Researchers in the Materials Department at UC Santa Barbara’s College of Engineering have discovered a major cause of efficiency limitations in a new generation of solar cells.
Various possible defects in the network of so-called hybrid perovskites had previously been considered the potential cause of these limitations, but it was assumed that the organic molecules (the components responsible for the “hybrid” nickname) would remain intact. State-of-the-art calculations have now revealed that the missing hydrogen atoms in these molecules can lead to massive efficiency losses. The results are published in an article titled “Minimizing Hydrogen Vacancies to Enable Highly Efficient Hybrid Perovskites”, in the April 29 issue of the journal Materials from nature.
The remarkable photovoltaic performance of hybrid perovskites has generated a lot of enthusiasm, given their potential to advance solar cell technology. “Hybrid” refers to the inclusion of organic molecules in an inorganic perovskite network, which has a crystal structure similar to that of the mineral perovskite (calcium titanium oxide). The materials exhibit power conversion efficiencies rivaling that of silicon, but are much cheaper to produce. However, defects in the perovskite crystal lattice are known to create unwanted energy dissipation in the form of heat, which limits efficiency.
A number of research teams have investigated such flaws, including the group of UCSB materials professor Chris Van de Walle, who recently made a breakthrough by discovering a damaging flaw in a place no one had looked at before: on the organic molecule.
“Methylammonium lead iodide is the prototypical hybrid perovskite,” explained Xie Zhang, principal investigator of the project. “We have found that it is surprisingly easy to break one of the bonds and remove a hydrogen atom on the methylammonium molecule. The resulting “hydrogen vacancy” then acts as a sink for the electrical charges that travel through the crystal after being generated by the falling light. on the solar cell. When these loads are taken at the vacancy level, they can no longer perform useful work, such as charging a battery or powering a motor, hence the loss of efficiency. “
The research was made possible by advanced computational techniques developed by the Van de Walle group. These state-of-the-art calculations provide detailed information on the quantum-mechanical behavior of electrons in the material. Mark Turiansky, a graduate student in Van de Walle’s group who participated in the research, helped develop sophisticated approaches to transform this information into quantitative values for charge carrier entrapment rates.
“Our group has created powerful methods for determining which processes cause a loss of efficiency,” said Turiansky, “and it is gratifying to see the approach provide such valuable information for such an important class of materials.”
“The calculations act like a theoretical microscope which allows us to examine the material with a much higher resolution than that which can be achieved experimentally,” Van de Walle explained. “They also form a basis for the rational design of materials. By trial and error, it has been found that perovskites in which the methylammonium molecule is replaced by formamidinium exhibit better performance. We can now attribute this improvement to the fact that hydrogen defects are less readily formed in the compound formamidinium.
“This idea provides a clear rationale for the empirically established wisdom that formamidinium is essential for the realization of high efficiency solar cells,” he added. “Based on this fundamental knowledge, the scientists who fabricate the materials can develop strategies to remove harmful defects, thereby increasing further efficiency improvements in solar cells.
Funding for this research was provided by the Bureau of Science and the Bureau of Basic Energy Sciences of the Department of Energy. The calculations were carried out at the National Center for Scientific Computing for Energy Research.
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Material provided by University of California – Santa Barbara. Original written by James Badham. Note: Content can be changed for style and length.