25 research outputs found

    Exploiting the Optical Limits of Thin‐Film Solar Cells: A Review on Light Management Strategies in Cu(In,Ga)Se 2

    Get PDF
    Light management strategies are of utmost importance to allow Cu(In,Ga)Se2 (CIGS) technology market expansion, as it would enable a conversion efficiency boost as well as thinner absorber layers, increasing sustainability and reducing production costs. However, fabrication and architecture constraints hamper the direct transfer of light management architectures from other photovoltaic technologies. The demand for light management in thin and ultrathin CIGS cells is analyzed by a critical description of the optical loss mechanisms in these devices. Three main pathways to tackle the optical losses are identified: front light management architectures that assist for an omnidirectional low reflection; rear architectures that enable an enhanced optical path length; and unconventional spectral conversion strategies for full spectral harvesting. An outlook over the challenges and developments of light management architectures is performed, establishing a research roadmap for future works in light management for CIGS technology. Following the extensive review, it is expected that combining antireflection, light trapping, and conversion mechanisms, a 27% CIGS solar cell can be achieved.Fundação para a Ciência e a Tecnologia (FCT) and Fundo Social Europeu (FSE) are acknowledged through the projects IF/00133/2015, UIDB/ 50025/2020, UIDP/50025/2020, UIDB/04730/2020, UIDP/04730/2020, and DFA/BD/4564/2020. This research was also supported by NovaCell—Development of novel Ultrathin Solar Cell Architectures for low-light, low-cost, and flexible optoelectronic device project (028075) cofunded by FCT and ERDF through COMPETE2020. This research was supported by InovSolarCells–Development of innovative nanostructured dielectric materials for interface passivation in thin-film solar cells project (029696) cofunded by FCT and ERDF through COMPETE2020. The authors acknowledge the financial support of the project Baterias 2030, with the reference POCI-01-0247-FEDER-046109, cofunded by Operational Programme for Competitiveness and Internationalization (COMPETE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF)info:eu-repo/semantics/publishedVersio

    Cd and Cu interdiffusion in Cu(In,Ga)Se2/CdS hetero-interfaces

    Get PDF
    We report a detailed characterization of an industry-like prepared Cu(In,Ga)Se2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy (STEM) and photoluminescence (PL). Energy dispersive x-ray spectroscopy (EDS) shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd-Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu-S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach we identified that this effect is independent of focused-ion beam sample preparation and of the TEM-grid. Furthermore, photoluminescence measurements before and after an HCl etch indicate a lower degree of defects in the post-etch sample, compatible with the segregates removal. We hypothesize that Cu2-xSe nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favourable. These results provide important additional information about the formation of the CIGS/CdS interface.publishe

    Optical lithography patterning of SiO2 layers for interface passivation of thin film solar cells

    Get PDF
    Ultrathin Cu(In,Ga)Se2 solar cells are a promising way to reduce costs and to increase the electrical performance of thin film solar cells. In this work, we develop an optical lithography process that can produce sub-micrometer contacts in a SiO2 passivation layer at the CIGS rear contact. Furthermore, an optimization of the patterning dimensions reveals constrains over the features sizes. High passivation areas of the rear contact are needed to passivate the CIGS interface so that high performing solar cells can be obtained. However, these dimensions should not be achieved by using long distances between the contacts as they lead to poor electrical performance due to poor carrier extraction. This study expands the choice of passivation materials already known for ultrathin solar cells and its fabrication techniques.publishe

    Secondary crystalline phases identification in Cu2ZnSnSe4 thin films: contributions from Raman scattering and photoluminescence

    Get PDF
    In this work, we present the Raman peak positions of the quaternary pure selenide compound Cu2ZnSnSe4 (CZTSe) and related secondary phases that were grown and studied under the same conditions. A vast discussion about the position of the X-ray diffraction (XRD) reflections of these compounds is presented. It is known that by using XRD only, CZTSe can be identified but nothing can be said about the presence of some sec- ondary phases. Thin films of CZTSe, Cu2SnSe3, ZnSe, SnSe, SnSe2, MoSe2 and a-Se were grown, which allowed their investigation by Raman spectroscopy (RS). Here we present all the Raman spectra of these phases and discuss the similarities with the spectra of CZTSe. The effective analysis depth for the common back-scattering geometry commonly used in RS measurements, as well as the laser penetration depth for photoluminescence (PL) were esti- mated for different wavelength values. The observed asymmetric PL band on a CZTSe film is compatible with the presence of CZTSe single-phase and is discussed in the scope of the fluctuating potentials’ model. The estimated bandgap energy is close to the values obtained from absorption measurements. In general, the phase identifica- tion of CZTSe benefits from the contributions of RS and PL along with the XRD discussion.info:eu-repo/semantics/acceptedVersio

    On the importance of joint mitigation strategies for front, bulk, and rear recombination in ultrathin Cu(In,Ga)Se2 solar cells

