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Tin sulphide solar cells by thermal evaporation
Full text linkProdurre energia elettrica dalla combustione di carburanti di origine fossile o dalla fissione di materiale radioattivo significa inquinare il pianeta, impoverirlo delle sue risorse e non garantire un futuro alle generazioni future. Sebbene nei suoi report annuali l’agenzia internazionale per l’energia (IEA) dipinga una situazione di espansione della produzione di energia “verde”, il crescente fabbisogno di energia pulita e a basso costo spinge la ricerca verso nuove frontiere. Nel panorama del fotovoltaico, di anno in anno si vedono maturare i frutti della ricerca negli annunci di nuovi record mondiali di efficienza per molte tecnologie: nuove tecnologie emergenti, basate su concetti o materiali innovativi, si stanno aggiungendo a tecnologie mature, quali quelle basate su Si, CdTe o CuInxGa(1-x)Se2. Un esempio di queste nuove tecnologie sono quelle basate sui semiconduttori composti totalmente da elementi chimici non tossici ed abbondanti nella crosta terrestre (quindi potenzialmente a basso costo), come Cu2ZnSnS4 o SnS. In questa tesi di dottorato, verranno studiati alcuni aspetti del solfuro di stagno, SnS, in vista di una sua possibile applicazione come strato assorbente per celle solari a film sottile. Il solfuro di stagno ha ottime proprietà optoelettroniche (band gap diretta nella regione di massima efficienza teorica, ottimo coefficiente di assorbimento della luce, e mostra intrinsecamente conduzione di tipo p), che lo rendono un ottimo candidato per il fotovoltaico del futuro. In questa tesi, verranno dapprima discusse le difficoltà incontrate con l’apparato di deposizione e come sono state superate. Successivamente verrà descritto e analizzato il dispositivo solare basato sul solfuro di stagno e caratterizzato dalla migliore prestazione: questo risultato è in linea con quanto pubblicato in letteratura. La difficoltà nel riprodurre questo risultato in modo sistematico ci spingerà poi ad indagarne le possibili cause: suggeriremo la presenza di una possibile correlazione tra la performance dei nostri dispositivi e la storia termica del materiale grezzo utilizzato per evaporare lo strato assorbente. Nel proseguo della tesi, dato che anche il miglior risultato ha comunque mostrato una performance lontana dal limite teorico per un materiale con la band gap dell’SnS, studieremo gli effetti di alcuni trattamenti post deposizione ideati per migliorare le caratteristiche optoelettroniche attraverso il miglioramento della qualità cristallina dell’assorbitore. Tali trattamenti post deposizione sono fondamentali in altre tecnologie, come nel caso del CdTe. Si studieranno due tipologie di trattamento termico: in atmosfera controllata o in aria, utilizzando vari composti a base di cloro e non, atti a favorire la ricristallizzazione dell’assorbitore. I risultati ottenuti verranno discussi caso per caso. Infine, anziché concentrarci ancora sulle caratteristiche dell’assorbitore stesso, ma sempre con lo scopo finale di migliorare le prestazioni dei nostri dispositivi, investigheremo delle alternative per gli altri strati che costituiscono nel loro insieme la cella solare: il contatto elettrico anteriore, quello posteriore, e infine il materiale semiconduttore di tipo n che completa la giunzione p-n.The production of electricity by the combustion of fossil fuels or by the fission of radioactive materials leads to the pollution of Earth’s environment, impoverishes Earth of its resources and does not secure the future for generations to come. Although International Energy Agency (IEA) in its annual reports depicts an increase of electricity production from renewable energy sources, the increasing need for low cost clean energy pushes research towards new frontiers. In photovoltaics, year after year, we see research coming to fruition with the announcements of new world record efficiencies for many technologies: new emerging technologies, based on innovative concepts or materials, are added to the mature ones, such as those based on Si, CdTe o CuInxGa(1-x)Se2. Examples of these innovations are those based on semiconductor compounds totally constituted by non-toxic and Earth’s crust abundant chemical species (which could potentially be low cost materials), such as Cu2ZnSnS4 or SnS. In this doctoral dissertation, we will investigate some aspects of SnS (tin sulphide), in view of its application as absorber layer for thin film solar cells. Tin sulphide is characterized by excellent optoelectronic properties (direct band gap in the region of the maximum theoretical efficiency, excellent absorption coefficient, and shows intrinsically p-type conduction), which makes SnS a promising candidate for the photovoltaic of the future. In the first part of this thesis, we will discuss the issues related to the deposition apparatus, and the strategies applied to solve them. Afterwards, the SnS based solar device, which exhibited the best performance, will be described and discussed: our result is consistent with similar processes from international laboratories. Since the reproducibility of this result has been observed to be a complex task, we will study its origin. A possible correlation between the performance of our devices and the thermal history of the SnS raw material used to evaporate the absorber layer has been suggested. Then, since even the best performing device exhibited a poor performance, i.e. far from the theoretical limit for a material with the SnS energy band gap, we will study the effects of several post deposition treatments, designed to enhance optoelectronic characteristics by improving the crystalline quality of the absorber material. Similar post deposition treatments are fundamental in other technologies, as in the CdTe case. We will study two types of thermal treatment: those taking place in a controlled atmosphere and those in air, by adding different compounds (with and without chlorine) to promote the absorber layer recrystallization process. The results will be discussed case by case. Up to now, we focused on the improvement of the absorber layer to enhance the performance of our devices. In the last part of this thesis, we will investigate some alternatives for the other layers constituting the solar device: the front and back contact, and the n-type semiconductor material which completes the p-n junction
Analysis of SnS Growth and Post Deposition Treatment by Congruent Physical Vapor Deposition
No full textIn recent times, new earth abundant and non toxic materials, like Cu2ZnSnS4 or SnS, have acquired a
considerable interest in the photovoltaic research field. In this work, SnS thin films solar cells were prepared in
superstrate configuration by vacuum evaporation of 99.99 % pure SnS compound on a glass/ITO/ZnO/CdS stack
substrate heated at several temperatures. The morphology, structure and stoichiometry of as deposited SnS absorber
layers were studied. Several post deposition treatments were performed in order to improve the SnS thin film
crystallinity. The SnS treated layers were also characterized. The J-V characteristics of both as deposited and treated
SnS thin film solar cells were determined after the gold back contact deposition. We report a best working device
with a power conversion efficiency of 1.51
Superior stability of ultra thin CdTe solar cells with simple Cu/Au back contact
No full textDue to its high scalability and low production cost, CdTe has shown a significant potential for high mass production,
resulting to be one of the cheapest photovoltaic technologies available. Efficiencies exceeding 20% have been
obtained by the application of high temperature CdTe deposition.
