November 7th, 2013

Cambridge physics professor David Mackay is scientific adviser to the UK Department of Energy and Climate Change. In his online best-nonseller Sustainable Energy – without the hot air, page 39, he thinks we are already at the practical limit of PV efficiency:

Typical solar panels have an efficiency of about 10%; expensive ones perform at 20%. (Fundamental physical laws limit the efficiency of photovoltaic systems to at best 60% with perfect concentrating mirrors or lenses, and 45% without concentration. A mass-produced [photovoltaic] device with efficiency greater than 30% would be quite remarkable.)

Accordingly his scenarios assume 20% panel efficiency for residential and 10% for utility. That’s till 2050. In 100% renewable scenarios, the volume of solar is constrained by total roof area and competing land uses. So if you could plug in a higher efficiency, the limits shift too.

The DECC online scenario calculator uses 10% PV efficiency throughout. This is plain wrong. In SRoeCo Solar’s online database comparing 12,622(!) panels, the median panel at position 6,311 has an efficiency of 14.50%, and only the bottom 3% are below 10%. The best panel is at 21.57%.

Here’s why I don’t believe Mackay’s 20% (or SunPower’s 21.57%) is a limit.

There’s a huge amount of research going on into solar PV, in many different avenues. I looked for innovations which:

  • enhance efficiency, not simply reduce costs,
  • look reasonably close to marketable products,
  • use cheap not exotic materials and straightforward methods of fabrication,
  • build on silicon, the dominant technology today.

Why the last condition? The steep learning curve for solar PV – 22% cost reduction for each doubling in volume – makes the course of the industry strongly path-dependent. Polymer cells, say, would have to beat today’s silicon module costs (60-70c$/watt) before getting any market share at all. Nobody’s going to stake polymer PV the €120 bn FIT liability it took Germany to get silicon down to this price. It would need to emerge from the lab at 50c/w, which is extraordinarily unlikely. It’s this mechanism that has kept concentrating PV a market curiosity and a financial black hole.

Here are four horses that look good to me.

1. Light-trapping channels

microgrroove Californian company Solar 3D has a scheme to gouge precisely shaped channels or pits in the surface of silicon panels, to trap more light, and light from more oblique angles. They claim to achieve 25.47% internal efficiency, and a much bigger gain in daily power output because of the wide-angle performance. The grooves can be made by etching, a standard method in the semiconductor industry.

This photo is not from the company, which is keeping its precise design a secret.





2. Aluminium nanodots
nanostudsNicholas Hylton of Imperial College London (et al) have a simpler idea to achieve the same result: deposit a Lego-style pattern of tiny 100nm aluminium dots on the top of the silicon or other PV wafer. He started with gold, but aluminium in fact works better. The dots bend light by quantum effects, trapping more of it in the semiconductor where it can give up its energy. Hylton claims a 22% gain in efficiency, say from 20% to 25%, but gives no information on the possible output gain from wider trapping angles.












1 and 2 seem to be alternatives. But I can’t see why they should not be combined with the others.

3. Perovskite thin-film

Perovskites are a class of crystals similar to the naturally occurring calcium-titanium minerals first found in the Urals in 1839. Michael Grätzel at Lausanne has championed the use of synthetic perovskites for PV, and has reached 15% efficiency.  His perovskites “have the formula (CH3NH3)PbX3 with X being iodine, bromine or chlorine”; they are deposited on a thin layer of titanium dioxide, the white in white paint. None of these elements are scarce (the lead is a worry). Another British team at Oxford – Henry Snaith and Michael Johnston, a fellow of my old college – have managed to get the same efficiency without any fancy nanostructures, merely with vapour deposition – another standard semiconductor technology. This brings perovskite cells into the realm of potential mass production.

Perovskite-solar15% is amazing for a new laboratory technology; it’s much higher than the 12% or so of production thin-film modules, which have benefited from years of improvement. By itself, it would still have a hard time catching up with silicon, and thin-film is overall slowly losing market share. However, perovskite films have a trump card: they are transparent. This means that in principle they could be combined with silicon.

