Photovoltaic application of O-doped Wittichenite-Cu 3 BiS 3: from microscopic properties to maximum efficiencies

The electronic properties and the low environmental impact of Cu 3 BiS 3 make this compound a promising material for low-cost thin film solar cell technology. From the first principles, the electronic properties of the isoelectronic substitution of S by O in Cu 3 BiS 3 have been obtained using two different exchange-correlation potentials. This compound has an acceptor level below the conduction band, which modifies the opto-electronic properties with respect to the host semiconductor. In order to analyze a possible efficiency increment with respect to the host semiconductor, we have calculated the maximum efficiency of this photovoltaic absorber material.

Estudio teórico de propiedades termodinámicas y optoelectronicas de nuevos materiales fotovoltaicos de banda intermedia = Theoretical study of thermodynamic and optoelectronic properties of new intermediate band photovoltaic materials

Los materiales de banda intermedia han atraido la atención de la comunidad científica en el campo de la energía solar fotovoltaica en los últimos años. Sin embargo, con el objetivo de entender los fundamentos de las células solares de banda intermedia, se debe llevar a cabo un estudio profundo de la características de los materiales. Esto se puede hacer mediante un modelo teórico usando Primeros Principios. A partir de este enfoque se pueden obtener resultados tales como la estructura electrónica y propiedades ópticas, entre otras, de los semiconductores fuertemente dopados y sus precursores. Con el fin de desentrañar las estructuras de estos sistemas electrónicos, esta tesis presenta un estudio termodinámico y optoelectrónico de varios materiales fotovoltaicos. Específicamente se caracterizaron los materiales avanzados de banda intermedia y sus precursores. El estudio se hizo en términos de caracterización teórica de la estructura electrónica, la energética del sistema, entre otros. Además la estabilidad se obtuvo usando configuraciones adaptadas a la simetría del sistema y basado en la combinatoria. Las configuraciones de los sitios ocupados por defectos permiten obtener información sobre un espacio de configuraciones donde las posiciones de los dopantes sustituidos se basan en la simetría del sólido cristalino. El resultado puede ser tratado usando elementos de termodinámica estadística y da información de la estabilidad de todo el espacio simétrico. Además se estudiaron otras características importantes de los semiconductores de base. En concreto, el análisis de las interacciones de van der Waals fueron incluidas en el semiconductor en capas SnS2, y el grado de inversión en el caso de las espinelas [M]In2S4. En este trabajo además realizamos una descripción teórica exhaustiva del sistema CdTe:Bi. Este material de banda-intermedia muestra características que son distintas a las de los otros materiales estudiados. También se analizó el Zn como agente modulador de la posición de las sub-bandas prohibidas en el material de banda-intermedia CuGaS2:Ti. Analizándose además la viabilidad termodinámica de la formación de este compuesto. Finalmente, también se describió el GaN:Cr como material de banda intermedia, en la estructura zinc-blenda y en wurtztite, usando configuraciones de sitios ocupados de acuerdo a la simetría del sistema cristalino del semiconductor de base. Todos los resultados, siempre que fue posible, fueron comparados con los resultados experimentales. ABSTRACT The intermediate-band materials have attracted the attention of the scientific community in the field of the photovoltaics in recent years. Nevertheless, in order to understand the intermediate-band solar cell fundamentals, a profound study of the characteristics of the materials is required. This can be done using theoretical modelling from first-principles. The electronic structure and optical properties of heavily doped semiconductors and their precursor semiconductors are, among others, results that can be obtained from this approach. In order to unravel the structures of these crystalline systems, this thesis presents a thermodynamic and optoelectronic study of several photovoltaic materials. Specifically advanced intermediate-band materials and their precursor semiconductors were characterized. The study was made in terms of theoretical characterization of the electronic structure, energetics among others. The stability was obtained using site-occupancy-disorder configurations adapted to the symmetry of the system and based on combinatorics. The site-occupancy-disorder method allows the formation of a configurational space of substitutional dopant positions based on the symmetry of the crystalline solid. The result, that can be treated using statistical thermodynamics, gives information of the stability of the whole space of symmetry of the crystalline lattice. Furthermore, certain other important characteristics of host semiconductors were studied. Specifically, the van der Waal interactions were included in the SnS2 layered semiconductor, and the inversion degree in cases of [M]In2S4 spinels. In this work we also carried out an exhaustive theoretical description of the CdTe:Bi system. This intermediate-band material shows characteristics that are distinct from those of the other studied intermediate-band materials. In addition, Zn was analysed as a modulator of the positions of the sub-band gaps in the CuGaS2:Ti intermediate-band material. The thermodynamic feasibility of the formation of this compound was also carried out. Finally GaN:Cr intermediate-band material was also described both in the zinc-blende and the wurtztite type structures, using the symmetry-adapted-space of configurations. All results, whenever possible, were compared with experimental results.

