Ing. Mgr. Neda Neykova, Ph.D.

Zinc oxide (ZnO) is a low-cost and environmentally friendly material with unique optical properties and a variety of nano and microstructures imposing challenges for energy conversion, scintillators, photocatalytic wastewater treatment, electrochemical energy storage, or sensing applications. In this work, the nominally undoped and Al-doped nanocrystalline ZnO thin layers were pulsed laser deposited (PLD) on fused silica glass. The samples were characterized by photothermal deflection (PDS) and photoluminescence (PLS) spectroscopy.

While all traditional photovoltaic materials are covalent compounds with coordination number 4, currently the most progressive materials - hybrid halide perovskites - have coordination number 6 and are ionic. This can be the key to success, especially regarding surface electronic properties, which are essential for polycrystalline semiconductors. This motivates us to explore completely new materials - cubic lithium thiospinels - and compare them with the former. Li thiospinels are prepared from stoichiometrically mixed precursors as a pellet in a nitrogen-filled glovebox. This tablet serves as a target in Pulsed Laser Deposition (PLD) and a special chamber is constructed that avoids air exposure of the precursor tablet during the loading.

Recent advances in the performance of perovskite solar cells (PSCs) have been strongly linked to advancements in interfacial engineering and the development of charge-selective contact. Self-assembled monolayers (SAMs) play an important role in inverted PSCs due to their distinctive and versatile ability to manipulate chemical and physical interface properties. Here we compared the dipole moments and energy levels of (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz)(4(3,6Dimethyl-9Hcarbazol-9yl)butyl)phosphonic acid (Me-4PACz), (4-(3,6-Dimethoxy-9H-carbazol-9-yl)butyl)phosphonic acid (MeO-4PACz), and (2-(3,7-Dibromo-10H-phenothiazin-10-yl)ethyl)phosphonic acid (Br-2EPT) molecules and theirimpact on the energy conversion of PSC devices.

The influence of Mo on the electronic states and crystalline structure as well as morphology, phase purity, luminescence and defects in the ZnO rods grown as free-standing nanoparticles is under study of the bundle of experimental techniques. Mo has almost no influence on luminescence of the as grown ZnO particles whereas shallow donors are strongly affected in ZnO rods.

Nanorods of erbium-doped zinc oxide (ZnO:Er) were fabricated using a hydrothermal method. One batch was prepared with and another one without constant ultraviolet (UV) irradiation applied during the growth. The nanorods were free-standing (FS) as well as deposited onto a fused silica glass substrate (GS). The goal was to study the atomistic aspects influencing the charge transport of ZnO nanoparticles, especially considering the differences between the FS and GS samples. We focused on the excitons; the intrinsic defects, such as zinc interstitials, zinc vacancies, and related shallow donors; and the conduction electrons. UV irradiation was applied for the first time during the ZnO:Er nanorod growth. This led to almost total exciton and zinc vacancy luminescence reduction, and the number of shallow donors was strongly suppressed in the GS samples. The effect was much less pronounced in the FS rods. Moreover, the exciton emission remained unchanged there. At the same time, the Er3+ content was decreased in the FS particles grown under constant UV irradiation while Er3+ was not detected in the GS particles at all. These phenomena are explained.

Molybdenum doped ZnO was hydrothermally grown as the arrays of nanorods deposited onto the fused silica glass substrate. The molybdenum doping level varied from 1 to 30%. The influence of Moon the electronic and crystalline structure as well as luminescence and defects in the ZnO nanorods isunder study of the bundle of experimental techniques complemented with density functional theorycalculations. The tendency of Mo to create energy states within the bandgap of ZnO and theirinfluence on the energy levels of native defects as well as excitons were proven by the synergy ofexperiment and theory. The improvement of the timing characteristics of the exciton- and zincvacancy-related emission bands upon Mo doping (1-10%) was observed. This paves the way for the defect engineering strategy in the search of effective and ultrafast scintillator with the improved lightyield as well as compared to other materials. The new concept is based on the combination of theexciton and the defect emission. It is expected to have the potential of application in the detection ofgamma rays implemented in time-of-flight positron emission tomography (TOFPET). The 20 and 30%Mo doping levels resulted in the zinc molybdates creation strongly outnumbering ZnO nanorods.