Since they absorb light in a different part of the electromagnetic spectrum to silicon, the two materials might be used together in so-called tandem cells in which a silicon device would be placed underneath a perovskite one. “Here, the perovskite top cell would absorb higher-energy photons and the lower-band-gap silicon the lower-energy ones,” explains Johnston.

He did not guess an efficiency for the tandem cell, but his cannot be the only team trying to make them today to find out.

Two-junction cells of this type escape the Shockley–Queisser theoretical limit of 33.7% for an ideal single-junction p-n cell. They are not fantasy. Panasonic and others already make panels with a layer of amorphous silicon on top of monocrystalline silicon. SFIK these are not true two-junction cells, but they show that multilayer fabrication is commercially as well as technically possible. Johann Harter, the COO of the very big Austrian solar developer Activ Solar is “a strong believer in thin-film on silicon as a potentially strong solar option in the future”.

4. Graphene conducting sheets
graphene structureAnd just in time for multilayer cells come transparent graphene conducting sheets to take away the electrons knocked loose. Unlike perovskites, graphene is easy to grasp: simply a flat hexagonal mesh of carbon atoms, the 2D version of buckyballs and nanotubes. Marc Gluba and Norbert Nickel in Berlin

grew graphene on a thin copper sheet, next transferred it to a glass substrate, and finally coated it with a thin film of silicon. … Even though the morphology of the top layer changed completely as a result of being heated to a temperature of several hundred degrees C, the graphene is still detectable.

Graphene is a fantastically good conductor. Their next problem is soldering bigger conductors to something one atom thick …


horse_teeth2Can you combine these? My WimTech® module would have the nanolego – it looks cheaper to fabricate than the grooves -, then the perovskite cell, then the graphene conductor to take away the juice, then a conventional 20% monocrystalline cell. Would it work? Could I get 35% efficiency? Would it be reliable? Can I keep fabrication costs in the same range per watt as plain silicon? (I can charge a small premium per watt, as with mono silicon today, because higher efficiency lowers BOS costs per watt.) Dunno, dunno, dunno, dunno. But there are many dumber ways of betting €10m on a really high-payoff idea.

Now you should not buy any horse from me, however shapely its teeth. There may be hidden problems with these ideas that will prevent their reaching mass deployment; and if not, they may be overtaken by swifter runners. I’m not asking you to believe that these specific innovations will bring more efficient panels.

The argument is that if I, an amateur searching using my patented method of filter-feeding on a few cleantech blogs, can easily identify four promising routes to overcoming Mackay’s limit, then there must be many more out there. The professional and economic incentives to find and deploy solutions are very, very strong: Nobel Prizes and fortunes. So they are much more likely to happen than not.

My predictions for 2023:

1. Technical progress in solar cells will be sufficient to maintain the learning curve in PV module cost, bringing it to ca. 25c$/w. (The production costs of the leading Chinese manufacturers are already down to 50c$/w today.)

2. The efficiency of good-quality mass-produced (not niche) panels will go up from <20% to >30%. This will help the (slower) reduction in BOS costs, most of which are proportional to panel area not power.


A couple of closing data points.

The net installed cost per watt of residential solar PV in Germany has fallen below $2/watt (€1.450/w).

An updated chart of global solar installations, now back on their long-term trend curve of 44% growth per annum (data sources EPIA, pdf Global Market Outlook for Photovoltaics 2013-2017, page 13, Figure/table 1, and trade forecasts. Spreadsheet.)

Solar PV installation trend

13 Responses to “Limits to solar efficiency?”

  1. Rob in CT says:

    If I recall correctly, the medium-grade panels on my roof right now are ~16% or so. They’re Siliken 305-watt panels… looking online it appears that 15.7% is the top of the range for them. We considered SunPower 20% efficient panels (which at the time I think was their max), but the additional expense didn’t seem worth it.

    Anyway, I’d challenge the “typical panels are 10% efficient” given that we hardly went with the bargain-basement option and are at ~15%.

