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

Bewertung der elektronischen und chemischen Oberflächeneigenschaften von Kesterit

Kesterite-based thin-film solar cells are of great interest due to non-toxic, earth-abundant constitutes. Kesterite can be a good substitute for chalcopyrite with a band gap in the same range of values. When going from pure sulfide to pure selenide composition, the band gap of kesterite is changing from 1.5 to 1 eV, respectively. Several questionsarose for us: how do the electronic and chemical surface properties of kesterite change when we change the [S]/([S]+[Se]) ratio? How does the band alignment change with a change in the buffer layer? What are the relative positions of conduction (CB) and valence band (VB) when going from CZTS to CZTSe? A comprehensive study of the kesterite absorbers was done using surface-sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS) and synchrotron-based spectroscopies, to investigate the chemical composition of kesterite surfaces. Samples with different [S]/([S]+[Se]) ratios were examined before and after special etching procedures. A strong deviation of the relative chemical composition of the as- received and etched surfaces was observed. Surfaces also differ strongly from the bulk. The detailed investigation of the relative positions of the valence band edge was done using Ultraviolet Photoelectron Spectroscopy (UPS). The results obtained with a laboratory source were confirmed using different excitation energies at the synchrotron. The well-reproduced shift value is in good agreement with theoretical values available in literature. The changes in anion composition cause a shift not only in the VBM, but also in the CBM. We have proven that using X-ray absorption spectroscopy, namely its special case- NEXAFS. A large set of absorption edges were obtained for CZTS and CZTSe samples: Cu, Zn K- and L-edges, and the Sn L-edge. The relative shifts between the spectra of one type were attributed to the shift in CBM. The value of the shift was the same within the error margin for all measured edges of polycrystalline thin film samples. The value of the shift was confirmed by the powdered samples used as references, due to their well-defined composition and the single phase nature. We have correlated experimental results with theoretical calculations and found a good agreement between them. Thus, the DFT calculations of the conduction band states have confirmed the trend seen in the experiments: the relative positions of the unoccupied states the CBM of selenides are shifted to lower energies compared to those of the sulfides. Using a so called GW approximation, the values of the shift of VBM and CBM were obtained, which overlap within the error with experimental findings. The observed shifts in the absorption spectra and in the calculated states have been correlated with the changes in the atomic-scale structure of the kesterite samples when substituting s atoms with bigger Se. The simulation package FDMNES allowed us to reproduce the NEXAFS edges. Cu, Zn K- and L- edges and Sn L-edge were calculated and all essential spectral features were well reproduced together with the shifts between the CZTS- and CZTSe-related spectra. The values of the shifts between simulated spectra are in the same range as the experimental values. The obtained knowledge was implemented to explain the influence of the surface and interface physics on the complete device performance.Kesterit-basierte Dünnschicht-Solarzellen mit der allgemeinen Zusammensetzung Cu2ZnSn(Se,S)4 sind in den letzten Jahren Gegenstand intensiver Forschungs-und Entwicklungsarbeiten gewesen. Kesterit kann wegen der ungiftigen und in der Erdkruste häufig vorkommenden Bestandteile und einer vergleichbaren Bandlücke ein guter Ersatz für Chalkopyrit (Cu(In,Ga)(Se,S)2) sein. Beim Übergang von der reinen Sulfid- zur reinen Selenidzusammensetzung verändert sich die Kesteritbandlücke von 1.5 auf 1 eV. In dieser Arbeit wurden die folgenden Fragen untersucht: wie ändern sich die elektronischen und chemischen Oberflächeneigenschaften von Kesterit, wenn wir das [S]/([S]+[Se])-Verhältnis ändern? Wie ändert sich die Bandanpassung mit der Pufferschicht? Was sind die relativen Valenzband- und Leitungsbandpositionen, wenn man von CZTS zu CZTSe geht? Um die elektronischen Eigenschaften von Kesterit zu untersuchen, wurden umfassende Analysen von Dünnschicht-Kesteritabsorbern unter Verwendung von oberflächensensitiven Techniken wie Röntgen-Photoelectronenspectroskopie (XPS) und Ultraviolett-Photoelektronenspectroskopie (UPS) sowie synchrotronbasierter Spektroskopie erfolgreich durchgeführt. Proben mit unterschiedlichen Konzentrationsverhältnissen von [S]/([S]+[Se]) wurden vor und nach der Anwendung spezieller nasschemischer Ätzverfahren untersucht. Es wurden starke Abweichungen zwischen der chemischen Zusammensetzung von unbehandelten und geätzten Oberflächen beobachtet. Generell unterscheiden sich Oberflächen und Volumenzusammensetzung. Die relativen Positionen der Valenzbandkante wurden mit UPS ausfühlich untersucht. Die dabei mit der Laborquelle erzielten Ergebnisse konnten mit Messungen mit unterschiedlichen Anregungsenergien am Synchrotron bestätigt wurden. Die experimentellen Ergebnisse stimmen gut mit theoretischen Werten zusammen, die in der Literatur verfügbar sind. Die Änderungen in der Anionenzusammensetzung bewirken eine Verschiebung nicht nur im Valenzbandmaximum (VBM), sondern auch im Leitungsbandminimum (LBM). Dies wurde mit Röntgennahkanten-Absorptionsspektroskopie (NEXAFS) gezeigt. Eine Serie von Absorptionskanten wurde für CZTS- und CZTSe-Proben erhalten: Cu, Zn K- and L-Kanten und die Sn L-Kante. Die relativen Verschiebungen zwischen den Spektren wurden der Verschiebung im LBM zugeschrieben. Der Wert der Verschiebung war für alle Kanten innerhalb der Fehlergrenze gleich. Neben den polykristallinen Dünnfilmproben wurden einphasige Pulverproben als Referenzen verwendet. Wir haben experimentelle Ergebnisse mit theoretischen Berechnungen korreliert und eine gute Übereinstimmung zwischen ihnen gefunden. Dichtefunktionaltheorie-Berechnungen der Leitungsbandzustände haben die in den Experimenten gesehene Tendenzbestätigt: Die relativen Positionen der unbesetzten-Zustände an der LB-Kante liegen in den Seleniden bei niedrigeren Energien als die des Schwefels. Unter Verwendung einer sogenannten GW- Annährung wurden die Werte der Verschiebung von VBM und LBM erhalten. Sie stimmen innerhalb der Fehlergrenzen mit den experimentellen Ergebnissen überein. Die beobachteten Verschiebungen in den Absorptionsspektren und in den berechneten Zuständen wurden mit den Änderungen in der Kesteritkristallstruktur korreliert. Mit dem Simulationspaket FDMNES konnten wir die NEXAFS-Kanten wiedergeben. Cu, Zn K- und L-Kanten und Sn L-Kante wurden berechnet. Alle wesentlichen Spektraleigenschaften wurden zusammen mit den Verschiebungen zwischen den Spektren gut nachgebildet. Die erhaltenen Informationen wurden eingesetzt, um den Einfluss der Bandanpassung und der Grenzflächen auf die Solarzelleneigenschaften zu erklären

