New fabrication method brings single-crystal perovskite devices closer
to viability

Date:
July 29, 2020
Source:
University of California - San Diego
Summary:
Nanoengineers have developed a new method to fabricate perovskites
as single-crystal thin films, which are more efficient for use in
solar cells and optical devices than the current state-of-the-art
polycrystalline forms of the material. Their fabrication method --
which uses standard semiconductor fabrication processes -- results
in flexible single-crystal perovskite films with controlled area,
thickness, and composition.



FULL STORY ========================================================================== Nanoengineers at UC San Diego developed a new method to fabricate
perovskites as single-crystal thin films, which are more efficient for
use in solar cells and optical devices than the current state-of-the-art polycrystalline forms of the material.


========================================================================== Their fabrication method -- which uses standard semiconductor fabrication processes -- results in flexible single-crystal perovskite films with controlled area, thickness, and composition. These single-crystal films
showed fewer defects, greater efficiency, and enhanced stability than
their polycrystalline counterparts, which could lead to the use of
perovskites in solar cells, LEDs, and photodetectors.

Researchers in Professor Sheng Xu's Jacobs School of Engineering nanoengineering lab published their findings on July 29 in Nature.

"Our goal was to overcome the challenges in realizing single-crystal
perovskite devices," said Yusheng Lei, a nanoengineering graduate student
and first author of the paper. "Our method is the first that can precisely control the growth and fabrication of single-crystal devices with high efficiency. The method doesn't require fancy equipment or techniques --
the whole process is based on traditional semiconductor fabrication,
further indicating its compatibility with existing industrial procedures." Perovskites are a class of semiconductor materials with a specific
crystalline structure that demonstrate intriguing electronic and
optoelectronic properties, which make perovskites appealing for use in
devices that channel, detect, or are controlled by light -- solar cells, optical fiber for communication, or LED-based devices, for example.

"Currently, almost all perovskite fabrication approaches are focused
on polycrystalline structures since they're easier to produce, though
their properties and stability are less outstanding than single-crystal structures," said Yimu Chen, a nanoengineering graduate student and
co-first author of the paper.



========================================================================== Controlling the form and composition of single-crystal perovskites during fabrication has been difficult. The method invented in Xu's lab was able
to overcome this roadblock by taking advantage of existing semiconductor fabrication processes including lithography.

"Modern electronics such as your cell phone, computers, and satellites
are based on single-crystal thin films of materials such as silicon,
gallium nitride, and gallium arsenide," said Xu. "Single crystals have
less defects, and therefore better electronic transport performance, than polycrystals. These materials have to be in thin films for integration
with other components of the device, and that integration process
should be scalable, low cost, and ideally compatible with the existing industrial standards. That had been a challenge with perovskites."
In 2018, Xu's team was the first to successfully integrate perovskites
into the industrial standard lithography process; a challenge, since lithography involves water, which perovskites are sensitive to. They got
around this issue by adding a polymer protection layer to the perovskites followed by dry etching of the protection layer during fabrication. In
this new research, the engineers developed a way to control the growth
of the perovskites at the single crystal level by designing a lithography
mask pattern that allows control in both lateral and vertical dimensions.

In their fabrication process, the researchers use lithography to etch
a mask pattern on a substrate of hybrid perovskite bulk crystal. The
design of the mask provides a visible process to control the growth of
the ultra-thin crystal film formation. This single-crystal layer is then
peeled off the bulk crystal substrate, and transferred to an arbitrary substrate while maintaining its form and adhesion to the substrate. A
lead-tin mixture with gradually changing composition is applied to the
growth solution, creating a continuously graded electronic bandgap of
the single-crystal thin film.

The perovskite resides at the neutral mechanical plane sandwiched between
two layers of materials, allowing the thin film to bend. This flexibility allows the single-crystal film to be incorporated into high-efficient
flexible thin film solar cells, and into wearable devices, contributing
toward the goal of battery-free wireless control.

Their method allows researchers to fabricate single-crystal thin films
up to 5.5 cm by 5.5 cm squares, while having control over the thickness
of the single-crystal perovskite -- ranging from 600 nanometers to 100
microns -- as well as the composition gradient in the thickness direction.

"Further simplifying the fabrication process and improving the transfer
yield are urgent issues we're working on," said Xu. "Alternatively,
if we can replace the pattern mask with functional carrier transport
layers to avoid the transfer step, the whole fabrication yield can
be largely improved." Instead of working to find chemical agents to
stabilize the use of polycrystalline perovskites, this study demonstrates
that it's possible to make stable and efficient single-crystal devices
using standard nanofabrication procedures and materials. Xu's team
hopes to further scale this method to realize the commercial potential
of perovskites.


========================================================================== Story Source: Materials provided by
University_of_California_-_San_Diego. Original written by Katherine
Connor. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yusheng Lei, Yimu Chen, Ruiqi Zhang, Yuheng Li, Qizhang Yan,
Seunghyun
Lee, Yugang Yu, Hsinhan Tsai, Woojin Choi, Kaiping Wang, Yanqi Luo,
Yue Gu, Xinran Zheng, Chunfeng Wang, Chonghe Wang, Hongjie Hu,
Yang Li, Baiyan Qi, Muyang Lin, Zhuorui Zhang, Shadi A. Dayeh,
Matt Pharr, David P. Fenning, Yu-Hwa Lo, Jian Luo, Kesong Yang,
Jinkyoung Yoo, Wanyi Nie, Sheng Xu. A fabrication process for
flexible single-crystal perovskite devices. Nature, 2020; 583
(7818): 790 DOI: 10.1038/s41586-020-2526-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200729114826.htm

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