Point Contacts in Chalcopyrite Solar Cells

Recent advancements in Cu(In,Ga)Se2 based solar cells with alkaline fluoride treatment have suggested the role of the recombination at the absorber/buffer interface as a limiting factor of open-circuit voltage (Voc ) in high-efficiency CIGSe solar cells. Therefore, this work focuses on the interface engineering at the CIGSe/buffer of 2 μm standard absorbers. Furthermore, as the influence of recombination at the back interface becomes noticeable with thin absorbers, the back interface Mo/CIGSe of thinner absorbers is also optimised. Both use the concept of point junctions through a passivation layer (PaL). A low-cost versatile technique for point contact fabrication is developed by using a nanolithographic technique employing a sacrificial template of low temperature synthesised, mono-dispersed, self-assembled and size-tunable CdS nanoparticels (NP’s), and a PaL of aluminium oxide (Al2O3), thereby achieving 60 nm point contact radius and a maximum of 89% PaL coverage on CIGSe. The impact of point contacts at the CIGSe/buffer interface on solar cell performance is theoretically analysed using three-dimensional simulations on the point contact radius, coverage area, defect density and interface quality. An efficient PaL should create positive surface charge, which induces band bending at the CIGSe/PaL and influences the contact junction properties; its beneficial effect on Voc and efficiency reaches a maximum when the coverage area of PaL is more than 95% and the interface charge density is greater than 10^12/cm2 . The point contact technology is experimentally validated by incorporating it into tangible CIGSe devices, featuring CdS and Zn(O,S) buffer layers. A positive impact of +10.4% is seen on the Voc of the point contact devices with sputtered Zn(O,S) compared to the unpassivated reference cells. However the power conversion efficiency (PCE) didn’t follow the same trend, which might be due to an upward bandbending created by Al2O3 at the interface, impeding the current flow. Nonetheless, at the Mo/CIGSe interface of thinner CIGSe absorbers, the technology led to a significant reduction in the surface recombination velocity, due to the back surface field from the Al2O3 layer. Consequently, all cell parameters of point contact devices showed a relative improvement to the unpassivated reference devices: open-circuit voltage (Voc : +21%), short-circuit current (Jsc : +2.6%), fill-factor (FF: +4.9%), and efficiency (η: +31%).Jüngste Fortschritte bei Cu(In,Ga)Se2 -basierten Solarzellen durch eine Alkalimetallfluorid Behandlung haben dazu geführt, dass die adungsträgerrekombination an der Absorber/Puffer-Grenzfläche als limitierender Faktor für die Leerlaufspannung (Voc) in hocheffizienten CIGSe-Solarzellen diskutiert wird. Daher konzentriert sich diese Arbeit auf die Optimierung der Grenzfläche CIGSe/Puffer von Solarzellen mit 2 μm Standard absorbern. Außerdem wird auch der Rückkontakt Mo/CIGSe von Solarzellen mit dünneren Absorbern optimiert. In beiden Fällen wird das Konzept der Punktkontakte durch eine Passivierungsschicht (PaL) verwendet. Eine vielseitige, kostengünstige nanolithografische Technik zur Herstellung von Punktkontakten wird unter Verwendung einer Opferschicht realisiert, die als Schablone eingesetzt wird. Diese besteht aus bei niedrigen Temperaturen synthetisierten, monodispersen und größenabstimmbaren CdS-Nanopartikeln (NP’s) und einem PaL aus Aluminiumoxid (Al2O3). Damit lässt sich ein Punktkontaktradius von 60 nm und eine Oberflächenbedeckung von maximal 89% PaL auf CIGSe erreichen. Der Einfluss von Punktkontakten an der CIGSe/Puffer-Grenzfläche auf die Leistung der Solarzelle wird mittels dreidimensionaler Simulationen unter Berücksichtigung des Punktkontaktradius, der Oberflächenbedeckung, der Defektdichte und der Qualität der Grenzfläche analysiert. Eine effiziente PaL sollte eine positive Oberflächenladung erzeugen, die am CIGSe/PaL eine Bandverbiegung induziert und damit die Eigenschaften der Grenzfläche beeinflusst; ihre positive Wirkung auf Voc und den Wirkungsgrad erreicht ein Maximum, wenn die Oberflächenbedeckung der PaL mehr als 95% beträgt und die Grenzflächenladungsdichte größer als 10^12/cm2 ist. Die Punktkontakttechnologie wird mithilfe von CIGSe-Solarzellen mit CdS- und Zn(O,S)- Pufferschichten experimentell validiert. Eine Steigerung der Voc um 10,4% gegenüber den unpassivierten Referenzzellen wird mit der Punktkontaktsolarzelle erzielt. Allerdings wird keine Erhöhung des gesamten Wirkungsgrads erreicht, was auf eine von Al2O3 erzeugte, nach oben gerichtete Bandverbiegung an der Grenzfläche hindeuten könnte, die den Stromfluss behindert. Am Mo/CIGSe-Rückkontakt von dünneren CIGSe absorbern führt die Einführung der Punktkontakte jedoch zu einer signifika nten Senkung der Oberflächen rekombinationsgeschwindigkeit, bedingt durch das durch Ladungen in der Al2O3 -Schicht induzierte elektrische Feld. Folglich zeigten alle Zellparameter von unktkontaktsolarzellen eine relative Verbesserung gegenüber den unpassivierten Referenzzellen: Leerlaufspannung (Voc : + 21%), Kurzschlussstrom (Jsc: + 2,6%), Füllfaktor (FF: + 4,9%) und Wirkungsgrad (FF+ 31%)

