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STUDY OF THE PHOTO-CONVERSION EFFICIENCY OF QUANTUM DOTS SENSITIZED TiO2 SOLAR CELLS WITH VISIBLE-INFRARED ABSORPTION

Diego Esparza (2016)

"Quantum Dots (QDs) are currently being investigated for application into solar cells to enhance the cell efficiency based on the photovoltaic parameters such as photocurrent, photovoltage and fill factor. Our aim is to increase the photocurrent using different QDs, which absorption in the visible and infrared region. The most common QDs with absorption in the visible range are the Cadmium Sulphide (CdS) and Cadmium Selenide (CdSe) QDs. The effect of different sensitization techniques in the configuration TiO2/CdS/CdSe/ZnS is analyzed; with this configuration a photoconversion efficiency (PCE) of 4.7% is obtained. A strong absorption band centered at 650 nm reveals the contribution of colloidal CdSe QDs.

The infrared region is studied with lead sulphide (PbS) QDs in the configuration TiO2/PbS/CdS/ZnS, where we find that the infrared absorption is for the PbS, several authors use the CdS for protected the oxidation of PbS. The TiO2/PbS/CdS/ZnS QDSSCs have been compared with the efficient electron transport of the TiO2/PbS/PFN/CdS/ZnS configuration, where PFN is poly[(9,9-bis(3′-(N,N dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)]. The PbS was used in TiO2/PbS/PFN/CdS/ZnS configuration to obtain a 3.6% photo conversion efficiency (PCE) by the utilization of a PFN active layer, which increases short circuit current by the light scattering strategy. Moreover, the less toxicity of Bismuth sulfide (Bi2S3) has been applied to TiO2/CdS/Bi2S3/ZnS configuration instead of PbS, we found a PCE of 2.52%. It is important to note that the enhancement in the cell efficiency was explained by broadening the absorption spectra and energy level diagram to reduce the transport losses because of superficial defects.

One of the most efficient colloidal materials, Cadmium Selenide Telluride (CdSeTe) QDs, was added into the TiO2/CdS/CdSe/CdSeTe/ZnS/SiO2 configuration, resulting in a record PCE of 7.4%. The general increase of absorption as well as a shift towards longer wavelengths up to 800 nm was observed clearly with the coating of colloidal CdSeTe QDs. Finally a preliminary result about perovskite solar cells and perovskite QDs is presented."

Doctoral thesis

QUANTUM DOTS, QUANTUM DOTS SENSITIZED SOLAR CELLS, PHOTOCONVERSION EFFICIENCY, PEROVSKITE SOLAR CELLS, PEROVSKITE QUANTUM DOTS CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ÓPTICA

Preparation of quantum dots hydrogel nanocomposites with improved cytotoxicity

ABRAHAM GONZALEZ RUIZ MIGUEL ANGEL CAMACHO LOPEZ MIRIAM VERONICA FLORES MERINO (2017)

Nanocomposites are materials with unique properties and a wide range of applications. The combination of different nanostructures with traditional materials gives a variety of possibilities that should be analyzed. Especially, functional fluorescent semiconductor quantum dots (QDs) embedded in polymeric matrices have shown promising fluorescence and biocompatibility properties. These hybrid materials can be used in medical applications such as biodiagnostic and bioimaging. In this study, two hydrogels, one of polyethylene glycol diacrylate (PEGDA) and other of polyacrylamide (PAAm), were prepared with quantum dots of CdTe (4 nm of diameter) and characterized. The aim of this research was to analyze the optical properties of the nanocomposites and their cell viability. QDs nanocomposites were fabricated by a free radical polymerization process. The optical studies showed that the nanocomposites have well defined properties of fluorescence. To study the biocompatibility of the nanocomposites, metastatic B16f10 cell line were used and MTT assay was performed. The nanocomposites had a significant improved cell viability compared with QDs solutions.

Article

quantum dots hydrogels cytotoxixity BIOLOGÍA Y QUÍMICA

Semi-classical model of a pair of coupled quantum dots

JULIO CESAR GARCIA MELGAREJO (2011)

Quantum dots are semiconductor structures whose size is in the order of 10 nm.

