Perovskite solar cells have only been on the scene for a few years and they’re already on track to catch up with — and surpass — silicon as the go-to material for the next generation of super efficient solar cells. In the latest development, a bi-national research team has figured out that perovskites have a unique ability to re-create their own photons and recycle them for an extra energy boost.
New Solar Cell Recycles Photons
For those of you new to the topic, perovskites are a class of synthetic crystalline minerals. The discovery of naturally occurring perovskite dates back to the 19th century, but it took researchers about 150 years to catch on to its high solar potential, and to realize that they could easily make their own perovskites in the lab.
Lead halide perovskite solar cells have emerged as the go-to technology, with researchers around the world charting an “exceptional” rise in conversion efficiency in just a few years of tinkering.
The key characteristics of lead halide solar cells include long charge carrier lifetimes and high emissions yields, leading researchers to wonder if the material is “recycling” photons.
The new study answers that question. It comes from a team at St John’s College at the University of Cambridge, partnering with the University of Oxford and Amsterdam’s FOM Institute AMOLF.
According to author Richard Friend, the recycling observation was conducted on a new solar cell created by co-author Luis Miguel Pazos Outón. The cell was the first demonstration of a back-contact solar cell using perovskite, and it had not been engineered specifically to demonstrate high energy production.
For their research, the team leveraged the fact that when light falls on perovskites, they emit light as well as absorb it.
They used a laser to measure photon activity within a nanoscale piece of lead-iodide perovskite, about 500 nanometers thick. As expected, they observed a high-energy light emission close to the laser point.
The surprise was that another high-energy light emission near the infrared end of the scale was appearing farther away. The farther-away emission was also accompanied by another emission consisting of lower-energy photons.
Those two types of farther-away emissions provided the team with enough evidence to conclude that the perovskite chip was “recycling” photons, combining them with incoming photons:
This single cell proved capable of transporting an electrical current more than 50 micrometers away from the contact point with the laser; a distance far greater than the researchers had predicted, and a direct result of multiple photon recycling events taking place within the sample.
The result, as described by co-author Outón, is to concentrate many charges in a small area, a quality that silicon and other materials “simply don’t have.” As Outón explains:
The low-energy component enables charges to be transported over a long distance, but the high-energy component could not exist unless photons were being recycled.
Lead-Free Perovskite Solar Cells — #thanksobama!
The findings provide researchers with a path forward to increasing the efficiency of lead-halide solar cells, and since lead has been in the news lately, it’s worth exploring the merits of introducing a new lead-based product into the market.
On a technology tour of Switzerland, CleanTechnica visited thin film solar cell pioneer Michael Graetzel, who proposed that without an alternative material for adding stability (that’s what the lead is there for) to perovskite solar cells, the technology could still be safely deployed at secure facilities where lifecycle issues associated with lead toxicity could be tightly controlled, and the potential for hazard practically eliminated.
As for non-secure facilities — for example, residential and commercial rooftops — that market might have to wait for a while.
Tin is being explored as one promising (and cheap) alternative material for perovskite solar cells, and the US Energy Department is funding research through the Obama Administration’s SunShot Initiative, so group hug for US taxpayers, #thanksobama, and stay tuned.
Image: “Depiction of photon recycling inside the crystalline structure of perovskite. The study shows that perovskite can reabsorb photons that are regenerated inside the material, a feature which could enable researchers to break through the efficiency limits of existing solar cells” by Criss Hohmann.