Clean Power Empa develops low cost thin film solar cell.

Published on August 16th, 2013 | by Tina Casey


How Low Can Solar Go? Check Out Empa’s New Thin Film Breakthrough

August 16th, 2013 by  

What a difference 34 years can make. When the Carter Administration installed solar panels at the White House back in 1979, photovoltaic cells were space age technology that most households could not afford, aside from the rare DIY-er. Now the price of solar power has been sinking like a stone, thanks partly to the introduction of low cost materials and inexpensive thin film fabrication methods. Switzerland’s Empa, the Swiss Federal Laboratories for Materials Science and Technology, has just come out with a low cost thin film solar cell breakthrough that demonstrates both in the form of a new high efficiency copper-doped cadmium telluride (CdTe) solar cell.

Low Cost Materials For High Efficiency Solar Cells

The new Empa solar cell boasts an efficiency of 11.5 percent, which might not sound like a big deal compared to last year’s announcement of a 44 percent efficiency mark by the company Solar Junction, but we’re talking about two distinctive technologies. The key takeaway is that today there are multiple paths to affordable solar cells. One of them is finding the most efficient way to collect and convert solar energy, another is finding the cheapest way to do it, and a third way is to find a balance between the two.

Empa develops low cost thin film solar cell.

Low cost solar cell (cropped) courtesy of Empa.

For its solar cell, the Empa team has set a near-term goal of achieving 15 percent efficiency and they’re looking at the potential for 20 percent or even greater, so the 11.5 percent rate is just the beginning.

They started with CdTe, which is already the least expensive of its kind in terms of production methods, and they focused on one obstacle to lowering the cost even farther. The problem with CdTe is that conventional production methods involve “growing” cells on a rigid glass sheet, and using an expensive transparent foil to enable sunlight to pass onto the CdTe layer.

The solution is to flip the cell around and have sunlight come in through the CdTe side, which opens up a whole new field of possibilities for flexible, low cost materials, including metal foil.

However, that left another obstacle. Where the record for CdTe solar cells on glass has reached 19.6 percent in the lab, the previous high for metal foil was less than eight percent.

The answer to that particular problem is to tweak or “dope” the semiconductor layer with a relatively cheap metal like copper, but that opens up another obstacle. Ayodha Nath Tiwari, head of Empa’s laboratory for Thin Films and Photovoltaics, explains:

“People have tried to dope CdTe cells in substrate configuration before but failed time and again.”

The basic problem is that CdTe is “notoriously hard to dope,” mainly because too much extra metal is almost as bad as adding too little, but it seems that nanoscale advances in lab technology saved the day. The team achieved precise control over the amount of copper added to the CdTe layer by using a high-vacuum evaporation process followed by a penetrative heat process, which they fine tuned to atomic level proportions.

The result was a monolayer of copper atoms, which enabled the rate of 11.5 efficiency on metal foils. When the team compared the same technique to CdTe on glass, their best value was a comparable 13.6 percent.

Another Path To Low Cost Solar Power

That brings us to yet another pathway to mainstreaming solar power. While glass-based solar cells have higher efficiency, the flexibility of thin film solar cells opens up a whole new range of applications, enabling the introduction of solar power where conventional glass panels are unfeasible.

That means, for example, integrating transparent thin film solar cells into window glass, so who knows, maybe some day in addition to solar panels at the White House you’ll have solar windows for every office in the U.S. House of Representatives and the Senate, too.

With that in mind, keep your eye on the transparent thin film solar cell under development by the company New Energy Technologies in partnership with the National Renewable Energy Laboratory (wait for it…hey, we built this!).

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About the Author

specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.

  • eject

    that sort of really cheap throw away PV is imho more laid out to be used instead of batteries in small devices like remote controls. It could also replace the need for stand by power for many appliances.

  • mds

    Ms. Casey,
    I like your articles. You pick up on neat renewable stuff and have some interesting insights to offer. I do have a nit-pick here. The title says “How low can solar go?”, but there is no information on EMPA’s projected PV panel production cost or price on the market. After all what does “lower cost” mean? As my father used to say: “everything in this life is relative.” Lower than what? No offense intended, just a nit-pick.

  • JamesWimberley

    Sharp sell solar windows already using conventional silicon technology. Don´t swallow the hype that thin films are the only route to low costs. What´s happening today is that they are being squeezed out. Flexibility is just a niche.

  • JamesWimberley

    Jimy Carter´s solar panels were thermal water heaters, not PV.

    • Bob_Wallace

      Correct. And I suspect few people realize it was a solar water heater, not PV.

      Greentech Media has an interesting piece about the WH and solar. PV was mounted on one of the White House buildings during G W Bush’s term.

      “Then, nine months after the start of the W. administration, Strong
      visited the site and spent the entire day with the White House
      architect. They looked at all the potential siting opportunities,
      excepting the main mansion, which is “covered with spook stuff.” Strong
      ended up helping design and install a 10-kilowatt photovoltaic system
      and two thermal solar systems within the compound. All the inverters
      had to go to the Secret Service warehouse for clearance, presumably for
      inspection for listening devices and explosives.”

