Anisotropy of the Optimally-Doped Iron Pnictide Superconductor Ba(Fe0.926Co0.074)2As2

Anisotropies of electrical resistivity, upper critical field, London penetration depth and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, $x$=0.074 and $T_c \sim$23 K. The normal state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio $\gamma_{\rho} = \rho_c /\rho_a$ is about 4$\pm$1 just above $T_c$ and becomes half of that at room temperature. The anisotropy of the upper critical field, $\gamma_{H} = H_{c2,ab} /H_{c2,c}$, as determined from specific heat measurements close to $T_c$, is in the range of 2.1 to 2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, $\gamma_{\lambda}=\lambda_{c}/\lambda_{ab}$, was recorded from TDR measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, $\gamma_j=j_{c,ab}/j_{c,c}$, as determined from both direct transport measurements ($\sim$1.5) and from the analysis of the magnetization data ($\sim$3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compound and bear little resemblance to cuprates

In-Situ Determination of Buildings’ Thermo- Physical Characteristics

Ever since the introduction of energy conversion systems in the built environment, buildings have become responsible for a considerable share of global energy consumption. Many countries have therefore aimed to invest on buildings’ energy efficiency plans to reduce the depletion rate of the fossil resources and the CO2 emissions associated with them. In this context, accurate determination of building’s thermo-physical characteristics is a necessity in the processes which lead to execution of energy conservation strategies in existing buildings. These characteristics are the essential inputs for buildings’ thermal modelling, quality control, energy audits, and energy labelling, the results of which are determinant for energy renovation decisions and policies. In practice, the values of these parameters are not always available because the current determination methods are time-and-effort-expensive, and consequently rarely used. In accordance with the large deviations observed between the in-lab and in-situ thermal behaviour of building components, a special attention is laid on in-situ methods. This thesis aims at developing and testing different in-situ determination methods and approaches at different levels. Theories, simulations, and experiments, are combined for determination of a number of buildings’ most important thermo-physical characteristics. Transmission losses through the façades are known to be responsible for a significant portion of heat loss in buildings and consequently are investigated in all standard energy calculation methods. Thus, the major part of the thesis is dedicated to the thermal behaviour of exterior walls. The exact construction of existing walls is generally unknown. Consequently, the estimation of their thermal resistance, thermal conductivity, and volumetric heat capacity can be erroneous. Later, the attention is upscaled to the building level where rather than local characteristics, global characteristics are determined. At the first stage, the walls’ in-situ determination of thermal resistance has been examined. Despite the advantages of the existing standard method, “ISO 9869 Average Method” for measuring this parameter, two problems have been pointed out: long duration and imprecision. Accordingly, this phase describes and demonstrates how the simplest modifications to this standard method can improve it in terms of solving these problems. Heat transfer simulations and experiments in a variety of wall typologies have been applied to show the effect of using an additional heat flux sensor, facing the first one, installed on the opposite side of the wall. Three estimations of thermal resistance based on either indoor or outdoor heat fluxes, and the average of the two values are then defined. It is shown that one of these values satisfies the convergence criteria earlier than the other two, leading to a quicker insitu determination of thermal resistance with a higher precision. To further shorten the measurement period, in the second phase, a new transient in-situ method, Excitation Pulse Method, EPM, is developed and examined experimentally on three walls. The method is inspired by the theory of thermal response factors. In EPM, a triangular surface temperature excitation is applied at one side of the wall and the heat flux responses at both sides are measured and converted into the wall’s corresponding response factors which then leads to the wall’s thermal resistance. To validate, the results are compared to the ones obtained following the ISO 9869. The good agreement of the results confirms the possibility of measuring the Rc-value within a couple of hours. Applying this method, the overestimation of around 400% between the actual and estimated values (in practice, often based on the construction year) of thermal transmittance was resolved. Thus, EPM is believed to significantly improve the required time and accuracy in determination of the thermal behavior of walls with unknown constructions. Experimental and practical details regarding the design and construction of the method’s prototype as well as its application range are demonstrated subsequently. EPM has been patented in the Dutch patent office (Patent No. 2014467) and can be applied on in-lab and in-situ circumstances. Following the success in the proof of principle, in the third phase, detailed conditions for correct application of EPM in heavy and multi-layered walls are further studied. Heat transfer theories, simulations, and experiments are combined to evaluate the method’s performance for different types of walls. A specific attention is devoted to the relationship between the walls’ thermal response time and the response factors’ time interval, affecting the accuracy of Rc-value determination. Additionally, other hidden information in the response factors of the walls such as the possible construction are revealed. It is moreover demonstrated that in addition to the thermal resistance, the two main thermo-physical properties of a wall, the thermal conductivity and the volumetric heat capacity, as well as the wall’s thickness can be determined using inverse modelling of the Response Factors. The accuracy and precision of the method is tested in many different ways, fortifying the confidence for future application of this method. In the last phase, the advancement of smart metering and monitoring systems in buildings are considered. Such smart technologies have led to utilization of the data from, for instance, home automation systems. This data acquisition is referred to as “on-board-monitoring” category of measurements, which removes the hassle, cost, and intrusion associated with locally-conducted experiments. The problem is then observed from a perspective wider than the component level. This time, the thermo-physical characteristics are studied for a whole building rather than just the walls. It is presumed that the current and future houses and their HVAC installations are by default, equipped with basic sensors, providing on-board monitored data. Therefore, the expected available data is measured and used as input parameters. A case study of an occupied apartment, in which air temperatures, humidity, and CO2 concentrations, gas consumption, and meteorological data have been measured for one year is investigated. Global characteristics such as the heat loss coefficient and thermal capacitance are estimated through inverse modelling of a 1st order circuit analogous to the thermal model of the building, and fed by the measurement data. In addition, using construction information, winter daily air change rates leading to ventilation and infiltration heat losses are estimated from the results of the inverse modelling. These results can be used to tailor the energy efficient use of the building. In summary, the in-situ determination of walls’ thermal resistance is conducted by two methods in this thesis. The first one calls for longer measurement methods (minimum three days), but includes a straight-forward, well-known procedure. This method is highly suitable for high temperature gradients across the wall. The second method, EPM, requires more complicated instrumentation, but in return, in addition to rapid (couple of hours) determination of the Rc-value, it provides the walls’ response factors which are required for a dynamic thermal building simulation. In addition, using the results of this method, the thermal conductivity and volumetric heat capacity can be determined. EPM is most suitable for light-to-medium weighted walls and for homogeneous walls of known thickness. Stable heat flux profiles at the surfaces of the wall increase the accuracy of the method, especially when the temperature gradients across the wall are lower. Finally, as a less intrusive approach, the data from the HVAC installations’ existing sensors can be used. Global characteristics including the heat loss coefficient and the global capacitance can be then determined for a whole building, followed by ventilation and infiltration losses. Despite the low accuracy, this process is more suitable when the smart meter data is available and measurements at component level are not desired. By introducing and testing new experimental and computational methods and approaches for reliable determination of buildings’ local and global thermo-physical characteristics, this thesis pays a significant contribution to the accuracy of the energy-related predictions and operations, especially within the built environment

