Fabrication of high-efficiency Cu(In,Ga)Se2 solar cells by Pulsed Electron Deposition technique

In the past few years, a continuous rise of gas/oil prices promoted the exploitation of renewable energy especially from photovoltaic. This field is today monopolized by Silicon, either mono- or crystalline-, solar cells, although thin film technologies are gaining increasing interest for the possibility to reduce the material quantity and cost and to use light and flexible substrates. Among thin films, CuInGaSe2 (CIGS) is the material with the highest conversion efficiency, close to 22%. Despite the fact that the performances of CIGS-based solar cells are comparable to the silicon ones, their spread on the market has been limited by the high production cost; long and expensive multistage processes (thermal co-evaporation and sputtering/selenization) must be adopted to solve the problems related to the CIGS complex composition and incongruent melting. An innovative deposition technique (PED, Pulsed Electron Deposition) has been developed at IMEM-CNR, aiming to simplify the growth of complex materials such as CIGS. PED is based on a non-thermodynamic equilibrium process, consisting in the ablation of a target with the same composition of the film to be grown, leading to a simple and “single stage” CIGS deposition. The work is carried on in the frame of an industrial project “PED4PV” (Pulsed Electron Deposition for PhotoVoltaic), coordinated by IMEM-CNR and financed by the Italian Economic Development Ministry, with the purpose of optimizing the PED technique for depositing high efficiency (>15%) CIGS-based solar cells. The first part of the thesis is focused to exploit the PED peculiarities (specifically, the optimal stoichiometric transfer from the target to the substrate) in order to obtain CIGS thin films of high crystalline quality and, remarkably, at a much lower temperature compared to the alternative growth techniques. The optimization of the CIGS absorber layer and its doping, by Na addition, allowed to obtain solar cell efficiencies of 18.75% on active area. The low temperature CIGS deposition process has been successfully tested also on crystalline substrates (GaAs and Ge), onto which monocrystalline CIGS films have been epitaxially deposited; the absence of structural defects such as grain boundaries could furtherly increase the efficiencies (up to a theoretical value of 28%). The second part of the thesis is dedicated to a pre-industrial development of the PED process. In particular, the solutions to overcame the main problems typical to the high energy techniques have been studied: i) reduction of the micrometric particulate on the film surfaces, caused by the interaction between the high-energy electronic beam and the target, by applying an appropriate electric field between target and substrate. ii) increase of the deposition area, by designing and assemble a pre-industrial deposition chamber prototype equipped with different PED sources, suitable to fabricate photovoltaic cells with large area (16x16cm2, same as the conventional Silicon-based cells) with high thickness uniformity. iii) Stability of the electronic beam during long deposition time, limiting the PED sources heating. This has been achieved by designing realizing and testing a new type of heater based on the Joule effect (flowing a current through the solar cell metal back contact), enabling the growth of high quality CIGS on thermolabile and flexible materials such as polymers. This thesis work contributed to the rapid development of CIGS-based thin film solar cells with efficiencies comparable to the highest values at international level, with a simple and potentially scalable industrial process.Negli ultimi anni il continuo aumento dei prezzi dei combustibili fossili ha favorito la diffusione di fonti energetiche rinnovabili, in particolare da solare. Il settore del fotovoltaico è tutt’oggi monopolizzato da celle solari a base di Silicio, tuttavia la tecnologia dei film sottili sta rapidamente guadagnando interesse, per la sua capacità di ridurre la quantità ed i costi rispetto alle tradizionali celle al Silicio, e per la possibilità di avere celle solari flessibili. Tra i film sottili, il Cu(In,Ga)Se2 (CIGS) è il materiale con la più alta efficienza di conversione fotovoltaica, vicina al 22%. Nonostante le performance delle celle basate sullo strato assorbitore di CIGS sia confrontabile con quelle del Si, la loro diffusione sul mercato è limitata dalla costosa tecnologia di produzione tradizionale, che per superare i problemi legati alla fusione incongruente del CIGS si basa su lunghi e complicati processi a multistadio (co-evaporazione termica e sputtering/selenizzazione). Presso l’Istituto IMEM-CNR di Parma è stata sviluppata una innovativa tecnica di deposizione (PED, Pulsed Electron Deposition), con l’obiettivo di semplificare la tecnologia di crescita del CIGS. Essa è basata su un processo fuori dall’equilibrio termodinamico, che consiste nell’ablazione di un target con la stessa composizione del film da ottenere permettendo così di depositare lo strato di CIGS in un unico stadio, riducendo i tempi e i costi di fabbricazione. Questo lavoro di tesi è nato all’interno di un progetto industriale “PED4PV” (Pulsed Electron Deposition for PhotoVoltaic), guidato da IMEM e finanziato dal Ministero delle Sviluppo Economico, con lo scopo di rendere la tecnica PED ottimale per depositare strati di CIGS come assorbitori in celle solari di efficienza > 15%. Il lavoro di tesi è stato diviso in due parti. La prima parte è stata dedicata a come sfruttare le peculiarità della PED (in particolare il trasferimento stechiometrico dal target al substrato) per ottenere film sottili in CIGS di alta qualità cristallina con un processo semplice ed a temperature di crescita più basse rispetto alla tecnologia tradizionale. L’ottimizzazione del processo di crescita e del controllo del “drogaggio” dello strato assorbitore di CIGS, ha permesso di ottenere efficienze dei dispositivi fino al 18.75% su area attiva. Il processo di deposizione del CIGS a bassa temperatura è stato testato con successo utilizzando substrati monocristallini (GaAs e Ge), che hanno consentito la crescita epitassiale di CIGS monocristallino, che potrebbero permettere di raggiungere efficienze teoriche superiori al 28%. La seconda parte della tesi è stata dedicata allo sviluppo pre-industriale della tecnica PED. In particolare si sono studiate delle soluzioni tecnologiche per limitare le criticità intrinseche delle tecniche ad alta energia come la PED: - Riduzione della presenza di particolato micrometrico sulla superficie del film. La presenza di questo particolato è dovuta all’interazione del fascio elettronico ad alta energia con il target. Sfruttando opportuni campi elettrici applicati tra target e substrato, i particolati di grosse dimensioni prodotti dall’ablazione vengono respinti dalla superficie del substrato stesso. - Aumento della superficie di deposizione. Lo studio della distribuzione del materiale ablato ha permesso la realizzazione di un modello di simulazione matematica utilizzato per la creazione di un prototipo pre-industriale in grado di realizzare celle fotovoltaiche di dimensioni commerciali (16x16 cm2) con uniformità superiori rispetto a quelle attualmente in commercio. - Stabilizzazione del fascio elettronico nel tempo. Con l’obiettivo di contenere il riscaldamento delle sorgenti PED. È stato disegnato un nuovo tipo di riscaldatore in grado di scaldare localmente per effetto Joule (facendo passare la corrente attraverso il contatto metallico della cella solare), permettendo la realizzazione di dispositivi fotovoltaici anche su materiali termolabili e flessibili come le plastiche. Questo lavoro di tesi ha contribuito in soli tre anni a sviluppare celle solari a film sottile a base di CIGS con efficienze paragonabili con i migliori valori internazionali con un processo semplice e potenzialmente scalabile a livello industriale

