The commercially available solar cells suffer from low conversion efficiency due to the thermalization and transmission losses arising from the mismatch between the band gap of the semiconductor materials and the solar spectrum. Advanced device architectures based on nanomaterial have been proposed and being successfully used to enhance the efficiency of the solar cells. Quantum dots (QDs) and nanowires (NWs) are the nanosclae structures that have been exploited for the development of the third generation solar cell devices and nanowire based solar cells, respectively. The optical and electrical properties of these materials can be tuned by their size and geometry; hence they have great potential for the production of highly efficient solar cell. Application of QDs and NWs with enhanced optoelectronic properties and development of low-cost fabrication processes render a new generation of economic highly efficient PV devices. The most significant contribution of this PhD study is the development of simple and cost effective methods for fabrication of nanowires and quantum dots for advanced solar cell architectures. In advanced silicon nanowires (SiNWs) array cell, SiNWs have been widely synthesised by the well-known vapor-liquid-solid method. Electron beam lithography and deep reactive ion etching have also been employed for fabrication of SiNWs. Due to the high price and complexity of these methods, simple and cost effective approaches are needed for the fabrication of SiNWs. In another approach, to enhance the cell efficiency, organic dyes and polymers have been widely used as luminescent centers and host mediums in the luminescent down shifting (LDS) layers. However, due to the narrow absorption band of the dyes and degradation of the polymers by moisture and heat, these materials are not promising candidates to use as LDS. Highly efficient luminescent materials and transparent host materials with stable mechanical properties are demanded for luminescent down shifting applications. In this project, simple fabrication processes were developed to produce SiNWs and QDs for application in advanced cell architectures. The SiNWs array were successfully fabricated, characterized and deployed in new cell architectures with radial p-n junction geometry. The luminescence down shifting of layers containing QDs in oxide and glass mediums was verified. The silica coated quantum dots which are suitable for luminescence down shifting, were also fabricated and characterized for deployment in new design architectures. Silicon nanowires were fabricated using two simplified methods. In the first approach, a maskless reactive ion etching process was developed to form upright ordered arrays of the SiNWs without relying on the complicated nano-scale lithography or masking methods. The fabricated structures were comprehensively characterized. Light trapping and photoluminescence properties of the medium were verified. In the second approach, combination of the nanosphere lithography and etching techniques were utilized for wire formation. This method provides a better control on the wire diameters and geometries in a very simple and cost effective way. The fabricated silicon nanowires were used for formation of the radial p-n junction array cells. The functionality of the new cell structures were confirmed through experimental and simulation results. Quantum dots are promising candidates as luminescent centers due to their tunable optical properties. Oxide/glass matrices are also preferred as the host medium for QDs because of their robust mechanical properties and their compatibility with standard silicon processing technology. Besides, the oxide layers are transparent mediums with good passivation and anti-reflection coating properties. They can also be used to encapsulate the cell. In this work, ordered arrays of QDs were incorporated in an oxide layer to form a luminescent down shifting layer. This design benefits from the enhanced absorption of a periodic QD structure in a transparent oxide. The down shifting properties of the layer after deployment on a crystalline silicon solar cell were examined. For this purpose, crystalline silicon solar cells were fabricated to use as test platform for down shifting. In order to examine the down-shifting effect, different approaches for formation of a luminescence down shifting layer were developed. The LDS layer consist of cadmium selenide- zinc sulfide (CdSe/ZnS) quantum dots in oxide and glass layers to act as luminescent centers and transparent host medium, respectively. The structural and optical properties of the fabricated layers were studied. The concept of spectral engineering was proved by the deployment of the layer on the solar cell. To further benefit from the LDS technique, quantum efficiency of the QDs and optical properties of the layer must be improved. Demand for the high quantum efficiency material with desired geometry leaded us to synthesis quantum dots coated with a layer of grown oxide. As the luminescence quantum efficiency of the QDs is correlated to the surface defects, one advantage of having oxide on the outer shell of the QDs, is to passivate the surface non-radiative recombination centers and produce QDs with high luminescent quantum yield. In addition, nanoparticles with desired size can be obtained only by changing the thickness of the oxide shell. This method also simplifies the fabrication of QD arrays for luminescence down shifting application, since it is easier to form ordered arrays from larger particles. QD superlattices in an oxide medium can be fabricated on a large area by a simple spin-coating or dip coating methods. The photonic crystal properties of the proposed structure can greatly increase the absorption in the QDs layer and enhance the effect of down shifting.