Conjugated BaI2:Eu(5 at.%) nanocrystalline particles were synthesized decorated with golden nanoparticles (GNP). GNP demonstrated relatively strong hydrophobic effect protecting the surface of the hygroscopic BaI2 from BaI2·H2O phase create the GNP/BaI2 composite – it was sped up. This was explained by the moderation of negative charge in the GNP leading to the repulsion with an electron in the excited Eu2+. The Eu2+ luminescence intensity was dropping upon the GNP content. This was due to the decreased number of Eu2+ ions in the BaI2, most probably, arising from the Au substitution for Ba competing with Eu

Heavily Er-doped zinc oxide (ZnO) microrods with nominal compositions, ZnO:Er(2, 10, 30%), wereprepared by the hydrothermal growth method. The crystallographic phases present in the as grown and annealed materials were determined. The presence of the hexagonal Wurtzite ZnO phase in the form of the hexagonal shape nanoplatelets and microrods was confirmed. Moreover, the Er2O3 phase in the form of nanosheets has been revealed as well. Erbium is partly incorporated into the ZnO hosts. It was thinning upon erbium doping level contributing to erbium oxide. This effect was even more pronouncedupon annealing in air. Moreover, the influence of erbium doping and annealing on paramagnetic shallow donor centers has been studied.

A unique set of undoped and Er doped ZnO nanorods that are grown by a hydrothermal method under the exactly same conditions in the form of 2D nanoarrays on SiO2/ZnO substrate or in a free-standing form on random nucleation seeds in solution were investigated. Their optoelectronic properties are characterized by a photo-, radio-, and cathodoluminescence in correlation with scanning electron microscopy, energy dispersive X-ray spectroscopy, electron paramagnetic resonance spectroscopy, resonance Raman spectroscopy and theoretical computing by using density functional theory. We demonstrate that erbium is incorporated at regular zinc site in the 2D arrays and as additional nucleation seeds in the free-standing nanorods. The deposited nanorods contain larger number of shallow donors (by about two orders of magnitude) and larger number of free carriers (by about one order of magnitude) as compared to the free-standing ones. It is related to the fact that the nanorods grow about one order of magnitude larger and in polycrystalline bunches on the random seeds in solution compared to the deposited arrays. Doping by Er slows down the excitonic emission further from 465 to 522 ps.

Mixed-halide perovskites are highly promising materials for tandem solar cells. The phenomenon of phase segregation, however hinders their application. Here, we combine Fourier-Transform photocurrent spectroscopy with photoluminescence and current density–voltage (J–V) measurements to study the effect of light soaking on such materials and devices. At first, we observe a gradual formation of an I-rich phase, which correlates with an increase in deep defect level concentration. We attribute these deep defects to charged iodide interstitials and associate phase segregation with iodide migration through interstitial positions. Upon further light soaking, the second less I-rich phase forms, while the deep level concentration simultaneously decreases. An empirical model describing the phase segregation mechanism is proposed to rationalize these observations. Further, we point to an important role of grain size in determining the degree and terminal phase of segregation.

Cationic doping of ZnO nanorods has gained increased interest as it can lead to the production of materials with improved luminescent properties, electrical conductivity and stability. We report on various Mo-doped ZnO powders of nanorods synthesized by the hydrothermal growth method. Further annealing or/and cold hydrogen or oxygen plasma modification was applied. The atomic structure of the as-grown and plasma-modified rods was characterized by X-ray diffraction. To identify any possible changes in morphology, scanning electron microscopy was used. Paramagnetic point defects were investigated by electron paramagnetic resonance. In particular, two new types of defects were initiated by the plasma treatment. Their appearance was explained, and corresponding mechanisms were proposed. The changes in the luminescence and scintillation properties were characterized by photo- and radioluminescence, respectively. Charge trapping phenomena were studied by thermally stimulated luminescence. Cold plasma treatment influenced the luminescence properties of ZnO:Mo structures. The contact with hydrogen lead to an approximately threefold increase in intensity of the ultraviolet exciton-related band peaking at ~3.24 eV, whereas the red band attributed to zinc vacancies (~1.97 eV) was suppressed compared to the as-grown samples. The exciton- and defect-related emission subsided after the treatment in oxygen plasma.