DIMENSÃO POLÍTICA DOS ASSISTENTES SOCIAIS NA GARANTIA DOS DIREITOS SEXUAIS E REPRODUTIVOS FEMININOS

O artigo apresenta um estudo sobre os direitos sexuais e reprodutivos, a partir do direcionamento político das entidades representativas dos assistentes sociais. Nesse sentido, os documentos elaborados pelas entidades da categoria demonstram a dimensão política da profissão na garantia desses direitos. O artigo apresenta alguns resultados da pesquisa realizada com esses documentos. A pesquisa foi qualitativa, exploratória e bibliográfica. Foi realizado um levantamento de dados nos documentos pautados pelos órgãos representativos da categoria Associação Brasileira de Ensino e Pesquisa em Serviço Social - ABEPSS, Conselho Federal de Serviço Social - CFESS, Executiva Nacional dos Estudantes de Serviço Social - ENESSO e o Conselho Regional de Serviço Social do Rio Grande do Sul - CRESS/RS. Foram levantados 38 documentos para o universo e 12 para a amostra. Embora a existência dos documentos seja fundamental para fomentar a discussão sobre os direitos sexuais e reprodutivos com a categoria, ainda são poucos com esse direcionamento

Efficiency enhancement in two-cell CIGS photovoltaic system with low-cost optical spectral splitter.

Spectrum splitting represents a valid alternative to multi-junction solar cells for broadband light-to-electricity conversion. While this concept has existed for decades, its adoption at the industrial scale is still stifled by high manufacturing costs and inability to scale to large areas. Here we report the experimental validation of a novel design that could allow the widespread adoption of spectrum splitting as a low-cost approach to high efficiency photovoltaic conversion. Our system consists of a prismatic lens that can be manufactured using the same methods employed for conventional CPV optic production, and two inexpensive CuInGaSe(2) (CIGS) solar cells having different composition and, thus, band gaps. We demonstrate a large improvement in cell efficiency under the splitter and show how this can lead to substantial increases in system output at competitive cost using existing technologies

On the temperature behavior of shunt-leakage currents in Cu(In,Ga)Se2 solar cells: The role of grain boundaries and rear Schottky contact

By comparing simulated and measured dark current-voltage (I-V) characteristics of CIGS cells at different temperatures, we investigate the temperature behavior of the shunt leakage current, and find that it can be explained by large donor trap concentrations at grain boundaries (GBs), and by a Schottky barrier at the backside contact where the GBs meets the anode metallization. We studied the I-V characteristics in the temperature range 280 K - 160 K achieving good fits of the measured I-V curves, especially for reverse bias and low forward bias, where the shunt leakage current dominates. The most important parameters determining the shunt leakage current value and its temperature dependence are the peak energy and density of the GB donor distribution, which control the inversion of GBs and the pinning of Fermi level at the anode/GB contact