These structures can confine inside themselves electrons. The study of these

structures are interesting because of their application range is quite wide. They

are used in the manufacture of lasers with a small bandwidth. In medicine they

are used to obtain highly-contrasted medical images. Also, they can be applied

in the manufacture of efficient solar cells.

Among quantum dots there is an interaction, called Foerster interaction; it

consists on the exciton transfer from a quantum dot to another in a non-radiative

transfer mechanism.

This thesis work consists of two sections, the first is devoted to developing a semi-

classical model of the interaction between a single quantum dot and a classical

electric field. Analytic expressions for the single QD-population inversion and

complex amplitude electric dipole is given. Later this analysis is generalized

into a semiclassical study of a pair of coupled quantum dots through their Foerster

interaction; each quantum dot is within its own cavity interacting with its own

classical electric field. We give analytic expressions for their inversions and for

their respective complex electric dipole.

Finally, we emphasize the weak coupling regime and in addition, we point out

the characteristics of that system in comparison to the single quantum dot. The

observation of those characteristics is a proof of the coupling and its analysis

allows obtaining information on the strength of the coupling.

Los puntos cuánticos son estructuras semiconductoras cuyo tamaño es del orden de

10 nm. Estas estructuras tienen la propiedad de confinar en su interior electrones.

El estudio de estas estructuras es interesante, ya que su campo de aplicación es

muy amplio. Son utilizados en la fabricación de láseres con un ancho de banda

muy pequeño. En la medicina son utilizados para obtener imágenes médicas de

alto contraste. También pueden ser aplicados en la construcción de celdas solares

eficientes.

Entre los puntos cuánticos se presenta una interacción, denominada interacción

de Foerster, la cual consiste en la transferencia de un excitón de un punto cuántico

al otro. Este es un mecanismo de transferencia de energía no radiativo.

Este trabajo de tesis consta de dos secciones, una dedicada a desarrollar el

modelo semiclásico de la interacción de un punto cuántico con un campo eléctrico

clásico. Se proporciona la expresión analítica para la inversión del punto cuántico;

de la misma forma se proporcionan expresiones analíticas para las oscilaciones de

la amplitud compleja del dipolo eléctrico.

Posteriormente este análisis es generalizado al estudio de un par de puntos

cuánticos acoplados mediante una interacción de Foerster; cada punto cuántico

está en su propia cavidad interactuando con su propio campo eléctrico clásico.

Proporcionamos expresiones analíticas para la inversión y para las oscilaciones

complejas del dipolo eléctrico.

Finalmente se pone énfasis en el régimen de acoplamiento débil comparado

con la Frecuencia de Rabi y se señalan las características que el sistema acoplado

presenta respecto al sistema de un punto cuántico. La observación de estas características es una prueba del acoplamiento y su análisis permite obtener información acerca de su fuerza.

Master thesis

Quantum dots Population inversion Fluoresience CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ÓPTICA

Preparación y caracterización de películas delgadas de dióxido de titanio (TiO2) como capa conductora de electrones para celdas fotovoltaicas de perovskitas orgánicas

Preparation and characterization of titanium dioxide (TiO2) thin films as electron transport layer for organic perovskite solar cells

EMMANUEL VILLA FLORES (2017)

