A hybrid compound of chlorine, lead and methylammonium cation (MAPbCl3) is one of the halide perovskites used in LED lamps. The crystals of such a perovskite are transparent, and when energy is transferred, they glow within the blue and near ultraviolet range.
Semiconductors Crystals are usually studied by irradiation them with an electron beam. The energy of the incident electrons converts into crystal excitation, and it begins to glow (luminescence occurs). In turn, to understand the luminescence mechanisms, the crystal should be cooled to low temperatures. This is exactly what the scientists from the Laboratory of Crystallophotonics at SPbSU did, who synthesized an MAPbCl3 crystal and studied its luminescence at the liquid nitrogen temperature (-196 ℃).
“The luminescence spectrum of MAPbCl3 halide perovskite is complex; we can distinguish three main spectral bands in this spectrum. Scientists noticed them before, but there was no understanding of what this luminescence is associated with and what it depends on. Our research has allowed us to understand this problem,” – quotes SPbSU quotes Associate Professor Yuri Kapitonov.
One of the spectral bands turned out to be luminescence of foreign impurities on the surface of the crystal, and the rest belonged to the perovskite itself. The scientists found out that one of these bands is the glow of excitons – “artificial atoms” available in semiconductors, and the second is associated with defects in the crystal. Semiconductors with defects usually do not glow, that’s why scientists have to make great efforts to obtain luminous crystals of a sufficient quality and purity. However, the experiment showed that defects in halide perovskites are able to emit a bright blue glow.
“Our unexpected finding was the ability to rearrange the glow color when the sample is irradiated with electrons. The luminescence color can change without decreasing intensity, indicating that the defect structure of the halide perovskite rearranges to a stable form. Such rearrangement can be used for fine-tuning of the finished products made of halide perovskite, for example, LEDs,” SPbSU quotes Associate Professor Yuri Petrov.
