| 1. |
- Erbing, Axel
(författare)
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In pursuit of next generation photovoltaics : An electronic structure study of lead-free perovskite solar cells
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The recent development of perovskite-based solar cells have shown a remarkably fast increase in power conversion efficiency making them a promising low-cost alternative to conventional cells. The most successful class of materials however, the lead-halide perovskites, are held back due to toxicity and stability issues significantly limiting their use. Because of this, the investigation of new, lead-free, light-absorber materials as a replacement is an important step towards improved solar cells. The focus of this licentiate thesis is the study of bismuth-based materials and their photovoltaic properties through electronic structure calculations. Specifically, the cubic-phase AgBi2I7 under gradual substitution of either bromine or antimony is investigated using density functional theory under periodic boundary conditions. This enables calculations of the system's energy levels and band structure. Furthermore, the energy variance of the employed model of the system is sampled with respect to its level of ion disorder to obtain a better understanding of the distribution of ions within the crystal. The materials are found to have good optical properties but comparatively low efficiencies. The introduced substitutions allow fine-tuning of the system's band gap and is shown to increase the overall performance of the solar cells. In addition, spin-orbit coupling effects are demonstrated to be important when treating these bismuth-based systems. The crystal structure is found to have a significant preference for separating its silver ions and cation vacancies.
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| 2. |
- Erbing, Axel, 1991-
(författare)
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In pursuit of next generation photovoltaics : An electronic structure study of emerging solar cell materials
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of a new generation of photovoltaic technologies is an important task in order to increase the production of clean energy. Perovskite solar cells, with an exceptionally rapid development over the last decade, have transformed into perhaps the most promising candidate to provide a low-cost alternative to conventional cells. While having excellent efficiency, the most successful category of photovoltaic perovskites, the class of hybrid lead-halide perovskites, suffers from poor stability in ambient conditions and gives rise to potential health concerns due to lead toxicity. Because of these issues, studies yielding a better understanding of lead-based perovskites and investigations of new, lead-free materials are likely meaningful steps towards better and more competitive solar cells. This thesis contains studies about established lead-based perovskites, CH3NH3PbI3 and CH(NH2)2PbI3, as well as the lead-free alternatives AgBi2I7 and Cs2AgBiI6. The main method employed is electronic structure calculations through density functional theory under periodic boundary conditions including band structure calculations and projected density of states. A particular focus is given to systems with mixed anion and related effects on the electronic structure.
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| 3. |
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| 4. |
- Erbing, Axel, 1991-, et al.
(författare)
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Spatial microheterogeneity in the valence band of mixed halide hybrid perovskite materials
- 2022
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Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 13:32, s. 9285-9294
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Tidskriftsartikel (refereegranskat)abstract
- The valence band of lead halide hybrid perovskites with a mixed I/Br composition is investigated using electronic structure calculations and complementarily probed with hard X-ray photoelectron spectroscopy. In the latter, we used high photon energies giving element sensitivity to the heavy lead and halide ions and we observe distinct trends in the valence band as a function of the I : Br ratio. Through electronic structure calculations, we show that the spectral trends with overall composition can be understood in terms of variations in the local environment of neighboring halide ions. From the computational model supported by the experimental evidence, a picture of the microheterogeneity in the valence band maximum emerges. The microheterogeneity in the valence band suggests that additional charge transport mechanisms might be active in lead mixed halide hybrid perovskites, which could be described in terms of percolation pathways.
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| 5. |
- Garcia Fernandez, Alberto, et al.
(författare)
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Experimental and Theoretical Core Level and Valence Band Analysis of Clean Perovskite Single Crystal Surfaces
- 2022
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Ingår i: Small. - : Wiley. - 1613-6810 .- 1613-6829. ; 18:13
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Tidskriftsartikel (refereegranskat)abstract
- A detailed understanding of the surface and interface properties of lead halide perovskites is of interest for several applications, in which these materials may be used. To develop this understanding, the study of clean crystalline surfaces can be an important stepping stone. In this work, the surface properties and electronic structure of two different perovskite single crystal compositions (MAPbI(3) and Cs(x)FA(1-)(x)PbI(3)) are investigated using synchrotron-based soft X-ray photoelectron spectroscopy (PES), molecular dynamics simulations, and density functional theory. The use of synchrotron-based soft X-ray PES enables high surface sensitivity and nondestructive depth-profiling. Core level and valence band spectra of the single crystals are presented. The authors find two carbon 1s contributions at the surface of MAPbI(3) and assign these to MA(+) ions in an MAI-terminated surface and to MA(+) ions below the surface. It is estimated that the surface is predominantly MAI-terminated but up to 30% of the surface can be PbI2-terminated. The results presented here can serve as reference spectra for photoelectron spectroscopy investigations of technologically relevant polycrystalline thin films, and the findings can be utilized to further optimize the design of device interfaces.