En el presente trabajo se optimizaron los parámetros de depósito para obtener películas delgadas de TiO2 para ser usadas como capa conductora de electrones en celdas solares de perovskita orgánica. Para dicho fin las películas deben estar formadas por una capa densa continua y otra mesoporosa. La capa densa continua se depositó sobre substratos de vidrio y óxido de estaño dopado con flúor (FTO), por dos técnicas: la primera, mediante el uso combinado de sol-gel y centrifugado; y la segunda fue la de erosión iónica; en ambas técnicas se determinaron los parámetros de depósito y se compararon las características de las películas (fase cristalina, espesor, rugosidad, uniformidad, brecha de energía y transmitancia). Por sol-gel y centrifugado, se encontró que la solución óptima es usando etanol, agua, ácido nítrico y tetraisopropóxido de titanio, mezclándolos y agitándolos por 20 minutos. Usando esta solución se determinaron los parámetros para crecer las películas, los que fueron de 0.5 ml de solución de TiO2, 5000 rpm, 20 s de tiempo de centrifugado, y calcinación a 500 °C durante 30 minutos. Para la optimización de la capa continua depositada por erosión iónica se obtuvo que usando una presión de oxígeno (9%) y argón de 2.5𝑥10−3 Torr, 380 °C y una potencia de 120 W con una fuente DC, se obtienen características similares a las películas crecidas por sol-gel. Por ambas técnicas se producen películas con una fase cristalina de anatasa, una superficie homogénea y continua, transmitancia de 88%-93%, y una banda prohibida de 3.38-3.39 eV. La capa mesoporosa se depositó sobre la continua, con el mismo procedimiento de sol-gel y centrifugado ya descrito, pero agregando una cantidad de 0.024 gr para PEG y 0.0038 g de Pluronic F-127 ® por ml de solución y un tiempo de agitación de 90 minutos, lo que produce poros menores a los 2nm. Así entonces se optimizaron los parámetros de depósito que proporcionaron tanto de la capa continua como la mesoporosa de TiO2, de modo que las películas obtenidas sirven para su aplicación como capa conductora de electrones en celdas solares de perovskita orgánica

In the present work, the deposition parameters were optimized in order to obtain TiO2 thin films for applications as electron transport layer in organic perovskite solar cells. For this purpose, the films must be constituted by a continuous dense and a mesoporous layer. The first one was deposited over a glass substrate covered with Fluorine-doped Tin oxide, by two techniques, sol-gel assisted by spin coating and sputtering; for both techniques the deposition parameters were determined, and the characteristics of the grown films were compared (crystalline phase, thickness, roughness, uniformity, band-gap and transmittance). Using sol-gel and spin coating, it was found that the optimal solution is made of ethanol, deionized water, nitric acid, and titanium tetraisopropoxide. After these compounds were mixed and stirred for 20 minutes, 0.5 ml of the solution were spin coated at 5000 rpm for 20s and then a heat treatment of 500 °C for 30 minutes was applied. With the sputtering technique, the optimal parameters were an Oxygen (9%) and Argon pressure of 2.5𝑥10−3 𝑇𝑜𝑟𝑟, 380 °C and a DC power of 120 W, obtaining similar characteristics as the films grown by sol-gel. The anatase TiO2 crystalline phase was obtained with both techniques, a homogeneous continuous surface, transmittance between 88%-93%, and a bandgap of 3.383.39 eV were also obtained. The mesoporous layer was deposited over the continuous layer; it was prepared using the same sol-gel and spin coating conditions, but adding 0.024 gr of PEG or 0.0038 gr of Pluronic F-127 ® per 1ml of the prepared solution, both with a stirring time of 90 minutes. Using this organic compounds, pores under 2 nm of diameter were obtained. Therefore, the optimal parameters for growing the dense continuous and mesoporous layer were achieved; consequently the films obtained by both methods can be used as electron transport layers in organic perovskite solar cells

Master thesis

TiO2, sol-gel, centrifugado, erosión iónica, perovskita, celdas solares Spin coating, sputtering, perovskite, solar cells INGENIERÍA Y TECNOLOGÍA CIENCIAS TECNOLÓGICAS TECNOLOGÍA DE MATERIALES PROPIEDADES DE LOS MATERIALES

STUDY OF INTERACTION EFFECTS OF QUANTUM DOTS AND GOLD NANOPARTICLES WITH CELLS BY RAMAN SPECTROSCOPY AND ADVANCED MICROSCOPY

Elisa Cepeda-Perez (2016)

"This dissertation is organized into five chapters. Chapter one corresponds to an introduction, which provides a brief explanation about the Raman spectroscopic technique, the use of CdTe for biomedical applications, the cellular uptake processes and finally the uses of SERS powdered substrates for tissue diagnosis. Chapter two explains synthesis and characterization of thioglycolic acid capped CdTe quantum dots (TGA@CdTe QDs), naked anisotropic gold nanoparticles (AuNPs) and Au/SiO2 powder. Chapter three describes the uptake, internalization and interaction of TGA@CdTe QDs and AuNPs with Haematococcus Pluvialis algae cells and human oral epithelial cells. In chapter four a systematic study of the SERS activity of Au/SiO2 powder and AuNPs for tissue diagnosis is presented and discussed. Finally, the dissertation is summarized in chapter five with the possible future works in SERS and cellular uptake for biomedical applications."

