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The key to the getting a full spectrum solar cell, hence increasing efficiency and cutting costs, is to select the right material, insulate the intermediate band, and exploit a common, affordable semiconductor manufacturing process.  (Source: LBNL)

A new cell design by LBNL team members, including staff scientist Kin Man Yu and team leader Wladek Walukiewicz, pictured here, offers full spectrum capture in a single-alloy cell created by common semiconductor manufacturing techniques. The resulting blend -- made from gallium, arsenic, and nitrogen -- also is advantageous in that it doesn't rely on overly scarce elements like indium.  (Source: LBNL)
Vapor deposition process will allow for less expensive mass production of the cells, LEDs

A significant fraction of visible light goes to waste at solar farms, as current generation cells generally can only take advantage of a very specific band of the spectrum.  Researchers have spent the last decade cooking up cells that could capture the entire spectrum and operate at higher efficiency, therefore lowering costs.  But those cells were hard to manufacture.

Now a research collaboration between researchers in the Solar Energy Materials Research Group in the Materials Sciences Division (MSD) at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), and a corporate partner -- Sumika Electronics Materials, Inc. in Phoenix, Arizona -- may have discovered [press release] the key to mass producing these more efficient cells.

The key was in cooking up the right semiconductor.  

I.  Finding the Right Material

Ideally a semiconductor would absorb the entire visible and non-visible spectrum of light, including infrared, the visible colors (white light), and ultraviolet light.  Unfortunately no single semiconductor can do that.

Wladek Walukiewicz [profile], head of Berkley Lab's MSD team explains, "Since no one material is sensitive to all wavelengths, the underlying principle of a successful full-spectrum solar cell is to combine different semiconductors with different energy gaps."

Many semiconductors used in the industry are actually blends of two or more elements -- one or more of which are semiconducting elements like silicon or gallium.  The blend composition (how much of each element) determines what wavelength of light is absorbed.  What Dr. Walukiewicz's team discovered in 2002 was that wiring layers of semiconductor could create a full spectrum cell with different blends (and thus different absorption wavelengths) in series.  That study used blends of indium and gallium in indium gallium nitride.

Then in 2004, his team discovered a way to make a single alloy have three bands (a valence band, an "intermediate" band, and a conduction band), thus absorbing the full spectrum in a single layer of alloy.  The alloy used was zinc (plus manganese) and tellurium, doped with oxygen.  While the results were good, the resulting material was too expensive and difficult to mass-produce.

Now the team has cooked a semiconductor blend that they feel is ready for prime time.  Like the zinc/manganese/tellurium/oxygen blend, the new mix is a tri-band device.  Taking gallium arsenide and replacing some of the arsenic atoms with nitrogen, to form the third band, form the semiconductor blend.  This replacement can be done using metalorganic chemical vapor deposition (MOCVD), one of the most common methods of fabricating compound semiconductors.

The findings are a beautiful example of how research that is far from production can iteratively evolve and mature into something commercially viable.  It is also a testament to theory pointing the way for manufacturing progress.  

The team used a special model developed in lab, called the band anticrossing model.  That model states, among other things, that the element replaced in the alloy must be from the same group in Mendeleev’s original periodic table.  It also gives clues as to what percent to use.  The original zinc/manganese/tellurium/oxygen blend was a so-called "II-VI" semiconductor alloy, while the new blend (gallium/arsenic/nitrogen) is a "III-V" alloy.

II.  Preparing the Blend for Production

Though the researchers had a good idea what material and what percent of it they wanted to use for this study, the key to building a working device was in doping the semiconductor so as to isolate the intermediate band.  Describes Dr. Walukiewicz, "The intermediate band must absorb light, but it acts only as a stepping stone and must not be allowed to conduct charge, or else it basically shorts out the device."

By adding additional doped layers the researchers were able to essentially isolate the central layer, and assured it was passing on its energy and not shorting out.

Part of the verification that the device worked was taken from when current was applied to it.  When a solar material is subject to current, it reverses from absorbing photons, to emitting them (goes from a photodiode to a light-emitting diode).  The resulting LED admitted emitted three peaks in the visible range and a peak in the infrared range.  This means that not only does the device promise high efficiency absorbance of sunlight, but it may also be promising as a light-emitting diode for applications such as lighting and lasers.

