Perovskite solar cells like this one, made by Xiwen Gong’s group, could make solar energy cheaper and more environmentally friendly, but they degrade more quickly than silicon. In a study published in the journal Matter, the team discovered how to extend the life of the black perovskite film. Photo credit: Zhengtao Hu, Gong Lab, University of Michigan.
A University of Michigan discovery aimed at preventing the rapid degradation of perovskite semiconductors could produce solar cells estimated to be two to four times cheaper than current thin-film solar panels.
The study is published in the journal Matter.
Perovskites can also be combined with the silicon-based semiconductors that predominate in today’s solar panels to create “tandem” solar cells that could exceed the maximum theoretical efficiency of silicon solar cells.
“Silicon solar cells are great because they are very efficient and can last a very long time, but their high efficiency comes at a high cost,” said Xiwen Gong, assistant professor of chemical engineering at UM. “To make high-purity silicon, temperatures above 1,000 degrees Celsius are required. Otherwise, the efficiency will not be as good.”
High temperature comes with higher economic and environmental costs. But while perovskites can be produced at lower temperatures, they degrade when exposed to heat, humidity and air. As a result, the lifespan of perovskite is currently too short to be commercially competitive in the field of solar panels.
Gong’s research aims to make stronger perovskite solar cells, and his new study in Matter suggests that bulky “defect pacifying” molecules are best for increasing the stability and overall lifetime of perovskites.
Perovskite crystals contain lead atoms that are not fully bonded to the other components of the perovskite. Such “undercoordinated sites” are defects often found on crystal surfaces and at grain boundaries where there is a break in the crystal lattice. These defects hinder the movement of electrons and accelerate the disintegration of the perovskite material.
Engineers already know that mixing defect-soothing molecules in perovskites can help lock in the undercoordinated wire, preventing further imperfections from forming at high temperatures. But until now, engineers didn’t know exactly how a given molecule affected the resistance of perovskite cells.
Xiwen Gong’s team designed these three molecular additives to study the impact of an additive’s size and configuration on the stability of perovskite films, a class of materials that could be used to make solar cells. high efficiency and low cost. Additives can prevent defects – which harm the efficiency of solar cells – from developing at breaks in the perovskite crystal lattice, called grain boundaries. The perovskite lattice is represented by an array of yellow diamonds while the defect sites are represented by dark blue dotted circles. The black dotted lines represent the bonds that can potentially form between the perovskite and the additives. The larger molecule covers most of the defects on the surface of the perovskite grains while also increasing the overall grain size during the manufacturing process. Larger perovskite grains result in lower grain boundary density throughout the film, reducing the number of places where defects can form. Image credit: Carlos A. Figueroa Morales, Gong Lab, University of Michigan.
“We wanted to determine which features of the molecules specifically improve the stability of the perovskite,” said Hongki Kim, a former postdoctoral researcher in chemical engineering and one of the first authors of the study.
To study the problem, Gong’s team created three additives of varying shapes and sizes and added them to thin films of perovskite crystals, which can absorb light and convert it into electricity. Each additive contained the same or similar chemical elements, making size, weight, and arrangement the main properties that differentiated them.
Next, the team measured the strength with which the different additives interacted with the perovskites and consequently influenced the formation of defects in the films. Molecules that are larger in terms of mass adhere better to the perovskite because they have more binding sites that interact with the perovskite crystals. As a result, they tended to better prevent the formation of defects.
But the best additives also had to take up a lot of space. Large but thin molecules gave rise to smaller perovskite grains during the manufacturing process. Smaller grains are not ideal because they also create perovskite cells with more grain boundaries or more areas of defect formation. In contrast, larger molecules forced the formation of larger perovskite grains, which reduced the density of grain boundaries in the film.
Heating the perovskite films to over 200 degrees Celsius confirmed that bulky additives helped the films retain more of their characteristic slate black color and develop fewer structural defects.
“Size and configuration are important when designing additives, and we believe this design philosophy could be implemented in various perovskite formulations to further improve the lifespan of perovskite solar cells, light-emitting devices and photodetectors,” said Carlos Alejandro Figueroa Morales, a Ph.D. student in macromolecular science and engineering and one of the first authors of the study.
More information:
Hongki Kim et al, Molecular design of defect passivators for thermally stable metal halide perovskite films, Matter (2024). DOI: 10.1016/j.matt.2023.12.003
Provided by University of Michigan
Quote: Bulky additives could extend the life of cheaper solar cells (January 11, 2024) retrieved January 11, 2024 from
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