    • Rob in CT says:

      Though to be fair, he uses 20% for residential, which is higher than ours.

      Eh, who knows? The efficiency of the panels has been creeping up, up, up. I suppose it’s possible to hit a wall, but I’ll believe that when I see it actually happen.

      • James Wimberley says:

        20% is a high figure for today. But MacKay’s and DECC’s purpose is to map scenarios to a sustainable future for the UK out to 2050. The crucial assumption isn’t the starting point, it’s the expected rate of improvement. Near-zero is not credible.

        • Rob in CT says:


          Also, I’d figure that utilities would have higher real-life efficiency than many residential setups. My panels sit on my roof. They’re not often in the perfect alignment to the sun for max efficiency. A utility (or commercial installation) might have a tracker array that squeezes more from the panels. Maybe he accounts for this, I dunno.

          Heck, my residential installer tried to talk me into a tracker array. For a couple of reasons, I didn’t go that route, but I’ve seen several other tracker units go up in the area. The owners are getting more from their panels than I’m getting from mine.

      • MobiusKlein says:

        why would utilities be satisfied at 10% efficiency? Are the 20% panels more than twice the cost, now and forever more? Even in a world where the maximum efficiency is capped at 25%, I’d envision the lower end panels getting closer to the peak panels.

        • Barry says:

          ” Are the 20% panels more than twice the cost, now and forever more? ”

          Given installation costs, even twice the purchase price for twice the power would be a good thing to do.

        • James Wimberley says:

          First Solar, the only really big company still in the thin-film business, makes a good living in the utility market. The average efficiency of its panels is only 13.3%, but its cost per watt is competitive with silicon. In parts of the world where land is cheap like Saudi Arabia that’s all the customer is interested in. Japanese and Anerican householders may be more concerned with maximum power from a limited roof area, so that’s where SunPower sells its premium 21% monocrystalline modules. The Pentagon and NASA will pay silly prices for 35% triple-junction cells for satellites and SEALs, the equivalent of $500 brass hammers. IMHO there will always be a spread of efficiencies selling into different customer tradeoffs, but it doesn’t matter. My question was whether the mean will rise, which does.

  2. Ken Doran says:

    An interesting and useful post. I would be interested in seeing something similar on the state of the developing art in battery technology/energy storage generally. This has always seemed to me to be the laggard of the industrial revolution, and progress continues to be less rapid than one would hope. Many important technologies will work better sooner with improved energy storage, including vehicles, but none more so than solar energy, which uses the most pervasive but most decisively intermittent source there is.

    • James Wimberley says:

      I’ll keep an eye open. But don’t hold your breath. It’s quite hard for an amateur to get any sort of handle on a field like this. There’s a flood of boosterish press releases; on the other hand, the comparative surveys tend to be late and conservative.

      Remember that there is a perfectly good and well-established technology for large-scale grid storage: pumped hydro. Japan already has 25GW, more than the US total of 21GW, and ten times Britain’s measly 2.5GW. So the storage need is for batteries closer to the consumer. I’m not worried that this nut won’t be cracked, as electric vehicles have reached a market scale where the quite capable car industry has strong incentives for steady improvement. That’s without the good bet by Chu to give priority to battery research.

      Good car batteries will carry over to cheaper home storage. This will be fun to watch, as self-consumption will rise to the point where many homeowners and businesses will be independent of the grid on many days of the year. The utilities hate net metering now, batteries are their nightmare.

  3. Maynard Handley says:

    I’m sorry, James, but this seems like the exact sort of pettifogging that is engaged in when people encounter more or less undeniable results that they don’t like. (cf How many people “died” in Iraq? or What will be the EXACT consequences of global climate change?)

    What difference does it make? If 20% efficiency is infeasible, you seriously think something IMPORTANT changes even if we assume a doubling of that limit, to 40%?
    Mackay’s point is ultimately that solar energy is very very very diffuse. If you want to capture it through PV to create large amounts of energy (ie a wedge) it’s going to imply SERIOUS modification of the environment we live in. And throwing in prices doesn’t change that fact.

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