Actually, don’t forget perovskite. We’ve been fangirling over perovskite all year as a new pathway to low cost solar power, and we’re not going to give it the old heave-ho just because some new kid shows up. However, this thing about iron pyrite just sailed across our radar for the first time in five years so naturally we had to drop everything and run after it.
Before we get into the meat, let’s note that many roads lead to low cost solar power, including a raft of “soft costs” such as designing, permitting, and installation. Factors like these currently account for more than half the installed cost of solar power, but hard costs — namely, the solar cell itself — still play a critical role.
The Last Time We Checked Into Iron Pyrite…
Yep, it was about five years ago. In February of 2009 one of our colleagues here at CleanTechnica reported on some findings out of Lawrence Berkeley National Laboratory, indicating that the yellowy mineral iron pyrite (aka fool’s gold) could be a cheaper alternative to the silicon currently used in conventional solar cells.
The finding turned up in a Berkeley Lab study that identified 23 different semiconductors that had the potential to replace silicon in terms of efficiency, but half of that group was knocked out of the competition because of supply issues.
Of the remaining group, Berkeley Lab tapped iron pyrite as the champ, partly because it won out in terms of abundance.
Sure enough, a couple of years later the Department of Energy issued a $1.4 million grant to the University of California-Irvine to exploit this pathway to low cost solar power:
…to build a prototype solar cell made from nontoxic, inexpensive, and earth-abundant iron pyrite (FeS2), also known as Fool’s Gold, with an efficiency of 10% or greater. The research team is developing a stable p-n heterojunction using innovative solution-phase pyrite growth and defect passivation techniques. A pyrite-based device offers a clear pathway to meeting SunShot cost targets, 20% module efficiency, and terawatt scalability using a proven, manufacturable geometry that is suitable for rapid scale-up by a U.S. thin-film photovoltaic industrial partner.
Cool, right? That goal of 10 percent is pretty ambitious, considering that the best-case scenario to date is about three percent.
…And Now There’s This
So, while UC-Irvine has been banging away, the Energy Department has also provided support to a research team at the University of Wisconsin-Madison. That team has been looking at iron pyrite from a different angle and it made an interesting discovery, which it announced just yesterday.
According to the team, one advantage of iron pyrite is that it can still absorb sunlight efficiently even when fabricated into sheets 1,000 times thinner than silicon. However, it falls way short in the voltage department, which basically kills the whole deal.
Until now, researchers investigating the voltage flaw have focused their efforts on problems with the surface of iron pyrite crystals. The UW-Madison team dug under the skin to find this:
The internal problems, called “bulk defects,” occur when a sulfur atom is missing from its expected place in the crystal structure. These defects are intrinsic to the material properties of iron pyrite and are present even in ultra-pure crystals. Their presence in large numbers eventually leads to the lack of photovoltage for solar cells based on iron pyrite crystals.
Here’s some more detail about the methods used in the study, available online at the Journal of the American Chemical Society:
We utilized electrical transport, optical spectroscopy, surface photovoltage, photoelectrochemical measurements in aqueous and acetonitrile electrolytes, UV and X-ray photoelectron spectroscopy, and Kelvin force microscopy to characterize the bulk [bulk refers to internal] and surface defect states and their influence on the semiconducting properties and solar conversion efficiency of iron pyrite single crystals.
The team suggests that the precise nature of the bulk defects likely has to do with an absence of sulfur, leading to interruptions in the distribution of the charge among other problems.
Okay, so now what? The path to low cost solar power via iron pyrite appears to be a long row to hoe, but at least the UW-Madison findings shed some light in the tunnel, so keep your ear to the ground.
What About Perovskite?
Since we did bring up perovskite as another pathway to low cost solar, a couple of the developments we spotted this year include a “coal-killing” low cost solar cell from Northwestern University based on tin, and a diesel-killing perovskite solar cell from a University of Oxford spinoff called Oxford PV.
The problem with perovskite solar cells is that until recently, research was focused on perovskite – lead combinations, which churns up a whole mess of toxicity and lifecycle issues. The new research is aimed at working around that issue.
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