In-Situ Determination of Buildings’ Thermo-Physical Characteristics:

Accurate determination of building’s critical thermo-physical characteristics such as the walls’ thermal resistance, thermal conductivity, and volumetric heat capacity is essential to indicate effective and efficient energy conservation strategies at building level. In practice, the values of these parameters, which determine not only possible energy savings, but also related costs, are rarely available because the current determination methods are time-and-effort-expensive, and consequently seldom used. This thesis combines theories, simulations, computations, and experiments to develop and improve methods and approaches for determination of a number of buildings’ most important thermophysical characteristics. First, a modification to the existing standard method, “ISO 9869 Average Method” is proposed to measure the walls’ thermal resistance. Two current problems are solved: long measurement duration (weeks) and imprecision. To further shorten the measurement period to a few hours, a new transient in-situ method, Excitation Pulse Method, EPM (Patent No. 2014467), is then developed and tested. This method allows the determination of the walls’ response factors which can be applied directly in dynamic models. More importantly, it is used to extract critical construction information including walls’ thermal resistance, thermal conductivity, volumetric heat capacity, and the possible layer composition. Finally, in an attempt to reduce the hassle, cost, and intrusion associated with locally-conducted experiments, the use of data from smart meters and home automation systems is explored. Building’s global characteristics including heat loss coefficient, global heat capacitance and daily air change rates are accordingly determined