    Get PDF
    Several optoelectronic issues, such as poor optical absorption and recombination limit the power conversion efficiency of ultrathin Cu(In,Ga)Se2 (CIGS) solar cells. To mitigate recombination losses, two combined strategies were implemented: a Potassium Fluoride (KF) Post-Deposition Treatment (PDT) and a rear interface passivation strategy based on an Aluminium Oxide (Al2O3) point contact structure. The simultaneous implementation of both strategies is reported for the first time on ultrathin CIGS devices. Electrical measurements and 1-D simulations demonstrate that, in specific conditions, devices with only KF-PDT may outperform rear interface passivated based devices. By combining KF-PDT and rear interface passivation, an enhancement in open-circuit voltage of 178 mV is reached over devices that have a rear passivation only and of 85 mV over devices with only a KF-PDT process. Time-Resolved Photoluminescence measurements showed the beneficial effects of combining KF-PDT and the rear interface passivation at decreasing recombination losses in the studied devices, enhancing charge carrier lifetime. X-ray photoelectron spectroscopy measurements indicate the presence of a In and Se rich layer that we linked to be a KInSe2 layer. Our results suggest that when bulk and front interface recombination values are very high, they dominate and individual passivation strategies work poorly. Hence, this work shows that for ultrathin devices, passivation mitigation strategies need to be implemented in tandem.publishe

    Coronary computed tomography angiography-adapted Leaman score as a tool to noninvasively quantify total coronary atherosclerotic burden

    Get PDF
    To describe a coronary computed tomography angiography (CCTA)-adapted Leaman score (CT-LeSc) as a tool to quantify total coronary atherosclerotic burden with information regarding localization, type of plaque and degree of stenosis and to identify clinical predictors of a high coronary atherosclerotic burden as assessed by the CT-LeSc. Single center prospective registry including a total of 772 consecutive patients undergoing CCTA (Dual-source CT) from April 2011 to March 2012. For the purpose of this study, 581 stable patients referred for suspected coronary artery disease (CAD) without previous myocardial infarction or revascularization procedures were included. Pre-test CAD probability was determined using both the Diamond-Forrester extended CAD consortium method (DF-CAD consortium model) and the Morise score. Cardiovascular risk was assessed with the HeartScore. The cut-off for the 3rd tercile (CT-LeSc ≥8.3) was used to define a population with a high coronary atherosclerotic burden. The median CT-LeSc in this population (n = 581, 8,136 coronary segments evaluated; mean age 57.6 ± 11.1; 55.8 % males; 14.6 % with diabetes) was 2.2 (IQR 0-6.8). In patients with CAD (n = 341), the median CT-LeSc was 5.8 (IQR 3.2-9.6). Among patients with nonobstructive CAD, most were classified in the lowest terciles (T1, 43.0 %; T2, 36.1 %), but 20.9 % were in the highest tercile (T3). The majority of the patients with obstructive CAD were classified in T3 (78.2 %), but 21.8 % had a CT-LeSc in lower terciles (T1 or T2). The independent predictors of a high CT-LeSc were: Male sex (OR 1.73; 95 % CI 1.04-2.90) diabetes (OR 2.91; 95 % CI 1.61-5.23), hypertension (OR 2.54; 95 % CI 1.40-4.63), Morise score ≥16 (OR 1.97; 95 % CI 1.06-3.67) and HeartScore ≥5 (OR 2.42; 95 % CI 1.41-4.14). We described a cardiac CT adapted Leaman score as a tool to quantify total (obstructive and nonobstructive) coronary atherosclerotic burden, reflecting the comprehensive information about localization, degree of stenosis and type of plaque provided by CCTA. Male sex, hypertension, diabetes, a HeartScore ≥5 % and a Morise score ≥16 were associated with a high coronary atherosclerotic burden, as assessed by the CT-LeSc. About one fifth of the patients with nonobstructive CAD had a CT-LeSc in the highest tercile, and this could potentially lead to a reclass

    Prevalence and predictors of coronary artery disease in patients with a calcium score of zero

    Get PDF
    The absence of coronary calcification is associated with an excellent prognosis. However, a calcium score of zero does not exclude the presence of coronary artery disease (CAD) or the possibility of future cardiovascular events. Our aim was to study the prevalence and predictors of coronary artery disease in patients with a calcium score of zero. Prospective registry consisted of 3,012 consecutive patients that underwent cardiac CT (dual source CT). Stable patients referred for evaluation of possible CAD that had a calcium score of zero (n = 864) were selected for this analysis. The variables that were statistically significant were included in a multivariable logistic regression model. From 864 patients with a calcium score of zero, 107 (12.4 %) had coronary plaques on the contrast CT (10.8 %, n = 93 with nonobstructive CAD and 1.6 %, n = 14 with obstructive CAD). By logistic regression analysis, the independent predictors of CAD in this population were age >55 years [odds ratio (OR) 1.63 (1.05-2.52)], hypertension [OR 1.64 (1.05-2.56)] and dyslipidemia [OR 1.54 (1.00-2.36)]. In the presence of these 3 variables, the probability of having coronary plaques was 21 %. The absence of coronary artery calcification does not exclude the presence of coron

    Diabetes as an independent predictor of high atherosclerotic burden assessed by coronary computed tomography angiography: The coronary artery disease equivalent revisited