PSYCHOLOGICAL AND PEDAGOGICAL INSTRUMENTS FOR THE TEACHER OF NATURE DISCIPLINES IN HIGHER MEDICAL EDUCATION

The article considers the main psychological and pedagogical tools of the teacher in the field of fundamental training of applicants for higher medical education in the discipline “Medical and Biological Physics”. The role of the teacher of the cycle of fundamental training in medical education and the pedagogical tools he or she uses deserve special attention. The stimulating influence and effect on the internal (why the student should do it) and external (in what conditions the student will do it) motivation of the student by the teacher is very important. The contemporary teacher must also restructure his/her activities because of new challenges in the education system and deviate from traditional methods, pursue permanent selfimprovement, adapt to the way of thinking of students, understand and analyze it to be able to moderate the educational process, more widely use interactive methods and creative, individual approach focused on students, be practically oriented, innovative, etc. The modern medicine has made a great step forward thanks to the development of new technologies based on the achievements of information, mathematical and natural sciences. Therefore, it is important for the future physician to know the basic laws of nature, including physical laws, which are basic for the disciplines of the clinical cycle. The discipline «Medical and Biological Physics» was formed at the intersection of physics, biology, chemistry and medicine and is one of the most important in the preparation of future physicians. The historical progress of medical education in our country gives convincing information about the close relationship between natural sciences and clinical departments, which is a guarantee of quality in fundamental training of graduates. Today’s medicine widely uses the results of theoretical and experimental achievements in the field of physics, so the study of biological physics is important in the professional development and growth of the future competitive specialist, in the development of his/her personality. If the motivation to study other disciplines of natural sciences is sometimes present, so to speak, “by default”, and an additional stimulus for such studies is their inclusion into the structure of licensed exams, the discipline “Medical and Biological Physics” requires additional stimuli and pedagogical tools, namely: – clear, logical structuring of the lesson to systematize the material; – modeling of problem situations; – moderation of the educational process; – encouragement of independent and in-depth study of some issues, and others. Finding and using effective incentives to motivate students is the key to improving their academic performance, which in turn is reflected in the future in their professional competence development. The main role in this process undoubtedly belongs to the teacher and well-chosen pedagogical tools. The additional stimuli and pedagogical tools described in the article increase the motivation to study biological physics and understand its importance. Often the students’ argument against studying this discipline is the lack of time, and this is another problem of first year students which needs to be addressed – their irrationality in planning their time

Comparative Analysis of the Formation of Professional Competencies for Healthcare-Students in the Study of Surface Tension