It was a long path to the full-spectrum cell, but the results in "Engineering the Electronic Band Structure for Multiband Solar Cells" certainly offer a convincing argument that a commercially viable solution has finally been reached.  The paper was written by team members Nair Lopez, Lothar Reichertz [profile], Kin Man Yu [profile], and Wladyslaw Walukiewicz, plus Sumika Electronic's Ken Campman, and published [abstract] in the January 10, 2011 edition of the peer-reviewed journal Physical Review Letters.

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love it
By joe4324 on 1/27/2011 5:50:41 PM , Rating: 5
As a dailytech reader living on solar power for sometime I enjoy these articles!

I see to many armchair-pundits on this site discrediting solar every chance they get. At current prices I think its a fantastic solution for residential power generation. You can buy it for roughly $2.00/watt right now in small lots. (I paid 2.25$) Go grid-tie without batteries and you could get a respectable amount of power feed back into the grid saving you $ for $5,000. Yeah sure its 5 grand but your going to pay it anyway one way or the other eventually. It could even be cheaper than that, the work is easy just do it yourself if you feel like it.

New tech is great, but solar already made sense for people a long time ago and only continues to do so. I would have done it sooner and even higher prices had I had the chance.

RE: love it
By semo on 1/27/2011 6:35:27 PM , Rating: 2
Many critics pick up on claims that solar panels reduce carbon dioxide emissions but don’t take into account that they still need to be manufactured. I think embodied CO2 describes what the problem is.

If you don’t care about that sort of thing then solar panels are great for sunny places but a bit of a luxury in colder countries.

RE: love it
By sleepeeg3 on 1/27/2011 9:59:51 PM , Rating: 5
5, huh? Mick thumbing up his own articles again?

$2.00/W? Too bad you have to take that 10-12% efficiency into account for thin-film technology. Woops. I see too many brainwashed ecocentrics who spend entirely too much time watching Jon Stewart and have no concept of how to calculate the actual cost of things. A pundit actually has knowledge, I suspect you bought into solar power based purely on emotions. For $5K, how much of your house is being powered by solar technology, how much was your monthly electric bill before, what square area was required and what state/region do you live in?

The simple fact is that if the technology were cost effective, we would all be using it.

RE: love it
By Jeffk464 on 1/27/2011 10:51:13 PM , Rating: 3
I went even cheaper than you with your solar. I just bought a new condo and put in the most efficient fridge, washer and dryer, and lighting I could find. Barely any added cost over the other stuff and I save big on electricity. I think the gas dryer and stove help a lot, but I guess that's cheating. :)

RE: love it
By Jeffk464 on 1/27/2011 10:57:45 PM , Rating: 2
FYI with my limited research Samsung seems to be beating everyone on appliance efficiency. They even put led light arrays in the fridge instead of the normal little light bulb.

RE: love it
By joe4324 on 1/28/2011 12:31:20 AM , Rating: 3
I am envious of your fridge without even needing to see it, unfortunately I wanted much higher efficiency that anything I could buy under $2k, so I made my own for a hundred dollars. You use a external thermostat and a relay to automatic turn on/off a chest freezer at whatever temp you want. Chest freezers are super efficient compared to their upright cousins. Of course its 35f all the time in my 3 season room so thats where my cold food storage is now. But in the other 8 months of the year the chest freezer *ahem* chest fridge only consumes between .2 and .35 Kilowatts a day. So 200-350 watts respectively.

It only needs to turn on for a few minutes hour-hour and a half. Sometimes less but I will be the first to admit its much more annoying bending over into a box, than opening a door and pulling stuff off of shelves. Its not a new some of the hard-core folks have been doing this for a long time now.

RE: love it
By Jeffk464 on 1/28/2011 2:40:20 PM , Rating: 3
You're pretty hard core.

RE: love it
By Jeffk464 on 1/28/2011 2:46:07 PM , Rating: 2
I'm just going to go for super efficiency on anything new that I buy. I have a V6 toyota tacoma that still has about 10 years life in it. Its my biggest energy hog, when its worn out I'm going to get something like a little civic coupe or something. Personally going solar and all that stuff doesn't make economic sense.

RE: love it
By joe4324 on 1/28/2011 12:48:11 AM , Rating: 4
I don't really understand most of this post or what you are driving at...

I'm off grid, solar IS my power source. If the sun doesn't shine for a few days its a backup generator (that does not run on 'much' fossil fuels) thats it. Pretty comfy too. My neighbors are even more extreme they have a 2kw windmill and only 800 watts of PV, and have only had to turn on a back up generator a few times in 8 years. They use less power though and only ran the generator to equalize the batteries and whats a few gallons of diesel a year as your total bill after RE deployment.

