Alternative Substrates for Sustainable and Earth-abundant Thin Film Photovoltaics

[eng] The development of sustainable energy sources with a high energy return on energy investment (EROI) that can substitute fossil fuels is a must in order to avoid the collapse of our current civilization. In this context, this work explores the feasibility of fabricating efficient Cu2ZnSn(Sx,Se1-x)4 (kesterite) solar cells on three strategic substrates: polyimide, ceramic and SnO2:F-coated soda-lime glass (SLG/FTO). These substrates present several advantages with respect to the standard SLG/Mo. Polyimide is compatible with roll-to-toll production processes and easily integrable in many applications thanks to its light weight and flexibility, ceramics have a direct application in building-integrated photovoltaics as solar tiles and the transparency of SLG/FTO enables advanced photovoltaic concepts like bifacial and tandem solar cells as well as the fabrication of semi-transparent devices. Their combination with a sustainable thin film photovoltaic technology based on Earth-abundant materials like kesterites has the potential of decreasing the energy fabrication cost and, thus, of increasing the EROI of photovoltaics through: 1) high throughput production, 2) integration and 3) advanced applications and functionality. However, these substrates also present several drawbacks. Alkalis, especially Na, are fundamental to achieve high efficiency devices but polyimide and ceramics are alkali-free materials. Likewise, FTO acts as a barrier for alkalis. In addition, polyimide presents a low thermal robustness that limits process temperatures below 500ᵒC, ceramics are very rough and possess detrimental impurities and the use of FTO as back contact leads to a non-optimum p-kesterite/n-FTO interface. This work focuses on the implementation of specific strategies to adapt the kesterite solar cell fabrication process to the characteristics of the different substrates. A combination of alkaline doping and low-temperature annealings is studied for the fabrication of Cu2ZnSnSe4 solar cells on polyimide. While doping with NaF and KF is found to lead to critical improvements, working at low temperature is linked to the formation of SnSe2. This phase decreases the open-circuit voltage of the devices and is the main factor controlling their performance. Further experimentation leads to a 4.9% efficiency record device by combining NaF and Ge doping and a 480ᵒC annealing. In the case of ceramic, vitreous enamels with controlled amounts of Na2O in their composition are used as surface smoothers, Na sources and impurity barriers, simultaneously. However, large amounts of Na2O in the enamel composition result in high densities of surface defects: undulations, pinholes and cracks. While undulations and pinholes are rather benign, cracks strongly deteriorate the back contact. In addition, the annealing time needs to be controlled to avoid the formation of SnSe2. Besides these issues, enamelled ceramic substrates are observed to behave similarly to SLG yielding a record Cu2ZnSnSe4 device with a 7.5% efficiency. Regarding SLG/FTO, the addition of transition metal oxides (TMOs) and/or Mo:Na nanolayers is studied as an approach to improve the back interface of the devices. Mo:Na is found to alleviate shunting and recombination issues and to protect FTO from degradation during annealing which leads to highly improved devices, especially for Cu2ZnSnS4. On the other hand, TMOs introduce a severe current blockage. However, the combination of the TiO2 and V2O5 with Mo:Na is observed to boost the beneficial effects of the latter in Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 devices. Although these multi-layered back interfaces exhibit a complex behaviour, this approach results in record efficiencies of 6.2%, 6.1% and 7.9% for Cu2ZnSnSe4, Cu2ZnSnS4 and Cu2ZnSn(S,Se)4 devices, respectively. These results represent the highest efficiencies ever reported for kesterite solar cells fabricated on polyimide, ceramic and transparent substrates and give proof of their large potential for sustainable kesterite-based photovoltaics.[spa] Esta tesis estudia el desarrollo de células solares de Cu2ZnSn(Sx,Se1-x)4 (kesterita) sobre sustratos de poliimida, cerámica y vidrio recubierto con SnO2:F (SLG/ITO). Estos sustratos presentan una serie de ventajas frente al estándar SLG/Mo como su compatibilidad con procesos de fabricación rollo a rollo (poliimida), con fotovoltaica integrada en edificios (todos) y/o la posibilidad de aplicaciones y funcionalidades avanzadas (poliimida y SLG/FTO). Sin embargo, también poseen características que dificultan la fabricación de células solares de kesterita como la falta de elementos alcalinos (todos), su baja resistencia térmica (poliimida), su alta rugosidad (cerámica) o su conductividad tipo n (SLG/FTO). Este trabajo se centra en la implementación de estrategias para la fabricación de células solares de kesterita de alta eficiencia atendiendo a las características de cada sustrato. En el caso de la poliimida, se utiliza una combinación de dopaje alcalino y procesos de recocido a baja temperatura la cual permite obtener un dispositivo Cu2ZnSnSe4 record con una eficiencia del 4.9% combinando dopaje con NaF y Ge y una temperatura de 480ᵒC. Los sustratos cerámicos se recubren con un esmalte vítreo que contiene una cantidad controlada de Na2O en su composición y que actúa como reductor de la rugosidad superficial, fuente de Na y barrera para impurezas. A pesar de que altas concentraciones de Na2O provocan defectos superficiales, los sustratos cerámicos esmaltados presentan un comportamiento muy similar al vidrio con un dispositivo Cu2ZnSnSe4 record de 7.5% de eficiencia. En cuanto a los sustratos SLG/FTO, se estudia el depósito de nanocapas de óxidos de metales de transición (TMOs) y de Mo:Na para mejorar la interficie trasera kesterita-p/FTO-n de los dispositivos. Los TMOs inducen una fuerte barrera eléctrica mientras el Mo:Na se muestra fundamental para la fabricación de dispositivos eficientes. Sin embargo, el uso conjunto de Mo:Na y TiO2 o V2O5 amplifica los efectos beneficiosos del Mo:Na lo que permite obtener un dispositivo Cu2ZnSn(Sx,Se1-x)4 record con una eficiencia de 7.9%. Estas son las eficiencias más altas reportadas para células solares de kesterita sobre poliimida, cerámica y SLG/FTO y demuestran el potencial de estos sustratos para el desarrollo de una fotovoltaica sostenible basada en kesterita

