M.Sc. / Thin film solar cell devices based on chalcopyrite absorber layers have reached a high performance level over the last few years, especially on laboratory scale. Despite this progress, there is still an urgent need to develop an industrial easily scalable deposition technology for depositing chalcopyrite thin films on a large scale. In this study, homogeneous single-phase quaternary Cu(In1-xGax)Se2 thin films were prepared with a reproducible two-step growth technique. The growth process is based on the controlled selenization of sputtered metallic CuIn0.75Ga0.25 alloys in a H2Se/Ar gas mixture at atmospheric pressure. Attention was mainly focused on the optimization of the reaction parameters such as the temperature profiles, gas concentrations and reaction periods. In an optimal reaction process, the reaction velocities of the binary selenide phases were carefully controlled to prevent the formation of stable group I-III-VI2 ternary alloys during the initial selenization step. The composite alloys were subsequently annealed in an inert atmosphere, followed by a second selenization step to promote the homogeneous alloying of gallium with partially formed CuInSe2. Glancing incident angle x-ray diffraction (GIXRD) at incident angles between 0.2º and 10º revealed virtually no shift in d-spacing with sample depth, which confirmed the monophasic nature of the quaternary alloys. Optical measurements revealed an increase in the band gap value of the chalcopyrite alloy due to the homogeneous incorporation of gallium into the CuInSe2 structure. Solar cell devices were fabricated by depositing cadmium sulphide (CdS) buffer layers and zinc oxide (ZnO) window layers onto the CuIn0.75Ga0.25Se2 absorber films. These devices were measure under standard A.M. 1.5 conditions and favorable conversion efficiencies were demonstrated. / Prof. V. Alberts

M.Sc. / As environmental and energy resource concerns have increased, greater stress has been placed on development of renewable energy resources such as photovoltaic electric generators. CuInSe2/ZnO heterojunction solar cells are currently one of the most promising technologies. CuInSe2 and its related alloys such as Cu(In,Ga)Se2 have been deposited by a number of techniques, including methods which have been demonstrated to be scalable to mass production volumes. In this study, attention was focused on (i) developing a relatively simple deposition technology for the production of chalcopyrite absorber films, (ii) detailed characterization of the semiconductor thin films in terms of the experimental parameters and (iii) production of completed CuInSe2/CdS/ZnO solar cell devices. A new two-stage growth process was developed which involved a low temperature precursor formation step and a subsequent high temperature selenization step. Selenium containing Cu-In-Ga and Cu-In-Ga-Se precursors were deposited by a thermal process in which the constituent elements were evaporated from a single graphite crucible onto heated substrates in presence of a selenium overpressure. These precursors were subsequently reacted in vacuum to elemental selenium vapour or to H2Se/Ar at atmospheric pressure in a separate diffusion reactor. In order to investigate the growth kinetics of the respective processes, the precursors were reacted to the Se in the temperature range between 300„aC and 600„aC. The structural features (morphology, presence of crystalline phases, bulk and in-depth compositional uniformity) of the respective films were compared and correlated against the growth parameters. From this systematic study, optimum growth parameters were determined for the production of completed solar cell devices. / Prof. V. Alberts

Ph.D. / The two-step growth process, involving the selenization and sulfurization of sputter deposited CuInGa alloys has been identified as a commercially viable method to produce large area Cu(In1-xGax)(Se1-ySy)2 absorber films for solar cell application. The success of this method is however limited by insufficient control over the lattice parameters and band gap of the compound due to phase segregation associated with non-uniform Ga and S incorporation. This study provides an approach to overcome this limitation by investigating the influence of process parameters on the structural features of the Cu(In1-xGax)(Se1-ySy)2 films. In this approach, films were partially selenized in optimum H2Se/Ar flow to produce composite alloys comprising of a mixture of binary selenides (InSe, CuSe and GaSe) and at least one group I-III-VI ternary alloy. The subsequent reaction step in H2S/Ar produced homogeneous Cu(In1-xGax)(Se1-ySy)2 films. The lattice constants of the resulting films varied linearly with an increase in the S/(S+Se) ratio in accordance with Vegard’s law. The Raman spectra of the films were characterized by the presence of the A1-Se mode near 180 cm-1 and a low intensity, A1-S mode around 290 cm-1. With an increase in the S/(S+Se) ratio of the films, the FWHM of the A1-Se mode increased and its frequency shifted linearly towards that of A1-S mode. A corresponding increase in the value of the Urbach energy, attributed to an increase in chalcopyrite crystal alloy disorder, was observed from the analysis of the transmission and reflectance data. 0.45 cm2 area devices with conversion efficiencies between 12% and 15%, were fabricated from absorber layers with the (112) x-ray diffraction peak position between 27.1°and 27.2°, corresponding to the S/(S+Se) ratio of about 0.18 to 0.20. The process scale up was demonstrated by the fabrication of large area, (30 x 40) cm2 modules, with conversion efficiencies of 10%.