People can say whatever they want. Few people including myself would say that solar IS the answer. Its not! I live in a pretty poor area for solar power (north east) its cloudy all the time. And even here I know a half dozen houses that get along just fine on RE including mine. Solar isn't going to take over the baseline standy power grid. But it sure as hell can whittle away at it as it becomes more widely adopted.

Its going to be a crazy conglomeration of power sources taking us into tomorrow, we use waaaaay to much power for the best options to work. Maybe instead of attacking solar all the time we should attack energy sucking lifestyles that make it non-viable. But whatever I'm doing my part and I'm already set for the next several years before mean-time-to-failure kicks on something expensive I like having.

The cost effective argument only exists based on the premise that the best way to do anything is the cheapest way possible no questions asked.

RE: love it
By Kiffberet on 1/28/11, Rating: 0
RE: love it
By B3an on 1/28/2011 9:49:08 AM , Rating: 2
...But iPhone and Apple are sh*t. But then i like choice, getting my moneys worth, and things like Flash/viewing the whole web.
I have nothing against solar power though, it's great.

RE: love it
By Kurz on 1/28/2011 9:42:28 AM , Rating: 2
So Glad you can live so far from civilization that Grid tie in is not cost effective when compared to installing Solar. :)

If you were on the Grid you'd see that it doesn't make sense to install solar power quite yet.

Cost effectiveness is one of the most important part of economics. We live in scarcity to pretend otherwise will just lead to miss use of scarce resources that have 'other' uses.

RE: love it
By 1prophet on 1/30/2011 12:11:07 AM , Rating: 2

What I find ironic is that if someone has the means and wants to spend for example 50,000 dollars toward being as energy independent as possible people will criticize them on the cost effectiveness, but if they took that money and bought a flashy new sports car or boat (money pit) that argument seldom if ever comes up.

Energy independence is not always cost effective but sometimes desirable.

RE: love it
By Spuke on 1/28/2011 12:45:14 PM , Rating: 2
My neighbors are even more extreme they have a 2kw windmill and only 800 watts of PV, and have only had to turn on a back up generator a few times in 8 years.
I would LOVE to be off grid. And I live in a perfect place to do it BUT I don't want to stay here.

Maybe instead of attacking solar all the time we should attack energy sucking lifestyles that make it non-viable.
Didn't you just make a point that solar was viable just to turn it around and make a point that it isn't?

RE: love it
By SPOOFE on 1/29/2011 1:34:19 AM , Rating: 2
Didn't you just make a point that solar was viable just to turn it around and make a point that it isn't?

To me it seemed that he was pointing out that it CAN be viable in some situations, but there's reasons why it's not for all situations.

RE: love it
By Laereom on 1/28/2011 9:13:04 PM , Rating: 2

Energy-sucking lifestyles do not make solar non-viable.

In fact, I'd say solar and nuclear are more or less the only two viable ways for us to continue cheaply sucking energy away like the progress and prosperity oriented civilization that we are.

RE: love it
By VahnTitrio on 1/28/2011 10:33:24 AM , Rating: 2
It also depends on where you live. I think I calculated that if I were to cover my entire roof in solar panels, I could save about $50 per month on electricity. Though almost all of that would come in the summer. Those of us in northern parts of the US receive only 1/4 the total solar energy in the winter as we do in the summer. Not to mention winter is usually much cloudier.

Wind energy is a much better option for anyone living above ~35 degrees latitude.

RE: love it
By SPOOFE on 1/29/2011 1:36:57 AM , Rating: 2
Those of us in northern parts of the US receive only 1/4 the total solar energy in the winter as we do in the summer.

How much greater is the demand for electricity in the summer versus winter, if at all?

RE: love it
By Laereom on 1/28/2011 9:07:35 PM , Rating: 2
Oh, damn! At 10-12% efficiency, $2/watt is almost...oh, yeah, $2/watt. Distractors do help with FUD, though.

In southern, warmer places, especially those with expensive electricity (say, Texas and California, two of the largest and most populous states in the union), solar absolutely makes sense to the individual consumer, especially in a grid-tie system.

Furthermore, every single panel that is built gets used...with growth rates of 40+% for several years now.