The commercially available solar cells suffer from low conversion efficiency due to the thermalization and transmission losses arising from the mismatch between the band gap of the semiconductor materials and the solar spectrum. Advanced device architectures based on nanomaterial have been proposed and being successfully used to enhance the efficiency of the solar cells. Quantum dots (QDs) and nanowires (NWs) are the nanosclae structures that have been exploited for the development of the third generation solar cell devices and nanowire based solar cells, respectively. The optical and electrical properties of these materials can be tuned by their size and geometry; hence they have great potential for the production of highly efficient solar cell. Application of QDs and NWs with enhanced optoelectronic properties and development of low-cost fabrication processes render a new generation of economic highly efficient PV devices. The most significant contribution of this PhD study is the development of simple and cost effective methods for fabrication of nanowires and quantum dots for advanced solar cell architectures. In advanced silicon nanowires (SiNWs) array cell, SiNWs have been widely synthesised by the well-known vapor-liquid-solid method. Electron beam lithography and deep reactive ion etching have also been employed for fabrication of SiNWs. Due to the high price and complexity of these methods, simple and cost effective approaches are needed for the fabrication of SiNWs. In another approach, to enhance the cell efficiency, organic dyes and polymers have been widely used as luminescent centers and host mediums in the luminescent down shifting (LDS) layers. However, due to the narrow absorption band of the dyes and degradation of the polymers by moisture and heat, these materials are not promising candidates to use as LDS. Highly efficient luminescent materials and transparent host materials with stable mechanical properties are demanded for luminescent down shifting applications. In this project, simple fabrication processes were developed to produce SiNWs and QDs for application in advanced cell architectures. The SiNWs array were successfully fabricated, characterized and deployed in new cell architectures with radial p-n junction geometry. The luminescence down shifting of layers containing QDs in oxide and glass mediums was verified. The silica coated quantum dots which are suitable for luminescence down shifting, were also fabricated and characterized for deployment in new design architectures. Silicon nanowires were fabricated using two simplified methods. In the first approach, a maskless reactive ion etching process was developed to form upright ordered arrays of the SiNWs without relying on the complicated nano-scale lithography or masking methods. The fabricated structures were comprehensively characterized. Light trapping and photoluminescence properties of the medium were verified. In the second approach, combination of the nanosphere lithography and etching techniques were utilized for wire formation. This method provides a better control on the wire diameters and geometries in a very simple and cost effective way. The fabricated silicon nanowires were used for formation of the radial p-n junction array cells. The functionality of the new cell structures were confirmed through experimental and simulation results. Quantum dots are promising candidates as luminescent centers due to their tunable optical properties. Oxide/glass matrices are also preferred as the host medium for QDs because of their robust mechanical properties and their compatibility with standard silicon processing technology. Besides, the oxide layers are transparent mediums with good passivation and anti-reflection coating properties. They can also be used to encapsulate the cell. In this work, ordered arrays of QDs were incorporated in an oxide layer to form a luminescent down shifting layer. This design benefits from the enhanced absorption of a periodic QD structure in a transparent oxide. The down shifting properties of the layer after deployment on a crystalline silicon solar cell were examined. For this purpose, crystalline silicon solar cells were fabricated to use as test platform for down shifting. In order to examine the down-shifting effect, different approaches for formation of a luminescence down shifting layer were developed. The LDS layer consist of cadmium selenide- zinc sulfide (CdSe/ZnS) quantum dots in oxide and glass layers to act as luminescent centers and transparent host medium, respectively. The structural and optical properties of the fabricated layers were studied. The concept of spectral engineering was proved by the deployment of the layer on the solar cell. To further benefit from the LDS technique, quantum efficiency of the QDs and optical properties of the layer must be improved. Demand for the high quantum efficiency material with desired geometry leaded us to synthesis quantum dots coated with a layer of grown oxide. As the luminescence quantum efficiency of the QDs is correlated to the surface defects, one advantage of having oxide on the outer shell of the QDs, is to passivate the surface non-radiative recombination centers and produce QDs with high luminescent quantum yield. In addition, nanoparticles with desired size can be obtained only by changing the thickness of the oxide shell. This method also simplifies the fabrication of QD arrays for luminescence down shifting application, since it is easier to form ordered arrays from larger particles. QD superlattices in an oxide medium can be fabricated on a large area by a simple spin-coating or dip coating methods. The photonic crystal properties of the proposed structure can greatly increase the absorption in the QDs layer and enhance the effect of down shifting.