У структурі медичної освіти ведеться підготовка за різними спеціальностями. Перший рік навчання, незалежно від спеціальності здобувачі освіти вивчають дисципліни природничого напрямку і гуманітарні дисципліни. Характерним для різних дисциплін є наявність у підготовці спеціаліста інваріантної (базові знання) та варіативної частин (знання, характерні для конкретного напрямку підготовки). Важливим є навчити студента знаходити взаємозв’язки між означеннями, закономірностями, явищами і фахово-орієнтованою компонентною. Цьому повинні сприяти вдало підібраний контент та педагогічні інструменти викладача вищої школи. У статті розглянуто специфіку інформаційного контенту за темою «Поверхневий натяг» у курсах «Медичної та біологічної фізики» та «Біофізики з фізичними методами аналізу» для студентів першого року навчання різних спеціальностей за напрямом «Охорона здоров’я», яка сприяє формуванню у студентів мотивації до вивчення дисципліни та навиків знаходження причинно-наслідкових зв’язків. Також на прикладі вивчення явища поверхневого натягу описано інформаційне наповнення кейсів практичних занять для різних спеціальностей, з акцентами на причинно-наслідкових зв’язках фізичне явище, показник, закономірність ↔ фахово-орієнтована ситуація, які практикуються на кафедрі біологічної фізики та медичної інформатики БДМУ. Наведені приклади орієнтованості навчального матеріалу у формуванні професійних компетентностей студентів напрямків підготовки «Медицина», «Стоматологія», «Фармація» в рамках вивчення дисциплін «Медична та біологічна фізика» та «Біологічна фізика з фізичними методами аналізу» сприяють підвищенню рівня внутрішньої мотивації до вивчення дисциплін, закладають основи самостійної неперервної освіти, професійного зростання. Студент усвідомлює необхідність вивчення даної теми для своєї майбутньої професії, що в свою чергу, призводить до підвищення рівня якості засвоєння вивчаємого матеріалу та успішності.In the structure of medical education, training is conducted in various directions. In the first year of study, regardless of the direction, students study natural sciences and a number of humanitarian disciplines. Characteristic for different disciplines is the presence of invariant (basic knowledge) and variable parts (knowledge specific to a specific field of training) for different topics. It is important to teach the student to find relationships between definitions, patterns, and phenomena that were studied during secondary education and the vocationally oriented component. This should be facilitated by well-chosen content and pedagogical tools of a higher school teacher. The article examines the specifics of the information content on the topic "Surface tension" in the courses "Medical and biological physics" and "Biophysics with physical methods of analysis" for students of the first year of study of various areas of training in the direction of "Health care", which contributes to the formation of students motivation to study the discipline and the formation of skills for finding cause-and- effect relationships. Also, using the example of the study of the phenomenon of surface tension, the information content of cases of practical classes for various specialties is described, with emphasis on the cause-and-effect relationships of a physical phenomenon, an indicator, a regularity ↔ a professional-oriented situation, which are practiced at the Department of Biological Physics and Medical Informatics of the BSMU. The given examples of orientation of the educational material in the formation of professional competences of students of the "Medicine", "Dentistry", "Pharmacy" fields of study within the framework of studying the disciplines "Medical and biological physics" and "Biological physics with physical methods of analysis" contribute to increasing the level of internal motivation to study the disciplines , lay the foundations of independent continuous education, professional growth. The student realizes the need to study this topic for his future profession, which, in turn, leads to an increase in the quality of mastering the studied material and success

Actualization of Physical and Mathematical Education in Doctor Training by Using Educational Cases

