Scientists Finally Uncovered a Major Efficiency Flaw Holding Back Solar Cells
Perovskite has a lot to offer in our search for a cheap, efficient way to harvest solar energy. By dusting organic molecules, these crystalline structures were able to convert more than a quarter of the light falling on them into electricity.
In theory, perovskite crystals made with the right mix of materials could exceed that limit by over 30 percent and outperform silicon-based solar cells (currently the most common solar panel technology), at a much lower cost. All is well on paper, but in reality something has held the technology back.
Combine calcium, titanium and oxygen under the right conditions and you create repeating cages of molecules that look like bundles of boxes joined together at their corners.
Regardless of the elements involved, this special crystal pattern is called the perovskite structure. Make one out of lead iodide, throw in an organic compound like methylammonium for a positive charge, sprinkle some sunshine on it, and you’re on your way to creating a current.
In order to achieve efficiencies in excess of 25 percent with this energy conversion, the engineers quickly learned that it is worth making sure that there is enough iodide, and apparently making sure that any defects in the perovskite crystal lattice are well and truly filled .
However, this assumption was never fully tested, so researchers at the University of California at Santa Barbara, USA, went back to first principles to see what was really going on.
The team used state-of-the-art computing to analyze the quantum behavior that affects the electrons as they migrate through the hybrid mixture of organic molecules and lead iodide structures.
It turned out that the bug in the system wasn’t where anyone expected it to be.
Instead of a flaw in the perovskite cages, it was the organic component – previously thought of as an unbreakable entity – that exhibited a rather frustrating weakness. It turns out that their hydrogen can break off instantly.
“Methylammonium lead iodide is the prototypical hybrid perovskite,” says senior researcher and materials engineer Xie Zhang.
“We found that it is surprisingly easy to break one of the bonds and remove a hydrogen atom on the methylammonium molecule.”
This hydrogen void creates a rather inconvenient pothole in the electric highway, hindering the current that is generated when sunlight releases electrons from the surrounding perovskite structure.
“If these charges get stuck empty, they can no longer do useful work, such as charging a battery or driving a motor, which leads to a loss of efficiency,” says Zhang.
Although the process is completely theoretical at this point, the calculations also allow the team to find ways to work around the error.
One way that is consistent with experimental evidence would be to find a middle ground in iodide concentrations.
Replacing the organic molecule with a different cation such as cesium, or better yet a similar type of organic compound such as formamidinium, could also lead to a radical improvement in efficiency.
Converting this theoretical work into a practical method of generating electricity requires a lot more testing and planning. What works in calculations would have to be woven into processes in which flawless perovskite wafers grow around formamidinium molecules.
In order for perovskite to have the hope of dominating the power generation market, it must prove itself on both a financial and functional level.
Projections onto silicon suggest that there is still a long way to go before it reaches its theoretical limits beyond 30 percent.
Given the advances perovskite has made over the past decade, perovskite solar cells may be responsible for their major rupture in the not too distant future.
This research was published in Nature Materials.