Technological processes for CIGS based solar cells

The solar photovoltaics (PV) market has been booming since the late 1990’s with an impressive 40% average annual growth rate and rapid growth has continued even in the last four years despite the bottlenecks in silicon feedstock availability. , which have determined a slow down in the decline of PV module prices. As an economic alternative to silicon a great deal of attention has been devoted to Cu(InxGa1-x)Se2 (CIGS) thin film based solar cells, whose development lead to demonstrate in 2010 a record efficiency of 20.3% on a 0.5 cm2 laboratory cell. The activity discussed in this thesis was performed in the frame of PED4PV project (“Industria 2015” programme of the Italian Ministry of Economic Development) aiming at the low cost production of thin film CIGS-based solar cells on conventional glass substrates as well as on alternative substrates suitable for building integration (such as ceramic and cement tiles) by using the relatively new, highly efficient, but yet largely unexplored technology called “Pulsed electron deposition”. This thesis reports about the set up of the technological processes that complement PED growth of the multilayer structure in the fabrication of the solar cells and their characterization. In particular the optimization of the Mo back contact deposition by sputtering is detailed, and the activity devoted to support Marazzi S.p.a and CTG S.p.a. in the development of ceramic and cement substrates, respectively, is described. A relevant part of the work has been devoted to the characterization of the cells. Current-voltage, capacitance-voltage and infrared thermography measurements gave the PED group the feedbacks necessary to improve the multilayer structure properties up to achieve an efficiency of 15.5% on lab-scale solar cells completely prepared at IMEM by depositing CIGS from a stoichiometric quaternary target at low temperature (270 °C) and without any post-growth treatment

Effect of temperature on the mutual diffusion of Ge/GaAs and GaAs/Ge

GaAs/Ge heterostructures are commonly used in high efficiency solar cell: structures such as InGaP/GaAs/GaAs or GaAs/Ge and triple junction InGaP/GaAs/Ge, are continuously improving their efficiency. Sharp heterointerfaces and abrupt dopant profiles are essential in order to obtain a good control of the heteroepitaxy. One common problem in the Ge/GaAs, GaAs/Ge, Ge/GaAs/Ge, Ge/GaAs/GaAs, GaAs/Ge/GaAs and GaAs/Ge/Ge heterostructures is the interdiffusion of the elements in the different layers, i.e. Ge in GaAs and Ga and As in Ge. Since Ge acts as dopant in GaAs, and vice-versa, the film/substrate interdiffusion of Ge and GaAs is able to change the carrier concentrations in the layers. In order to assess this problem we grew Ge/GaAs and GaAs/Ge heterojunctions by metal-organic vapour phase epitaxy (MOVPE) using iso-butylgermane, arsine and trimethylgallium in hydrogen atmosphere at low pressure, varying the deposition temperature. The use of low temperature GaAs and Ge buffer layers was investigated in order to limit the interdiffusion. Different experimental techniques, including Secondary Neutral Mass Spectrometry (SNMS), High Resolution X-ray Diffraction (HR-XRD), Transmission Electron Microscopy (TEM), Rutherford Backscattering Spectrometry combined with Channeling technique (RBS/C) and Atomic Force Microscopy (AFM) have been used to investigate the samples. HR-XRD profiles show the good crystalline quality of the epitaxial layers, with a lattice mismatch between the layer and the substrate as calculated form the peak separation corresponding to perfectly adapted layers RBS/channeling spectra show no significant difference between the samples grown at different temperatures, indicating that the presence of extra defects, strain, or misorientation at the interface is below the detection limit. The depth profile analysis of samples measured by SNMS show remarkable interdiffusion of all components at the heterointerfaces between the layers and substrates, indicates a variation with temperature. The results confirmed that a low temperature GaAs buffer layer could efficiently reduce GaAs/Ge mutual diffusion. The same is not true for a low temperature Ge buffer layer in Ge/GaAs epitaxy. TEM was used to assess the crystal quality of the grown layers and composition distribution by X-ray microanalysis, that confirmed elements interdiffusion as measured by SNMS

