There are a couple of new perovskite solar cells breakthroughs this week which we think should be talking to one another. These solar cells are built around a perovskite structured compound, often with lead or tin halide as the light-harvesting active layer. This is the pioneering edge of solar, and perovskite is advancing rapidly with the promise of very low production costs.
One of the issues with any “disruptive” technology is how will it get off the ground? The Crystalline Silicon solar process has not only had a lot of money spent perfecting and improving it, but it has made companies a lot of money. When you reinvest a percentage of revenues into the minutiae of process detail, and that revenue is large, then the results are as close as you can get to perfect.
When a new technology comes along academics try to push it ahead, and academic or VC funding has to pick the perfect moment when it can find initially a niche and then later the mainstream, and begin to get supported by real revenues. Funding for a revolution cannot get close to the amount of funding for evolution, but somehow some inherent advantage of the new process eventually comes through and just enough money gets spent to see it into the mainstream. The question for perovskite solar cells is whether or not that moment is now.
The first announcement we want to bring to your attention is Power Roll, a new cell design from the UK company of the same name working with scientists from Sheffield University, published this week in the journal, Energy and Environmental Science. It is essentially a new physical architecture for solar based on a surface embossed with ‘micro-grooves’ which allow it to absorb more light.
The sides of the grooves are coated with electrical contacts, and the bottom is filled with a solar ink, to create a back contacted solar cell (see picture).
This removes many of the manufacturing process steps required by existing PV modules and when you couple this with the low cost of the perovskite material and its flexibility and light weight, this results in solar panels produced for 20% of the cost of the current market leader and under 10% of existing flexible solar panels. The end result should be cheaper electricity and accelerated roll-out.
Power Roll claims that a solar module using this approach will weigh only 4% of a conventional solar module of the same power and that it weighs less than 0.5 kg per square meter compared to 12 kg per square meter for existing silicon PV panels. This is particularly interesting for systems in front of the meter, where often the weight upon for instance a rooftop, has to be taken into account.
There are other benefits to the Power Roll architecture, reduced power losses due to shading or defects; removal of expensive transparent conductive oxides; simple and low-cost electrical interconnections; lower carbon footprint and the ability to tune electrical output to match user requirements.
At present this is about a panel which can only convert 7% of the power falling onto its surface. The team expects to beat this over time and approach the 21% or so that is a viable economic target to replace any solar cell.
Power Roll has recently produced mini demonstrators and is now focusing on scaling up the technology ready for commercialization. These panels use methylammonium lead iodide.
Meanwhile in the US at the National Renewable Energy Laboratory (NREL) a new compound has been discovered for producing a tandem perovskite solar cell which pushes the technology closer to maximum efficiency
Most perovskite research efforts focus on lead-based chemicals as our friends at Power Roll did. This is because it has a wide bandgap. The NREL idea is to have multiple layers each coated with a different chemical, which each absorb different bandwidths of spectrum. We’re not sure if Power Roll could find a process to lay this on its groove architecture, but it’s worth thinking about.
The reason this has not worked in the past is because low bandgap perovskites have traditionally had large energy losses. NREL has replaced lead with Tin in the perovskite structure and managed to achieve a 20.5% efficiency. But using Tin will create pin-holes and other defects and now NREL has found that by adding a specific concentration of guanidinium thiocyanate (GuaSCN) this process is largely defect free.
The results are detailed in the new paper, “Carrier lifetimes of >1μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells,” which appears in Science Magazine.
NREL believes that a tandem solar cell using this approach can reach the theoretical maximum efficiency of 30%.
The improved low-bandgap, single-junction solar cell with its 20.5% efficiency was then coupled with a conventional wide-bandgap perovskite cell. The researchers achieved a 25% efficient four-terminal and a 23.1% efficient two-terminal perovskite thin film tandem cell.
It may not be possible to combine the two idea we have talked about in this article, but we can see that there is genuine momentum behind the development of mass-produced and cheap perovskite PV devices, and it is only a matter of time before one hits the generalized market place.