Perovskites are a set of materials with a crystalline structure that can be organic, inorganic or a mix of the two. Researchers at Stanford have been studying the materials, exploring their potential for use in solar applications in the hope of finding an alternative to silicon-based solar cells.
“A perovskite refers to a crystal structure, a crystalline material,” explains Stanford University professor of materials science and engineering Reinhold Dauskardt. “An example would be salt; sodium chloride has a perovskite crystal structure. Some compositions that happen to have perovskite crystal structures are also good photovoltaic materials, in other words, they absorb light and produce charge carriers that can then be used to power a solar cell. The most typical example of that is methylammonium lead iodide.”
Interest in perovskites has been limited for a long time due to their unstable nature; however, breakthroughs in 2009 signalled change, with research growing steadily over the last few years. Perovskites are highly efficient and easy to process. Their power conversion efficiency has now reached >20%, competitive with the 26.6% world record efficiency of a silicon photovoltaic cell. This efficiency can be achieved whilst processing costs dramatically drop because perovskites can be processed simply by making them into a precursor solution and depositing them on a substrate, thereby creating a solar cell.
Too unstable to succeed
Despite the clear advantages of using perovskites in photovoltaics, they have been held back by the material’s instability. They are both mechanically and chemically unstable, so the conditions that affect and alter it are wide-ranging.
“[Perovskite is] also mechanically extremely fragile, it has one of the lowest fracture energies, which means resistance to failure either by delamination or by simply just cracking in the perovskite layer itself,” says Dauskardt.
The brittle nature of perovskites has made them inappropriate for solar panels, as they must be able to operate outside for years despite wind, rain and other environmental factors. If perovskites lack the strength to endure even the slightest knock, there is no economic case for using them instead of traditional silicon-based photovoltaics.
Chemically, perovskites have been seen as less appealing than conventional solar cells as they degrade very quickly. They are particularly affected by heat, making them unsuitable for the sunny and warm environments in which solar cells generally operate best.
“You don’t have to go to super-high temperatures; you just have to go to tens of degrees above room temperature for the material to become increasingly unstable,” says Dauskardt. “In addition, it’s very sensitive to moisture, so if moisture were to diffuse into the solar cell then it would interact with the perovskite and basically cause it to decompose.” Ambient conditions alone can cause the efficiency of perovskites to reduce by over 80% within just 28 hours.
Taking inspiration from nature
It was the instability, especially the mechanical instability, of perovskite that Dauskardt and his team at Stanford University set out to tackle, finding inspiration in nature and ceramics.
“Well, I had been thinking for a long time that we couldn’t change the inherent properties of the perovskite material, and that means we couldn’t make it into a more stable material by itself because in doing that you would basically lose all of the optoelectronic properties, so it just wouldn’t be an effective photo absorber anymore,” explains Dauskardt.
“In the case of ceramics, if you did this by trying to change the inherent properties then you would just turn it into another material and you’d lose all of the high-temperature properties or the corrosion-resistant properties,” he continues. “So the key was to extrinsically toughen or extrinsically shield the material. They did things like put fibres in it, the fibres could carry all of the mechanical stresses and the ceramic would be in some ways kind of shielded from the mechanical environment.”
This concept of extrinsically shielding a material was crucial to the team’s breakthrough with perovskites. It was here that nature offered a helping hand, after Dauskardt noticed the scaffolding structure of a fly’s eye. Within each of a fly’s two eyes, there are thousands of ommatidia, or simple eyes. These ommatidia are exceptionally fragile, but being compounded and protected within a scaffold makes them durable.
“So nature had this beautiful example of extrinsic shielding and the notion of a compound eye, and that’s what led us to think of building a scaffold that was tough and durable to mechanical loads, and then to create these individual solar cells,” says Dauskardt. “And it worked, it worked extremely well.”
Building a scaffold for solar cells
Dauskardt and his team set about testing the concept of a reinforced scaffold to solve the mechanical instability of perovskites. Using photolithographic methods, they were able to construct a hexagonal scaffold just 500 microns wide out of a durable polymer. Within this, they built the perovskite cells.
“What we’ve found is that not only is it mechanically much more durable, but it’s also chemically more stable because, as I mentioned earlier on, if you heat up perovskite materials there are certain components in the perovskite that become volatile,” says Dauskardt. “By essentially containing each individual cell, we could help slow down the loss of this volatile component of the material.”
To test the structures, the perovskite cells were kept at 85°C and 80% humidity for six weeks. And in these conditions, the cells continued to generate electricity at a high efficiency.
Although this represents a milestone for perovskites’ viability as a solar cell material, there are still challenges to overcome if they are to compete with existing photovoltaic cells. “We’ve taken this basic proof of concept and we’re now taking them further and developing more efficient scaffold cells,” says Dauskardt. “We’re learning to deal with the illumination and making sure that we don’t lose any light in the scaffold walls themselves because obviously they take up space, and in taking up space they would absorb some of the light that actually shines onto the solar cell.”
To combat the potential loss of light that comes with using scaffolds, the team is looking at ways to use the structure itself to help scatter light into the cells. As they continue to develop the stability of perovskites, it appears that the compounds may be a significant step closer to reaching their solar potential.