from their atoms, allowing them to flow through the semiconductor. The presence of a p-n junction acts as a diode, only allowing the electrons to move in a single direction. (Electrons and holes move in opposite directions, but the electrical current only moves in one.) Metal electrodes then transfer the light-generated electron flow into an electric circuit for use. A p-n junction is the interface between a region of the semiconductor where the dominant charge carriers are holes and a region where the carriers are electrons.

How a solar cell works (Image: Brian Dodson)
How a solar cell works (Image: Brian Dodson)

A conventional solar cell is made of a thin wafer of a semiconductor with a metallic electrode deposited on its rear side. The side facing the light source is polished more finely than any optical lens, cleaned to the atomic level, and then dopant atoms are deposited onto the front side, whereupon the entire wafer is placed in a high-temperature diffusion furnace.

The purpose of a dopant is to change the dominant charge carrier in the semiconductor from hole-rich to electron-rich, or v.v. In this process the p-n junction that converts incident light into a flow of electrons is formed. Following diffusion, the wafer is again cleaned, and a metallic electrode is grown on the front surface, using arcane rituals to ensure an ohmic contact with the active semiconductor material. (An ohmic contact is an electrical contact that obeys Ohm's law, having no rectifying or diode-like properties.)

The efficiency of conventional solar cells is also limited by the semiconducting materials which are suitable for the manufacture of solar cells by some approximation of the above process. It must be possible for the dominant charge carrier of the semiconductor to be changed between p (hole) and n (electron)-dominated conduction by introduction of chemical dopants, so that a well-behaved p-n junction is formed. It must also be possible to make a satisfactory electrical contact between the electrodes and the semiconductor.

There are many semiconductor materials with optical properties and electronic band-gaps well suited to conversion of light to electricity for which one or another of the rather rigid criteria for manufacture of conventional solar cells fails. These include many metal oxides, sulfides and phosphides, which are plentiful and inexpensive, but have been considered unsuitable because it is so difficult to alter their electronic structure chemically (e.g., through doping). For example, zinc oxide is a semiconductor well suited for capturing violet and near ultraviolet light, which is wasted by most conventional solar cells.

We now have Zettl and Wang's unconventional solar cells. Broken down, the expensive parts of making regular solar cells are the semiconductor wafer, forming a high-quality p-n junction under the surface of the wafer, and making ohmic electrical contact with the front and back of the wafer. Aside from providing a semiconductor wafer (usually of a cheaper material), the new solar cells require none of this.

The new technology is called "screening-engineered field-effect photovoltaics" (SFPV). An electrode is deposited on the front of the semiconductor wafer, which partially screens the semiconductor from an electric field generated between the front and rear electrodes. Assume the semiconductor is naturally p-type, so that it has an excess of holes. The applied electric field then penetrates the semiconductor surface slightly, attracting electrons toward the surface and repelling holes. As a result, the semiconductor near the surface changes from p-type to n-type (electron-rich), and a buried p-n junction has been generated – not by chemistry, but by use of carefully tailored electric fields. An extra bonus is that the front electrode automatically forms an ohmic contact with the semiconductor wafer.

“Our technology requires only electrode and gate deposition, without the need for high-temperature chemical doping, ion implantation, or other expensive or damaging processes,” said lead author of a paper describing the new technology, William Regan. “The key to our success is the minimal screening of the gate field which is achieved through geometric structuring of the top electrode. This makes it possible for electrical contact to and carrier modulation of the semiconductor to be performed simultaneously.”

Two approaches to engineering the screening properties of SFPV electrodes - fingered electrodes on the left, graphene electrodes on the right
Two approaches to engineering the screening properties of SFPV electrodes - fingered electrodes on the left, graphene electrodes on the right

Two electrode configurations that exhibit the SFPV concept have been developed. In one, the electrode in contact with the semiconductor wafer is formed of a row of narrow fingers, while in the other the partial screening is accomplished by placing a layer of graphene atop the semiconductor wafer. In both cases, high-quality p-n junctions are formed in semiconductors for which this structure was previously impossible.

Low cost, high efficiency solar cells? Sounds like a winner here – let's see when and if it hits the market.

The team's research is published in the journal Nano Letters.