M.Sc. / CuInSe2 (CIS) is considered to be one of the most promising candidates for high efficiency thin film solar cells. The reaction of metallic alloys to a reactive selenium atmosphere (H2Se/Ar or elemental Se vapour) is a promising growth technique to produce CIS thin films of high crystalline quality. However, up to now, the control of the final film quality has been critically influenced by the loss of material and subsequent formation of detrimental binary phases during the high temperature selenization stages. In this study, it is shown that this phenomenon is strongly related to the selenization temperature and, in particular, the ramping procedure followed to the final selenization temperature. Metallic alloys which were selenized in H2Se/Ar at 400°C or slowly heated in 20 minutes to temperatures around 400°C were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) to have nonuniform surface morphologies, highly defected 0.8-2 !lm sized grains and to contain Cuselenide binary phases. Energy dispersive X-ray spectroscopy (EDS) analysis confirmed the generally reported sharp increase in the Cu/In atomic ratio for these classes of samples. In contrast, rapid heating (in 2 minutes) of identical metallic alloys to temperatures above 400°C, resulted in uniform, dense films with low defect density 1 !lm sized grains void of any evidence of secondary phases. X-ray fluorescence (XRF) Kal,2 measurements of metallic alloys at different stages of selenization revealed no evidence of material losses. XRF depth profiles, however, explained this discrepancy by revealing a pronounced segregation of In towards the Mo back contact when the samples were selenized at 400°C, or slowly heated to temperatures around 400°C. This segregation was dramatically reduced in films rapidly heated and selenized at temperatures above 400°C. For the purpose of comparison, metallic alloys were also reacted to elemental Se vapour. The structural features (grain size and preferred orientation) ofthese films differed significantly from those selenized under similar conditions in H2Se/Ar. The results from this study, including photoluminescence (PL) measurements obtained from these films, were used to affect the fabrication of CIS absorbers with excellent material properties and solar cell devices with moderate conversion efficiencies.

No description available.

Due to their unique properties, nanoparticles have been a focus of significant research interest for use in various opto-electronic applications, particularly in the field of solar energy generation. In order to realize a nanoparticle-based solar cell, it is important to be able to create thin films of organised nanoparticles and to be able to control their surface properties. In this work the use of a novel synthesis technique involving reaction at the interface between two immiscible liquids to synthesise thin films of lead sulfide nanoparticles on the order of ~10 nm in diameter is reported. The use of the liquid-liquid interface allows the synthesis of particles without the use of stabilising ligands, with sizes and morphologies determined by the conditions present at the interface. Variations in the precursor used, solvent height, and precursor concentration were explored. Films synthesised at various solvent heights displayed a decrease in particle size with increasing solvent height. This trend was seen to vary depending on the lead-containing precursor used. Changes in the precursor concentration resulted in changes in the morphology of the resulting particles as observed with transmission electron microscopy (TEM). Preferential growth along certain planes was observed for particles synthesised with the highest lead precursor concentration. Experiments with precursors with differing organic chain length displayed an increase in particle size with increasing chain length, as well as an increase in preferential growth observed by X-ray diffraction (XRD). Surface ageing was investigated using X-ray photoelectron spectroscopy (XPS) techniques, which showed that all samples followed a similar oxidation mechanism. Oxidised lead species, attributed to hydrated lead oxide, were determined to be the initial oxidation product, formed within a week of exposure to air. Sulfoxy species were observed to form over a greater length of time, with sulfate being determined to be the final oxidation product. An oxidation mechanism is proposed based on XPS analysis of films exposed to air for up to nine months.

