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Study Published in Nature Communications Reveals That Lead Halide Perovskites, Even When Produced in Solution and Filled with Structural Defects, Use Internal Domain Wall Networks to Separate Charges and Allow Transport Over Hundreds of Micrometers, Approaching Their Efficiency to Silicon-Based Technology
Physicists from the Institute of Science and Technology Austria explained why lead halide perovskites, even filled with defects, achieve efficiency close to silicon, revealing that perovskites use internal domain wall networks to transport charges over long distances.
In the last 15 years, lead halide perovskites have emerged as promising materials for next-generation solar cells. Processed in solution and manufactured using low-cost techniques, they exhibit photovoltaic performance close to silicon, the established industry standard.
The fundamental difference between the two technologies has always intrigued researchers. While silicon solar cells depend on ultra-pure, nearly defect-free monocrystalline wafers, perovskites are grown in solution and are naturally filled with impurities and structural failures.
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In a study published in Nature Communications, postdoctoral researcher Dmytro Rak and assistant professor Zhanybek Alpichshev presented the first comprehensive physical explanation for the mechanism behind the efficiency of perovskites. The main conclusion indicates that, unlike silicon, structural defects are an essential part of the functioning of these materials.
According to the authors, it is precisely the natural defect network that allows the long-range charge transport necessary for the efficient conversion of solar energy into electricity. The discovery answers an age-old debate on the source of the superior performance of perovskites in photovoltaic harvesting.
Perovskites and the Enigma of Charges Traveling Hundreds of Micrometers
An efficient solar cell must absorb light and convert it into charges, formed by negatively charged electrons and positively charged holes. These charges need to be collected at the electrodes to generate usable current.
The challenge is that electrons and holes must travel hundreds of micrometers within the material, equivalent to hundreds of kilometers on a human scale, without becoming trapped by defects before reaching the electrodes.
In silicon-based technology, this obstacle is overcome by virtually eliminating all defects that could capture charges. In perovskites, however, the abundant presence of defects seemed to contradict their high efficiency.
There was evidence that electrons and holes form excitons and recombine quickly. Still, experiments showed they remained separated for long periods within perovskites, allowing efficient charge transport. This apparent paradox motivated the investigation.
The researchers conjectured that unexplained internal forces within the perovskites were responsible for separating the newly formed electron-hole pairs, preventing their immediate recombination and allowing prolonged movement.
Optical Tests Reveal Internal Forces Even Without Applied Voltage
To test the hypothesis, the team introduced electrons and holes into a perovskite sample using nonlinear optical methods. The technique allowed the observation of charge behavior within the crystal.
With each new batch of electrons and holes introduced, a finite current flowing in the same direction inside the material was detected, even without the application of external voltage. The result indicated the presence of internal forces separating opposite charges.
According to Alpichshev, the observation demonstrated that, even in unmodified and as-grown perovskite monocrystals, there are internal electric fields capable of promoting charge separation.
However, previous characterizations indicated that such behavior would not be compatible with the intrinsic crystal structure of the material. To resolve the contradiction, the team proposed that the separation does not occur uniformly.
Domain Walls Form Microscopic Network Inside the Crystal
The hypothesis suggested that charge separation occurs locally at the so-called domain walls, regions of modified structure that can form microscopic networks spanning the entire sample.
These domain walls would function as zones where local electric fields are established, creating favorable conditions for the separation of electrons and holes immediately after their generation by light absorption.
The next challenge was to visualize this internal network, as most available local probes are sensitive only to the surface of the material, where properties can differ significantly from the interior.
Rak drew upon his training in chemistry to circumvent the obstacle. Noting that perovskites also exhibit good ionic conductivity, he developed a strategy based on the introduction of marker ions.
“Angiography” Technique with Silver Reveals Internal Structure
The team developed an electrochemical staining technique that allows visualization of the domain walls inside the crystal. Silver ions were diffused into the perovskite, preferentially accumulating in these regions.
Subsequently, the ions were electrochemically transformed into metallic silver, enabling direct visualization of the internal network under a microscope. The approach was likened to an angiography applied to the microstructure of a crystal.
The technique allowed the observation that the domain wall network densely extends throughout the material’s depth. This structure functions as an internal transport system for charge carriers.
According to Rak, when an electron-hole pair is created near a domain wall, the local electric field pulls the charges to opposite sides. Prevented from recombining immediately, they can move along these regions.
These walls act as true expressways for charge carriers. The phenomenon explains how perovskites can sustain efficient transport even in a structurally defective environment.
Implications for the Next Generation of Solar Cells
The authors claim that the work provides the first coherent physical explanation for the photovoltaic properties of lead halide perovskites. The approach reconciles previously conflicting observations.
Until now, much of the research has focused on adjusting the chemical composition of perovskites, with limited success. The new understanding points to the importance of microstructure and domain walls.
With the identification of these internal networks as a central element of photovoltaic performance, researchers will be able to seek ways to optimize efficiency without compromising the low-cost solution production process.
The results could accelerate the transition of perovskite solar cells from the laboratory to real-world applications. The discovery reinforces that, in this case, defects do not represent failure but rather an essential part of the functional mechanism.
By demonstrating that the natural domain wall network is responsible for long-range charge transport, the study redefines the understanding of perovskites and establishes a physical basis for future innovations in solar energy generation.
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