However tellurium scarcity is a limitation for mass production and one of the possibilities to overcome this is the
reduction of absorber thickness.
We have already demonstrated efficiencies above 11% for devices with 1.5 μm thick CdTe. Nowadays we have
fabricated ultra-thin absorber devices performing more than 13% efficiencies. But what is most interesting is
that we have observed a different electrical operation and stability, connected to the fact that the depletion region
takes a very large part of the device.
In this work many CdTe solar cells with a standard Cu/Au back contact, made with different absorber thicknesses,
were prepared, stored in dark and tested at different aging times, showing different reactions to the aging and in
particular a remarkable stability as CdTe thickness reduces
Analysis of the influence on the performance degradation of CdTe solar cells by the front contact
No full textCdTe thin film solar cells have shown to be competitive in terms of scalability and efficiency, the latest record efficiency exceeds 22%. Because of the CdTe high electron affinity the insertion of copper has demonstrated to deliver the highest efficiency device, however degrading the cell. For this reason other possible degradation effects are generally neglected.In this work CdTe solar cells were prepared with different transparent conductive oxides (TCOs), delivering similar conversion efficiencies. However aged cells show that stability is strictly connected with the different TCOs. Analysis of capacitance-voltage and drive level capacitance profiling shows a different defects distribution for solar cells with less stable front contact and admittance spectroscopy highlights the different defects formation after aging
Study of MgCl2 activation treatment on the defects of CdTe solar cells by capacitance-voltage, drive level capacitance profiling and admittance spectroscopy techniques
No full textCdTe solar cells have recently reached its highest conversion efficiencies, comparable to CuInGaSe2 and Si devices. One of the key factors for the success of these devices is the so-called “activation” treatment, which typically consists in depositing a CdCl2 film on the CdTe absorber layer and in a subsequent annealing in air or nitrogen atmosphere. Nevertheless CdCl2 is a carcinogenic and water-soluble compound; for this reason recent studies have found MgCl2 as a good alternative. In this work we have studied devices (exceeding 14% efficiency) prepared by low-substrate temperature CdTe deposition and activated with MgCl2 treatment (applied by wet deposition). The devices were characterized by means of current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopy techniques. Carrier concentration and also concentration and distribution of shallow and deep defects are presented and compared with data obtained from analogous devices but activated with the standard CdCl2. Despite CdTe layers treated with the two different processes exhibit similar structural properties, the distribution of deep and shallow defects results to be different and moreover admittance spectroscopy technique reveals the presence of different defects, at 344 and 358 meV, which are present only in MgCl2 treatment cas
Effects of Temperature and Post Deposition Annealing on SnS Polycrystalline Thin Film Growth
No full textThe limited amount in nature of Te and In and the use of toxic elements like Cd and Se in CdTe and CuInGaSe2 solar devices (despite they have no effect on the environment) could be overcome with alternative thin films. SnS is one of the most promising materials due to its electrical and physical properties, but it is already demonstrated that high quality SnS thin films are required to enhance solar cells efficiency. In the present work, we have found out a threshold temperature, above which the absorber grows highly crystallized and compact avoiding the porous structure observed previously. The properties of this differently grown film is presented and compared with low substrate temperature layers. Moreover the study of a new post deposition treatment, which merges our CdCl2 methodologies with new non-toxic salts (KI or SnCl2), more suitable for SnS, is presented
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
CdTe thin film solar cells by pulsed electron deposition
No full textIn this paper we present a fabrication process of complete CdTe thin-film solar cells where the CdTe absorber layer is made by pulsed electron deposition (PED) method. PED is a high-density pulsed energy thin film deposition system that allows high-energy growth at lower substrate temperatures. In order to analyze and optimize the fabrication process we started from our baseline where CdTe is deposited in a vacuum chamber and then annealed after CdCl2 deposition (by dipping in methanol solution). Cells with CdTe absorbers made by PED and by vacuum evaporation have been compared in terms of electrical and physical properties. Efficiency for thin CdTe devices demonstrates that PED allows fabricating a very compact and dense film
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