| 1. |
- Ghorbani, Elaheh, et al.
(author)
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Energy level alignment of Cu(In,Ga)(S,Se)₂ absorber compounds with In₂S₃, NaIn₅S₈, and CuIn₅S₈ Cd-free buffer materials
- 2019
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In: Physical Review Materials. - 2475-9953. ; 3:7
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Journal article (peer-reviewed)abstract
- Motivated by environmental reasons, In2S3 is a promising candidate for a Cd-free buffer layer in Cu(In, Ga)(S, Se)(2) (CIGSSe)-based thin-film solar cells. For an impactful optimization of the In-2 S-3 alternative buffer layer, however, a comprehensive knowledge of its electronic properties across the absorber-buffer interface is of foremost importance. In this respect, finding a favorable band offset between the absorber and the buffer layers can effectively reduce the carrier recombination at the interface and improve open-circuit voltage and fill factor, leading to higher conversion efficiencies. In this study, we investigate the band alignment between the most common CIGSSe-based absorber compounds and In2S3. Furthermore, we consider two chemically modified indium sulfide layers, NaIn(5)S(8 )and CuIn5S8, and we discuss how the formation of these secondary phases influences band discontinuity across the interface. Our analysis is based on density functional theory calculations using hybrid functionals. The results suggest that Ga-based absorbers form a destructive clifflike conduction-band offset (CBO) with both pure and chemically modified buffer systems. For In-based absorbers, however, if the absorber layer is Cu-poor at the surface, a modest favorable spikelike CBO arises with NaIn5S8 and CuIn5S8.
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| 2. |
- Ghorbani, Elaheh, et al.
(author)
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New insights on the nature of impurity levels in V-doped In₂S₃: why is it impossible to obtain a metallic intermediate band?
- 2019
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In: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7496 .- 2050-7488. ; 7:13, s. 7745-7751
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Journal article (peer-reviewed)abstract
- Generation of metallic intermediate band (IB) in a semiconductor material is a key challenge for increasing the efficiency of solar cells. The formation of a partially filled IB inside the band gap of V-doped In2S3 (In2S3:V) was first predicted by first principles calculations, based on density functional theory (DFT) [Palacios et al., Phys. Rev. Lett.101, 046403 (2008)]. It is well established, however, that DFT severely underestimates the band gap and overestimates the exchange coupling constant of semiconductors and insulators. As a consequence, predictions of impurity-induced gap levels and their splitting can be flawed. In this work, we revisit In2S3:V, using a band gap corrected method (hybrid functional) and explain that the optimistic but erroneous DFT-predicted partially filled IB was caused by neglecting the strongly correlated nature of d-electrons and the present Jahn-Teller effect. Furthermore, recalling the fact that transition metals embedded in semiconductors tend to change their oxidation state, we analyze rehybridization of V d-orbitals with S p-orbitals for different oxidation states of the incorporated V. Our results demonstrate that in the presence of a reducing agent (for instance, H+), a totally filled IB can appear in In2S3:V. Successful operation of the IB solar cell is, however, strongly correlated to the metallic character of the formed IB, simply due to its ability in both receiving and promoting electrons through absorption of lower energy photons. Though, since this filled IB is non-metallic, these levels can act as active recombination centers and deteriorate the efficiency of device, which is opposite the primary goal of obtaining them.
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| 3. |
- Ghorbani, Elaheh, et al.
(author)
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Self-consistent calculations of charge self-trapping energies: A comparative study of polaron formation and migration in PbTiO₃
- 2022
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In: Physical Review Materials. - 2475-9953. ; 6:7
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Journal article (peer-reviewed)abstract
- This study presents a systematic assessment of the behavior of self-trapped electrons in PbTiO3, which is a prototypical ferroelectric material with a wide range of technological applications. Since modeling of polarons depends sensitively on the applied method, the goal of this work is to identify the parameters used in density functional theory (DFT), which allow to predict the properties of polarons with high accuracy. The DFT+U method is employed to benchmark how the choice of k-mesh grids, lattice parameters, and pseudopotential (PP) affects the polaron trapping energy. Then, the appropriate parameters were used to study polaron trapping energy and its optical transition using the HSE06 hybrid functional. It is shown that the magnitude of the trapping energy is highly sensitive to the choice of the PP and the applied lattice parameters. A comparison of polaron trapping energies using the two functionals indicates proximity of the DFT+U result to the HSE06 result. Finally, configuration coordinate diagrams for the polaron-associated absorption and luminescence peaks in PbTiO3 are presented and compared to experiments.
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| 4. |
- Klein, Andreas, et al.
(author)
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The Fermi energy as common parameter to describe charge compensation mechanisms: A path to Fermi level engineering of oxide electroceramics
- 2023
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In: Journal of Electroceramics. - 1573-8663 .- 1385-3449. ; 51
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Journal article (peer-reviewed)abstract
- Chemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering.
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