Solar researchers have been going nuts over perovskites, a class of synthetic crystals that could far surpass conventional silicon solar cells with lower costs and higher efficiency. As a result of all the attention, perovskite solar cell efficiency has been zooming upwards, and two new findings from Oak Ridge National Laboratory and Stanford University could push things along even further.
As for how fast things are zooming along, in 2006 the early attempts at perovskite solar cells clocked a conversion efficiency of 2-3 percent. By 2015, that figure was up to 20 percent.
Perovskite crystals are based on the structure of the naturally occurring mineral perovskite, and lately attention has been focused on a group called organometallic halide perovskites (organometallics combine carbon and a metal, and halides are compounds of a halogen and another element).
Though much has been accomplished in terms of efficiency in just a few years, the Energy Department has targeted a number of key challenges for organometallic halide perovskite solar cells including stability (they don’t like humidity), materials toxicity (namely, lead), and kinks in the manufacturing process.
The new Oak Ridge perovskite solar cell research digs into the nitty-gritty of the kinetic activity that occurs when organometallic halide perovskite crystals are synthesized. With a better understanding of the process in hand, researchers have a pathway for creating a crystalline form that maximizes solar conversion efficiency, and that can be sprayed onto a thin film for high volume, low cost manufacturing.
Part of the problem is that there seems to be a lot of random kinetic activity. The focus seems to be halogen ions, which bounce around and “jockey” for position as the crystal grows, and the result is a negative impact on solar conversion efficiency.
The Oak Ridge researchers used X-ray diffraction to take a real-time look at what happens when you expose a thin lead-iodide film to a mixed-halide vapor, then used ion mass spectrometry to gather in the data. The results indicate that a kind of atomic-level teamwork goes on during crystal growth:
…while bromine, chlorine and iodine ions facilitate growth in a developing organometallic perovskite structure, only iodine gets a spot in the final crystal. However, though they are left out of the final structure, the molecules build “team morale” as they help promote overall crystal growth.
To extend Oak Ridge’s sports metaphor into opera, now that the prima donna has been identified, the staging and choreography can be more precisely tuned to maximize solar conversion efficiency. The new insights will also help researchers engineer crystals that lend themselves to spray-on manufacturing for thin film solar cells.
As for the lead issue, CleanTechnica has been keeping an eye on that issue and we had a chance to speak with leading thin film solar innovator Michael Graetzel on that topic last summer. In his view, lead-based perovskite solar cells could find their way into markets where security and supply chain oversight effectively eliminate environmental concerns. That would probably mean limited usage for small scale distributed solar, though secure facilities could be candidates.
Do Squeeze The Perovskite
Turning now to the new Stanford solar cell research, it seems that the research team has been inspired by Mr Whipple. They decided that it would be interesting to see what effect pressure would have on the solar conversion efficiency of perovskites.
They created pressure on their perovskite sample using a tiny anvil-type device with two diamonds at either end:
The results were visible. One sample, which is normally orange, turned lighter in color under compression, an indication that the perovskite was absorbing higher-energy light waves. But as the pressure increased, the sample darkened, indicating that lower-energy light was also being absorbed.
Okay, so incorporating tiny diamond studded anvils into your solar cells is not going to work in the open market. However, the research team notes that compression does not necessarily have to be mechanical. Now that this “tuning knob” has been identified, the hunt is on for chemicals that can do the trick as well.
More Trouble For Fossil Fuels
Even with hot competition from natural gas, solar costs have been dropping rapidly and the solar market is already exploding in the US. Given the recent trajectory of solar research (here’s another example), it’s clear that solar costs will continue to plummet, while the beneficial public policies that have supported low-cost fossil fuels are gradually being rolled up.
As for the issue of pumping taxpayer dollars into energy research, that’s been settled public policy in the US for generations. For example, a recent Deloitte study has charted the extent to which the current US fossil energy boom is the consequence of innovations based on Energy Department patents.
Images: top via Oak Ridge National Laboratory by Jill Hemman, bottom via Stanford University by Adam Jaffe and Yu Lin/Stanford & SLAC.