arXiv subjects

Veysel Çelik

Publications and source records attributed to Veysel Çelik.

5 recordsLinked to original sources

arXiv2025

Comparative Investigation of MAPbBr$_x$I$_{1-x}$ and SbH$_4$Br$_x$I$_{1-x}$ Perovskites: Electronic and Structural Properties

This paper comparatively investigates the structural and electronic properties of hybrid perovskites MAPbBr$_x$I$_{1-x}$ and SbH$_4$PbBr$_x$I$_{1-x}$ by means of DFT-based calculations. The main aim is to check if the increase in band gap due to substitution of I$^-$ ions with Br$^-$ ions can be overcome by introducing the inorganic SbH$_4^+$ cation. Since the Br$^-$ ions merely enhance structural stability of the perovskite framework and SbH$_4^+$ not only sustains that stability but also reduces the band gap to nearly ideal values and thereby improves electronic performance, these are the leading candidates among them. Of them, the reduced band gap SbH$_4$PbI$_3$ ($\sim$1.37 eV) and the perfectly matched SbH$_4$PbBrI$_2$ with its band gap being exactly 1.51 eV are top prospects for being stable and having high-efficiency solar cell applications. The findings show that SbH$_4^+$-based perovskites have potential for future photovoltaic devices.

cond-mat.mtrl-sci↗

arXiv2023

Theoretical investigation of the role of the organic cation in methylammonium lead iodide perovskite

The hybrid halide perovskite CH$_3$NH$_3$PbI$_3$ is easy to manufacture and inexpensive. Despite these, its efficiency as a solar cell is comparable to today's efficient solar cells. For these reasons, it is attracting a lot of attention today. However, the effects of the CH$_3$NH$_3^+$ (MA) molecule in the perovskite structure on the electronic and structural properties are still a matter of debate. Previous studies have generally focused on the rotation of the MA molecule. In this study, from a different perspective, the effects of the movement of the MA molecule along the C-N axis are investigated. With this method, the effects of the MA molecule were examined in a more controlled way. In this study, density functional theory (DFT) that accounts for van der Waals (vdW) interactions was used in the calculations for the cases. According to the data obtained, H-I ionic bonds are formed between the MA molecule and the inorganic framework. Within the structure, the H-I bond length tends to be preserved, although the position of the MA changes. In this mechanism, the I ion plays an important role by moving away from its place in the Pb-I-Pb alignment. The position of the I ion determines the nature of the band gap transition. Another effect is on the value of the band gap. Depending on the position of the I ion, the band gap may narrow by about 0.26 eV. The separation of the I ion from the Pb-I-Pb alignment by the effect of the MA molecule breaks the inverse symmetry. According to the data obtained from this study, this mechanism in the band gap is due to the breaking of the inverse symmetry in the crystal structure.

cond-mat.mtrl-sci↗

arXiv2020

Use of local density approximation within range separated hybrid exchange-correlation functional to investigate Pb doped SnO$_2$ as an electron transport layer

In this study, the structural, electronic and optical properties of Pb doped rutile SnO$_2$ were investigated using the range separated hybrid exchange-correlation functional method. In the calculations, LDA functional was used instead of PBE functional. The electronic structure of SnO$_2$ obtained by this method is quite compatible with the experimental data. The SnO$_2$ has an important usage area in optoelectronic devices due to its transparent and conductive nature. One of these important areas is the use of SnO$_2$ as an electron transport layer (ETL) in perovskite solar cells. Therefore, the energy level of the conduction band of the SnO$_2$ is important. In the Pb doped SnO$_2$ cases, the band gap narrows as the Pb doping rate increases. The bandgap of SnO$_2$ can be narrowed from 3.60 eV to 3.02 eV with a %12.5 Pb doping ratio, and this narrowing is proportional to the amount of Pb. The calculation results obtained in this study show that the decrease in the energy level of the bottom of the conduction band plays an important role in the narrowing of the band gap and there is no significant change in the energy level of the top of the valence band. Due to this effect of the Pb atom, the energy level of the conduction band can be adjusted by using the doping ratio of the Pb atom and the band gap can be narrowed in a controlled manner. With the Pb doping, the energy levels of the SnO$_2$ ETL can be adjusted in a range according to the type of perovskite used in solar cell. In addition, the doping with Pb does not create electron traps in the band gap, which is important in the transport process of electrons.

cond-mat.mtrl-sci↗

arXiv2018

Screened Coulomb Hybrid Density Functional Investigation of Oxygen Point Defects on ZnO Nanowires

In this study, oxygen vacancies and adatoms have been considered on the surface of both hexagonal and triangular ZnO nanowires. Their effect on the electronic structure and optical spectra of the nanowires have been investigated using the exact exchange hybrid density functional theory calculations. A surface oxygen vacancy gives rise to appearance of a band gap state at almost 0.7 eV above the valence band of the both types of the nanowires while an oxygen adatom show bulk-like electronic properties. A shape dependence is also indicated by the calculated physical quantities of oxygen related point defects on ZnO nanowires.

cond-mat.str-el↗

arXiv2012

Range separated hybrid exchange-correlation functional analyses of W and/or N(S) (co)doped anatase TiO_2

Electronic properties and atomic structures of W, N, S, W/N, and W/S dopings of anatase TiO_2 have been systematically investigated using the density functional theory (DFT). The exchange and correlation effects have been treated with Heyd, Scuseria and Ernzerhof (HSE) hybrid functional. Mixing traditional semi-local and non-local screened Hartree-Fock (HF) exchange energies, the HSE method corrects the band gap and also improves the description of anion/cation derived gap states. Enhanced charge carrier dynamics, observed for W/N codoped titania, can partly be explained by the passivative modifications of N 2p and W 5d states on its electronic structure. Following this trend we have found an apparent band gap narrowing of 1.03 eV for W/S codoping. This is due to the large dispersion of S 3p states at the valance band (VB) top extending its edge to higher energies and Ti--S--W hybridized states appearing at the bottom of the conduction band (CB). W/S-TiO_2 might show strong visible light response comparable to W/N codoped anatase catalysts.

cond-mat.str-el↗