Zooming in on tiny defects

Date:
December 13, 2021
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
Researchers have looked at what limits the efficiency of a promising
solar material to reveal the nature of multiple different kinds
of defects, their varied roles in device efficiency and their
responses to treatment.



FULL STORY ==========================================================================
The expansion of solar energy is a necessity to sustainably meet the
world's energy demands. To achieve this, researchers have focused on
the development of semiconducting energy materials. One such material,
hybrid perovskites, consists of both organic and inorganic ions. These
are promising candidates to form the next generation of solar cells,
which could be more cost effective and more easily mass produced than
the conventional ones currently available.


==========================================================================
But, before they can be commercialized, their efficiency needs to be
expanded and their limits properly understood. Writing in Energy & Environmental Science, researchers from the Femtosecond Spectroscopy
Unit, led by Professor Keshav Dani, at the Okinawa Institute of Science
and Technology Graduate University (OIST) and Optoelectronics Materials
and Device Spectroscopy Group, led by Dr Sam Stranks at the University
of Cambridge identified three different kinds of defect clusters in state-of-the-art perovskite thin films, which likely form during the perovskite's fabrication and may impede efficiency.

"If you have a solar cell, you want the whole material to contribute
to converting sunlight to electricity, otherwise you are not utilizing
its full potential, which is not desirable for commercial purposes,"
said Sofiia Kosar, PhD candidate in the OIST Unit and first author of
the research paper.

The perovskite material lies at the heart of the solar cell, which
consists of many different layers. When the sun hits the solar cell, its
energy is absorbed by the perovskite, causing electrons to jump into a
higher energy level and leaving holes behind. All the electrons then move
in one direction through the layers of the solar cell to the electrical contact. The holes move in the other direction, thus generating a current.

"If there are defects within the material this might impact the generation
of electron-hole pairs or their collection at contacts and mean that
the perovskite solar cell loses efficiency," Sofiia said. She went on to explain that, theoretically, perovskite solar cells could convert about
30% of incident light to electricity. "Right now, hybrid perovskites
regularly output between 20% and about 25%." Experts in the field
were aware of the existence of defects that might impede efficiency,
but it was previously unclear whether these defects all had the same characteristics and could thus be removed using one strategy. In this
study, by using state-of-the-art equipment, the defects were imaged and characterized with a nanoscale resolution to reveal three distinct kinds.

The most detrimental kind the researchers found was the grain boundary
defect.

These defects are tiny and, as the name suggests, sit at the boundary
between different crystal grains of perovskite. They seemed to actively
trap photogenerated holes and deplete efficiency from the regions far
exceeding their size, thus causing huge issues for the performance of
the perovskite.

Then there are polytype defects. These occur when the precursor material crystallizes, not in the typical cubic perovskite structure, but a
hexagonal one. This type of defect cluster is relatively big and also negatively impacts efficiency by trapping photogenerated holes.

"If there are a lot of polytype defects in a film, their impact can become
just as detrimental as the grain boundary ones," Sofiia explained. "Both
the grain boundary and the polytype defects need to be targeted by
developing specific strategies." Finally, the study revealed the lead
iodide defects. These form from precipitated lead iodide -- an important
part of perovskite material that is used during the fabrication. However,
this research suggests that they are benign in terms of trapping charges
and have little impact on efficiency.

The researchers decided to use an approach that is often used to reduce
defect density in perovskites -- treatment with light and oxygen -- to see
how these defects would respond. They did this by exposing the perovskite
to visible light and a mildly oxygenated atmosphere. Interestingly,
they found that the defects reacted in different ways. For example,
the most detrimental grain boundary defects were healed and stopped
trapping the holes. However, the effect on the other defect types was
more nuanced and not necessarily beneficial.

"This research shows that we likely need targeted approaches to address
the undesired effects of the different defect types, and thus improve
the performance of perovskite solar cells," concluded Prof. Dani.

========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Lucy Dickie. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Sofiia Kosar, Andrew J. Winchester, Tiarnan A. S. Doherty, Stuart
Macpherson, Christopher E. Petoukhoff, Kyle Frohna, Miguel Anaya,
Nicholas S. Chan, Julien Made'o, Michael K. L. Man, Samuel
D. Stranks, Keshav M. Dani. Unraveling the varied nature and
roles of defects in hybrid halide perovskites with time-resolved
photoemission electron microscopy. Energy & Environmental Science,
2021; 14 (12): 6320 DOI: 10.1039/d1ee02055b ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211213121834.htm

--- up 1 week, 2 days, 7 hours, 13 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)