Doctoral thesis

NANOPARTICLES, SERS, QUANTUM DOTS, RAMAN SPECTROSCOPY, CELLULAR INTERACTIONS CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ÓPTICA LÁSERES

Dynamic behavior of collective and reduced Quantum dots system

SERGIO SANCHEZ SANCHEZ (2011)

In this dissertation we show a theoretical study on quantum nanostructures of 3D

confinement better known as Quantum Dots (QDs). We explore the collective and

cooperative potential of such systems under a quantum scheme that is all quantum

i.e. radiation-matter quantization (QDs and EM field), also we make a semiclassical

approach for solve the collective system derived of the Heisenberg

picture. We make use of Excited Atomic Coherent States in order to facilitate the

theoretical study of nonlinear Hamiltonian. Subsequently we reduce our system to

a simple system of two QDs in its own cavity for research the quantum

entanglement necessary for producing qubits (bits of quantum information

processing) for quantum computing and information.

En esta tesis se presenta un estudio teórico sobre nano-estructuras cuánticas de

confinamiento tridimensional mejor conocidas como Puntos Cuánticos. Se exploran

las potencialidades colectivas y cooperativas de dichos sistemas bajo un esquema

todo cuántico es decir cuantización de la materia-radiación (Puntos y Campo EM),

además hacemos una aproximación semi-clásica para la solución del sistema

colectivo derivada de la imagen de Heisenberg. Hacemos uso de Estados Atómicos

Coherentes Excitados para facilitar el estudio teórico no lineal del Hamiltoniano.

Posteriormente reducimos nuestro sistema a un sistema simple de dos puntos en

su propia cavidad para estudiar el entrelazamiento cuántico necesario para

producir qubits (bits para procesamiento de información cuántica) para

computación e información cuántica.

Doctoral thesis

Quantum optics Quantum dots Quantum entanglement CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ÓPTICA

STUDY OF THE PHOTOCONVERSION EFFICIENCY ON NANOSTRUCTURED SOLAR CELLS BASED ON TIO2 FILMS SENSITIZED WITH QDS AND DECORATED WITH AU NANOPARTICLES AND P3OT

ISAAC ZARAZUA MACIAS (2013)

"Novel composite titanium dioxide (TiO2) films of 10 µm thickness have been prepared and characterized with emphasis on evaluating their photovoltaic properties. The films contain TiO2 nanocrystals (NCs) with anatase crystalline phase deposited on a substrate with a thin conductor oxide film, composited with CdSe quantum dots (QDs), Au nanoparticles (NPs), and poly(3-octylthiophene) (P3OT) in different configurations. In the case of single sensitized films, Au NPs considerably increases the FF of titania films, indicating that such NPs help to the charge carriers transport. P3OT slightly increases the photocurrent and FF of the TiO2 films, indicating that the polymer may acts as a photogenerator and as a hole conductor giving a relatively good cell performance. However, CdSe QDs sensitized films exhibited the largest photocurrent (237 µA/cm2) giving a photoconversion efficiency of 0.149%, overcoming the efficiency of the other two materials and giving a four-fold efficiency increase of not sensitized TiO2 (0.034%).