Desarrollo de la autonomía en niños de 18 a 24 meses de edad según la filosofía de Reggio Emilia aplicada en el nido La Casa Amarilla

La presente tesis tiene como objetivo identificar la manera en que se fomenta la autonomía en niños y niñas de 18 a 24 meses de edad, según la filosofía de Reggio Emilia aplicada en el Nido La Casa Amarilla. La investigación profundiza en la autonomía como concepto y en su desarrollo en los niños y niñas de edad temprana; así como también, brinda a profesionales en educación una serie de valores y estrategias de acción para fomentar la autonomía en la escuela. A partir de un enfoque cualitativo de nivel exploratorio y a través de un estudio de caso, este estudio demuestra las implicancias que tiene la filosofía de Reggio Emilia en el despliegue continuo de dicha facultad en los niños. La información recopilada evidencia que el maestro promueve la autonomía a nivel físico y desde el soporte emocional que brinda al niño, desde el lenguaje verbal y no verbal que emplea, y desde la empatía para entender sus necesidades, decisiones y tiempos. Asimismo, el ambiente físico, considerado como un “tercer maestro”, propicia dicha facultad a partir del orden, estética y distribución de los materiales en el espacio, lo cual invita al niño a explorar de manera segura y por iniciativa propia. Finalmente, la investigación confirma que la rutina de los niños, al ser organizada y al contemplar transiciones amables en las que son conscientemente partícipes, suscita en ellos seguridad para actuar y tomar decisiones acordes a su edad.Tesi

The deployment of popular solidarity economy program on development by the SENAES at Rio Grande do Sul

A Economia Popular Solidária surgiu no Brasil, nas últimas décadas do século XX, como uma das alternativas de gerar trabalho e renda aos sujeitos exclusos do mercado formal de trabalho. Faz-se necessário o incentivo do Estado, mediante políticas e programas sociais, que financiem e assessorem os empreendimentos coletivos. O governo brasileiro criou, em 2003, o Programa de Economia Solidária em Desenvolvimento, que possui como intencionalidade fortalecer e viabilizar as experiências de geração de trabalho e renda. O governo está tentando materializar este objetivo por meio de parcerias estabelecidas com instituições da sociedade civil, vinculadas à Secretaria Nacional de Economia Solidária, que, por intermédio de assessorias e processos de incubação, juntamente com outros setores governamentais e incubadoras universitárias, operacionalizam e materializam o referido Programa Social. Pretende-se, neste artigo, apresentar como este Programa vem sendo implantado no Rio Grande do Sul/Brasil, juntamente com a parceria das instituições de apoio mencionadas.The Popular Solidarity Economy surged in Brazil, on the last decades of 20th century, as one of the alternatives to generate work and income to the formal market job excluded people. It’s required the incentive of the State, with politics and social programs which finance and assessors the collective ventures. The Brazilian government created, on 2003, The Popular Solidarity Economy Program on Development, that has as intent strengthen and enable the experiences of generating work and income. The government is trying to materialize this objective through partnerships established with civil society institutions, bound to the Solidarity Economy National Secretary, that, by intermediate of advisory services and processes of incubation, with other governmental sectors and academic incubators, operate and materialize the social program.It’s the objective, on this article, show how this program has been deployed at Rio Grande do Sul/Brazil, whit the partnership of support institutions mentione

Implementing a robotic liver resection program does not always require prior laparoscopic experience

Background: Preliminary experience in laparoscopic liver surgery is usually suggested prior to implementation of a robotic liver resection program. Methods: This was a retrospective cohort analysis of patients undergoing robotic (RLR) versus laparoscopic liver resection (LLR) for hepatocellular carcinoma at a center with concomitant initiation of robotic and laparoscopic programs RESULTS: A total of 92 consecutive patients operated on between May 2014 and February 2019 were included: 40 RLR versus 52 LLR. Median age (69 vs. 67; p = 0.74), male sex (62.5% vs. 59.6%; p = 0.96), incidence of chronic liver disease (97.5% vs.98.1%; p = 0.85), median model for end-stage liver disease (MELD) score (8 vs. 9; p = 0.92), and median largest nodule size (22 vs. 24 mm) were similar between RLR and LLR. In the LLR group, there was a numerically higher incidence of nodules located in segment 4 (20.0% vs. 16.6%; p = 0.79); a numerically higher use of Pringle's maneuver (32.7% vs. 20%; p = 0.23), and a shorter duration of surgery (median of 165.5 vs. 217.5 min; p = 0.04). Incidence of complications (25% vs.32.7%; p = 0.49), blood transfusions (2.5% vs.9.6%; p = 0.21), and median length of stay (6 vs. 5; p = 0.54) were similar between RLR and LLR. The overall (OS) and recurrence-free (RFS) survival rates at 1 and 5 years were 100 and 79 and 95 and 26% for RLR versus 96.2 and 76.9 and 84.6 and 26.9% for LLR (log-rank p = 0.65 for OS and 0.72 for RFS). Conclusions: Based on our results, concurrent implementation of a robotic and laparoscopic liver resection program appears feasible and safe, and is associated with similar oncologic long-term outcomes