Obtaining an intermediate band photovoltaic material through the Bi insertion in CdTe

Defect interaction can take place in CdTe under Te and Bi rich conditions. We demonstrate in this work through first principles calculations, that this phenomenon allows a Jahn Teller distortion to form an isolated half-filled intermediate band in the host semiconductor band-gap. This delocalized energy band supports the experimental deep level reported in the host band-gap of CdTe at a low bismuth concentration. Furthermore, the calculated optical absorption of CdTe:Bi in this work shows a significant subband-gap absorption that also supports the enhancement of the optical absorption found in the previous experimental results.

Optical absorption analysis of quaternary molybdate- and tungstate-ordered double perovskites

Quaternary-ordered double perovskite A2MM’O6 (M=Mo,W) semiconductors are a group of materials with a variety of photocatalytic and optoelectronic applications. An analysis focused on the optoelectronic properties is carried out using first-principles density-functional theory with several U orbital-dependent one-electron potentials applied to different orbital subspaces. The structural non-equivalence of the atoms resulting from the symmetry has been taken in account. In order to analyze optical absorption in these materials deeply, the absorption coefficients have been split into inter- and intra-non-equivalent species contributions. The results indicate that the effect of the A and M’ atoms on the optical properties are minimal whereas the largest contribution comes from the non-equivalent O atoms to M transitions.

Microscopic Analysis and Applications of the Cu(Sb,Bi)S2 High Optical Absorption

Ternary Cu(Sb,Bi)S2 semiconductors are a group of materials with a wide variety of applications, especially photovoltaic. An analysis of the structural, electronic, and optical properties obtained from first-principles is presented. The microscopic justification of the high absorption coefficients is carried out by splitting the optical properties on chemical species contributions according to the symmetry. Focusing on photovoltaic applications, and from first-principles results, the efficiencies for several solar spectra are obtained as a function of the device thickness. This study indicates the great potential of these materials for photovoltaic and other optical devices.

Optical absorption and applications of the ABO4 (A=Ca, Pb and B= Mo, W) semiconductors

Ternary molybdates and tungstates ABO4 (A=Ca, Pb and B= Mo, W) are a group of materials that could be used for a variety of optoelectronic applications. We present a study of the optoelectronic properties based on first-principles using several orbitaldependent one-electron potentials applied to several orbital subspaces. The optical properties are split into chemical-species contributions in order to quantify the microscopic contributions. Furthermore, the effect of using several one-electron potentials and orbital subspaces is analyzed. From the results, the larger contribution to the optical absorption comes from the B-O transitions. The possible use as multi-gap solar cell absorbents is analyzed.

Photovoltaic application of the V, Cr and Mn-doped cadmium thioindate

The CdIn2S4 spinel semiconductor is a potential photovoltaic material due to its energy band gap and absorption properties. These optoelectronic properties can be potentiality improved by the insertion of intermediate states into the energy bandgap. We explore this possibility using M = Cr, V and Mn as an impurity. We analyze with first-principles almost all substitutions of the host atoms by M at the octahedral and tetrahedral sites in the normal and inverse spinel structures. In almost all cases, the impurities introduce deeper bands into the host energy bandgap. Depending on the site substitution, these bands are full, empty or partially-full. It increases the number of possible inter-band transitions and the possible applications in optoelectronic devices. The contribution of the impurity states to these bands and the substitutional energies indicate that these impurities are energetically favorable for some sites in the host spinel. The absorption coefficients in the independent-particle approximation show that these deeper bands open additional photon absorption channels. It could therefore increase the solar-light absorption with respect to the host.