These results are attributed to the ability of QDs to photogenerate charge carriers efficiently giving a great amount of electrons to increase the photocurrent. With the introduction of Au NPs or P3OT into the TiO2/QDs films, the photocurrent increases up to ~85% and ~150% while the photoconversion efficiency increases by ~167% and ~177%, respectively. An interesting synergistic effect was observed when Au NPs and P3OT were used in conjunction. The configuration of TiO2/Au/QDs/P3OT film exhibits a photocurrent of 906 !A (an enhancement of ~285%) and the photoconversion efficiency of 0.661% (an enhancement of ~600%) compared to that of TiO2/QDs films. Such significant enhancement is attributed to the ability of Au NPs to facilitate charge separation and improve electron injection as well as P3OT’s ability to inject electrons and enhance hole transport avoiding electron recombination. Such ability when combined with the QD’s strong photoabsorption in the visible, lead to the overall increase in photocurrent generation, fill factor, and consequently photoconversion efficiency."

Doctoral thesis

ORGANIC SOLAR CELLS, IMPEDANCE MODELING, SPECTROSCOPY, PHOTOCONVERSION EFFICIENCY, PHOTOVOLTAIC APPLICATIONS CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA FÍSICA DE ALTAS ENERGÍAS FÍSICA TEÓRICA ALTAS ENERGÍAS

DIRECT OBSERVATION OF ORBITAL ANGULAR MOMENTUM CONSERVATION IN COLLINEAR TYPE-I SPDC

Carlos Andres Sevilla Gutierrez (2018)

"We implement a direct measurement of the spatial modes distribution generated in a collinear type I spontaneous parametric down-conversion (SPDC) process using a triggered intensified CCD camera. This type of experiments have been realized previously. However in those cases the measurement was an indirect one, where both photons were collected into one-pixel detectors. In this thesis, to the best or our knowledge, it is the first time that a direct two dimensional measurement of the spatial modes distribution is made. These are well described by Laguerre-Gaussian modes. In our experiments we could observe up to mode l=+/-10. By exploiting the capabilities of an ICCD camera, it is possible to visualize the orbital angular momentum (OAM) conservation of the unprocessed SPDC bi-photon state. This is performed by observing the diffraction pattern by a triangular aperture. We show the results using this method for modes l=+/-1,+/-2. This conservation of OAM leads to entanglement in OAM, since one photon carries the anti-correlated topological charge with respect to the other one. Therefore, they are conditioned to the state of the other photon."

Master thesis

OAM Conservation SPDC Laguerre-Gauss Quantum Entanglement CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA FÍSICA TEÓRICA TEORÍA CUÁNTICA DE CAMPOS TEORÍA CUÁNTICA DE CAMPOS

Desarrollo de celdas solares de heterounión c-Si / a-Si:H

JOSE DE JESUS MARTINEZ BASILIO (2011)

This work is focused on the design, manufacture and characterization of pin and nip heterojunction solar cells. These solar cells were made on crystalline silicon with depositions of intrinsic and doped hydrogenated amorphous silicon thin films, at low temperatures (< 200 ºC) by means of the Plasma Enhanced Chemical Vapor Deposition (PECVD) Technique.

Nip solar cells were made on p-type crystalline silicon with an intrinsic buffer layer and n-type hydrogenated amorphous silicon thin film. A layer of Oxide Tin Film was deposited as a transparent conductive oxide (TCO). The metal contacts were formed by aluminum evaporation. Pin solar cells were made on n-type crystalline silicon with an intrinsic buffer layer and p-type hydrogenated amorphous silicon thin film. The characterization was carried out by obtaining the current - voltage curves under illumination using a lamp with a known irradiance. In this way, the short circuit current density (Jsc), the open circuit voltage (Voc) and efficiency were obtained.

Also, an analysis about the effect of varying the flow rates of phosphine and diborane in the deposition of the amorphous films (n and p type, respectively) was performed, aiming to study their influence in the characteristics of the performance of the solar cells. It has found that the best efficiency was achieved with the deposition of an a-Si:H thin film doped with diborane with flow of 10 sccm. It was found that the main limiting factor in the efficiency of the cells was a high resistance due to the non ohmic metal-semiconductor contacts. Several techniques were used to reduce the series resistance in the cells and therefore increase the fill factor and the efficiency. However, that was not achieved in a satisfactory way.