Effects of ITO based back contacts on Cu(In,Ga)Se2 thin films, solar cells, and mini-modules relevant for semi-transparent building integrated photovoltaics

This study presents the results of the development of semi-transparent Cu(In,Ga)Se2 (CIGSe) mini-modules for the application in building integrated photovoltaics (BIPV). Applying in-situ X-ray diffraction in real-time during CIGSe growth we find that the bulk of indium-tin-oxide (ITO), acting as the transparent back contact, is chemically stable in CIGSe processing. CIGSe layers grown on reactively sputtered ITO (Ar/O2 flux ratio = 60:1) or on ITO annealed in ambient air have a pro-portionally higher (220/204) orientation compared to CIGSe layers grown on as fabricated ITO sputtered solely by Ar. However, independent from the fabrication and annealing state of the ITO back contact, after CIGSe deposition at high substrate temperatures >= 600 degrees C accumulation of Ga at the CIGSe/ITO back contact interface combined with reduced solar cell efficiency is observed. This Ga accumulation visible in elemental depth profiles is attributed to the formation of gallium -oxide (GaOx). Applying a very thin (approximate to 10-30 nm) functional molybdenum layer in between CIGSe and the ITO back contact inhibits the formation of GaOx. Based on this Mo/ITO back contact configuration semi-transparent 10 x 10 cm2 mini-modules with 14 cells interconnected in series have been fabricated. Module parameters resulted in a fill factor of 63% and >12% in efficiency. The solar active coverage of the modules amounts to approximate to 70%, and the average visible transmittance (in the range 380-780 nm) of the transparent sections was 27.6% (9.6% for the total area of the device). Optimisation of the Mo/ITO contact allows increasing this transparency to values > 50%. Long-term outdoor testing of a semi-transparent module prototype reveals no degradation in electric output power for 3 months, demonstrating the device stability under changing climatic conditions

Insights into interface and bulk defects in a high efficiency kesterite-based device

This work provides a detailed analysis of a high efficiency Cu2ZnSnSe4 device using a combination of advanced electron microscopy and spectroscopy techniques. In particular, a full picture of the different defects present at the interfaces of the device and in the bulk of the absorber is achieved through the combination of high resolution electron microscopy techniques with Raman, X-ray fluorescence and Auger spectroscopy measurements at the macro, micro and nano scales. The simultaneous investigation of the bulk and the interfaces allows assessing the impact of the defects found in each part of the device on its performance. Despite a good crystalline quality and homogeneous composition in the bulk, this work reports, for the first time, direct evidence of twinning defects in the bulk, of micro and nano-voids at the back interface and of grain-to-grain non-uniformities and dislocation defects at the front interface. These, together with other issues observed such as strong absorber thickness variations and a bilayer structure with small grains at the bottom, are shown to be the main factors limiting the performance of CZTSe devices. These results open the way to the identification of new solutions to further developing the kesterite technology and pushing it towards higher performances. Moreover, this study provides an example of how the advanced characterization of emergent multilayer-based devices can be employed to elucidate their main limitations.Peer ReviewedPostprint (author's final draft

