Volodymyr Kapustianyk

This work evaluates the feasibility of an X-ray-voltaic element based on a CH₃NH₃PbI₃ perovskite single crystal, which combines excellent scintillation and photovoltaic properties. It was demonstrated that the X-ray-voltaic effect in this material can only be achieved if the sample is transformed from its initial antiferroelectric state to a ferroelectric state through exposure to intense UV light. The observed generation of free current carriers is explained by a combination of two effects─the direct X-ray-to-current conversion and the conventional PV effect generated due to excitonic X-ray luminescence. The crucial role of the second mechanism is confirmed by the observed temperature changes in the photocurrent density, which was found to be proportional to the intensity of the excitonic luminescence band. It was also shown that ionic conductivity arising in the sample mostly under the influence of light photons partially compensates the flow of the free electrons and holes generated due to the conventional PV and X-ray-voltaic effects. Although the value of the generated power at room temperature in the proposed device was found to be considerably lower in comparison with those for the best X-ray-voltaic batteries based on the semiconductor p-n-junctions, it has been shown that its photoresponse increases 2 orders of magnitude at low temperatures. Besides, the technology of its manufacturing is considerably simpler and cheaper. Moreover, the significant increase in the photocurrent at cryogenic temperatures offers a crucial advantage when the proposed power cell is applied in outer space.

Due to the crystalline acentricity leading to the bulk photovoltaic effect (PV) the ferroelectrics (FEs) are considered as important candidates for creation of the PV cells overcoming the Shockley–Queisser limit of semiconductors. However, this research direction still requires more investigations to develop reliable pathways for PV efficiency optimization. The recent progress in the power conversion efficiency of the cells based on the organic-based compounds such as CH₃NH₃PbI₃ perovskite attracted much attention of the scientists. Unfortunately, manufacturing of these multilayer cells implies a very complicated technology and very high price of the devices. Under such circumstances investigations of the PV effect in the single crystals of FE perovskites look very promising. In this paper we report that due to the sample illumination with intensive UV light, CH₃NH₃PbI₃ single crystal is transformed from the pristine antiFE into the FE state. As a result, the PV effect characteristic of the FEs is realized in this material. The theoretically maximal value of the power conversion efficiency in this case was found to be one of the largest among the single crystals of this class of ferroics. We also considered the ways allowing to increase the PV efficiency of the potential solar cells based on such materials.

Pb(Mg1/3Nb2/3)O3-PbTiO3 perovskite-based crystals attract considerable scientific interest due to their interesting properties and possible use in piezoelectricity and photovoltaics. To understand the local structure and fundamental properties of such materials, in this work, we focused on the study within the density functional theory of structural, electronic, and optical properties of Pb[(Mg1/3Nb2/3)0.75Ti0.25]O3. Using GGA(PBEsol) approximation for structure optimization gives a good agreement with experimental data. Through the variation in Hubbard U parameters to GGA(PBEsol) functional, we achieve the bandgap for the Pb[(Mg1/3Nb2/3)0.75Ti0.25]O3 which is in good agreement with the experimental results. The study of the bond populations showed that the Mg-O bond demonstrates no covalency, whereas there is a significant Ti-O and Nb-O covalent bonding. Such different bonding characteristics must be responsible for the relaxor properties of Pb[(Mg1/3Nb2/3)0.75Ti0.25]O3 compound. In addition, the investigations of the optical properties of the Pb[(Mg1/3Nb2/3)0.75Ti0.25]O3 by adopting Hubbard U corrections, modifying the error of the GGA approximation, and confirming the electronic analysis, were performed.