Assessment of Chemical and Electronic Surface Properties of the Cu2ZnSn(SSe)4 after Different Etching Procedures by Synchrotron-based Spectroscopies

Kesterite Cu2ZnSn(S,Se)4 absorber layers with different [S]/([S]+[Se]) ratios were studied using XPS, UPS, Hard X-ray (HIKE) photoemission and the Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The samples were prepared by IREC using sequentially sputtered metallic precursor stacks with metal ratios of [Cu]/([Zn]+[Sn])=0.80, [Zn]/[Sn]=1.20 followed by annealing under S+Se+Sn atmosphere. Different etching procedures were used depending on the sample's composition. It is shown that the surface composition varies from that of the bulk, especially for the Se-rich samples. Contamination with sulfur is detected after using a Na2S etching solution for the pure Se kesterite. A Cu-depleted surface was found for all samples before and after etching. HIKE measurements show a higher [Zn]/[Sn] ratio in the near surface region than on the very surface. This is explained by the fact, the etching procedure removes secondary phases from the very few surface layers, while some of ZnS(e) is still buried underneath. In order to investigate the band gap transition from the pure sulfide (1.5 eV) to the pure selenide (1eV), the valence and conduction band of the respective absorbers were probed. According to UPS and HIKE measurements, the relative distance between Fermi level (Ef) and valance band maximum (VBM) for sulfide sample was 130 meV larger than for selenide. Using NEXAFS on the copper, zinc and tin edges, the development of the conduction band with increasing [S]/([S]+[Se]) ratios was studied. Stoichiometric powder samples were used as reference materials. © 2015 Published by Elsevier Ltd.Peer ReviewedPostprint (published version

Point contacts at the copper indium gallium selenide interface a theoretical outlook

For a long time, it has been assumed that recombination in the space-charge region of copper-indium-gallium-selenide (CIGS) is dominant, at least in high efficiency solar cells with low band gap. The recent developments like potassium fluoride post deposition treatment and point-contact junction may call this into question. In this work, a theoretical outlook is made using three-dimensional simulations to investigate the effect of point-contact openings through a passivation layer on CIGS solar cell performance. A large set of solar cells is modeled under different scenarios for the charged defect levels and density, radius of the openings, interface quality, and conduction band offset. The positive surface charge created by the passivation layer induces band bending and this influences the contact (CdS) properties, making it beneficial for the open circuit voltage and efficiency, and the effect is even more pronounced when coverage area is more than 95%, and also makes a positive impact on the device performance, even in the presence of a spike at CIGS/CdS heterojunction

Assessment of Chemical and Electronic Surface Properties of the Cu2ZnSn(SSe)4 after Different Etching Procedures by Synchrotron-based Spectroscopies

