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Thin-film solar cells consist of several layers. The interfaces between these layers can provide critical recombination paths and consequently play a vital role in the efficiency of the solar cell. One of the main challenges for polycrystalline semiconductor absorber materials is the absorber-buffer interface. The Cu(In, Ga)Se-2 system is particularly interesting in this context, since Cu-rich absorbers are dominated by recombination at the interface, while Cu-poor ones are not. This paper unveils the root cause of the challenge in the interface of Cu-rich solar cells in terms of a Se-related defect with an activation energy of 200 +/- 20 meV. This defect causes interface recombination and is responsible for the deficiency of open-circuit voltage in Cu-rich cells. Moreover, this paper demonstrates that the origin of this defect is due to the etching step necessary to remove secondary phases. Postdeposition surface treatments or modified buffer layers are shown to passivate this defect, to reduce interface recombination, and to increase the efficiency. This paper has been funded by the Luxembourgish Fonds National de la Recherche in the framework of the Cu-rich CIS treated with potassium (K) (CURI-K), Surface passivation for thin film photovoltaics (SURPASS), and Materials for Sensing and Energy Harvesting (Massena) projects, which are gratefully acknowledged. Christian Andreas and Stephan Bucheler from EMPA are acknowledged for providing CdS buffer layer deposition.

In this work, the fabrication and properties of a Ge-based Kesterite Cu2ZnGeSe4 solar cell have been discussed. The substitution and the existence of the quaternary compound has been verified by physical methods. The device has a power conversion efficiency of 5.5% under AM1.5G illumination which is among the highest reported for pure Ge substitution. In depth electrical and optical analysis show that the Cu2ZnGeSe4 absorber has less bulk defects, less or no band tailing and no sub band gap emissions, which are all characteristic of Cu2ZnGeSe4 devices. These beneficial opto-electronic properties also result in a high open circuit voltage (V-oc) of 744 mV which is amongst the highest reported for Kesterite materials. Flemish government, Department Economy, Science and Innovation; European Union's Horizon 2020 research and innovation program [640868]

Ultrathin single- and three-stage Cu(In,Ga)Se-2 absorber layers were analyzed with room temperature photoluminescence (PL) spectra. An anomalous blueshift was observed upon increasing carrier injection for both samples. This blueshift was attributed to the presence of bandgap fluctuations that are of the same order as the minority carrier diffusion length. From time resolved measurements, a diffusion length of a few 100nms was deduced. The single-stage spectrum consists of two peaks, and the sample was, therefore, also analyzed by hyperspectral imaging, providing lateral PL and reflectance data with 1 mu m resolution. Marginal variations were observed in the PL yield and spectra. This homogeneity could again be attributed to an intermediate scale of the bandgap fluctuation with an upper limit of 1 mu m for the scale of the lateral bandgap fluctuations. The two peaks in the PL spectra of the single-stage sample could be attributed to interference, and correction methods were applied. The bandgap fluctuations were extracted for the three-stage and single-stage sample and were 45meV and 72 +/- 3meV, respectively. It is suggested that this difference is attributed to the smaller grains and larger amount of grain boundaries in the single-stage sample. This work received funding from the European Union's H2020 Research and Innovation Program under Grant Agreement No. 715027. We acknowledge Photon etc. and Professor Delphine Bouilly's research group from IRIC for the hyperspectral measurements and Erik Spaans for the python program.

