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| Simulated atomic structure of perovskite after calcium doping. Illustration: Danil Bukhvalov |
UrFU scientists have found a way to protect perovskite solar cells based on lead-methylammonium iodide (a promising alternative to traditional silicon photovoltaic cells) from degradation by water, such as rain. They found that partial replacement of lead with other alkaline earth metals protects them from such degradation, and also increases the parts of the visible spectrum of radiation involved in the process of generating electrons. An article on the results of the study was published in the Journal of Solid State Chemistry. The research was financially supported by the Ministry of Education and Science of Russia under the Priority 2030 development program of Ural Federal University.
Perovskite solar cells based on lead-methylammonium iodide are superior to silicon cells in performance and ease of synthesis. They are also capable of effectively generating electricity in cloudy or foggy conditions, so they are ideal for use in Russia or countries with similar climates. However, a complete switch to perovskite solar panels is not possible due to a number of reasons causing instability of such photovoltaic cells.
One of the causes of instability is that the compound is unstable to contact with water or other organic solvents. If it rains on the photocell, the compound begins to degrade rapidly, destroying its structure. Scientists determined that replacing lead with metals such as calcium, barium, or strontium would protect the compound from rapid degradation.
"The performance of solar cells depends largely on the high level of electrical polarization of the compound. The organic part, methylammonium, is unstable to organic solvents, including ordinary water. When exposed to rain, the entire compound begins to gradually "dissolve" and lose its electrical properties. This problem can be solved by complicating the breakdown of the molecules by partially replacing lead with alkaline earth metals, such as calcium. It is common and readily available. In this case, we have observed certain improvements in the properties of the compound without losing the performance of the solar cell," explains Danil Bukhvalov, Senior Researcher at the Department of Theoretical Physics and Applied Mathematics at UrFU.
The scientist notes that the amount of lead cannot be reduced by 100%. The complete replacement of lead atoms in the perovskite hybrid lattice makes them virtually unusable as light absorbers in photovoltaic devices. This happens because of an increase in the bandgap width, which is the minimum energy required to move an electron captured from a light source into the conduction zone of electricity. When the band gap width is too large, the compound stops capturing parts of the visible spectrum of radiation and only converts ultraviolet rays. Partial replacement of the lead allowed not only to preserve the compound's photovoltaic characteristics, but also slightly improved them: the compound began to absorb more sunlight for conversion into electricity.
"It is optimal to replace lead by only 5%, but even this seemingly insignificant figure yields several positive results. The second positive effect of the substitution is that the bandgap width increases, but is still able to absorb visible light. The impurity allows you to cover even more parts of the visible spectrum, to involve more radiation in the process of generating electrons. The more light it is able to cover, the higher the efficiency of power generation," adds Danil Bukhvalov.
Scientists also report that the disposal of lead-containing photovoltaic cells is a notable environmental problem. Although perovskite cells based on lead-methylammonium iodide are more attractive and organically safer than silicon cells, they require proper disposal so that when they decay, lead will not poison the soil. Reducing the number of toxic elements in the compound will make it much easier to dispose of.
The improvement of lead iodide-methylammonium perovskite solar cells require a comprehensive approach and further study. This study is one step toward their improvement. Previously, researchers at UrFU and the Institute of Chemical Physics Problems of the Russian Academy of Sciences (Chernogolovka) proposed a way to extend the lifetime of perovskite photovoltaic cells.
Source/Credit: Ural Federal University
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