Researchers at the National Institute of Standards and Technology (NIST) and the University of Nebraska have found evidence for a property of perovskites that may affect their long-term stability as solar cells.
The unexpected feature that the team found is known as ferroelasticity, a spontaneous rearrangement of the internal structure of perovskites in which each crystal subdivides into a series of tiny regions, or domains, that have the same atomic arrangement but which are oriented in different directions. This rearrangement creates a spontaneous strain in each domain that exists even in the absence of any external stress (force).
At high temperatures, perovskite crystals do not subdivide and have the same cubic arrangement of atoms throughout. At room temperature, however, their crystal structure changes from cubic to tetragonal, in which one axis of the cube elongates. That’s where the ferroelastic property of the material appears.
“To transform from a cubic to a tetragonal arrangement, one axis of the cube must elongate. In the process, each crystal subdivides into smaller domains in which the elongated axis can point in a different direction, leading to spontaneous internal strain,” explained a team member.
At present, it remains unknown whether ferroelasticity is a property that improves or hinders the performance and stability of perovskite solar cells. But the very fact that perovskites have this internal structure, breaking up single crystals into domains, is important to investigate, the team stated. Boundaries between crystals' inter-grain boundaries are known to be weak points, where structural defects concentrate. Similarly, the boundaries between the newly discovered ferroelastic domains inside a single crystal intra-grain boundaries might also affect the stability of perovskites and their performance as solar cells.
The researchers discovered that by bending the crystals, they could reliably create and manipulate the ferroelastic grain boundaries. In their study, the team found no evidence that the perovskites were ferroelectric; in other words, that they formed domains where the separation of the center of positive and negative electric charges is aligned in different directions in the absence of an external electric field. This finding is significant, because some researchers had speculated that ferroelectricity might be the underlying property that makes perovksites promising candidates for solar cells.