When a defect can be beneficial

In atomic-resolution electron microscopy, Arashdeep Singh Thind, a graduate student at Rohan Mishra's laboratory, studied grain boundaries in crystals (see arrows). Credit: Washington University in St. Louis

In order to design more efficient solar cells and light-emitting diodes (LEDs), a team of engineers has analyzed different types of semiconductor material defects allowing these devices to determine if and how they affect performance.

Rohan Mishra, an assistant professor of mechanical engineering and materials science at the McKelvey School of Engineering at Washington University in St. Louis, led a large team of researchers, including at Washington University, Tennessee's Oak Ridge National Laboratory, and at the University of Missouri. Columbia – who has studied the structure and properties of plane defects commonly encountered at the atomic scale, which covers only a few tenths of a nanometer.

The Mishra team has studied lead-halide perovskites, a new class of high-performance semiconductor study for the next generation of low-cost solar cells enabling energy conversion. Solar electricity with high efficiency.

When these materials are manufactured, defects can occur at the meeting of different crystals, called grain boundaries. In conventional semiconductors, these defects can decrease their electrical conductivity and the efficiency of converting solar energy into electricity; however, in lead-halide perovskites, there are different experimental reports on the activity of grain boundaries. In some cases, they are harmful, while in others they have no impact on performance or are even beneficial. But, to date, no one has understood why. The Mishra team explained why Advanced MaterialsDecember 3

"A small defect at the atomic scale has a big impact on the solar cell," said Mishra. "If a specific atom is missing at these grain boundaries, your cell will not work well."

At the Oak Ridge National Lab, Arashdeep Singh Thind, a PhD student at the Institute of Materials and Materials Engineering of the University of Washington and working in the Mishra lab, has realized the following: imaging with one of the most powerful electron microscopes to observe the atomic structure of grain boundaries. Guangfu Luo, a former researcher in Mishra's laboratory and assistant professor at the Southern University of Science and Technology in Shenzen, China, then used quantum mechanics calculations on some of the fastest supercomputers to understand the electronic properties of these supercomputers. grain boundaries.

In silicon semiconductors, grain boundaries are devastating, but not in lead-halide perovskites. And it depends on the concentration of halide ions, an essential element of the properties.

"If you grow the crystals in a halide-poor environment, the grain boundaries are terrible for performance," said Mishra. "But if you can grow them or anneal them [heat and recombine] in an atmosphere rich in halides, the grain boundaries are good. "

Thind also examined another type of planar fault known as the Ruddlesden-Popper Fault, in which the crystal planes pile up incorrectly; for example, instead of being aligned in ordered rows, one of the rows is shifted slightly to the left or right of an atomic column. Again, using quantum mechanics calculations, Luo and Mishra have discovered that by having a high density of such stacking defects, it would be possible to obtain a bright optical emission from large nanoparticles and more stable some lead perovskites, which could potentially way for LEDs with a longer life.

"The challenge for the experimenters is to develop overlapping faults at periodic intervals," said Mishra.

In a related research published in ACS Applied Nano Materials on Oct. 16, the Mishra team worked with researchers from the University of Missouri-Columbia, who found a new chemical pathway to promote perovskite growth. lead with a high density of such stacking defects. By eliminating surface ligands, an ion or a molecule that binds to a surface atom of a nanocrystal, smaller lead halide-based perovskite nanocrystals have merged and gone from one to another. about 8 nanometers at 60 nanometers in 48 hours.

These new nanocrystals had significantly improved optical properties because of the stacking errors formed during the fusion process, discovered by Thind using atomic resolution transmission scanning electron microscopy. In addition, the nanocrystals were more stable when exposed to light, had sharper emission lines, and higher quantum yield. With these defects, the new nanocrystals should improve the light-emitting properties of lead-halide perovskite nanocrystals, which would improve LEDs and other optoelectronic devices.

This new information provides engineers such as Mishra and Thind with more information to find lead alternatives in solar cells, which not only contain toxic lead, but are also unstable in the light, at the same time. moisture and heat and dissociate in just a few days. groundwater. Mishra studies whether a non-toxic element – bismuth, the neighbor of lead in the periodic table – is a safer and equally effective substitute for lead in perovskites.

Explore further:
New approach improving the stability and optical properties of perovskite films

More information:
Arashdeep Singh Thind et al. Atomic structure and electrical activity of grain boundaries and Ruddlesden-Popper defects in cesium bromide perovskite, Advanced Materials (2018). DOI: 10.1002 / adma.201805047

Maria V. Morrell et al. Significantly improved emission stability of CsPbBr3 nanocrystals by chemically induced melting growth for optoelectronic devices, ACS Nano Applied Materials (2018). DOI: 10.1021 / acsanm.8b01298

Journal reference:
Advanced Materials

Provided by:
University of Washington at St. Louis

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