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| 6. |
- Källquist, Ida, et al.
(författare)
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Operando ambient pressure photoelectron spectroscopy of solid/liquid interfaces in Li-ion batteries
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Ambient pressure photoelectron spectroscopy (APPES) is combined with electrochemistry (EC) to investigate the interface between the liquid electrolyte and the solid electrode in Li-ion battery (LIB) cells. The combination of these techniques is promising for further understanding the functionality of LIB interfaces, but it is also associated with several experimental challenges. In this work a functional EC-cell which allows for probing the solid/liquid interface is achieved by the dip-and-pull method. Two systems consisting of a 1M LiClO4 in propylene carbonate electrolyte and a sputter deposited lithium cobalt oxide (LCO) or lithium nickel manganese cobalt oxide (NMC) thin film electrode are investigated. A methodology for combined EC/APPES measurements is proposed, where continuously changing the measurement spot is necessary to avoid accumulation of surface species during X-ray exposure. The APPES spectra from the LCO and NMC electrodes show binding energy (BE) shifts depending on applied voltage. It is argued that this is related to the lithiation of the material, as the BE shifts are found to coincide with expected phase transitions to more conductive phases. The experimental data is compared to results from supercell DFT calculations modelling the bulk material. The opposite trends observed in the experimental and computational approaches indicate the importance of an accurate treatment of the exchange for a proper description of the oxidation states of the Co atoms and their corresponding core-level shifts.
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| 7. |
- Marks, Kess, 1987-, et al.
(författare)
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Naphthalene Dehydrogenation on Ni(111) in the Presence of Chemisorbed Oxygen and Nickel Oxide
- 2024
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Ingår i: Catalysts. - : MDPI AG. - 2073-4344. ; 14:2
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Tidskriftsartikel (refereegranskat)abstract
- Catalyst passivation through carbon poisoning is a common and costly problem as it reduces the lifetime and performance of the catalyst. Adding oxygen to the feed stream could reduce poisoning but may also affect the activity negatively. We have studied the dehydrogenation, decomposition, and desorption of naphthalene co-adsorbed with oxygen on Ni(111) by combining temperature-programmed desorption (TPD), sum frequency generation spectroscopy (SFG), photoelectron spectroscopy (PES), and density functional theory (DFT). Chemisorbed oxygen reduces the sticking of naphthalene and shifts H2 production and desorption to higher temperatures by blocking active Ni sites. Oxygen increases the production of CO and reduces carbon residues on the surface. Chemisorbed oxygen is readily removed when naphthalene is decomposed. Oxide passivates the surface and reduces the sticking coefficient. But it also increases the production of CO dramatically and reduces the carbon residues. Ni2O3 is more active than NiO.
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| 8. |
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| 9. |
- Wilks, Regan G., et al.
(författare)
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Dynamic Effects and Hydrogen Bonding in Mixed-Halide Perovskite Solar Cell Absorbers
- 2021
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185 .- 1948-7185. ; 12:16, s. 3885-3890
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Tidskriftsartikel (refereegranskat)abstract
- The organic component (methylammonium) of CH3NH3PbI3-xClx-based perovskites shows electronic hybridization with the inorganic framework via H-bonding between N and I sites. Femtosecond dynamics induced by core excitation are shown to strongly influence the measured X-ray emission spectra and the resonant inelastic soft X-ray scattering of the organic components. The N K core excitation leads to a greatly increased N-H bond length that modifies and strengthens the interaction with the inorganic framework compared to that in the ground state. The study indicates that excited-state dynamics must be accounted for in spectroscopic studies of this perovskite solar cell material, and the organic-inorganic hybridization interaction suggests new avenues for probing the electronic structure of this class of materials. It is incidentally shown that beam damage to the methylamine component can be avoided by moving the sample under the soft X-ray beam to minimize exposure and that this procedure is necessary to prevent the creation of experimental artifacts.
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| 10. |
- Wu, Hua, et al.
(författare)
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Bandgap Tuning of Silver Bismuth Iodide via Controllable Bromide Substitution for Improved Photovoltaic Performance
- 2019
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 2:8, s. 5356-5362
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Tidskriftsartikel (refereegranskat)abstract
- In this work, silver-bismuth-halide thin films, exhibiting low toxicity and good stability, were explored systemically by gradually substituting iodide, I, with bromide, Br, in the AgBi2I7 system. It was found that the optical bandgap can be tuned by varying the I/Br ratio. Moreover, the film quality was improved when introducing a small amount of Br. The solar cell was demonstrated to be more stable at ambient conditions and most efficient when incorporating 10% Br, as a result of decreased recombination originating from the increased grain size. Thus, replacing a small amount of I with Br was beneficial for photovoltaic performance.
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