A group of researchers from the Institute of Science and Technology Austria (ISTA) claims to have identified a mechanism that helps explain why lead halide perovskites (LHPs) are highly efficient for photovoltaic applications despite their ‘messy’ structure.
They found that flexoelectric polarization plays a key role at voltage-induced domain walls, generating internal electric fields that separate charge carriers, suppress recombination, and enable long-distance transport despite rapid intrinsic exciton decay.
The scientists explained that their work was initially motivated by the question of why perovskite solar cells can match the performance of silicon despite being fabricated using simple, low-cost solution processes, while photovoltaic silicon dioxide requires ultra-pure materials and energy-intensive growth of virtually defect-free single crystals.
“There were many conjectures about the origin of charge separation in perovskites,” said corresponding author Zhanybek Alpichshev pv magazine. “And since most practical PV devices have been based on methylammonium lead iodide (MAPbI₃), which is in the tetragonal phase at room temperature, there have been several attempts to attribute charge separation to the non-cubic phase.”
Previous research had often suggested that MAPbI₃ could be ferroelectric, meaning it would have built-in, switchable electrical polarization. However, this idea is controversial because ferroelectricity is incompatible with a perfectly cubic crystal structure, as cubic symmetry does not allow for a permanent directional dipole. In other words, the assumed symmetry violates the conditions required for ferroelectric behavior.
“This naive explanation completely failed to take into account the fact that certain cubic perovskites such as MAPbBr₃ also exhibit similar performance in PV and other perplexing properties common to lead-halide perovskites (LHPs),” Alpichshev said. “We chose to closely study single-crystal large cubic methylammonium lead bromide (MAPbBr₃) to ensure that what we observe is an intrinsic property of LHPs, and not the result of a phase with low symmetry or finite size or surface effects of the sample.”
In the study “Flexoelectric domain walls enable charge separation and transport in cubic perovskites”, published in communication about naturethe researchers explained that they used nonlinear optical excitation to generate electrons and holes deep within the bulk of a perovskite crystal. They then measured a reproducible current that flowed consistently in the same direction every time a new population of charge carriers was created, despite the absence of any applied voltage. “This observation clearly indicated that even deep within the individual crystals of unmodified, mature perovskites, internal forces exist that separate opposite charges,” says Alpichshev.
Using polarized light and temperature-dependent measurements, the scientists found that solution-grown MAPbBr₃ exhibits intrinsic structural distortion even in the high-temperature phase, questioning the assumption of true cubic symmetry. Polarization measurements also showed that the material behaves as a ferroelastic system in which structural non-cubicity is confined to domain walls rather than representing a uniform lattice deformation.
Using localized two-photon excitation in MAPbBr₃ single crystals, the researchers discovered a photocurrent without bias that varies spatially, confirming the presence of internal electric fields without external bias. The data are consistent with polarization confined to ferroelastic domain walls, which create electrostatic potential differences while preserving bulk inversion symmetry. Similar behavior observed in MAPbI3 suggests that flexoelectric domain walls may be a general mechanism for local symmetry breaking in lead-halide perovskites.
Overall, charged domain walls act as dynamic, tunable transport channels that simultaneously separate carriers and control their recombination, linking the mesoscopic voltage structure to photovoltaic performance.
“The key finding is that while silicon-based technology relies on the absence of impurities, the opposite is true for perovskites,” said co-author Dmytro Rak. “Our work provides a physical explanation for these materials while explaining most, if not all, of their documented properties.”