The problem at this point is not cost, it is production capacity. Manufacturers can't just instantly materialize solar cells out of nothing. Currently, production capacity is at around 20GW/year. It's expected to continue to rise steadily as costs decrease with economies of scale and technology improvements. With a 10-year amortization schedule (which isn't long at all), cost is already below that of coal.

RE: love it
By kattanna on 1/28/2011 11:11:54 AM , Rating: 2
I see to many armchair-pundits on this site discrediting solar every chance they get.

i dont have a problem with solar per se, but i DO have a problem with me having to help YOU pay for it via those tax breaks you got to install them. if you didnt use any tax breaks, then kudos for you and enjoy your panels.

New tech is great, but solar already made sense for people a long time ago and only continues to do so

new tech is great, but it sounds like your in a more rural area. that neighbor of yours with the windmill tells me so, as that would not fly within most city limits, unfortunately IMO.

RE: love it
By Laereom on 1/28/2011 9:11:12 PM , Rating: 2
I'm a big proponent of solar, but I'm with you all the way. Screw subsidies. We don't want or need that crap.

Of course, the massive amount of subsidy fossil fuels get dwarfs renewables per watt-hour generated by far in most localities, but I'm willing to let that one go.

All I know for sure is
By YashBudini on 1/27/2011 7:49:33 PM , Rating: 1
I won't be buying any solar panels manufactured by scumbag BP.

RE: All I know for sure is
By delphinus100 on 1/27/2011 8:31:48 PM , Rating: 2
Isn't that behavior you want to encourage? Just don't buy their gas.

I mean, if a tobacco company came up with a cure for lung cancer, I will praise them...but I still won't start smoking.

RE: All I know for sure is
By YashBudini on 1/27/11, Rating: 0
RE: All I know for sure is
By DanNeely on 1/28/2011 6:51:11 AM , Rating: 3
You do realize that the only people you hurt with that sort of boycott are the small business owners who signed long term contracts with BP to use their logo. A drop in sales of gas at BP franchises just means that BP sells more of their gas to non-BP logo gas stations instead. It has zero impact on BP corporate.

RE: All I know for sure is
By invidious on 1/27/2011 8:34:44 PM , Rating: 5
If you think you are boycotting BP i suggest you do some research. You support them every day with or without your knowledge/consent.

If your opinions/feelings get in the way of you capitalizing on new technology then its nobody's loss except your own. I can assure you that BP will get continue to get rich and their customers will get their modern conveniences.

BP cut some corners and paid the price. They they got their comeuppance, and there are plenty of other companies who haven't gotten theirs. So if you are going to have irrational emotional resentment at least direct it as someone more appropriate.

RE: All I know for sure is
By Kiffberet on 1/28/2011 8:33:59 AM , Rating: 2
I bet if they were cheap enough, you'd be queuing round the block to get your hands on them...

Just like all the cheap Chinese goods everyone goes for, ignoring the fact that the environment and polution those companies produce every year is far worse than the BP spill.

60W per sq foot
By Shadowmaster625 on 1/28/2011 9:54:17 AM , Rating: 2
A typical monocrystalline silicon solar panel delivers roughly 10 watts per square foot at roughly 15% efficiency. The best we can do is 100% efficiency, which is 60 watts per sq foot. Realistically the best we can ever hope for is 40 watts per square foot. Which during average daylight will yield about 0.100 KWh per day on a typical US roof. (Double that in AZ or SoCal.) That is 2 KWh per day for an area of 20 sq feet. Clearly there is just not that much energy density in sunlight.

RE: 60W per sq foot
By Jaybus on 1/28/2011 11:24:49 AM , Rating: 2
Exactly. It can never supply enough energy, even at 100% efficiency. This is why so many eco-nuts advocate a return to the stone age. There simply is no suitable technology. Well, there is, but nuclear power is taboo. At some point, perhaps we will face reality and build more nuclear reactors. Until then, we will, like President Obama, embrace wishful thinking, hoping beyond reason that some scientist will discover cold fusion and the perfect battery.

RE: 60W per sq foot
By SPOOFE on 1/29/2011 2:22:08 AM , Rating: 2
At some point, perhaps we will face reality and build more nuclear reactors.

Step one: Repeal the law that makes reprocessing illegal. Hey, holy crap, suddenly all this nuclear waste becomes something useful again!

RE: 60W per sq foot
By bh192012 on 1/28/2011 6:37:22 PM , Rating: 2
You do realize how ridiculously small 20 sq feet is right? No, you can't live on 2 panels, duh.

