Physicists from St Petersburg University measure one of the key parameters of a new semiconductor

Perovskites are materials with the chemical formula ABX3. Their ability to interact with light is determined by their composition, i.e. which atoms of which elements are used as A, B and X. Crystals of perovskites containing halogen atoms as X, e.g. iodine, bromine or chlorine, are called halide crystals. Their application in various devices such as solar cells and photodetectors, diodes or lasers is being actively developed.

The research findings are published in The Journal of Physical Chemistry C.

One such material is the MAPbI3 perovskite — an effective absorber and emitter of light. An important parameter of perovskites is the refractive index in the so-called transparency region, i.e. the energy region where the material does not absorb light. This index depends directly on the energy of the light and is therefore unstable, and excitons — quasiparticles that ’migrate’ through the material — play an important role in the absorption and emission of light. 

The physicists from St Petersburg University have studied the optical properties of MAPbI3 at low temperatures (from the temperature of liquid helium and above) near the absorption edge, where its properties are particularly determined by excitons and remain poorly understood. 

"We carried out an experiment to measure the reflection spectrum of the MAPbI3 perovskite at −269°C (or 4 K on the Kelvin scale). That is, we divided the spectrum of light reflected from the material by the spectrum of incident light and obtained the spectrum of the material’s reflection coefficient, which is related to the refractive index. This enabled us to measure for the first time the refractive index dispersion of the MAPbI3 perovskite in the transparency region near the exciton resonance at a temperature of 4 K. We also measured the temperature dependence of this dispersion up to a temperature of 80 K," said Anna Samsonova, an research engineer in the Photonics of Crystals laboratory at St Petersburg University.

The research was carried out at the Photonics of Crystals laboratory at St Petersburg University, established under the mega-grant programme of the Ministry of Science and Higher Education of the Russian Federation (project No 075 15 2022 1112). The researchers also used the equipment of the resource centres of the St Petersburg University Research Park: the Nanophotonics Resource Centre and the Resource Centre for X-ray Diffraction Studies.

Anna Samsonova noted that in some studies, the refractive index of perovskites in the transparency region is taken as a constant. However, the scientists from St Petersburg University believe that it is necessary to take into account the dispersion of the refractive index of perovskites when calculating the parameters of devices based on them. This is particularly important at low temperatures, where the interaction of light with excitons is strong, because it leads to a rapid change in the refractive index near the exciton resonance. 

The results of this study will help to understand the limits of perovskite applications, as well as its behaviour under some non-standard conditions.

St Petersburg University, the oldest university in Russia, was founded on 28 January (8 February) 1724. This is the day when Peter the Great issued a decree establishing the University and the Russian Academy of Sciences. Today, St Petersburg University is an internationally recognised centre for education, research and culture. In 2024, St Petersburg University celebrates its 300th anniversary. 

The plan of events during the celebration of the anniversary of the University was approved at the meeting of the Organising Committee for the celebration of St Petersburg University’s 300th anniversary. The meeting was chaired by Dmitry Chernyshenko, Deputy Prime Minister of the Russian Federation. Among the events are: the naming of a minor planet in honour of St Petersburg University; the issuance of bank cards with a special design; and the branding of the aircraft of the Rossiya Airlines to name just a few. To mark the 300th anniversary of St Petersburg University, a postage stamp depicting the Twelve Collegia building and the monument to Count Sergey Uvarov was issued. Also, a Soyuz rocket bearing the symbols of the University was launched from the Baikonur Cosmodrome. 

By the decision of the Governor of St Petersburg Alexander Beglov, 2024 is a year of the 300th anniversary of St Petersburg University in St Petersburg. On the day of the University’s 300th anniversary torches were lit on the Rostral Columns on the Spit of Vasilyevsky Island. St Petersburg University flags were raised on the Palace Bridge. The city public transport was decorated with the University’s symbols. During St Petersburg’s City Day celebrations in May 2024, St Petersburg University acted as a participating venue. Additionally, the University has launched a website dedicated to the anniversary. The website contains information about outstanding University staff, students, and alumni; scientific achievements; and details of events held as part of the celebration of the 300th anniversary of the University.