KF and NaF treatments were done for single-stage co-evaporated CuIn0.7Ga0.3Se2. The absorber layers were grown on a substrate with an alkali barrier layer and NaF was either added before or after absorber layer growth. No differences were found on the device performance amongst the procedures to add Na. This is expected if the single-stage process does not have a copper rich stage or a Ga gradient, which is likely since there was no change of the elemental fluxes during absorber layer growth and no Ga profile was measured. KF was added by post-deposition only. Current-voltage characteristics were measured and net doping concentrations were determined from capacitance-voltage measurements (CV). We see an improvement of the open-circuit voltage (Voc) with increasing KF amount, and a marginal increase of the fill factor. CV measurements showed increasing net acceptor concentration with increasing KF amount. Time resolved photoluminescence (PL) showed an increased decay time for KF treated cells and the PL peak shape changed. Without KF treatment the PL peak is symmetric, after KF treatment a further peak appears at higher energy in the PL spectrum. This higher energy peak increases in intensity with increasing KF concentration. The same effects were seen in a sample without Na, but here the Vocwas limited due to large tailing. Hence both Na and K are required for good cell efficiencies.

We have studied the crystallization reaction of polycrystalline Cu2ZnGeSe4 solar cell absorbers fabricated by H2Se selenization of sequentially deposited metal layer stacks. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent X-ray diffraction patterns. We have found that mainly Cu3Ge and ZnSe phases form very rapidly at temperatures below 350°C. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds. Putting the Ge layer in the bottom and the Cu layer on top leads to a very fast nucleation reaction of Cu2ZnGeSe4, leading to small grains that have obtained their final size already after 3 minutes of selenization at 460°C. The inverse stack with Ge on top and Cu in the bottom delays the nucleation of Cu2ZnGeSe4, leading to a somewhat slower formation reaction and larger Cu2ZnGeSe4 grains, which obtain their final grain size only after 15 minutes of selenization at 460°C.

We have studied the crystallization reaction of polycrystalline Cu2ZnGeSe4 solar cell absorbers fabricated by H2Se selenization of sequentially deposited metal layer stacks. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent X-ray diffraction patterns. We have found that mainly Cu3Ge and ZnSe phases form very rapidly at temperatures below 350 °C. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds. Putting the Ge layer in the bottom and the Cu layer on top leads to a very fast nucleation reaction of Cu2ZnGeSe4, leading to small grains that have obtained their final size already after 3 min of selenization at 460 °C. The inverse stack with Ge on top and Cu in the bottom delays the nucleation of Cu2ZnGeSe4, leading to a somewhat slower formation reaction and larger Cu2ZnGeSe4 grains, which obtain their final grain size only after 15 min of selenization at 460 °C. This research is partially funded by the Flemish government, Department Economy, Science and Innovation. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 640868. B. Vermang acknowledges the financial support of the Flemish Research Foundation FWO (mandate 12O4215N).

Room temperature photoluminescence (PL) and intensity dependent PL measurements were used as a tool to investigate alkali treatment (KF and RbF) of thin (< 500 nm) single-stage absorber layers and compared with similar 3-stage samples. Single-stage absorber layers have a clear distinguishable PL peak below the band gap, which is absent in 3-stage absorber layers. We attribute this peak to a defect band arising due to the planar defects and the small grains. We find that this low energy peak can be largely reduced with alkali treatment, leading to similar decay times and PL yield as thin 3 stage samples. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027

Nick Szabo defined smart contracts as, “….a set of promises, specified in digital form, including protocols within which the parties perform on the other promises.” The essence of a smart contract is that it is self-executing and has a protocol to effect this, i.e. the mechanism for communicating with the smart contract. In addition, Szabo distilled four basic objectives of any contract that should be fulfilled by smart contracts. Two of these, verifiability and enforceability, are given more attention in this paper. Szabo, however, did not address what types of contracts would be most suited to be executed as smart contracts. This paper describes first the unique aspects of financial contracts that make them the most promising candidates for implementation as smart contracts. Secondly, we analyze the conditions under which the use of distributed ledgers and smart financial contracts are most likely to prove successful. This analysis concludes that unless a distributed ledger is combined with an algorithmic financial contract standard and a standard protocol, the potential benefits of smart contracts will not be realized. The corollary to this conclusion is that it is essential for FinTech to adopt such a standard in order to be able to realize its promise of a paradigm shift in finance. The combination of distributed ledger technology and an open, well documented and well tested algorithmic financial contract standard is the next logical step in the development of FinTech.