La connaissance approfondie sur la luminescence des dispositifs photovoltaïques (PV) en a fait un outil de caractérisation puissant qui capte l'intérêt de la recherche et des industries du PV. Dans cette thèse, nous nous concentrons sur la luminescence des cellules solaires photovoltaïques à base de Cu(In,Ga)Se2. En particulier, nous explorons et revisitons ses dépendances temporelles, spectrales et spatiales. Cela a abouti dans un premier temps à la mise au point de nouvelles méthodes de caractérisation basée sur la luminescence de cette technologie PV en particulier. Nous montrons d’abord que par l’intermédiaire d'une méthode sans contact toute optique, nous sommes en mesure de détecter et de localiser les métastabilités de cette technologie. En utilisant une approche numérique basée sur des résultats expérimentaux de photoluminescence résolue en temps (TRPL) nous avions réussi à quantifier la densité des défauts de piégeage qui sont derrière ces métastabilités. Une fois quantifiée, nous traduisons cette densité en pertes absolues de performance PV de la cellule solaire. Ensuite, en explorant la dépendance spatiale de la luminescence des cellules solaires à base de Cu(In,Ga)Se2, nous avions corrélé avec succès, ses aspects temporels et spectrales en se basant sur la microscopie confocale à balayage et l’imagerie hyperspectrale. Cela nous a permis de généraliser nos résultats précédents à l'échelle globale des cellules solaires. Cette partie de la thèse nous a aidés à mieux comprendre une des origines fondamentales derrière l’inhomogénéité spatiale de la luminescence de ce type de dispositifs photovoltaïques.La dernière partie de la thèse était essentiellement technique et exploratoire. En particulier, nous introduisons une nouvelle technique optique dans le domaine de la caractérisation des dispositifs PV. Cette technique est dédiée à l’imagerie résolue en temps du temps de vie de fluorescence (TR-FLIM). Le principe de cette technique consiste essentiellement en acquisition d'images de luminescence du dispositif PV qui sont résolues temporellement. Avec ce nouveau dispositif expérimental, nous sommes maintenant en mesure de résoudre spatialement, et en temps réel la dynamique des porteurs de charge d'une technologie photovoltaïque donnée et accéder à ses propriétés électroniques clés. Une première démonstration a été faite sur une cellule solaire à base de GaAs, et pour laquelle nous avions extrait optiquement, la longueur de diffusion, la mobilité et le temps de vie de ses porteurs. De plus, nous avions pu estimer le coefficient de diffusion du matériau et son taux de dopage. / The extensive knowledge on the luminescence of photovoltaic (PV) devices has made it a powerful characterization tool that captures the interest of both research and industrial PV communities. In this thesis, we focus on the luminescence of Cu(In,Ga)Se2-based solar PV. In particular, we explore and revisit the luminescence temporal, spectral and spatial dependencies. This resulted in the development of new luminescence-based characterization methods for this particular PV technology. We show initially that by means of an all-optical, contactless methodology, we are able to detect and localize the metastabilities of this technology. Using a numerical approach based on experimental time-resolved photoluminescence (TRPL) we managed to quantify the trapping defects that are behind these metastabilities. Once quantified, we translated it into absolute losses in the PV performance of the solar cell. By exploring the spatial dependence of the luminescence of Cu(In,Ga)Se2 solar cells, we successfully correlated its temporal and spectral aspects based on scanning confocal microscopy and hyperspectral imaging. This allowed us to generalize our previous findings at the global solar cell scale. This part of the thesis helped us better understand one of the fundamental origins behind the spatially inhomogeneous luminescence of Cu(In,Ga)Se2 PV devices. The final part of the thesis was mainly technical and exploratory. In particular, we introduced a new optical technique to the field of PV characterization. It is dedicated to time-resolved fluorescence lifetime imaging (TR-FLIM) which basically consists of acquiring time-resolved luminescence images of the PV device. With this new setup we are now able to spatially resolve, in real-time the charge carrier dynamics of a given PV technology and access its key electronic properties. A first application was made on a GaAs-based solar cell, for which we were able to optically extract the mobility, diffusion length and lifetime of its carriers. Finally, we were also able to estimate the diffusion coefficient of the material and its doping density.

The objective of this dissertation is the development of the necessary processing steps needed to manufacture infrared photodiodes on InAs1-xSbx material. Preliminary surface preparation steps were performed on both InAs and InSb material, thus covering both possible extremes of the antimony mole fraction. The first experiments endeavoured to characterise the effect of several possible etchants with regards to etch rate, repeatability, limitations for photolithographic patterning and the resultant surface roughness. The etchants investigated include a lactic acid based etchant, a sulphuric acid based etchant, an acetic acid based etchant, an ammonium based etchant, a hydrochloric acid based etchant as well as an organic rinse procedure. These cleaning and etching steps were evaluated at several temperatures. Measurements were performed on an Alpha Step stylus profiler as well as an atomic force microscope. Metal-insulator-semiconductor capacitor devices were manufactured, on both InAs and InSb material, in order to investigate the effects of the above-mentioned etchants combined with surface passivation techniques in terms of surface state densities. Capacitance-versus-bias voltage measurements were done to determine the resultant surface state densities and to compare these to the surface state density of an untreated reference sample. The surface passivation techniques included KOH, Na2S as well as (NH4)2S anodisation. Auger electron spectroscopy measurements were done on InAs and InSb material in order to examine possible surface contamination due to the etchants as well as combinations of these etching and anodisation procedures. The extent of surface coverage by contaminants as well as by the intrinsic elements was measured. The results of the cleaning and etching as well as the surface passivation studies were used to manufacture photovoltaic infrared diodes on an MOCVD (metal oxide chemical vapour deposition) grown p-InAs0.91Sb0.09/i- InAs0.91Sb0.09/n-GaSb structure. Current-versus-voltage and electro-optical measurements were performed on the these diodes in order to evaluate the effect of sulphuric acid based etching combined with KOH, Na2S or (NH4)2S anodisation on the detector performance. The results of surface passivated structures were compared to those of an unpassivated reference detector.