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

Cosmic Rays from the Knee to the Highest Energies

This review summarizes recent developments in the understanding of high-energy cosmic rays. It focuses on galactic and presumably extragalactic particles in the energy range from the knee (10^15 eV) up to the highest energies observed (>10^20 eV). Emphasis is put on observational results, their interpretation, and the global picture of cosmic rays that has emerged during the last decade.Comment: Invited review, submitted to Progress in Particle and Nuclear Physic

First Observation of the Production and Decay of the sigma c

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Deviation from Regular Shape in the Early Stages of Formation of Strain-Driven 3D InGaAs/GaAs Micro/Nanotubes

Single-crystalline InGaAs/GaAs semiconductor micro/nanotubes have been obtained by the strain-driven self-rolling mechanism. This approach combines the advantages of bottom-up (epitaxial growth) and top-down (postgrowth processing) techniques, offering an exceptional opportunity to realize complex three-dimensional nanoarchitectures by using conventional photolithography and wet-etching processes. The method employed to obtain micro/nanotubes with selected orientation and length is described in detail. By means of high-resolution scanning electron microscopy characterization, we show a clear shape difference between single-wall and multiwalls tubes and we discuss it on the basis of strain release, taking into account also possible shape deformations induced during micro/nanotubes drying. We analyse the In-segregation profile in the nominal In0.20Ga0.80As/GaAs bilayer and we show its effect on the actual diameter of the tubes, concluding that a more accurate description of the structure should consider an In0.20Ga0.80As/In0.10Ga0.90As/GaAs trilayer. This work will be useful to set up reliable methodologies for the realization of strain-driven micro/nanotubes with controlled properties, necessary for their implementation in a large number of application fields

Sero-survey on long-term care facility residents reveals increased risk of sub-optimal antibody response to BNT162b2: implications for breakthrough prevention

Background The impact of coronavirus disease 2019 (COVID-19) caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) on residents of long-term care facilities (LTCFs) has been dramatic on global scale as older age and comorbidities pose an increased risk of severe disease and death. Methods Aim of this study was to evaluate SARS-CoV-2 Spike-specific IgG (S-IgG) antibody titers in 478 residents and 649 health care workers of a large Italian long-term care facility two months after complete vaccination with BNT162b2. Associations among resident-related factors and predictors of humoral response were investigated. Results By stratifying levels of humoral responses, we found that 62.1%, 21.6%, 12.1% and 4.2% of residents had high (>1,000 BAU/ml), medium (101-1,000), low (1-100) and null (<1 BAU/mL) S-IgG titers, respectively. Residents with documented previous COVID-19 and those with SARS-CoV-2 nucleocapsid-specific IgG (N-IgG) positive serology showed higher level of serological response, while significant associations were observed for cancer with suboptimal response (p = 0.005) and the administration of corticosteroid for suboptimal response (p = 0.028) and a null one (p = 0.039). According to multivariate logistic regression, predictors of an increased risk of null response were advanced age (Odd ratio, OR: 2.630; Confidence interval, CI: 1.13-6.14; p = 0.025), corticosteroid therapy (OR: 4.964; CI: 1.06-23.52; p = 0.042) and diabetes mellitus (OR:3.415; CI:1.08-10.8; p = 0.037). In contrast, previous diagnosis of COVID-19 was strongly associated with a reduced risk of null response to vaccination (OR:0.126; CI:0.02-0.23; p < 0.001). Conclusions SARS-CoV-2 specific antibodies in elderly individuals should be consider when deciding the need of a third dose of vaccine for prevention of reinfections in LTCFs despite the maintenance of barrier measures

Balloon aortic valvuloplasty before noncardiac surgery in severe aortic stenosis: a single-center experience