Ionization energies of amphoteric-doped Cu2ZnSnS4: Photovoltaic application

The substitution of Cu, Sn or Zn in the quaternary Cu2ZnSnS4 semiconductor by impurities that introduce intermediate states in the energy bandgap could have important implications either for photovoltaic or spintronic applications. This allows more generation–recombination channels than for the host semiconductor. We explore and discuss this possibility by obtaining the ionization energies from total energy first-principles calculations. The three substitutions of Cu, Sn and Zn by impurities are analyzed. From these results we have found that several impurities have an amphoteric behavior with the donor and acceptor energies in the energy bandgap. In order to analyze the role of the ionization energies in both the radiative and non-radiative processes, the host energy bandgap and the acceptor and the donor energies have been obtained as a function of the inward and outward impurity-S displacements. We carried out the analysis for both the natural and synthetic CZTS. The results show that the ionization energies are similar, whereas the energy band gaps are different.

Electronic and optical properties of substitutional V, Cr and Ir impurities in Cu2ZnSnS4

The substitution of cation atoms by V, Cr and It in the natural and synthetic quaternary Cu2ZnSnS4 semiconductor is analyzed using first-principles methods. In most of the substitutions, the electronic structure of these modified CZTS is characterized for intermediate bands with different occupation and position within of the energy band gap. A study of the symmetry and composition of these intermediate bands is carried out for all substitutions. These bands permit additional photon absorption and emission channels depending on their occupation. The optical properties are obtained and analyzed. The absorption coefficients are split into contributions from the different absorption channels and from the inter- and intra-atomic components. The sub bandgap transitions are significant in many cases because the anion states contribute to the valence, conduction and intermediates bands. These properties could therefore be used for novel optoelectronic devices.

Optoelectronic property analysis of MCrO4 (M=Ba, Sr) with a response to visible light irradiation

Ternary MCrO4 (M = Ba, Sr) semiconductors are materials with a variety of photocatalyst and optoelectronic applications. We present detailed microscopic analyses based on first principles of the structure, the electronic properties and the optical absorption in which the difference between symmetrically non-equivalent atoms has been considered. The high absorption coefficients of these materials are split into chemical species contributions in accordance with the symmetry. The high optical absorption in these materials is mainly because of the Cr–O inter-species transitions.

Quantification of the hydrophobic interaction by simulations of the aggregation of small hydrophobic solutes in water

The hydrophobic interaction, the tendency for nonpolar molecules to aggregate in solution, is a major driving force in biology. In a direct approach to the physical basis of the hydrophobic effect, nanosecond molecular dynamics simulations were performed on increasing numbers of hydrocarbon solute molecules in water-filled boxes of different sizes. The intermittent formation of solute clusters gives a free energy that is proportional to the loss in exposed molecular surface area with a constant of proportionality of 45 ± 6 cal/mol⋅Å2. The molecular surface area is the envelope of the solute cluster that is impenetrable by solvent and is somewhat smaller than the more traditional solvent-accessible surface area, which is the area transcribed by the radius of a solvent molecule rolled over the surface of the cluster. When we apply a factor relating molecular surface area to solvent-accessible surface area, we obtain 24 cal/mol⋅Å2. Ours is the first direct calculation, to our knowledge, of the hydrophobic interaction from molecular dynamics simulations; the excellent qualitative and quantitative agreement with experiment proves that simple van der Waals interactions and atomic point-charge electrostatics account for the most important driving force in biology.

A simple test of the vicarious trial-and-error hypothesis of hippocampal function.

