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

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

Progress on Low-Temperature Pulsed Electron Deposition of CuInGaSe2 Solar Cells

The quest for single-stage deposition of CuInGaSe2 (CIGS) is an open race to replace very effective but capital intensive thin film solar cell manufacturing processes like multiple-stage coevaporation or sputtering combined with high pressure selenisation treatments. In this paper the most recent achievements of Low Temperature Pulsed Electron Deposition (LTPED), a novel single stage deposition process by which CIGS can be deposited at 250 °C, are presented and discussed. We show that selenium loss during the film deposition is not a problem with LTPED as good crystalline films are formed very close to the melting temperature of selenium. The mechanism of formation of good ohmic contacts between CIGS and Mo in the absence of any MoSe2 transition layers is also illustrated, followed by a brief summary of the measured characteristics of test solar cells grown by LTPED. The 17% efficiency target achieved by lab-scale CIGS devices without bandgap modulation, antireflection coating or K-doping is considered to be a crucial milestone along the path to the industrial scale-up of LTPED. The paper ends with a brief review of the open scientific and technological issues related to the scale-up and the possible future applications of the new technology

Equivalent Circuit Model for Cu(In,Ga)Se₂ Solar Cells Operating at Different Temperatures and Irradiance

The modeling of photovoltaic cells is an essential step in the analysis of the performances and characterization of PV systems. This paper proposes an experimental study of the dependence of the five parameters of the one-diode model on atmospheric conditions, i.e., irradiance and temperature in the case of thin-film solar cells. The extraction of the five parameters was performed starting from two sets of experimental data obtained from Cu(In,Ga)Se2 solar cells fabricated by the low-temperature pulsed electron deposition technique. A reduced form approach of the one-diode model has been adopted, leading to an accurate identification of the cell. It was possible to elaborate suitable relations describing the behavior of the parameters as functions of the environmental conditions. This allowed accurately predicting the trends of the parameters from a pair of curves, instead of a whole set of measurements. The developed model describing the dependence on irradiance and temperature was validated by means of a large set of experimental measurements on several Cu(In,Ga)Se2 (CIGS) devices built with the same technological process

Advanced age and high American Society of Anesthesiologists’ risk score do not increase perioperative mortality in pancreatic resections: a view from a tertiary care center

Background and Objectives: To evaluate outcomes in elderly patients with high anaesthesiology risk (ASA 4) who underwent pancreatic resection compared to younger patients and elderly patients with lower anaesthesiology risk. Materials and Methods: A consecutive series of 345 patients who underwent pancreatic resection at our tertiary care centre between 2010 and 2017 was reviewed. We compared three groups based on age at the time of surgery:<65 years (group A), 65-74 years (group B), and ³75 years (group C). In addition, patients in group C were split into two subgroups, ASA 1-3 versus ASA 4, and compared. Prospectively collected data regarding pre-, intra-, post-operative course and follow up of patients belonging to these two subgroups were retrospectively analysed and compared. Results: The group A consisted in 117 (34%) patients, the group B in 128 (37%) patients, and group C in 100 (29%) patients. Group C patients had a significant higher incidence of comorbidities and ASA 4 respect the other two groups (p<0.05). The incidence of the overall post-operative complications was significantly higher in the group C (p<0.01), due to the higher incidence of medical complications. No difference in term of overall surgical complications was reported between the three groups. No difference was documented for post-operative mortality between the three groups. The mean overall survival was significantly lower for group C (p<0.01), but no difference in mortality for cancer was reported between the three groups. Within Group C, the comparison between patients with ASA score 1-3 and ASA 4 showed no significant differences regarding surgical complications (p=0.59), reoperation rate (p=0.45), mortality (p=0.34) and mean overall survival (p=0.53). Conclusion: Although elderly patients presented a higher rate of postoperative complications and a lower mean overall survival, they did not show a higher perioperative mortality. Furthermore, mortality due to cancer in operated patients was not different between the three groups. For these reasons, the advanced age should not be considered a reason to preclude the surgical option to elderly patients with pancreatic cancer . Furthermore, no differences were found in short-term and long-term survival in elderly patients with different operative risk factors (ASA score) , so the higher anesthesiological risk in subjects aged ≥ 75 years should not be considered an absolute contraindication to surgical treatment

Robot Assisted Trans-Gastric Drainage of Walled-Off Pancreatic Necrosis with Robotic Endowrist Stapler for the Da Vinci XI

