I love this chart – it’s a simple way to get a look at where different solar technologies stand and how far they’ve come from the past. For instance check out the very best of the best – the purple triangles get up to 43% efficiencies! That’s pretty sweet – except that this chart leaves out one other important piece of information: cost. For instance, those purple triangles are triple-junction single crystal solar cells which are north of $1000 per square centimeter. Yes you read that right, per square centimeter. So – even with those high efficiencies, these babies won’t be sitting on top of your roof anytime soon because they’d probably cost more than your house…
There’s more interesting things to be gleaned from this chart – read on to find out!
Most Technologies are Flat
Another thing to note is how some technologies have seemed to reach their limits. CIGS solar cells (Cu(In,Ga)Se2) [solid green circles] saw a huge jump in the early 90’s but seem to have leveled off since then. Similarly dye sensitized cells [open orange circles] were first developed in 1991, saw a huge improvement in efficiency and have been stuck on 11% since then. Same with Amorphous Si [open green circles], single crystal Si [solid blue square], and multicrystalline Si [open blue square]. It appears that while we may be able to get a little bit more efficiencies out of these technologies, there isn’t much room for improvement if the chart is any indication.
However, one of my favorite types of solar cell technologies, organic photovoltaics (OPVs) [solid orange circles], has recently seen some surprising surges in efficiencies. Developed recently (only 10 years or so ago) they seemed to be stuck in the 3-4% efficiency range – interesting but not really good enough to be market ready. However, you can see that they’ve improved to 8.3% – and some reports are claiming devices of over 9%! The majority of that improvement has come within the last two years or so indicating that hopefully these technologies can soon top 10% efficiencies – a milestone long considered necessary for any technology to be considered as a viable option.
OPVs – or plastic solar cells as they are also called – have a number of advantages over the higher efficiency solar cells shown in the chart, such as silicon, CdTe, and multi-junction devices.
- Abundant Elements – In a previous post I pointed out the relative abundance of elements in the Earth’s crust. While many of the materials in the chart above get great efficiencies, they use rare and/or exotic elements such as indium, gallium, and tellurium. Unless we can find better supplies of these elements (unlikely given their scarcity) it will be difficult to scale up production of these types of solar technologies. However, the organic cells are typically only made up of carbon, hydrogen, oxygen, and sometimes sulfur. Take a look at where these elements fall in the Earth abundance chart and you’ll see that they are all within the green area of most abundant elements on the planet! With these elements readily available their availability will not limit the production of OPV devices.
- Easy Processing – OPVs are typically made of polymers instead of metals, and they can be manufactured from a solution – the active components can be printed out like ink from a printer. This is because the polymers can be dissolved in a solvent for processing. This is in contrast to the current expensive techniques required for silicon technologies. Silicon needs to be heated up, shaped into ingots, and carefully cooled to achieve the correct crystalline structure. This is important not just because printing and solution processing is cheaper, it also means that the technique can be scaled up to huge production quantities, which will be necessary if we wish to have wide-scale implementation of solar power.
- Flexible – Again, because they are not made of crystalline materials OPVs do not need to be kept rigid. This allows them to be used for a wider range of applications.
- Cheap – The combination of abundant elements and easy processing means that OPVs have the potential to be extremely cheap to produce.