For the first time in the world, an international group of scientists led by specialists from the National Research Nuclear University MEPhI (NRNU MEPhI) was able to demonstrate the recently predicted quantum electrodynamic effect. According to the authors of the work, the results obtained will allow several times to increase the efficiency of solar cells, organic light-emitting diodes and other photovoltaic equipment. The article was published in the journal Chemical Science.
An exciton is a quasiparticle (an auxiliary object of quantum theory), the behavior of which describes the bound state of a pair of carriers of opposite charges, an electron and a hole. The concept of "exciton", as the scientists of NRNU MEPhI explained, allows one to describe with high accuracy, for example, the electrical properties of organic semiconductors when interacting with light.
The birth or destruction of an exciton - that is, a resonant transformation of energy in an organic semiconductor - is accompanied, according to scientists, by the absorption or emission of a photon (a quantum of electromagnetic radiation), respectively. In a new article by the research team, the possibility of controlling the properties of exciton transitions using the "strong coupling" effect is demonstrated.
"The effect of" strong coupling "consists in the formation of a hybrid state of energy between excitation in a substance, which is described using the concept of an exciton, and localized electromagnetic excitation. To create such conditions, special resonators are used, which are based on a pair of mirrors located opposite each other on distance of the order of the wavelength of light ", - said Igor Nabiev, a leading scientist of the Laboratory of Nano-Bioengineering (LNBE) of the National Research Nuclear University MEPhI, professor at the University of Reims in Champagne-Ardenne (France).
Lossless energy transfer
One of the effects in organic semiconductors, for which the term "exciton" is used, is Forster resonant energy transfer (FRET), which is used in medical technology. It consists in the transfer of energy without losses between two exciton states in different molecules located at a small distance from each other.
Under standard conditions, the transfer occurs in a certain direction, from the donor molecule to the acceptor molecule. To make wider use of the potential of this phenomenon in photovoltaics, it was necessary to experimentally record and study the so-called carnival effect, which consists in a controlled change in the direction of energy transfer in the FRET mode between excitons of different molecules.
It was theoretically predicted about three years ago by physicists from the United States. Employees of the Laboratory of Nano-Bioengineering of NRNU "MEPhI" became the first in the world who managed to demonstrate it.
Multiple increase in efficiency
The closest practical result of the work, according to the authors, is the ability to dramatically increase the efficiency of photovoltaic devices that convert light energy into electrical energy. This can be realized by collecting energy from those exciton states that traditionally turned out to be channels of energy losses, the scientists noted.
“The opened possibility of collecting energy from long-lived states due to the formation of hybrid states of exciton-photon will greatly increase the efficiency of electroluminescent and photovoltaic devices,” explained Dmitry Dovzhenko, a researcher at the LNBE NRNU MEPhI, a researcher at the University of Southampton (Great Britain).
The authors of the study used a previously developed microcavity to create a strong coupling between excitons in a pair of organic fluorophores and light localized in the cavity. According to NRNU MEPhI scientists, in this system it is possible to artificially control a number of parameters of energy transfer between the donor and acceptor, up to the change of the direction of transfer.
The system created at NRNU MEPhI can, according to scientists, be used for precise remote control of chemical reactions, as well as in the development of optically controlled imaging technologies in medical diagnostics and other areas.
“In addition to increasing the efficiency of FRET, which is widely used in biomedical diagnostics, the 'carnival effect' can be used to control other physicochemical processes - for example, to greatly increase the efficiency of charge transfer controlled by an external resonator or singlet fission of excitons,” Igor Nabiev noted.
The work was attended by specialists from the Moscow Institute of Physics and Technology, Sechenov University, Institute of Bioorganic Chemistry named after V. I. academicians M. M. Shemyakin and Yu. A. Ovchinnikov, University of Southampton (UK), University of Reims in Champagne-Ardenne (France), Donostia International Physics Center (Spain) and Basque Science Foundation (Spain). The research was carried out with the support of the Russian Science Foundation, grant No. 21-79-30048.