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

1D TiO2 nanostructures probed by 2D transmission electron microscopy

Hybrid solar cells based on nanoparticulate TiO2, dye and poly(3-hexylthiophene) are a common benchmark in the field of solid-state dye-sensitized solar cells. One-dimensionally nanostructured titanium dioxide is expected to enhance power-conversion efficiency (PCE) due to a high surface area combined with a direct path for electrons from the active interface to the back electrode. However, current devices do not meet those expectations and cannot surpass their mesoporous counterparts. This work approaches the problem by detailed investigation of diverse nanostructures on a nanoscale by advanced transmission electron microscopy (TEM). Anodized TiO2 nanotubes are analyzed concerning their crystallinity. An unexpectedly large grain size is found, and its implication is shown by corresponding solar cell characteristics which feature an above-average fill factor. Quasi-single crystalline rutile nanowires are grown hydrothermally, and a peculiar defect structure consisting of free internal surfaces is revealed. A growth model based on Coulombic repulsion and steric hindrance is developed to explain the resulting V-shaped defect cascade. The influence of the defects on solar cell performance is investigated and interpreted by a combination of TEM, electronic device characterization and photoluminescence spectroscopy, including lifetime measurements. A specific annealing treatment is proposed to counter the defects, suppressing several loss mechanisms and resulting in an improvement of PCEs by 35 %. Simultaneously, a process is developed to streamline electron-tomographic reconstruction of complex nanoparticles. Its suitability is demonstrated by the reconstruction of a gold nanostar and a number of iron-based particles distributed on few-layered graphene

Characterization of alternative carrier selective materials and their application to heterojunction solar cells