У цій статті представлено навчальний кейс, присвячений розвитку компетентностей лікаря щодо використання ультразвуку (УЗ) в медицині, який застосовується на практичному занятті дисципліни природничого циклу «Медична та біологічна фізика» для студентів першого року навчання з напрямку підготовки «Медицина». Кейс-метод особливий тим, що одночасно розглядається конкретна практична ситуація й відбувається процес реалізації конкретних знань. Студентам-медикам важливо навчитись нестандартно мислити, діяти, вміло оперувати отриманими знаннями. Даний метод стимулює активну діяльність студентів, сприяє підвищенню мотивації до навчання, розвитку самостійного мислення, вміння прислуховуватись до думки інших (стимулює командний вид діяльності), швидко реагувати на вирішення конкретної поставленої задачі, зменшує кількість незалучених до обговорення теми конкретного заняття студентів, творчо підходити до розв’язання проблем, готовність до дії в нестандартних умовах, що є однією із важливих складових майбутньої професії. Кейси були практично апробовані й отримали схвальні відгуки студентів. Студенти впевнені, що вони випробували модель прийняття рішення, яку зможуть використати у реальному житті. Отримані знання з фізико-математичної дисципліни в майбутньому дозволять їм швидше зорієнтуватися в клінічних випадках. На нашу думку, і це є предметом подальших наукових досліджень, ґрунтовність фізико-математичних знань в рамках вивчення фундаментальних дисциплін до переліку яких входить «Медична та біологічна фізика», підвищує ефективність засвоєння клінічних дисциплін на достатньому рівні професійної компетенції. Враховуючи результати спостережень та аналіз успішності студентів з використанням кейсів та за рахунок занурення студентів у реалії майбутньої професії, їх застосування – перспективний напрямок актуалізації фізикоматематичної освіти в підготовці медичних працівників.This article presents a case study on the development of physician competencies in the use of ultrasound (US) in medicine and which is used in the practical lesson of the discipline of natural cycle "Medical and Biological Physics" for students of the first year of training in "Medicine". The case method is valuable because at the same time a specific practical situation is considered and the process of realization of specific knowledge takes place. Medical cases, in contrast to economic, pedagogical, and others, are purposefully applied and contribute to the formation of professional competencies by analyzing a specific situation. This method stimulates active activity of students, increases motivation to study, development of independent thinking, ability to listen to opinion of others (i.e. stimulates team activity), to react quickly to the decision of the concrete task, reduces quantity of students not involved in discussion of a subject of concrete employment, creative approach to solve problems, readiness to act in non-standard conditions, which is one of the important components of the future profession. It is important for medical students to learn to think outside the box, to act, to skillfully operate with the acquired knowledge. The cases were practically tested and received positive feedback from students. Students are confident that they have experienced a decision-making model that they can use in real life that the knowledge gained will allow them to navigate more quickly in clinical cases that will be considered in the future. In our opinion, this is the subject of further scientific research, to consider a possible clinical case in the study of clinical disciplines in senior courses, medical student approaches having mastered a sufficient level of professional competencies in the study of fundamental disciplines, which includes "Medical and Biological Physics". Given the results of observations and analysis of student performance using cases, their application is a promising area for improving and enhancing the effectiveness of teaching medical students by immersing students in the realities of the future profession

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

Impact of Na Dynamics at the Cu₂ZnSn(S,Se)₄/CdS Interface During Post Low Temperature Treatment of Absorbers

Cu₂SnZn­(S,Se)₄ (CZTSSe) solar cells based on earth abundant and nontoxic elements currently achieve efficiencies exceeding 12%. It has been reported that, to obtain high efficiency devices, a post thermal treatment of absorbers or devices at temperatures ranging between 150 and 400 °C (post low temperature treatment, PLTT) is advisable. Recent findings point toward a beneficial passivation of grain boundaries with SnO_<i>x</i> or Cu-depleted surface and grain boundaries during the PLTT process, but no investigation regarding alkali doping is available, even though alkali dynamics, especially Na, are systematically reported to be crucial within the field. In this work, CZTSSe absorbers were subjected to the PLTT process under different temperatures, and solar cells were completed. We found surprisingly behavior in which efficiency decreased to nearly 0% at 200 °C during the PLTT process, being recovered or even improved at temperatures above 300 °C. This unusual behavior correlates well with the Na dynamics in the devices, especially with the in-depth distribution of Na in the active CZTSSe/CdS interface region, indicating the key importance of Na spatial distribution on device properties. We present an innovative model for Na dynamics supported by theoretical calculations and additional specially designed experiments to explain this behavior. After optimization of the PLTT process, a Se-rich CZTSSe solar cell with 8.3% efficiency was achieved

Complex Surface Chemistry of Kesterites: Cu/Zn Reordering after Low Temperature Postdeposition Annealing and Its Role in High Performance Devices

A detailed study explaining the beneficial effects of low temperature postdeposition annealing combined with selective surface etchings for Cu₂ZnSnSe₄ (CZTSe) based solar cells is presented. After performing a selective oxidizing surface etching to remove ZnSe secondary phases typically formed during the synthesis processes an additional 200 °C annealing step is necessary to increase device performance from below 3% power conversion efficiency up to 8.3% for the best case. This significant increase in efficiency can be explained by changes in the surface chemistry which results in strong improvement of the CdS/CZTSe heterojunction commonly used in this kind of absorber/buffer/window heterojunction solar cells. XPS measurements reveal that the 200 °C annealing promotes a Cu depletion and Zn enrichment of the etched CZTSe absorber surface relative to the CZTSe bulk. Raman measurements confirm a change in Cu/Zn ordering and an increase in defect density. Furthermore, TEM microstructural investigations indicate a change of grain boundaries composition by a reduction of their Cu content after the 200 °C annealing treatment. Additionally, insights in the CdS/CZTSe interface are gained showing a significant amount of Cu in the CdS buffer layer which further helps the formation of a Cu-depleted surface and seems to play an important role in the formation of the pn-heterojunction