Kesterite Cu2ZnSn(S,Se)4 absorber layers with different [S]/([S]+[Se]) ratios were studied using XPS, UPS, Hard X-ray (HIKE) photoemission and the Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The samples were prepared by IREC using sequentially sputtered metallic precursor stacks with metal ratios of [Cu]/([Zn]+[Sn])=0.80, [Zn]/[Sn]=1.20 followed by annealing under S+Se+Sn atmosphere. Different etching procedures were used depending on the sample's composition. It is shown that the surface composition varies from that of the bulk, especially for the Se-rich samples. Contamination with sulfur is detected after using a Na2S etching solution for the pure Se kesterite. A Cu-depleted surface was found for all samples before and after etching. HIKE measurements show a higher [Zn]/[Sn] ratio in the near surface region than on the very surface. This is explained by the fact, the etching procedure removes secondary phases from the very few surface layers, while some of ZnS(e) is still buried underneath. In order to investigate the band gap transition from the pure sulfide (1.5 eV) to the pure selenide (1eV), the valence and conduction band of the respective absorbers were probed. According to UPS and HIKE measurements, the relative distance between Fermi level (Ef) and valance band maximum (VBM) for sulfide sample was 130 meV larger than for selenide. Using NEXAFS on the copper, zinc and tin edges, the development of the conduction band with increasing [S]/([S]+[Se]) ratios was studied. Stoichiometric powder samples were used as reference materials. © 2015 Published by Elsevier Ltd.Peer Reviewe

Health-status outcomes with invasive or conservative care in coronary disease

BACKGROUND In the ISCHEMIA trial, an invasive strategy with angiographic assessment and revascularization did not reduce clinical events among patients with stable ischemic heart disease and moderate or severe ischemia. A secondary objective of the trial was to assess angina-related health status among these patients. METHODS We assessed angina-related symptoms, function, and quality of life with the Seattle Angina Questionnaire (SAQ) at randomization, at months 1.5, 3, and 6, and every 6 months thereafter in participants who had been randomly assigned to an invasive treatment strategy (2295 participants) or a conservative strategy (2322). Mixed-effects cumulative probability models within a Bayesian framework were used to estimate differences between the treatment groups. The primary outcome of this health-status analysis was the SAQ summary score (scores range from 0 to 100, with higher scores indicating better health status). All analyses were performed in the overall population and according to baseline angina frequency. RESULTS At baseline, 35% of patients reported having no angina in the previous month. SAQ summary scores increased in both treatment groups, with increases at 3, 12, and 36 months that were 4.1 points (95% credible interval, 3.2 to 5.0), 4.2 points (95% credible interval, 3.3 to 5.1), and 2.9 points (95% credible interval, 2.2 to 3.7) higher with the invasive strategy than with the conservative strategy. Differences were larger among participants who had more frequent angina at baseline (8.5 vs. 0.1 points at 3 months and 5.3 vs. 1.2 points at 36 months among participants with daily or weekly angina as compared with no angina). CONCLUSIONS In the overall trial population with moderate or severe ischemia, which included 35% of participants without angina at baseline, patients randomly assigned to the invasive strategy had greater improvement in angina-related health status than those assigned to the conservative strategy. The modest mean differences favoring the invasive strategy in the overall group reflected minimal differences among asymptomatic patients and larger differences among patients who had had angina at baseline

Initial invasive or conservative strategy for stable coronary disease

BACKGROUND Among patients with stable coronary disease and moderate or severe ischemia, whether clinical outcomes are better in those who receive an invasive intervention plus medical therapy than in those who receive medical therapy alone is uncertain. METHODS We randomly assigned 5179 patients with moderate or severe ischemia to an initial invasive strategy (angiography and revascularization when feasible) and medical therapy or to an initial conservative strategy of medical therapy alone and angiography if medical therapy failed. The primary outcome was a composite of death from cardiovascular causes, myocardial infarction, or hospitalization for unstable angina, heart failure, or resuscitated cardiac arrest. A key secondary outcome was death from cardiovascular causes or myocardial infarction. RESULTS Over a median of 3.2 years, 318 primary outcome events occurred in the invasive-strategy group and 352 occurred in the conservative-strategy group. At 6 months, the cumulative event rate was 5.3% in the invasive-strategy group and 3.4% in the conservative-strategy group (difference, 1.9 percentage points; 95% confidence interval [CI], 0.8 to 3.0); at 5 years, the cumulative event rate was 16.4% and 18.2%, respectively (difference, 121.8 percentage points; 95% CI, 124.7 to 1.0). Results were similar with respect to the key secondary outcome. The incidence of the primary outcome was sensitive to the definition of myocardial infarction; a secondary analysis yielded more procedural myocardial infarctions of uncertain clinical importance. There were 145 deaths in the invasive-strategy group and 144 deaths in the conservative-strategy group (hazard ratio, 1.05; 95% CI, 0.83 to 1.32). CONCLUSIONS Among patients with stable coronary disease and moderate or severe ischemia, we did not find evidence that an initial invasive strategy, as compared with an initial conservative strategy, reduced the risk of ischemic cardiovascular events or death from any cause over a median of 3.2 years. The trial findings were sensitive to the definition of myocardial infarction that was used