OBJECTIVES: The aim of this study is to investigate the role and short-term results of balloon aortic valvuloplasty (BAV) before noncardiac surgery in a high selected cohort of patients. BACKGROUND: Aortic stenosis is one of the most common valvular heart diseases and a well recognized risk factor for perioperative mortality. METHODS: Between May 2012 and July 2013 we enrolled 15 consecutive patients with severe aortic stenosis to allow urgent major noncardiac surgery. They had been excluded from surgical aortic valve replacement and transcatheter aortic valve implantation. RESULTS: Fifteen patients underwent BAV as a bridge to noncardiac surgery. They were elderly (mean age 81?±?5 years) and predominantly men (66%) with high surgery risk (mean logistic EuroSCORE: 31.1?±?18.2%). Three patients underwent vascular surgery, five underwent thoracic surgery, five were subjected to major abdominal surgery and in the last two patients orthopedic surgery and mastectomy were performed. No adverse events were observed in the perioperative period. Six patients (40%) were in New York Heart Association class III or IV. Mean aortic valve area was 0.52?±?0.1?cm/m; mean aortic pressure gradient was 55.6?±?10.8?mmHg. BAV was performed successfully in all patients. The mean peak-to-peak gradient assessed by catheterization significantly reduced after BAV (from 69.0?±?22.1 to 29.7?±?9.3?mmHg; P?<?0.0001). The echocardiographic mean gradient was also significantly improved (from 55.6?±?10.8 to 33.8?±?7.9?mmHg; P?<?0.0001). Survival at 30?days and at 6?months’ follow-up respectively was 100%. At 6?months’ follow-up clinical status according to New York Heart Association class was significantly improved (P?<?0.0001). CONCLUSION: BAV is well tolerated and effective in high-risk patients with severe aortic stenosis undergoing noncardiac surgery with good short-term survival. It could represent a valid choice of treatment prior to noncardiac surgery in these high-risk patients

Irreversible evolution of eumelanin redox states detected by an organic electrochemical transistor: en route to bioelectronics and biosensing

Org. electrochem. transistors (OECTs) are currently emerging as powerful tools for biosensing, bioelectronics and nanomedical applications owing to their ability to operate under liq. phase conditions optimally integrating electronic and biol. systems. Herein we disclose the unique potential of OECTs for detecting and investigating the elec. properties of insol. eumelanin biopolymers. Gate current measurements on fine aq. suspensions of a synthetic eumelanin sample from 5,6-dihydroxyindole (DHI) revealed a well detectable hysteretic response similar to that of the pure monomer in soln., with the formal concn. of the polymer as low as 10-6 M. Induction of the gate current would reflect electron transfer from solid eumelanin to the Pt-electrode sustained by redox active catechol/quinone components of the polymer. A gradual decrease in gate current and areas subtended by hysteretic loops were obsd. over 5 cycles both in the eumelanin- and DHI-based devices, suggesting evolution of the polymer from a far-from-the-equil. redox state toward a more stable electronic arrangement promoted by redox exchange with the gate electrode. OECTs are thus proposed as valuable tools for the efficient heterogeneous-phase sensing of eumelanins and to gauge their peculiar elec. and redox behavior

Durability of Humoral Responses after the Second Dose of mRNA BNT162b2 Vaccine in Residents of a Long Term Care Facility

Residents of long-term care facilities (LTCFs) have been dramatically hit by the COVID-19 pandemic on a global scale as older age and comorbidities pose an increased risk of severe disease and death. The aim of the study was to assess the quantity and durability of specific antibody responses to SARS-CoV-2 after the first cycle (two doses) of BNT162b2 vaccine. To achieve this, SARS-CoV-2 Spike-specific IgG (S-IgG) titers was evaluated in 432 residents of the largest Italian LTCF at months 2 and 6 after vaccination. By stratifying levels of humoral responses as high, medium, low and null, we did not find any difference when comparing the two time points; however, the median levels of antibodies halved overtime. As positive nucleocapsid serology was associated with a reduced risk of a suboptimal response at both time points, we conducted separate analyses accordingly. In subjects with positive serology, the median level of anti-S IgG slightly increased at the second time point, while a significant reduction was observed in patients without previous exposure to the virus. At month 6, diabetes alone was associated with an increased risk of impaired response. Our data provide additional insights into the longitudinal dynamics of the immune response to BNT162b2 vaccination in the elderly, highlighting the need for SARS-CoV-2 antibody monitoring following third-dose administration