No description available.

India’s geographical location renders it with ample solar-energy potential ranging from 4-7 kWh/m2 daily and 2,300–3,200 sunshine hours annually. The diverse nature of human settlements (scattered low-rise to dense high-rise) in India is one of the unexplored avenues of harnessing solar energy through electricity generation using photovoltaic (PV) technology. Solar energy is a promising alternative that carries adequate potential to support the growing energy demands of India’s burgeoning population. A previous study estimates, by the year 2070, with 425 million households (of which utilizing only 20 %), about 90 TWh of electrical energy can be generated utilizing solar energy. PV is viable for onsite distributed (decentralized) power generation offering advantages of size and scale variability, modularity, relatively low maintenance and integration into buildings (no additional demand land). The application of solar PV technology as the building envelope viz., walls, façade, fenestration, roof and skylights is termed Building Integrated Photovoltaic (BIPV). Apart from generating electricity, PV has to also function as a building envelope, which makes BIPV systems unique. Even with a gradual rise in the number of BIPV installations across the world over the years, a common consensus on their evaluation has not yet been developed. Unlike PV in a ground mounted system, its application in buildings as an envelope has huge implications on both PV and building performance. The functions of PV as a building material translates well beyond electricity generation alone and would also have to look into various aspects like the thermal comfort, weather proofing, structural rigidity, natural lighting, thermal insulation, shading, noise protection safety and aesthetics. To integrate PV into a residential building successfully serving the purpose (given the low energy densities of PV and initial cost), would also mean considering factors like the buildings electricity requirement and economic viability. As many studies have revealed, 40% of electricity consumed in a building is utilized for maintaining indoor thermal comfort. Tropical regions, such as India, are generally characterized by high temperatures and humidity attributed to good sunlight, therefore, the externality considered for this study has been the impact of BIPV on the thermal comfort. Passive designs need to regulate the buildings solar exposure by integrating a combination of appropriate thermal massing, material selection, space orientation and natural ventilation. On the other hand, PV design primarily aims to maximize solar to generate maximum energy. The design requirements for climate-responsive building design may thus infringe upon those required for optimal PV performance. Regulating indoor thermal comfort in tropical regions poses a particular challenge under such conditions, as the indoor temperature is likely to be sensitive to external temperature variations. In addition, given current performance efficiencies for various PVs, high initial cost and space requirement, it is also crucial to ascertain PV’s ability to efficiently support buildings energy requirement. Thus, BIPV would require addressing, concurrently, design requirements for energy-efficient building performance, effective PV integration, and societal feasibility. A real time roof integrated BIPV system (5.25 kW) installed at the Center for Sustainable Technologies at the Indian Institute of Science, Bangalore has been studied for its PV and building thermal performance. The study aims at understanding a BIPV system (based on crystalline silicon) from the technical (climate-responsiveness and PV performance), social (energy requirement and energy efficiency) and economical (costs and benefits) grounds and identifies relevant factors to quantify performance of any BIPV system. A methodology for BIPV evaluation has been proposed (Integrability Methodology), especially for urban localities, which can also be adopted for various PV configurations, building typologies and climatic zones. In the process, a novel parameter (thermal comfort energy) to evaluate the thermal performance of naturally ventilated buildings combining climate-responsiveness and thermal comfort aspects has also been developed. An Integrability Index has also been devised, integrating various building performance factors, to evaluate and compare the performance of BIPV structures. The methodology has been applied to the 5.25 kW BIPV system and the index has been computed to be 0.17 (on a scale of 0 – 1). An insulated BIPV system (building applied photovoltaic system) has been found to be favorable for the climate of Bangalore than BIPV. BIPV systems have also been compared across three different climates (Bangalore, Shillong and Delhi) and given the consideration of the same system for comparison, the system in Delhi is predicted to have a higher Integrability than the other two systems. The current research work is a maiden effort, that aims at developing and testing a framework to evaluate BIPV systems comprising technical, social and economic factors.