  • anti_banker

    Couldn’t electroplating in certain circumstances also achieve: “precise control over the amount of copper added to the CdTe layer” ?

  • wattleberry

    Nobody has noticed the most momentous breakthrough here-something cheap coming from Switzerland.

    • Bob_Wallace

      Cheap enough to overcome the costs of mounting and installing lower efficiency cells? Cell price is a bit over half of the module price and module price is well less than half of total system cost.

      • wattleberry

        Thanks, Bob. Should have known it was too good to be true! I got carried away by the headline.

      • anti_banker

        For certain applications it may still be worth it. But will it lead to mass produced cells that of cheaper than the similar CdTe cells of First Solar or are the more efficient than those of First Solar. (can’t remember FS’s efficiency). Would be nice if the author included a comparison like this.

        • Bob_Wallace

          FS has demonstrated 16.2% efficiency and has set a target of 15% to 16.2% for its panels in 2015.

          Here’s what they state on their web site…

          World-record holder for CdTe thin film module (14.4%) and cell (18.7%) efficiency

          Manufacturing cost leader at $.68/watt (Q4 2012)

        • mds

 – July 2013
          “First Solar Advances 162 MW of Unsubsidized Solar in Chile”
          “First Solar shipped 370 megawatts in Q1, down sequentially and year-to-year, amounting to a 75 percent factory utilization. Best cost was $0.62 per watt, but the
          company average was $0.69 per watt. The best line is running at 13.3 percent efficiency, and the company claimed that the best line would be at 14 percent by year-end after incorporating improved back-contact technology.” I don’t count claims like “14 percent by year-end” or 15% by 2015 until they happen.

          CdTe and CIGS were the savior PV technologies when purified silicon prices were high five years ago, or so. It is now hard to see how they will best the cost-efficiency performance combination of silicon. Silicon PV panels are now down to $0.70/W and are reportedly headed to $0.50/W. Some Silicon PV panels are already at 20% efficiency. If silicon PV companies can achieve $0.50/W and 20% efficiency by 2015, then I’m not sure how CdTe or CIGS PV companies will compete.

          Here is another link on FSLR that provides a different view:
 – August 2013
          “First Solar to manufacture new crystalline silicon line from end of 2014”

          “First Solar will start a 100MW manufacturing line for crystalline silicon cells for the residential distributed
          market from the end of next year with production scaling from 2015, it was revealed yesterday.” “In May this year, the leader in thin film CdTe acquired the lower-cost, high efficiency crystalline silicon startup founded in 2009 by Denis de Ceuster after 12 years at Sunpower.” “By
          2017, distributed residential and commercial generation is expected to account for more than half of the market [see slide 1], a lucrative, growing market that is more suited to higher efficiency crystalline technologies than thin film which typically require larger installation areas.”
          ” ‘Obviously the motivation is very clear,’ said de Ceuster, who is now director of research and development of
          c-Si at First Solar. ‘First Solar’s CdTe technology has been
          dominating the market for utility scale and large commercial applications. That technology isn’t suitable for residential or small commercial – that’s half the global market.’ “
          “Tetrasun has achieved a third party confirmed efficiency of 21.4% [see slide 2] with its n-type silicon cells and aims to achieve these efficiencies again when the 100MW capacity production starts in the last quarter of 2014. Metal
          fingers and busbars are copper-plated reducing metallisation costs to $0.01/Wp as opposed to costly silver paste.”

          I think they are ramping their newly acquired low-cost and high-efficiency silicon PV production too slowly, like a big company afraid to gamble on a new tech, but what do I know? They may be right to be cautious. One thing I do know is solar PV is getting so cheap, and that is already set to continue for a few more years at a minimum, so demand is going to rebound in a big way and we will actually go into an over-demand, under-supply market for solar PV. (The market expands exponentially with a linear drop in price.) Maybe all of them, CdTe, CIGS, and Silicon PV, will be selling out by 2015? PV panels are cheap now. It is more about bringing the soft costs, installation and other BOS costs, down now. High efficiency is a factor, but….

  • J_JamesM

    Hmm, this is important. I know a certain Canadian company called Solar Ship that will be thrilled to hear this.

    They are testing a hybrid cargo airship that flies partially or fully on solar power. They only have 220 square meters on top of the ship with which to put panels which are used to recharge the aircraft’s lithium-ion battery banks. Obviously, they need the cells to be lightwieght, and also flexible because they are being affixed to the top of an inflatable balloon. Their electric wing motors are both 35 kW, plus the main gasoline engine.

    If their current off-the-shelf cells are about 6% or 8% efficient, then how much more power could a 15% efficient cell provide? Currently, the ship’s cargo capacity and speed is hampered if it wants to fly on solar power alone. Their largest planned ship can carry 30 tons of cargo at 120 kph for 6,000 km on hybrid power, but on pure solar it is restricted to 12 tons at 85 kph, albeit with an unlimited range.

