The group of so-called metal halide perovskites as materials has revolutionized the field of photovoltaics in recent years. In general, metal halide perovskites are crystalline materials that follow the structure ABX3 with different compositions. Here A, B and X can represent a combination of different organic and inorganic ions. These materials have a number of properties that are ideal for use in solar cells and that could help make optoelectronic components such as lasers, light emitting diodes (LEDs) or photodetectors much more efficient. With regard to the development of a resource- and energy-efficient technology, the relevance of research on these materials is very high.
The beneficial properties of metal halide perovskites include their high light harvesting capacity and their remarkable ability to efficiently convert solar energy into electrical energy. Another special feature of these materials is that both charge carriers and ions are mobile in them. While charge carrier transport is a fundamental process required for the photovoltaic operation of the solar cell, ion defects and ion transport often have undesirable consequences for the performance of these devices. Despite significant advances in this area of research, many questions about the physics of ions in perovskite materials remain open.
The Technical Universities of Chemnitz and Dresden have now taken a big step forward on the way to a better understanding of these structures. In a joint study by the research groups led by Prof. Dr. Yana Vaynzof (Chair for Emerging Electronic Technologies at the Institute for Applied Physics and Center for the Promotion of Electronics in Dresden, TU Dresden) and Prof. Dr. Carsten Deibel (optics and photonics of condensed matter, Chemnitz University of Technology) under the direction of the Chemnitz University of Technology, the two teams discovered the ion defect landscape in metal halide perovskites. They were able to identify key properties of the ions that make up these materials. The migration of the ions leads to the presence of defects in the material, which have a negative effect on the efficiency and stability of perovskite solar cells. The working groups found that the movement of all observed ions, despite their different properties (such as positive or negative charge), follows a common transport mechanism and also enables the assignment of defects and ions. This is known as the Meyer-Neldel rule. The results were published in the journal Nature Communications.
“Investigating the ion defect landscape of perovskite materials is not an easy task,” says Sebastian Reichert, research assistant at the Chair for Optics and Photonics of Condensed Matter at Chemnitz University of Technology and main author of the publication. “We had to perform a comprehensive spectroscopic characterization of perovskite samples in which the defects were deliberately introduced and their type and density gradually adjusted. Therefore, the expertise of both teams was invaluable,” explains Reichert.
Clarification of basic transport mechanisms
“One of the most important results of our study is the complex interplay between ionic and electronic landscapes in perovskite materials,” adds Prof. Vaynzof. “By changing the density of the various ion defects in perovskite materials, we are finding that the built … The potential and open circuit voltage of the devices are affected.” This underscores that defect engineering is a powerful tool in improving the performance of perovskite To improve solar cells beyond the state of the art.
The joint study also found that all ion defects conform to the so-called Meyer-Neldel rule. “This is very exciting because it provides basic information about the jumping processes of ions in perovskites,” says Prof. Deibel. “We currently have two hypotheses as to the origin of this observation and plan to investigate these in our future studies.”
Background: Cooperation between Chemnitz and Dresden in the SPP 2196 of the DFG
Carsten Deibel’s research group is a leader in the field of impedance and deep-level transient spectroscopy, powerful methods for the investigation of defects in semiconductor materials. Yana Vaynzof’s group developed a method for influencing and controlling the type and density of defects in perovskite materials by deliberately modifying the stoichiometry of the solution from which they are deposited. These materials are then used to manufacture solar cells so that their spectroscopic characterization can be directly correlated with their photovoltaic performance.
The two teams are working on their joint project Perovskite Defects: Physics, Evolution and Stability (PERFECT PVs) as part of the Priority Program (SPP) 2196 Perovskite Semiconductors of the German Research Foundation (DFG): From Basic Properties to Application.
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