Vicarious trial-and-error (VTE) is a term that Muenzinger and Tolman used to describe the rat's conflict-like behavior before responding to choice. Recently, VTE was proposed as a mechanism alternative to the concept of "cognitive map" in accounts of hippocampal function. That is, many phenomena of impaired learning and memory related to hippocampal interventions may be explained by behavioral first principles: reduced conflicting, incipient, pre-choice tendencies to approach and avoid. The nonspatial black-white discrimination learning and VTE behavior of the rat were investigated. Hippocampal-lesioned and sham-lesioned animals were trained for 25 days (20 trials per day) starting at 60 days of age. Each movement of the head from one discriminative stimulus to the other was counted as a VTE instance. Lesioned rats had fewer VTEs than sham controls, and the former learned much more slowly or never learned. After learning, VTE frequency declined. Male and female rats showed no significant differences in VTE behavior or discrimination learning.

Propriedades magnéticas de trímeros de FexCo1-x depositados em Pt(111)

Quais propriedades magnéticas são modificadas quando se agrupam átomos de Fe/Co para formar estruturas quasi-2D, se comparadas aos nanofios (quasi-1D) de FexCo1-x? E como estas propriedades reagem com a variação da proporção de Fe/Co nos aglomerados? A fim de responder a estas questões, trímeros de FexCo1-x depositados em Pt(111) são investigados utilizando o método de primeiros princípios Real Space-Linear Muffin-Tin Orbital-Atomic Sphere Approximation (RS-LMTO-ASA) no âmbito da Teoria do Funcional da Densidade (DFT). Diferentes configurações de trímeros triangulares são consideradas, variando-se as posições e a concentração dos átomos de Fe/Co. Neste trabalho, demonstra-se a ocorrência de uma tendência não-linear estritamente decrescente dos momentos orbitais médios como função da concentração de Fe, distinta do encontrado tanto para os nanofios de FexCo1-x (dependência linear) quanto para a monocamada correspondente (dependência não-linear). Os resultados obtidos mostram ainda que os momentos orbitais variam com o ambiente local e com a direção de magnetização, especialmente quando associados aos átomos de Co, em concordância com publicações anteriores. A mudança de dimensionalidade quasi-1D (nanofios) para quasi-2D (trímeros compactos) não afeta o comportamento dos momentos de spin, que permanecem descritos por uma função linear com respeito à proporção de Fe/Co. Ambos o formato e a concentração de Fe nos sistemas apresentam um papel importante nos valores de energia de anisotropia magnética. Em adição, observou-se que o subtrato de Pt opera ativamente na definição das propriedades magnéticas dos aglomerados. Embora todas as configurações lineares e compactas dos aglomerados de FexCo1-x sejam estáveis e exibam interações fortemente ferromagnéticas entre os primeiros vizinhos, nem todas revelaram o ordenamento colinear como estado fundamental, apresentando uma interação de Dzyaloshinskii-Moriya não-desprezível induzida pelo acoplamento spin-órbita. Estes casos específicos são: o trímero triangular de Co puro e o trímero linear (nanofio) de Fe puro, para o qual foi verificado o acoplamento do tipo Ruderman-Kittel-Kasuya-Yosida entre os átomos de Fe constituintes. Os resultados obtidos contribuem para o entendimento de quais mecanismos definem o magnetismo nos trímeros de FexCo1-x/Pt(111), e discutem as questões presentes atualmente na literatura no contexto destes sistemas.

Nanocontacts: probing electronic structure under extreme uniaxial strains

Nanometer-sized metallic necks have the unique ability to sustain extreme uniaxial loads (about 20 times greater than the bulk material). We present an experimental and theoretical study of the electronic transport properties under such extreme conditions. Conductance measurements on gold and aluminum necks show a strikingly different behavior: While gold shows the expected conductance decrease with increasing elastic elongation of the neck, aluminum necks behave in the opposite way. We have performed first-principles electronic-structure calculations which reproduce this behavior, showing that it is an intrinsic property of the bulk band structure under high uniaxial strain.

Formation of a metallic contact: jump to contact revisited

The transition from tunneling to metallic contact between two surfaces does not always involve a jump, but can be smooth. We have observed that the configuration and material composition of the electrodes before contact largely determine the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through a combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimers, or double-bond contacts.