Impact of COVID-19 on cardiovascular testing in the United States versus the rest of the world

Objectives: This study sought to quantify and compare the decline in volumes of cardiovascular procedures between the United States and non-US institutions during the early phase of the coronavirus disease-2019 (COVID-19) pandemic. Background: The COVID-19 pandemic has disrupted the care of many non-COVID-19 illnesses. Reductions in diagnostic cardiovascular testing around the world have led to concerns over the implications of reduced testing for cardiovascular disease (CVD) morbidity and mortality. Methods: Data were submitted to the INCAPS-COVID (International Atomic Energy Agency Non-Invasive Cardiology Protocols Study of COVID-19), a multinational registry comprising 909 institutions in 108 countries (including 155 facilities in 40 U.S. states), assessing the impact of the COVID-19 pandemic on volumes of diagnostic cardiovascular procedures. Data were obtained for April 2020 and compared with volumes of baseline procedures from March 2019. We compared laboratory characteristics, practices, and procedure volumes between U.S. and non-U.S. facilities and between U.S. geographic regions and identified factors associated with volume reduction in the United States. Results: Reductions in the volumes of procedures in the United States were similar to those in non-U.S. facilities (68% vs. 63%, respectively; p = 0.237), although U.S. facilities reported greater reductions in invasive coronary angiography (69% vs. 53%, respectively; p < 0.001). Significantly more U.S. facilities reported increased use of telehealth and patient screening measures than non-U.S. facilities, such as temperature checks, symptom screenings, and COVID-19 testing. Reductions in volumes of procedures differed between U.S. regions, with larger declines observed in the Northeast (76%) and Midwest (74%) than in the South (62%) and West (44%). Prevalence of COVID-19, staff redeployments, outpatient centers, and urban centers were associated with greater reductions in volume in U.S. facilities in a multivariable analysis. Conclusions: We observed marked reductions in U.S. cardiovascular testing in the early phase of the pandemic and significant variability between U.S. regions. The association between reductions of volumes and COVID-19 prevalence in the United States highlighted the need for proactive efforts to maintain access to cardiovascular testing in areas most affected by outbreaks of COVID-19 infection

Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe

MUSiC : a model-unspecific search for new physics in proton-proton collisions at root s=13TeV

Results of the Model Unspecific Search in CMS (MUSiC), using proton-proton collision data recorded at the LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb(-1), are presented. The MUSiC analysis searches for anomalies that could be signatures of physics beyond the standard model. The analysis is based on the comparison of observed data with the standard model prediction, as determined from simulation, in several hundred final states and multiple kinematic distributions. Events containing at least one electron or muon are classified based on their final state topology, and an automated search algorithm surveys the observed data for deviations from the prediction. The sensitivity of the search is validated using multiple methods. No significant deviations from the predictions have been observed. For a wide range of final state topologies, agreement is found between the data and the standard model simulation. This analysis complements dedicated search analyses by significantly expanding the range of final states covered using a model independent approach with the largest data set to date to probe phase space regions beyond the reach of previous general searches.Peer reviewe

Measurement of prompt open-charm production cross sections in proton-proton collisions at root s=13 TeV

The production cross sections for prompt open-charm mesons in proton-proton collisions at a center-of-mass energy of 13TeV are reported. The measurement is performed using a data sample collected by the CMS experiment corresponding to an integrated luminosity of 29 nb(-1). The differential production cross sections of the D*(+/-), D-+/-, and D-0 ((D) over bar (0)) mesons are presented in ranges of transverse momentum and pseudorapidity 4 < p(T) < 100 GeV and vertical bar eta vertical bar < 2.1, respectively. The results are compared to several theoretical calculations and to previous measurements.Peer reviewe

Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at root s=13 TeV

A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 137 fb(-1) collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on themodel, the combined result excludes a top squarkmass up to 1325 GeV for amassless neutralino, and a neutralinomass up to 700 GeV for a top squarkmass of 1150 GeV. Top squarks with masses from 145 to 295 GeV, for neutralino masses from 0 to 100 GeV, with a mass difference between the top squark and the neutralino in a window of 30 GeV around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 GeV

Probing effective field theory operators in the associated production of top quarks with a Z boson in multilepton final states at root s=13 TeV

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Measurement of B-c(2S)(+) and B-c*(2S)(+) cross section ratios in proton-proton collisions at root s=13 TeV

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