You need to be thinking more like 200 sq ft @ 20 watts per sq ft. at the same cost as existing systems. Vs. tiered pricing here in Ca, an array like that would pay for itself in 7 (ish) years after rebates etc.

RE: 60W per sq foot
By Kurz on 1/29/2011 8:06:27 PM , Rating: 2
That is when I'll buy Solar Panels when it takes less than 10 years to recoup the costs.

RE: 60W per sq foot
By namechamps on 2/2/2011 7:54:38 PM , Rating: 2
Not sure where you got your bogus numbers.

Solar irradiance ~1kw per square meter (m2) at peak sunlight. Average Insolation (hours of peak sunlight equivelent) is about 6 globally and 4.5 in the US.

So on average each m2 in the US receives about 1642 annually.

Another way to look at it is the average American household uses about 25 kWh per day. With 4.5 hours of peak sunlight that requires roughly 5.5KW system.

At 100% efficiency that would require 5.5 m2 of roof space.
At 50% efficiency that would require 11 m2 of roof space.
At 25% efficiency that would require 22 m2 of roof space.
At 20% efficiency that would require 28 m2 of roof space.
At 15% efficiency that would require 38 m2 of roof space.

38 m2 is roughly 400ft.

Now how large is average single family residence roof in US. Know clue but the avg square footage is 1800ft. Assume two stories that is 900ft of roof. Say half of it faces south that is around 450ft.

Space is a non-issue. Cost and ROI% is the only issue affecting small scale (up to 30% of grid capacity) solar deployment. For deployment past 30% some effective means of storage becomes another issue.

Full spectrum
By Jeremy87 on 1/27/2011 4:42:50 PM , Rating: 2
The resulting LED admitted emitted three peaks in the visible range and a peak in the infrared range.

How is this a full spectrum? What about everything between these peaks?

RE: Full spectrum
By Jaybus on 1/28/2011 11:39:19 AM , Rating: 2
It is not a really sharp curve. The peak is just where it absorbs the most. The absorption falls off on either side of the peak. But if the peaks are close enough together, or the curves are not real steep, then the wavelengths in between still get absorbed, just not as much as at the peak.

Keep in mind that the solar radiation does not have the same intensity at all wavelengths either. It too has peaks. So if you design the cell to have its absorption peaks at the same wavelengths as the solar radiation, then you can have a fairly "full spectrum" device.

Good news
By NanoTube1 on 1/27/2011 6:36:05 PM , Rating: 2
Here in Israel we can make a serious use of these types of developments. We have a lot of sunny days with blue skys. We use solar power but not enough.

RE: Good news
By kattanna on 1/28/2011 11:18:23 AM , Rating: 2
if i am recalling right, isreal also requires the use of solar water heaters which is something i would like to see implemented more here in the US.

By Hulk on 1/27/2011 9:07:33 PM , Rating: 2
I read the whole article and we don't know the efficiency improvement!

That's what really matters right? How good and how much?

By sleepeeg3 on 1/27/2011 9:38:54 PM , Rating: 2
Details, details. Did you really expect concrete facts from climate change eccentrics?

Original article gives no information, but another company that is developing full spectrum, thin film solar cells is claiming a 3 fold increase (10-12% -> 35%):
If it could be produced at the same cost as current thin-film solar cell technology, that would reduce the cost to about twice as much as traditional power sources.

Not exactly true
By MrTeal on 1/27/2011 4:30:47 PM , Rating: 2
A significant fraction of visible light goes to waste at solar farms, as current generation cells generally can only take advantage of a very specific band of the spectrum.

This isn't exactly true. If you look at the concentrator cells used in many solar farms, even single junction GaAs ones, they can absorb higher energy (lower wavelength) photons than their bandgap energy, the extra energy goes to create heat. While they definitely aren't full spectrum, it's not really a very limited band. Triple junction cells can be even more efficient, Spectrolab would be more than happy to sell you some concentrator cells that absorb light from UV to IR.

Wait why?
By RugMuch on 1/27/11, Rating: -1
RE: Wait why?
By quiksilvr on 1/27/2011 4:12:55 PM , Rating: 5
Your mom has seen better break throughs in community college. lololololol

See how my comment contributed to the discussion?

"The Space Elevator will be built about 50 years after everyone stops laughing" -- Sir Arthur C. Clarke

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