Down-conversion (DC) and down-shifting (DS) layers are optical layers mounted on the top surface of a solar cell that can potentially increase the solar cell efficiency. The effect of DC and DS layers to enhance the performance of single-junction solar cells has been studied by means of simulation and experimental work. In this thesis a model is developed to study the effects of DC and DS layers by modifying the incident spectrum. The effect of the layers on ideal cells as well as commercial grade silicon and CIGS solar cells that are modeled in a device simulator is examined. Silicon nanocrystals (Si-nC) embedded in a silicon dioxide matrix to act as a DS layer were fabricated and characterized at McMaster University as part of this project. The measured optical properties as well as the photoluminescence measurements are used as input parameters to the optical model. The enhancement due to the Si-nC when coupled to silicon and CIGS solar cells is explored. Beside the DC and DS effects, there is also disturbance to the surface reflections due to the addition of a new layer to the top surface and is referred to as antireflection coating (ARC) effect. For the simulated silicon solar cell under the standard AM1.5G spectrum (1000W/m2), a maximum increase in Jsc of 8.4% is achieved for a perfect DS layer as compared to a reference cell, where 7.2% is due to ARC effect and only 1.2% is due to DS effect. On the other hand, there is an increase in Jsc of 19.5% for the CIGS solar cell when coupled to a perfect DS layer. The DS effect is dominant with 18%, while the ARC effect contributes only 1.5% to the total Jsc enhancement. Accurately characterizing DS layers coupled to solar cell requires knowledge of optical properties of the complete structure. Internal quantum efficiency is an important tool for characterizing DS systems, nevertheless, it is rarely reported. In addition, the ARC effect is not experimentally decoupled from the DS effect. In this work, a straightforward method for calculating the active layer contribution that minimizes error by subtracting optically-modeled electrode absorption from experimentally measured total absorption.

<p>The aim of third-generation photovoltaics (PV) is ultimately to achieve low-cost, high-efficiency devices. This work focused on a third-generation PV concept known as down-shifting, which is the conversion of high-energy photons into low-energy photons which are more useful for a typical solar cell. Silicon nanocrystals (Si-NCs) fabricated using electron-cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) were studied as a down-shifting material for single-junction silicon cells. A calibration was done to determine optimal deposition parameters for Si-NC formation. An experiment was then done to determine the effect of film thickness on emission, optical properties, and photoluminescence quantum efficiencies.</p> <p>Photoluminescence (PL) peaks varied depending on the stoichiometry of the films, ranging from approximately 790 nm to 850 nm. Variable-angle spectroscopic ellipsometry was used to determine the optical constants of the Si-NC films. The extinction coefficients indicated strong absorption below 500 nm, ideal for a down-shifting material. Transmission Electron Microscopy (TEM) was used to determine the size, density, and distribution of Si-NCs in two of the films. Si-NCs were seen to have an average diameter of approximately 4 nm, with larger nanocrystals more common near the surface of the film. A density of approximately 10⁵ nanocrystals per cubic micron was approximated from one of the TEM samples.</p> <p>The design and implementation of a PL quantum efficiency measurement system was achieved, using an integrating sphere to measure the absolute efficiency of Si-NC emission. Internal quantum efficiencies (IQE) as high as 1.84% and external quantum efficiencies (EQE) of up to 0.19% were measured. The EQE was found to increase with thicker films due to more intense photoluminescence; however the IQE remained relatively independent of film thickness.</p> / Master of Applied Science (MASc)

Le but de cette thèse était de concevoir des matériaux à base de nanoparticules de ZnO qui puissent être utilisés de manière efficaces comme couche de down-shifting sur la face avant des cellules solaires photovoltaiques. Le défi principal a donc été d’obtenir des nanoparticules de ZnO avec un rendement de photoluminescence (PL QY) aussi élevé que possible. Diverses méthodes ont été et comparées utilisées pour la synthèse de nanoparticules de ZnO. Nous avons en premier lieu étudié des particules synthétisées par voie physique (le dépôt par jet d’agrégats de basse énergie, LECBD). Les particules résultantes démontrent une faible PL QY. Nous avons par la suite étudié des particules commerciales qui se sont comportées comme celles issues de la LECBD. Par conséquent, nous ne les avons pas retenues. Enfin, nous nous sommes concentrés sur des particules produites par voies chimique humide: la co-précipitation