Moreover, a simulation study was performed in the cells, varying the resistance value of the back metal-semiconductor contact. It was found that a resistance value of above 10 - 20 Ωcm2 results in a fill factor very small and consequently in a very low efficiency of the solar cells. Finally, we can conclude that the developed process in the present thesis, has the potential to fabricate cells with better characteristics and larger efficiencies, once that the large series resistance has been reduced.

En el presente trabajo se realizó la fabricación y caracterización de celdas solares de hetero unión nip y pin. Las cuales fueron fabricadas sobre sustratos de silicio cristalino, donde posteriormente se depositaron películas delgadas de silicio amorfo intrínseco y silicio amorfo dopado, a bajas temperaturas (≤ 200ºC) por medio de la técnica de Depósito Químico en Fase Vapor Asistido por Plasma (PECVD).

Para la fabricación de celdas nip se utilizaron sustratos de silicio cristalino tipo p, sobre el cual se depositó una película delgada de a-Si:H intrínseco y, posteriormente, una película delgada de silicio amorfo hidrogenado dopado con fósforo, es decir a-Si:H tipo n, como capa anti reflejante se utilizó una película de Óxido de Indio Estaño ITO (Indium Tin Oxide) y finalmente, los contactos metálicos se realizaron por medio de evaporación de aluminio. Las celdas pin se fabricaron sobre silicio cristalino tipo n, sobre el cual se depositaron películas delgadas de silicio amorfo intrínseco y silicio amorfo dopado con boro, es decir a-Si:H tipo p . La caracterización se llevó a cabo por medio de la obtención de curvas corriente voltaje bajo iluminación, usando una lámpara de irradiancia conocida. De esta manera se extrajeron los parámetros de la densidad de corriente de corto circuito (Jsc), el voltaje de circuito abierto (Voc) y la eficiencia del dispositivo.

También se realizó un análisis sobre el efecto al variar los flujos de los gases dopantes en las películas amorfas (tipo n y p, respectivamente), con el objetivo de estudiar su influencia en las características de desempeño de las celdas fabricadas. Los resultados obtenidos muestran que el mejor desempeño de la celda se obtuvo al depositar una película de a-Si:H dopado

con boro con un flujo de 10 sccm, logrando obtener una eficiencia del 5.8%. Se encontró que la principal limitante de la eficiencia de las celdas solares fabricadas fue una alta resistencia en serie debido a los contactos no óhmicos metal-semiconductor y/o al efecto de tener una película intrínseca de un grosor mayor a 5 nm. Se utilizaron distintas técnicas para reducir la resistencia en serie en las celdas y de esta manera lograr un mayor factor de llenado y una mayor eficiencia. Sin embargo no fue posible reducirla.

Para confirmar que la resistencia en serie estaba limitando el funcionamiento de las celdas solares, se realizaron diversas simulaciones.

Master thesis

Solar cells Pn junctions Design CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ELECTRÓNICA

Efficient ITO–Si solar cells and power modules fabricated with a low temperature technology: Results and perspectives

OLEKSANDR MALIK FRANCISCO JAVIER DE LA HIDALGA WADE CARLOS ZUÑIGA ISLAS GUILLERMO RUIZ TABOADA (2008)

ITO–SiOx–nSi semiconductor–insulator–semiconductor (SIS) structures have been produced with a simple spraying technique. It is shown that the structures obtained in such a way may be considered as an induced p–n diode, in which the polycrystalline tin–doped indium oxide (ITO) layer spray deposited on the preliminary treated silicon surface leads to an inversion p-layer at the interface. Solar cells with an active area of 1–4 cm2 have been fabricated based on ITO–SiOx–nSi structures and studied. Under AM0 illumination conditions, the efficiency is nearly 11%, whereas it exceeds 12% for AM1.5 illumination conditions. The theoretical analysis provided in this work shows a good agreement with experimental results and allows for predicting the efficiency of the cells depending on the silicon electro-physical properties.

Article

Silicon Solar cells Photovoltaics Indium tin oxide and other transparent conductors CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ELECTRÓNICA