Source: Lawrence Berkeley National Laboratory

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29 comments
Mzungu_Mkubwa
Excellent explanation, Brian! Good article! The only thing I would crit is the conclusion: you left many things implied which I think should be explicitly stated, such as the notion that this new technique can be applied to the wide variety of potential semi-conductor materials successfully. A more "robust" wrap-up to your very detailed (and informative) explanation would have been more appropriate. Very nice, tho - and exciting news for this industry! Now, could we expedite commercialization of this tech this time? :-)
nutcase
Oh no. Yet another reason to wait before buying solar panels
usugo
graphene and low cost are not exactly on the same page. And for what we know now, it will be this way for many years to come.
Joseph Boe
"Low cost, high efficiency solar cells? Sounds like a winner here – let's see when and if it hits the market."
I read the article - twice. Where is the "high-efficiency"? What I read states the achievement is in allowing the use of less costly materials to achieve what expensive materials currently do.
That may be less costly, but it doesn't make it "high-efficiency". To date, solar technology is horribly inefficient. On average, solar operates at between 10-25% efficiency (converting light to useable energy). The MOST efficient (and most expensive) make it to 40% efficiency.
Gasoline, Coal, Hydro-electric - even relatively inefficient natural gas - are all roughly 3X more efficient than solar.
Don't get me wrong, I love the idea of solar and I support R&D to get it where it needs to be - and this breakthrough IS a breakthrough - but, "High-efficiency"? No.
attoman
Any technology that permitted energy to be captured from the far blue and UV spectrum is very desirable and worth further development. Just obtaining efficiency close to existing PV would net much more POWER since this region of light has more energy per photon.
This approach may well improve space based PV also.
Manufacturing and testing means at best the material is five to ten years away from the market.
jocco
If the source is free, the sun, efficiency is not as important as with high priced oil.
Njall
"Despite their ability to generate clean, green electricity, solar panels aren't as commonplace as the could be. The main sticking point, of course, is price. "
Don't get me wrong, I love hearing about new technology which might just breakthrough. However, in the world of solar based electricity the issue is not the price of the solar cells. The issue is the price of acceptable storage. Generating power at noon which is really needed after dark is the issue I believe. It's certainly why I've never invested in solar power.
Florin Nicoara
@ Joseph Boe "Gasoline, Coal, Hydro-electric - even relatively inefficient natural gas - are all roughly 3X more efficient than solar."
First you question the numbers in the article, and than to counter argue... you pull "3x" out of your ass showing you have no concept of the laws of thermodynamics.
Efficiency in photovoltaics refers to their proficiency at turning photon energy into electrical energy, NOT cost relative to something else!!!!!!!!!!!!!!!!!!!
A one meter square solar cell can only convert about 20% of the total energy output of a ONE SQUARE METER "cut" of the light hitting the earth. On the other hand to get hydrocarbon energy out of one square meter of light, first it has to shine on vegetation, which converts it to carbohydrates, which then goes trough the long process of sequestration over millions of years, before it can turn into hydrocarbons... so exactly how efficient is this process??????
All energy comes from the sun and when measured side by side, square meter of light against square meter of light, hydrocarbons are least efficient of all. We have just been lucky over the last hundred years because those hydrocarbons have already been "pre-processed" by nature for us, but it took millions of years, and millions of hours, for plants to turn light into carbohydrates, get eaten by animals, bacteria, and then decay into hydrocarbons. Using solar directly bypasses this million year process and gives us energy directly from the sun for as long as the sun shines, without relying on evolution and its million year process. Plus hydrocarbons will run out sooner than later(since evolution takes millions of years to do its thing) and then we won't have such easy "pre-made" energy on demand as we had in our lifetimes. Please stop drinking the corporate cool-aid. How many Jonestowns do we need before people wake up to reality!!!
Knowledge Thirsty
@jocco, I disagree that efficiency is not as important as oil. It may not be important to you as you may be sitting on 20 acres of land in Arizona. But it's very important if you consider how much sun is achievable in the target area as well as its size in oirder to generate the energy that is needed for one's purpose. If I need 10KWh per day of electricity to run my house, I don't want to have to cover my roof AND my yard with solar panels to get it. Efficiency is ALWAYS important.
Cheers KT
PrometheusGoneWild.com
I actually agree with all the comments so far. But you could argue that these new style solar panels are efficient in the fact they will take less energy and effort to produce..... Here in Connecticut we have Net Metering. There is no storage of solar energy (unless you want to get completely off the grid). Most people are connected to the grid and send their excess solar energy out to the grid and receive credit. If the prices of solar panels came down enough I would throw a bunch of them up on my property. Putting them up on your roof is expensive and complicated your home. Overall I look forward to seeing this technology make solar panels cheap and common. This may be the tipping point we have been waiting for.