de l’acétate ou du sulfate de zinc en présence d’hydroxyde alcalin. Pour chaque cas, les paramètres de synthèses ont été variés pour optimiser les propriétés optiques en vue de l’effet de down-shifting. Avec un choix approprié de la nature (Li+) et de la quantité d’ions alcalins, le PL QY a été accru à 13 %. Nos résultats reproduisent l’état de l’art concernant cette technique. Cependant, la technique par hydrolyse s’est révélée bien plus intéressante. La seule réaction d’hydrolyse n’a pas initialement conduit à des particules très brillantes. Nous avons donc proposé une approche originale : l’ajout d’un acide faible, l’acide polyacrylique (PAAH), durant la synthèse. Alors que le PAAH a déjà été utilisé comme agent passivant de la surface de ZnO, son utilisation pendant la synthèse n’a jamais été tentée. Notre travail montre que en contrôlant la quantité et le poids moléculaire (longueur de chaine) du PAAH introduit pendant la croissance, un nanocomposite hybride très efficace à base de nanoparticules de ZnO et de PAAH peut être obtenu, avec un PL QY aussi élevé que 20 %. En mélangeant le PAAH avec son sel de sodium, PAANa, le nanocomposite présente un PL QY record de 50%, qui augmente jusqu’à 70 % après un mois. Les raisons physico-chimiques de cet accroissement sont discutées dans le manuscrit. Nos explications pointent vers un effet combiné de la taille, de la morphologie et de la composition. Dans la partie suivante, des nanoparticules de ZnO pouvant être dispersées dans l’eau ont été obtenues avec succès tout en maintenant leur rendement quantique entre 20 % et 34 % ; ce en utilisant un mélange de PAAH/PAANa de ratio volumique, de concentration et de volume réactionnel optimaux. Nous insistons sur la nécessité d’obtenir un compromis entre une bonne capacité de dispersion et un fort PL QY. Cette partie de la thèse pave la voie vers des applications industrielles ultérieures.Finalement, l’effet de down-shifting des nanoparticules luminescentes de ZnO a été simulé pout déterminer le gain potentiel de rendement de cellules photovoltaïques. / In this thesis, we aim at designing mechanically stable ZnO nanoparticle based materials as a luminescent down-shifting layer that can be processed on a scalable amount and deposited on standard solar cells at a reduced cost. The main challenge was thus to get ZnO nanoparticles with as high photoluminescence quantum yield (PL QY) as possible. Different methods have been used and compared to synthesize ZnO nanoparticles. We have first studied particles synthesized by a physical route (the Low Energy Cluster Beam Deposition relying on the adiabatic expansion of a plasma). The resulting particles did not exhibit a PL QY high enough to be interesting for down-shifting. We next investigated commercial particles which behaved as the LECBD ones. We consequently discarded them. Eventually, we concentrated on nanoparticles produced by wet chemistry. Two routes were explored: the conventional co-precipitation method of Zn acetate or sulfate in presence of an alkaline hydroxide and the hydrolysis of ZnEt2. For both cases the synthesis parameters have been tuned to optimize the optical properties for down-shifting process. When appropriately choosing the alkaline ion (Li+ instead of K+) nature and amount, the PL QY has been increased to 13 % in the co-precipitation method. Our results reproduce the state-of-the-art knowledge concerning this technique. The hydrolysis route proved to be even more interesting. The sole hydrolysis reaction did not lead to very bright particles. We have thus proposed an original strategy: the addition of a weak acid, the polyacrylic acid (PAAH) during the synthesis. If PAAH has been used previously as a passivating capping agent of ZnO, its use during the synthesis has never been tempted. Our work shows that by tuning the amount and molecular weight (chain length) of PAAH introduced during the synthesis, a very efficient hybrid nanocomposite consisting of ZnO nanopaerticles in a PAAH matrix can be obtained with PL QY as high as 20 %. When mixing PAAH to its sodium salt PAANa, the nanocomposite exhibits record values of PL QY of 50 %, increasing to 70 % over a month. The physico-chemical reasons for this enhancement are discussed in the manuscript. Our explanations point to a combined effect of the size, morphology and composition. In the subsequent part, ZnO NPs dispersible in water have been successfully achieved while maintaining their PL QY high, between 20 % - 34 %, using a PAAH/PAANa mixture at the optimal volume ratio, concentration, lengths and volume. We highlight the need to get a compromise between a good dispersibility and a high PL QY. This part of the thesis paves the way for the further industrial applications. Finally, the down-shifting effect of luminescent ZnO nanoparticles on solar cells has been simulated to obtain a potential enhancement of solar cell efficiency by the ZnO NPs down-shifting layer.

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