Crystalline silicon (c-Si) solar cells can be considered a highly industrialized and mature product with a record conversion efficiency of 26.6%, not far from the practical limit of 29.4% (for single p/n junction devices). Accordingly, current research and development are addressing some remaining efficiency and cost limitations, including the reduction of (1) carrier recombination in highly doped materials, (2) parasitic absorption by narrow band gap films and (3) high temperature energy-intensive processing (especially critical for wafer thicknesses below 100 µm). In parallel, thin-film PV (e.g. organics and perovskites) have introduced a large number of dopant-free, hole- or electron-selective materials with optoelectronic properties that are comparable or superior to standard p- and n-doped layers in c-Si. Consequently, this thesis work explores novel heterojunctions between c-Si and these carrier-selective contact materials, putting special emphasis on TMO thin films whose wide energy band gap (>3 eV), surface passivation and large work function (>5 eV) characteristics permit their utilization as transparent/passivating/hole-selective front contacts in n-type c-Si (n-Si) solar cells. To this purpose, a comparative study among three thermally evaporated TMOs (V2O5, MoO3 and WO3) allowed correlating their chemical composition with thin film conductivity, optical transmittance, passivation potential and contact resistance on n-Si substrates. The variation of these properties with film thickness, air exposure or temperature annealings was also studied. Overall, V2Ox outperformed the other oxides by obtaining higher implied open-circuit voltages and lower contact resistances, translating into higher selectivities. Next, a thorough study of the TMO/c-Si interface was performed by electron microscopy, secondary ion-mass spectrometry and x-ray photoelectron spectroscopy, identifying two separate contributions to the observed passivation: (1) a chemical component, as evidenced by a thin SiOx interlayer naturally-grown by chemical reaction during TMO evaporation; and (2) a "field-effect" component, a result of a strong inversion (p+) of the n-Si surface, induced by the large work function difference between both materials. Considering all this, an energy band diagram for the TMO/SiOx/n-Si heterojunction was proposed, reflecting the possible physicochemical mechanisms behind c-Si passivation and carrier transport. Then, the characterized TMO/n-Si heterojunctions were implemented as front hole contacts in complete solar cell devices, using thin TMO films (15 nm) contacted by an indium-tin oxide (ITO) anti-reflection/conductive electrode and a silver finger grid. As rear electron contacts, n-type a-SiCx:H thin films (20 nm) were used in localized (laser-doped) and full-area configurations, the former contacted by titanium/aluminum while the latter by ITO/silver electrodes. The best performance solar cells were obtained for V2Ox/n-Si heterojunctions, characterized by an open-circuit voltage (VOC) close to 660 mV and a maximum conversion efficiency of 16.5%. Additional characterization confirmed the good quality of the induced p+/n-Si junction, with ideality factors close to 1 and built-in potentials above 700 mV. Moreover, a photocurrent gain of ~1 mA/cm2 (300-550 nm wavelength range) was directly attributed to the difference in energy band gaps between TMOs (>2.5 eV) and the a-SiCx:H reference (~1.7 eV). On a sideline, hole-selective contacts based on PEDOT:PSS polymer solutions were also characterized, resulting in a moderate conversion efficiency of 11.6% in ITO-free devices. Finally, it is worth emphasizing the high degree of innovation in this thesis project, reporting for the first time the properties of these alternative contact materials in the context of c-Si photovoltaics and contributing to a more generic understanding of solar cell operation and design.Las celdas solares de silicio cristalino (c-Si) pueden ser consideradas un producto maduro y altamente industrializado, con eficiencias de conversión record de 26.6% muy cercanas al límite práctico de 29.4%. En consecuencia, la investigación y desarrollo actuales están abordando las limitantes restantes en eficiencia y costes, incluyendo la reducción de (1) la recombinación de portadores en materiales altamente dopados, (2) la absorción parásita debido a energías de banda prohibida insuficientes y (3) los procesos térmicos (un factor crítico para obleas delgadas de 100 micras o menos). En paralelo, tecnologías de capa delgada (e.g. orgánicos y perovskitas) han introducido un gran número de materiales selectivos a electrones o huecos, libres de dopantes y cuyas propiedades optoelectrónicas son comparables o superiores a las capas dopadas tipo-n o tipo-p usadas de manera estándar en c-Si. Es así que esta tesis explora heterouniones novedosas entre c-Si y dichos materiales de contacto selectivos, poniendo especial énfasis en capas delgadas de TMOs cuya energía de band prohibida (>3 eV), pasivación superficial y alta función de trabajo (>5 eV) permiten su utilización como contactos frontales, transparentes, pasivantes y selectivos a huecos en celdas con substrato tipo-n (n-Si). Con este propósito, se realizó un estudio comparativo entre tres TMOs evaporados térmicamente (V2O5, MoO3 and WO3) que permitió correlacionar su composición química con la conductividad, transmitancia óptica, pasivación y resistencia de contacto de capas delgadas sobre sustratos de n-Si. La variabilidad de estas propiedades con el grosor de las capas, su exposición al aire o a recocidos de alta temperatura también fue estudiada. En general, V2Ox tuvo un mejor desempeño que el resto de los óxidos al obtener mayores pseudo-voltajes de circuito abierto y menores resistencias de contacto, traduciéndose en una mayor selectividad. En seguida, un estudio detallado de la interface TMO/c-Si fue llevado a cabo mediante microscopia de electrones, espectrometría de masas de iones secundarios y espectroscopia fotoelectrónica de rayos-x, identificando dos contribuciones a la pasivación superficial: (1) un componente químico, demostrado por la presencia de una inter-capa de SiOx formada mediante reacción química durante el depósito del TMO; y (2) un componente de "efecto de campo", que es resultado de la fuerte inversión de la superficie (p+/n-Si) inducida por la gran disparidad en funciones de trabajo entre ambos materiales. Bajo esta consideración, se propuso un diagrama de bandas para la heterounión TMO/SiOx/n-Si que refleja los posibles mecanismos de pasivación y transporte de cargas. Acto seguido, se implementaron dichas heterouniones como contactos tipo-p frontales en celdas solares finalizadas, con la estructura Ag/ITO(80 nm)/TMO (15 nm)/n-Si, donde el ITO "óxido de indio y estaño" sirve de capa antirreflejo conductora y la plata como electrodo. Para el contacto tipo-n trasero, capas de a-SiCx:H dopado (20 nm) fueron utilizadas en dos configuraciones (dopado puntual por láser y contacto en área completa). El mejor desempeño se obtuvo para las celdas de V2Ox/n-Si, caracterizadas por voltajes de circuito abierto (Voc) cercanos a 660 mV y una eficiencia máxima de 16.5%. La caracterización adicional de estos dispositivos reveló factores de idealidad cercanos a 1 y una barrera interna de potencial mayor a 700 mV, comprobando la buena calidad de la unión p+/n-Si inducida. Además, ganancias en fotocorriente de ~1 mA/cm2 (para el rango de longitudes de onda de 300-550 nm) fueron directamente atribuidas a las diferencias en energías de banda prohibida entre el TMO (>2.5 eV) y la capa referencia de a-SiCx:H (~1.7 eV). Finalmente, vale la pena enfatizar el alto grado de innovación en este proyecto de tesis, reportando por primera vez las propiedades de estos materiales de contacto alternativos en el contexto de la fotovoltaica de silicio