    Get PDF
    (1) To study the prevalence and severity of coronary artery disease (CAD) in diabetic patients. (2) To provide a detailed characterization of the coronary atherosclerotic burden, including the localization, degree of stenosis and plaque composition by coronary computed tomography angiography (CCTA). Single center prospective registry including a total of 581 consecutive stable patients (April 2011-March 2012) undergoing CCTA (Dual-source CT) for the evaluation of suspected CAD without previous myocardial infarction or revascularization procedures. Different coronary plaque burden indexes and plaque type and distribution patterns were compared between patients with (n = 85) and without diabetes (n = 496). The prevalence of CAD (any plaque; 74.1 vs. 56 %; p = 0.002) and obstructive CAD (≥50 % stenosis; 31.8 vs. 10.3 %; p<0.001) were significantly higher in diabetic patients. The remaining coronary atherosclerotic burden indexes evaluated (plaque in LM-3v-2v with prox. LAD; SIS; SSS; CT-LeSc) were also significantly higher in diabetic patients. In the per segment analysis, diabetics had a higher percentage of segments with plaque in every vessel (2.6/13.1/7.5/10.5 % for diabetics vs. 1.4/7.1/3.3/4.4 % for nondiabetics for LM, LAD, LCx, RCA respectively; p<0.001 for all) and of both calcified (19.3 vs. 9.2 %, p<0.001) and noncalcified or mixed types (14.4 vs. 7.0 %; p<0.001); the ratio of proximal-to-distal relative plaque distribution (calculated as LM/proximal vs. mid/distal/branches) was lower for diabetics (0.75 vs. 1.04; p = 0.009). Diabetes was an independent predictor of CAD and was also associated with more advanced CAD, evaluated by indexes of coronary atherosclerotic burden. Diabetics had a significantly higher prevalence of plaques in every anatomical subset and for the different plaque composition. In this report, the relative geographic distribution of the plaques within each subgroup, favored a more mid-to-distal localization in the diabetic patients

    Secondary crystalline phases identification in Cu2ZnSnSe4 thin films: contributions from Raman scattering and photoluminescence

    Get PDF
    In this work, we present the Raman peak positions of the quaternary pure selenide compound Cu2ZnSnSe4 (CZTSe) and related secondary phases that were grown and studied under the same conditions. A vast discussion about the position of the X-ray diffraction (XRD) reflections of these compounds is presented. It is known that by using XRD only, CZTSe can be identified but nothing can be said about the presence of some secondary phases. Thin films of CZTSe, Cu2SnSe3, ZnSe, SnSe, SnSe2, MoSe2 and a-Se were grown, which allowed their investigation by Raman spectroscopy (RS). Here we present all the Raman spectra of these phases and discuss the similarities with the spectra of CZTSe. The effective analysis depth for the common back-scattering geometry commonly used in RS measurements, as well as the laser penetration depth for photoluminescence (PL) were estimated for different wavelength values. The observed asymmetric PL band on a CZTSe film is compatible with the presence of CZTSe single-phase and is discussed in the scope of the fluctuating potentials’ model. The estimated bandgap energy is close to the values obtained from absorption measurements. In general, the phase identification of CZTSe benefits from the contributions of RS and PL along with the XRD discussion.info:eu-repo/semantics/publishedVersio

    Project goals, target selection, and stellar characterization

    Get PDF
    The detection of habitable worlds is one of humanitya-s greatest endeavors. Thus far, astrobiological studies have shown that one of the most critical components for allowing life to develop is liquid water. Its chemical properties and its capacity to dissolve and, hence, transport other substances makes this constituent a key piece in this regard. As a consequence, looking for life as we know it is directly related to the search for liquid water. For a remote detection of life in distant planetary systems, this essentially means looking for planets in the so-called habitable zone. In this sense, K-dwarf stars are the perfect hosts to search for planets in this range of distances. Contrary to G-dwarfs, the habitable zone is closer, thus making planet detection easier using transit or radial velocity techniques. Contrary to M-dwarfs, stellar activity is on a much smaller scale, hence, it has a smaller impact in terms of both the detectability and the true habitability of the planet. Also, K-dwarfs are the quietest in terms of oscillations, and granulation noise. In spite of this, there is a dearth of planets in the habitable zone of K-dwarfs due to a lack of observing programs devoted to this parameter space. In response to a call for legacy programs of the Calar Alto observatory, we have initiated the first dedicated and systematic search for habitable planets around these stars: K-dwarfs Orbited By habitable Exoplanets (KOBE). This survey is monitoring the radial velocity of 50 carefully pre-selected K-dwarfs with the CARMENES instrument over five semesters, with an average of 90 data points per target. Based on planet occurrence rates convolved with our detectability limits, we expect to find 1.68 ± 0.25 planets per star in the KOBE sample. Furthermore, in half of the sample, we expect to find one of those planets within the habitable zone. Here, we describe the motivations, goals, and target selection for the project as well as the preliminary stellar characterization. © 2022 EDP Sciences. All rights reserved
    corecore