Aim: Walled-off pancreatic necrosis (WOPN) is a late complication of acute pancreatits. The mortality rate with WOPN is high but less than infected pancreatic necrosis. Management depends on the patient's symptoms and the location of the WOPN. The da Vinci surgical System has been developed to allow an easy, mini-invasive and fast surgery, also in challenging abdominal procedures. We present here a case of a WOPN treated with a trans-gastric drainage using the da Vinci Xi. Methods: A 63-years old man with an episode of acute necrotizing pancreatitis was referred to our center. The patient developed a massive fluid collection determinates obstruction of the gastrointestinal tract and the TC-scan, performed 6 weeks after the acute episode, confirmed the presence of a walled-off pancreatic necrosis that compresses the stomach and the first duodenal portion. The patient was operated with the use of the da Vinci Xi surgical system. Results: The procedure was successfully completed in 130 min. Firstly, guided by intraoperative ultrasound, was performed an anterior ideal gastrotomy. Then, through the gastrotomy, the best location on the posterior gastric wall for drainage was again US-identified. The anastomosis between the posterior gastric wall and the walled-off necrosis wall was carried out with the new EndoWrist stapler. Necrosectomy and washing of the cavity through the anastomosis was performed. Finally, the anterior gastrotomy was closed with three layers of 3-0 V-Lock running sutures and the cholecystectomy was performed. No conversion or intra-operative complications were recorded. The postoperative course was uneventful and a post-operative TC scan showed the collapse of the fluid collection. Conclusions: In this case the da Vinci Surgical System has been safely used in the surgical treatment of walled-off necrosis. The Tile-Pro multi-input display consent the surgeon to view 3D video of the operative field along with ultrasound exam, allows to identify the best location where perform the drainage. Furthermore the enhanced surgical dexterity could give some advantages by facilitating the sutures

ROBOTIC ASSISTED VERSUS PURE LAPAROSCOPIC SURGERY OF THE ADRENAL GLANDS: A CASE-CONTROL STUDY COMPARING SURGICAL TECHNIQUES

AIM: The role of da Vinci® System in adrenal gland surgery is not well-defined yet. This case-controlled study aims to compare robotic assisted surgery with pure laparoscopic surgery, in the authors’ monocentric experience. MATERIALS AND METHODS: 116 patients underwent minimally invasive adrenalectomies in our Department between June 1994 and December 2014: 75 with pure laparoscopic surgery (LS), whereas 41 with da Vinci® robotic system (RS). This case-controlled study was performed comparing 19 patients operated with RS and 19 operated with LS according to BMI, age, laterality and neoplasia dimensions. The two groups were compared for clinical and surgical data. Patients of each group were divided in subgroups according to nodule dimensions (< 3 cm, between 3 and 6 cm, ≥ 6 cm). Statistical analysis was performed with Student’s t-test for independent samples. Value of p < 0.01 was considered significant. RESULTS: The laparoscopic group of this case-controlled study showed a significant increase of operative time in patients with malignancy, in those with BMI ≥ 30 kg/m2 and with nodules > 6 cm (p < 0.01). This trend was not evidenced in the robotic group (p = NS). The direct comparison between RS and LS did not reveal differences in the operative time with nodules < 3 cm or between 3 and 6 cm, whereas with nodules ≥ 6 cm the robotic group operative time resulted significantly lower compared to the laparoscopic group (163.3 vs. 276.4 minutes; p < 0.01). Conversions to open surgery were 2 for the laparoscopic group and 0 for the robotic group. Postsurgical complications were 2 and 0 respectively. No reoperations or deceased patients occurred. CONCLUSIONS: In our experience, robotic system in adrenal gland surgery showed potential benefits compared to classic laparoscopy in patients with malignancy, BMI ≥ 30 kg/m2 and neoplasia > 6 cm

CHIRURGIA ROBOT ASSISTITA E LAPAROSCOPICA DEL SURRENE: CONFRONTO TRA TECNICHE MEDIANTE STUDIO CASO-CONTROLLO

CHIRURGIA ROBOT ASSISTITA E LAPAROSCOPICA DEL SURRENE: CONFRONTO TRA TECNICHE MEDIANTE STUDIO CASO-CONTROLL

Robotic assisted versus pure laparoscopic surgery of the adrenal glands: a case-control study comparing surgical techniques

Purpose: The role of the da Vinci Robotic System® in adrenal gland surgery is not yet well defined. The goal of this study was to compare robotic-assisted surgery with pure laparoscopic surgery in a single center. Methods: One hundred and 16 patients underwent minimally invasive adrenalectomies in our department between June 1994 and December 2014, 41 of whom were treated with a robotic-assisted approach (robotic adrenalectomy, RA). Patients who underwent RA were matched according to BMI, age, gender, and nodule dimensions, and compared with 41 patients who had undergone laparoscopic adrenalectomies (LA). Statistical analysis was performed using the Student’s t test for independent samples, and the relationship between the operative time and other covariates were evaluated with a multivariable linear regression model. P &lt; 0.05 was considered significant. Results: Mean operative time was significantly shorter in the RA group compared to the LA group. The subgroup analysis showed a shorter mean operative time in the RA group in patients with nodules ≥6 cm, BMI ≥ 30 kg/m2 and in those who had previous abdominal surgery (p &lt; 0.05). Results from the multiple regression model confirmed a shorter mean operative time with RA with nodules ≥6 cm (p = 0.010). Conversion rate and postoperative complications were 2.4 and 4.8 % in the LA group and 0 and 4.8 % in the RA group. Conclusions: In our experience, RA shows potential benefits compared to classic LA, in particular on patients with nodules ≥6 cm, BMI ≥ 30 kg/m2, and with previous abdominal surgery