Characterization of alternative carrier selective materials and their application to heterojunction solar cells

Crystalline silicon (c-Si) solar cells can be considered a highly industrialized and mature product with a record conversion efficiency of 26.6%, not far from the practical limit of 29.4% (for single p/n junction devices). Accordingly, current research and development are addressing some remaining efficiency and cost limitations, including the reduction of (1) carrier recombination in highly doped materials, (2) parasitic absorption by narrow band gap films and (3) high temperature energy-intensive processing (especially critical for wafer thicknesses below 100 µm). In parallel, thin-film PV (e.g. organics and perovskites) have introduced a large number of dopant-free, hole- or electron-selective materials with optoelectronic properties that are comparable or superior to standard p- and n-doped layers in c-Si. Consequently, this thesis work explores novel heterojunctions between c-Si and these carrier-selective contact materials, putting special emphasis on TMO thin films whose wide energy band gap (>3 eV), surface passivation and large work function (>5 eV) characteristics permit their utilization as transparent/passivating/hole-selective front contacts in n-type c-Si (n-Si) solar cells. To this purpose, a comparative study among three thermally evaporated TMOs (V2O5, MoO3 and WO3) allowed correlating their chemical composition with thin film conductivity, optical transmittance, passivation potential and contact resistance on n-Si substrates. The variation of these properties with film thickness, air exposure or temperature annealings was also studied. Overall, V2Ox outperformed the other oxides by obtaining higher implied open-circuit voltages and lower contact resistances, translating into higher selectivities. Next, a thorough study of the TMO/c-Si interface was performed by electron microscopy, secondary ion-mass spectrometry and x-ray photoelectron spectroscopy, identifying two separate contributions to the observed passivation: (1) a chemical component, as evidenced by a thin SiOx interlayer naturally-grown by chemical reaction during TMO evaporation; and (2) a "field-effect" component, a result of a strong inversion (p+) of the n-Si surface, induced by the large work function difference between both materials. Considering all this, an energy band diagram for the TMO/SiOx/n-Si heterojunction was proposed, reflecting the possible physicochemical mechanisms behind c-Si passivation and carrier transport. Then, the characterized TMO/n-Si heterojunctions were implemented as front hole contacts in complete solar cell devices, using thin TMO films (15 nm) contacted by an indium-tin oxide (ITO) anti-reflection/conductive electrode and a silver finger grid. As rear electron contacts, n-type a-SiCx:H thin films (20 nm) were used in localized (laser-doped) and full-area configurations, the former contacted by titanium/aluminum while the latter by ITO/silver electrodes. The best performance solar cells were obtained for V2Ox/n-Si heterojunctions, characterized by an open-circuit voltage (VOC) close to 660 mV and a maximum conversion efficiency of 16.5%. Additional characterization confirmed the good quality of the induced p+/n-Si junction, with ideality factors close to 1 and built-in potentials above 700 mV. Moreover, a photocurrent gain of ~1 mA/cm2 (300-550 nm wavelength range) was directly attributed to the difference in energy band gaps between TMOs (>2.5 eV) and the a-SiCx:H reference (~1.7 eV). On a sideline, hole-selective contacts based on PEDOT:PSS polymer solutions were also characterized, resulting in a moderate conversion efficiency of 11.6% in ITO-free devices. Finally, it is worth emphasizing the high degree of innovation in this thesis project, reporting for the first time the properties of these alternative contact materials in the context of c-Si photovoltaics and contributing to a more generic understanding of solar