Researchers in Austria found that unusually high photovoltaic efficiency in lead-halide perovskites can be explained by internal electric fields generated at strain-induced domain walls, which separate charge carriers and enable long-range transport. Their experiments indicated that these effects arise from mesoscopic structural inhomogeneities rather than a uniform crystal structure, providing a general mechanism for charge separation in these materials.
A lead-halide perovskite crystal sample
Image: ISTA
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 a key role is played by flexoelectric polarization at strain-induced domain walls, which generates internal electric fields that separate charge carriers, suppress recombination, and enable long-range 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 silicon performance despite being fabricated using simple, low-cost solution processing, whereas silicon photovoltaics require ultra-pure materials and energy-intensive growth of nearly defect-free single crystals.
“There were many conjectures about the origin of charge separation in perovskites,” corresponding author Zhanybek Alpichshev told pv magazine. “And since most practical PV devices were based on methylammonium lead iodide (MAPbI₃), which happens to be in the tetragonal phase at room temperature, there were various attempts to attribute charge separation to the non-cubic phase.”
Previous research had often suggested that MAPbI₃ might be ferroelectric, meaning it would have a built-in, switchable electric polarization. However, this idea is controversial because ferroelectricity is incompatible with a perfectly cubic crystal structure, since cubic symmetry does not allow a permanent directional dipole. In other words, the assumed symmetry conflicts with the conditions required for ferroelectric behavior.
“This naive explanation completely failed to account for the fact that certain cubic perovskites like MAPbBr₃ also exhibit comparable performance in PV and other bewildering properties common to lead-halide perovskites (LHPs),” Alpichshev said. “We chose to study large single-crystal cubic methylammonium lead bromide (MAPbBr₃) precisely to ensure that what we observe is an intrinsic property of LHPs, not a result of a low-symmetry phase 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 nature communications, the researchers explained 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 consistently flowed in the same direction each time a new population of charge carriers was created, despite the absence of any applied voltage. “This observation clearly indicated that even deep inside single crystals of unmodified, as-grown perovskites, there are internal forces that separate opposite charges,” said Alpichshev.
Through polarized-light and temperature-dependent measurements, the scientists found that solution-grown MAPbBr₃ exhibits an intrinsic structural distortion even in its high-temperature phase, challenging 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 distortion.
Using localized two-photon excitation in MAPbBr₃ single crystals, the researchers detected a zero-bias photocurrent 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 mesoscopic strain structure to photovoltaic performance.
“The key finding is that while silicon-based technology relies on the absence of impurities, the opposite is true in perovskites,” co-author Dmytro Rak said. “Our work provides a physical explanation of these materials while accounting for most—if not all—of their documented properties.”
 
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