    • Bob_Wallace

      The top of the aircraft does not have to be flexible. The business part on the bottom isn’t.

      • J_JamesM

        Actually, it does, for a variety to reasons.

        The first, and most obvious, is weight. Off-the-shelf rigid panels tend to be very heavy, which reduces the aircraft’s payload.

        The second has to deal with a series of choices one has to make in the aircraft’s design- whether to make it non-rigid, like a blimp, or rigid, like a Zeppelin. Now, being an airplane-hybrid and therefore much more compact than either of those, the difference in cost, structural weight and complexity is heightened.

        A rigid structure on something so small would carry a heavy weight penalty in and of itself. The volume for a given size would be smaller. Balancing heat distribution and gas expansion would be made more complicated.

        All of this translates into a much larger aircraft relative to the payload, and greatly increased costs. By comparison, a simple inflatable hull is much cheaper, lighter, and easier to build, at least at such a “small” scale. This goes well with flexible thin-film panels, because they could lie flush with the hull, without the need for structural bracing, and bend with it if it moves.

        • Bob_Wallace

          You are assuming that one would mount “off-the-shelf” panels to a rigid structure. There is no reason to make that assumption.

          Think about a top made of something like carbon-fiber, as light or lighter than the flexible material it would replace. Bond the silicon and substrate to the carbon-fiber and cover it with a protective plastic material.

          • J_JamesM

            I think you misunderstand the goal of the design. Their main priority is not to be innovative, technologically advanced, or even “green.” They’re primarily concerned with making their ships cost extremely little to build and operate.

            That’s why they chose a very simple, very small lifting-body delta shape for their hull. It’s why they bought off-the-shelf thin film panels. It’s why they modified existing bushplanes to make the aircraft’s fuselage. The solar panels are there out of simple convenience; they greatly extend the aircraft’s range, provide power where infrastructure is lacking, and make it so that it doesn’t have to buy extremely expensive, dangerously tainted fuel from third-world locations.

            Their goal is to make an aircraft that costs much less than a plane, carries far more cargo, travels farther, and is not limited by landing sites. They can’t outcompete planes and trucks if they commit to fussing around with expensive, complicated composite materials and space-age designs. They want it simple, rugged, and cheap.

    • anti_banker

      Interesting idea!!! With an inflatable balloon, you don’t need any power in order to gain the lift that you need. (unlike with regular planes, or helicopters). You only need power (electrical motor/fan, or chemical ICE) to go forwards, or backwards. Thin film solar would be ideal if cheap enough.

      • J_JamesM

        Not exactly. The aircraft is still gets over 50% of its lift from aerodynamics (the wind passing over the delta-shaped hull), so if you shut off the engines and even if you take everything off, it’ll still just stay on the ground.

        The helium lift is there in order to enhance the bushplane’s performance. The top surface is the only thing big enough to hold the amount of solar cells needed to drastically increase the bushplane’s range, the dual sources of lift and reduced structural weight increases payload about five-fold, and the helium lift also reduces the aircraft’s stall speed to practically nothing- giving it terrific short takeoff and landing (STOL) performance, which is coveted by bushplanes. Really, the balloon was only a cheap, pragmatic solution.

  • Ivor O’Connor

    When exposed to sunlight thin film plastic has a very short life. I’d be surprised to see these products last 5 decades or even 5 years out in the hot elements.

    • mds

      Good point. I’m thinking you can protect plastics from UV degradation, or use plastics that are more resistant to this. Maybe not. Maybe the answer is
      There is more of a problem than this with flexible PV in general. Let’s say you successfully manufacture roll-to-roll thin-film PV with 15% efficiency at $0.30/W. (The goal Nanosolar promised, but never achieved with CIGS.) Then what? The big market, the most cost effective, is distributed end-of-grid solar for businesses and homes, residential. How do you mount those flexible PV sheets onto roofs? There have been several CIGS companies with flexible PV that failed to thrive. I don’t think they’ve thought it all through for successful application in the field. Same here. Hand waving. A system engineering solution is needed and maybe there isn’t one for flexible thin-film …or maybe there is. I haven’t seen this yet.
      Maybe the answer is roll-to-roll and then onto sheets of solar glass, for very low-cost panel production. Then even at $0.30/W you’d better be pushing for 20% efficiency soon, because that is were silicon PV is going …and maybe they can get down to $0.30/W too.
      It remains a very interesting contest. The winners will own a huge market.

      • shinyhalo

        The solution I am waiting for is simply an affordable adhesive backed thin film about 16 inches wide that I can roll out on my aluminum roof. The peaks that join the sheets of roofing further prevent wind lifting. It would be perfect. Wire them at the apex of the roof via the vent so the wires are hidden too.

        • Bob_Wallace

          There was a company that was making stick on thin film solar for raised seam metal roofs but they went out of business.

          I’ve been watching for them as well. But steel roof.

  • David Fuchs

    Any guess on the cost per kWh?

    • anti_banker

      I guess we won’t know till it’s being mass produced. (it’s only a prototype).

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