Despite being riddled with impurities and defects, solution-processed lead halide perovskites continue to defy expectations as highly efficient solar cell materials, with performance now approaching that of industry-standard silicon-based devices. A new experimental study from the Institute of Science and Technology Austria (ISTA) provides a comprehensive physical explanation for this efficiency puzzle and points the way to more effective next-generation photovoltaic technologies.
Over the past fifteen years, lead halide perovskites have risen from relative obscurity to become leading candidates for low-cost, high-performance solar cells. Originally cataloged in the 1970s for their hybrid organic-inorganic crystal structures and then largely forgotten, these compounds were rediscovered in the early 2010s when researchers discovered that they exhibit exceptional photovoltaic behavior, as well as strong performance in light-emitting diodes and X-ray detection and imaging.
Perovskites also exhibit striking quantum mechanical properties, including quantum coherence at room temperature, making them attractive testing grounds for the complex physics of condensed matter. Yet their practical promise is overshadowed by a fundamental paradox. Unlike silicon solar cells, which rely on ultra-pure, carefully grown monocrystalline wafers to minimize defects, perovskite devices are typically made using low-cost, solution-based processes, leaving them filled with structural imperfections.
In conventional silicon technology, such defects are carefully eliminated because they trap charge carriers and prevent them from passing through the hundreds of microns needed to reach the electrodes and generate usable current. The question for perovskites has therefore been how electrons and holes manage to move over long distances and survive for long periods of time in a material that, by standard criteria, should be littered with traps and recombination centers.
ISTA postdoctoral researcher Dmytro Rak and assistant professor Zhanybek Alpichshev tackled this problem by focusing on how charges behave in the bulk of a perovskite crystal. Previous studies had shown that when electrons and holes form tightly bound excitons in these materials, they typically recombine quickly. Nevertheless, experiments also indicated that the charges in working devices often remain separated for extended periods, an apparent contradiction that suggested that an internal force must be acting to pull the pairs apart.
To investigate this idea, the team used nonlinear optical methods to inject electrons and holes deep into single-crystal perovskite samples and then monitor the resulting electrical response. Each time they generated a new population of charges, they found that a finite current flowed in the same direction through the material even though no external voltage was applied. This behavior indicated built-in internal fields capable of separating opposite charges far away from the electrodes.
However, standard characterizations of the intrinsic crystal structure of lead halide perovskites had indicated that such large-scale photovoltaic effects should not be present uniformly throughout the material. Rak and Alpichshev proposed instead that the crucial fields are located on domain walls, thin regions of altered structure that form a microscopic network that extends through the crystal. In these walls, local distortions can break symmetry and create strong internal electric fields.
Visualizing such a domain wall network deep within a crystal posed a major experimental challenge, because many common probes are mainly sensitive to the surface where properties can differ from the bulk. Drawing on his chemistry background, Rak devised an electrochemical coloring method that uses ionic conduction in the perovskite to highlight its internal structure. He allowed silver ions to diffuse into the crystal, where they preferentially accumulated at the domain walls, and then converted the ions into metallic silver so that the resulting filaments could be imaged by optical microscopy.
The resulting images revealed a dense, sample-spanning network of silver-enriched pathways that follow the skeleton of the crystal’s domain wall. Alpichshev compares the method to angiography in living tissue, but applied to the microstructure of a solid, because it reveals the internal channels along which charges can move. The qualitative technique, invented and implemented at ISTA, provides direct evidence that perovskites harbor complex internal architectures rather than being uniformly disordered.
Rak and colleagues interpret these domain walls as natural highways for charge carriers. When light absorption creates an electron-hole pair near one of these walls, the local electric field pulls the negatively charged electron and the positively charged hole to opposite sides, preventing immediate recombination. Once separated, the carriers can drift along the extended domain wall network over distances that would be extraordinary on a microscopic timescale, eventually reaching the electrodes and contributing to the current.
In this image, the defects that would be harmful in a conventional semiconductor become functional elements that enable efficient energy harvesting. Rather than aiming for defect-free crystals, perovskite technology takes advantage of the presence and connectivity of flexoelectric domain walls that cross the bulk and support charge transport over long distances under illumination.
The authors claim that their framework reconciles a series of previously conflicting experimental observations of lead halide perovskites, including fast exciton recombination in some measurements and long lifetimes and diffusion lengths in others. By highlighting the role of internal fields and domain wall networks, their model provides a unified understanding of how these materials achieve high energy conversion efficiencies despite apparent structural disorder.
Until now, much of the research effort in perovskite solar photovoltaics has focused on tweaking the chemical composition to improve stability and performance, often with only incremental gains. The ISTA findings suggest a complementary path that focuses on engineering the density, orientation, and connectivity of domain walls to optimize charge separation and transport, while retaining the solution-based, low-cost manufacturing routes that make perovskites attractive for large-scale deployment.
Research report:Flexoelectric domain walls enable charge separation and transport in cubic perovskites
