Announcements Research

May 31, 2019 at 10:50 am

OHIO Researchers Characterize Surface Defect States for Solar Device Development

Dr. Jixin Chen in white lab coat

Dr. Jixin Chen

A team of Ohio University researchers recently published a paper in ACS Applied Materials & Interfaces that describes the mechanism by which lead halide perovskite nanoparticles blink under light illumination. This research may aid in the development of more efficient and more stable solar cell and LED devices.

The paper, titled “Single-Particle Organolead Halide Perovskite Photoluminescence as a Probe for Surface Reaction Kinetics,” was authored by OHIO chemistry graduate students Juvinch R. Vicente, Ali Rafiei Miandashti, Kurt Waldo Sy Piecco and Joseph R. Pyle as well as two Nanoscale and Quantum Phenomena Institute members, Physics & Astronomy professor Martin Kordesch and Chemistry & Biochemistry professor Jixin Chen.

Lead halide perovskites are a group of materials that have been under extensive examination by researchers for their potential uses in solar cells, LED devices, and lasers. Chen said these materials are known to blink under light illumination; however, the mechanism behind this phenomenon is unclear.

“The most popular theory is that a photo-induced charge gets trapped on a defect in the crystal or on the surface that causes the dark state of the nanoparticle,” Chen said. “When a light photon strikes on the materials, it causes split of one bound electron from the core of an atom, which is called an exciton or an electron-hole pair.”

When the negatively charged electron is recombined with the positively charged core, it may yield a new photon (i.e., radiative decay) or it may relax as heat (i.e., nonradiative decay). When a charge is trapped by a defect of the crystal, the nonradiative decay pathway is stimulated.

Because the photoluminescence of lead halide perovskites is sensitive to surface conditions, the team hypothesized that “photoblinking” would serve as an effective way to report surface states. At the single-particle level, the team observed that these materials photoblink at rates dependent on both the excitation intensity and oxygen concentration. Their proposed model predicts that photodarkening in a nitrogen atmosphere functions through a different mechanism than conventional photoluminescence quenching (i.e., photo-knockout).

It is extremely difficult to observe a surface defect and whether or not it has trapped a charge, Chen said. This paper provides a tool to examine the surface defect states indirectly via kinetic study. This, in turn, will help the scientists to understand and alter the properties of these states using chemical methods.

Abstract: Photoluminescence (PL) of organolead halide perovskites (OHPs) is sensitive to OHPs’ surface conditions and is an effective way to report surface states. Literature has reported that at the ensemble level, the PL of photoexcited OHP nanorods declines under an inert nitrogen (N2) atmosphere and recovers under subsequent exposure to oxygen (O2). At the single-particle level, we observed that OHP nanorods photoblink at rates dependent on both the excitation intensity and the O2 concentration. Combining the two sets of information with the charge-trapping/detrapping mechanism, we are able to quantitatively evaluate the interaction between a single surface defect and a single O2 molecule using a new kinetic model. The model predicts that the photodarkening of OHP nanorods in the N2 atmosphere has a different mechanism than conventional PL quenching, which we call photo-knockout. This model provides fundamental insights into the interactions of molecular O2 with OHP materials and helps design a suitable OHP interface for a variety of applications in photovoltaics and optoelectronics.

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