A Theoretical Exploration of Emerging Solar Absorber Materials

Renewable energy sources are the only sustainable solutions that can address the increasing worldwide demand for energy without significantly furthering anthropogenic climate change and damage to our environment. Photovoltaics are able to harness the massive amount of solar radiation onto the earth each day through direct conversion to electricity, however have historically been expensive to produce and deploy on a large scale. In this thesis, we examine some of the challenges facing current photovoltaic technologies and how recently-developed materials, such as the inorganic-organic lead halide perovskites, have inspired the search for materials that may be used to provide highly-efficient, yet also cheap, mass-producible and flexible solar cells. Through ab initio density functional theory, we examine three families of compounds and, through the calculation of their electronic, optical and defect properties, are able to assess their suitability and potential as absorbers within photovoltaic devices. The caesium silver bismuth halides are lead-free analogues of the lead halide perovskites, however our calculations demonstrate that they are limited in comparison to their lead counterparts due to a mismatch in orbital angular momentum in their electronic structure, weakening their absorption. The silver copper sulfides have also shown recent promise as solar absorber materials, although we show that consideration of the optical properties is essential in successfully predicting the potential of such emergent materials. Finally, our survey of the lead bismuth sulfides predicts a promising compound for solar absorption, including the cell architecture that would be necessary to produce high device efficiencies. Through this study, we can accurately calculate properties of these materials but also hope to provide guidance in the future search for new photovoltaic technologies at the atomic scale

Understanding the electronic structure of Y2Ti2O5S2 for green hydrogen production: a hybrid- DFT and GW study

Combined hybDFT and GW study reveals surface properties and optoelectronic behaviour of Y2Ti2O5S2 for green hydrogen production

Understanding the Photocatalytic Activity of La₅Ti₂AgS₅O₇ and La₅Ti₂CuS₅O₇ for Green Hydrogen Production:Computational Insights

[Image: see text] Green production of hydrogen is possible with photocatalytic water splitting, where hydrogen is produced while water is reduced by using energy derived from light. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of La(5)Ti(2)AgS(5)O(7) and La(5)Ti(2)CuS(5)O(7)—two emerging candidate materials for water splitting. The electronic structure of both bulk materials was calculated by using hybrid DFT, which indicated the band gaps and charge carrier effective masses are suitable for photocatalytic water splitting. Notably, the unique one-dimensional octahedral TiO(x)S(6–x) and tetragonal MS(4) channels formed provide a structural separation for photoexcited charge carriers which should inhibit charge recombination. Band alignments of surfaces that appear on the Wulff constructions of 12 nonpolar symmetric surface slabs were calculated by using hybrid DFT for each of the materials. All surfaces of La(5)Ti(2)AgS(5)O(7) have band edge positions suitable for hydrogen evolution; however, the small overpotentials on the largest facets likely decrease the photocatalytic activity. In La(5)Ti(2)CuS(5)O(7), 72% of the surface area can support oxygen evolution thermodynamically and kinetically. Based on their similar electronic structures, La(5)Ti(2)AgS(5)O(7) and La(5)Ti(2)CuS(5)O(7) could be effectively employed in Z-scheme photocatalytic water splitting

Spatial Electron-hole Separation in a One Dimensional Hybrid Organic-Inorganic Lead Iodide.

The increasing efficiency of the inorganic-organic hybrid halides has revolutionised photovoltaic research. Despite this rapid progress, the significant issues of poor stability and toxicity have yet to be suitably overcome. In this article, we use Density Functional Theory to examine (Pb2I6) · (H2DPNDI) · (H2O) · (NMP), an alternative lead-based hybrid inorganic-organic solar absorber based on a photoactive organic cation. Our results demonstrate that optical properties suitable for photovoltaic applications, in addition to spatial electron-hole separation, are possible but efficient charge transport may be a limiting factor

Cu₂SiSe₃ as a promising solar absorber: harnessing cation dissimilarity to avoid killer antisites

Copper-chalcogenides are promising candidates for thin film photovoltaics due to their ideal electronic structure and potential for defect tolerance. To this end, we have theoretically investigated the optoelectronic properties of Cu₂SiSe₃, due to its simple ternary composition, and the favourable difference in charge and size between the cation species, limiting antisite defects and cation disorder. We find it to have an ideal, direct bandgap of 1.52 eV and a maximum efficiency of 30% for a 1.5 μm-thick film at the radiative limit. Using hybrid density functional theory, the formation energies of all intrinsic defects are calculated, revealing the p-type copper vacancy as the dominant defect species, which forms a perturbed host state. Overall, defect concentrations are predicted to be low and have limited impact on non-radiative recombination, as a consequence of the p–d coupling and antibonding character at the valence band maxima. Therefore, we propose that Cu₂SiSe₃ should be investigated further as a potential defect-tolerant photovoltaic absorber

Frenkel Excitons in Vacancy-Ordered Titanium Halide Perovskites (Cs₂TiX₆)

[Image: see text] Low-cost, nontoxic, and earth-abundant photovoltaic materials are long-sought targets in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional, and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancy-ordered halide perovskites (A(2)TiX(6); A = CH(3)NH(3), Cs, Rb, or K; X = I, Br, or Cl) for photovoltaic operation, following the initial promise of Cs(2)SnX(6) compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps, a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment, a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies

Frenkel Excitons in Vacancy-Ordered Titanium Halide Perovskites (Cs₂TiX₆)

Low-cost, nontoxic, and earth-abundant photovoltaic materials are long-sought targets in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional, and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancy-ordered halide perovskites (A2TiX6; A = CH3NH3, Cs, Rb, or K; X = I, Br, or Cl) for photovoltaic operation, following the initial promise of Cs2SnX6 compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps, a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment, a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies