Researchers at St. Petersburg State University have investigated the luminescence of a promising perovskite semiconductor.

Perovskite was synthesized in the Laboratory of Crystallo-Photonics at St. Petersburg State University, established as part of the mega-grant program by the Ministry of Science and Higher Education of Russia. The research results have been published in The Journal of Physical Chemistry Letters.

The familiar white light found in LED bulbs can be produced by applying a yellow-orange phosphor to a tiny semiconductor crystal that emits ultraviolet or blue light. Thus, the "heart" of any LED lamp consists of a semiconductor.

Typically, the manufacturing process for such semiconductors is expensive, as it requires pure starting materials and production at high temperatures. About ten years ago, research began worldwide into new semiconductors—halide perovskites. The production of perovskite crystals is significantly cheaper than that of "classical" analogs, as they are grown from a solution.

One such halide perovskite—MAPbCl₃—is a hybrid compound of chlorine, lead, and a small organic cation of methylammonium. The crystals of this perovskite are transparent, and when energy is supplied to them, they emit light in the blue and near-ultraviolet range.

To study wide-bandgap semiconductors, the examined crystal is irradiated with an electron beam in an electron microscope equipped with an optical spectrometer. The energy of the incoming electrons is transferred to the excitation of the crystal, causing it to emit light, a process known as luminescence.

Semiconductors emit light at room temperature; however, cooling the crystal to low temperatures aids in understanding the processes and mechanisms behind its luminescence. Scientists from the Laboratory of Crystallo-Photonics at St. Petersburg State University synthesized the MAPbCl₃ crystal and studied its cathodoluminescence at the temperature of liquid nitrogen (-196^оC).

“The luminescence spectrum of the halide perovskite MAPbCl₃ is complex, with three main spectral bands identifiable. Scientists have observed them before, but there was no understanding of what this luminescence is associated with and what factors influence it. Our research has clarified this issue,” said Yuri Kapitonov, an associate professor at the Department of Photonics at St. Petersburg State University.

One of the spectral bands was found to be due to luminescence from foreign impurities on the surface of the crystal. The others belong to the perovskite itself. Researchers established that one of these bands corresponds to the luminescence of excitons—“artificial atoms” that exist in semiconductors—while the second is associated with crystal defects. As noted by the scientists, typically, semiconductors with defects do not emit light, and significant efforts are required to obtain luminescent crystals of sufficient purity and quality. However, defects in halide perovskites possess the ability to emit bright blue light themselves.

“Our unexpected finding was the ability to tune the color of luminescence when the sample is irradiated with electrons. The color of the luminescence can change without a decrease in intensity, indicating a restructuring of the defect structure of the halide perovskite into a stable form. This restructuring could be used for fine-tuning finished products made from halide perovskite, such as LEDs,” explained Yuri Petrov, an associate professor at the Department of Solid-State Electronics at the Physical Faculty of St. Petersburg State University.

The research was conducted in the Laboratory of Crystallo-Photonics at St. Petersburg State University, established under a mega-grant from the Ministry of Science and Higher Education of Russia, using equipment from the resource center of St. Petersburg State University “Nanotechnology” at the Scientific Park of St. Petersburg State University.