cell operation and design.Las celdas solares de silicio cristalino (c-Si) pueden ser consideradas un producto maduro y altamente industrializado, con eficiencias de conversión record de 26.6% muy cercanas al límite práctico de 29.4%. En consecuencia, la investigación y desarrollo actuales están abordando las limitantes restantes en eficiencia y costes, incluyendo la reducción de (1) la recombinación de portadores en materiales altamente dopados, (2) la absorción parásita debido a energías de banda prohibida insuficientes y (3) los procesos térmicos (un factor crítico para obleas delgadas de 100 micras o menos). En paralelo, tecnologías de capa delgada (e.g. orgánicos y perovskitas) han introducido un gran número de materiales selectivos a electrones o huecos, libres de dopantes y cuyas propiedades optoelectrónicas son comparables o superiores a las capas dopadas tipo-n o tipo-p usadas de manera estándar en c-Si. Es así que esta tesis explora heterouniones novedosas entre c-Si y dichos materiales de contacto selectivos, poniendo especial énfasis en capas delgadas de TMOs cuya energía de band prohibida (>3 eV), pasivación superficial y alta función de trabajo (>5 eV) permiten su utilización como contactos frontales, transparentes, pasivantes y selectivos a huecos en celdas con substrato tipo-n (n-Si). Con este propósito, se realizó un estudio comparativo entre tres TMOs evaporados térmicamente (V2O5, MoO3 and WO3) que permitió correlacionar su composición química con la conductividad, transmitancia óptica, pasivación y resistencia de contacto de capas delgadas sobre sustratos de n-Si. La variabilidad de estas propiedades con el grosor de las capas, su exposición al aire o a recocidos de alta temperatura también fue estudiada. En general, V2Ox tuvo un mejor desempeño que el resto de los óxidos al obtener mayores pseudo-voltajes de circuito abierto y menores resistencias de contacto, traduciéndose en una mayor selectividad. En seguida, un estudio detallado de la interface TMO/c-Si fue llevado a cabo mediante microscopia de electrones, espectrometría de masas de iones secundarios y espectroscopia fotoelectrónica de rayos-x, identificando dos contribuciones a la pasivación superficial: (1) un componente químico, demostrado por la presencia de una inter-capa de SiOx formada mediante reacción química durante el depósito del TMO; y (2) un componente de "efecto de campo", que es resultado de la fuerte inversión de la superficie (p+/n-Si) inducida por la gran disparidad en funciones de trabajo entre ambos materiales. Bajo esta consideración, se propuso un diagrama de bandas para la heterounión TMO/SiOx/n-Si que refleja los posibles mecanismos de pasivación y transporte de cargas. Acto seguido, se implementaron dichas heterouniones como contactos tipo-p frontales en celdas solares finalizadas, con la estructura Ag/ITO(80 nm)/TMO (15 nm)/n-Si, donde el ITO "óxido de indio y estaño" sirve de capa antirreflejo conductora y la plata como electrodo. Para el contacto tipo-n trasero, capas de a-SiCx:H dopado (20 nm) fueron utilizadas en dos configuraciones (dopado puntual por láser y contacto en área completa). El mejor desempeño se obtuvo para las celdas de V2Ox/n-Si, caracterizadas por voltajes de circuito abierto (Voc) cercanos a 660 mV y una eficiencia máxima de 16.5%. La caracterización adicional de estos dispositivos reveló factores de idealidad cercanos a 1 y una barrera interna de potencial mayor a 700 mV, comprobando la buena calidad de la unión p+/n-Si inducida. Además, ganancias en fotocorriente de ~1 mA/cm2 (para el rango de longitudes de onda de 300-550 nm) fueron directamente atribuidas a las diferencias en energías de banda prohibida entre el TMO (>2.5 eV) y la capa referencia de a-SiCx:H (~1.7 eV). Finalmente, vale la pena enfatizar el alto grado de innovación en este proyecto de tesis, reportando por primera vez las propiedades de estos materiales de contacto alternativos en el contexto de la fotovoltaica de silicio.Postprint (published version