This paper provides an overview of the physical vapor technologies used to synthesize Cu2ZnSn(S,Se)4 thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films have been fabricated using sequential stacking or co-sputtering of precursors as well as using sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition, pulsed laser deposition of composite targets and monograin growth by the molten salt method were developed as alternative methods for kesterite layers deposition. This review presents the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical description of the main issues limiting this efficiency and of the experimental pathways designed to prevent or limit these issues is provided and discussed as well. A final section is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiency, such as alkali doping and bandgap grading.

On the way to achieving high efficiencies for CuIn0.7Ga0.3Se2 (CIGS) thin film solar cells, alkali doping has become a necessary step. However, it is expected that a migration of the various alkali atoms over the lifetime of the cell will lead to efficiency losses [1]. Logically, ensuring the reliability of a solar cell material is a paramount step towards the commercialization of a product. Indeed, it is necessary for companies to able to guarantee the efficiencies of the “as-sold” panels over periods of time that expand well over 20 or even 25 years. Experiments studying the reliability of the CIGS solar cells and their different components have already been performed in the past [1-4], nevertheless, the exact effects of alkali, and more generally, the precise reasons for, and effect of, the degradation, are still widely unknown. In order to evaluate the viability of some of the most common techniques used to produce highly efficient thin-film solar cells, ultra-thin coevaporated CIGS solar cell absorber material is produced and doped with various alkali atoms (Na, K, ...). The absorber material is deposited in ultra-thin (400-500nm) layers on soda-lime glass (SLG) equipped with a Si(O,N) diffusion barrier. By reducing the amount of bulk material, the aim is to reduce the concentration of defects in the material and limit their effects as much as possible. The barrier prevents the diffusion of alkali atoms from the SLG into the CIGS and allows for a better control of the concentration of alkali atoms in the layer. Additionally, it allows for a better isolation of the effects of the different alkalis by preventing an uncontrolled mix of alkali from being present in the absorber material. All alkalis are deposited in their fluoride form (NaF, KF, etc.) either by evaporation, prior to CIGS deposition, or by a post deposition treatment. We submitted the produced cells to highly degrading conditions in a damp heat environment. This aggressive setting is expected to simulate standard outdoor condition equivalent to 20 years in an experimentally reasonable timespan of a 1000 hours. We monitored the effects of the degradation by regular current-voltage (IV) measurements all along the 1000h experiment. These measurements showed a clear, and alkali concentration dependent, degradation of the solar cells as time increases. Using Atom Probe Tomography (APT) on degraded as well as on undegraded (reference) cells, it was possible to show that in the absence of a potential, the alkali atom migration is minimal, and that K can be mostly found in the grain boundary region of the absorber material. Given the lack of the expected alkali migration, the main reason for the degradation of the cells seems to be the presence of water that seeps into the grain boundaries of the solar cell material, which could also be observed using APT. The possibility to reverse that degradation mechanisms involving water inclusion is currently under investigation. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027

The goal of this study was to analyse the frequency of ultrasonographic findings in 129 calves with bronchopneumonia and to determine how often multiple abnormalities occur in individual calves. The frequency of abnormal ultrasonographic findings ranged from 4 to 88%. Comet-tail artifacts were the most common finding (88%) followed in decreasing order by scattered echogenic foci (69%), air bronchograms (44%), superficial alveolograms (29%), pleural effusion (26%), hepatisation (23%), pleural lesions (18%), fluid bronchograms (14%), lung abscesses (6%) and fibrin deposits or fibrin strands (4%). Thoracic ultrasonography yielded a mean of 3.3 ± 1.55 abnormal findings (range, 1-6) per calf. Ultrasonography of the lungs in calves with bronchopneumonia is a useful adjunct to clinical examination and allows the determination of the type and severity of lesions.In der vorliegenden Arbeit wurde die Häufigkeit der bei 129 Kälbern mit Bronchopneumonie erhobenen Ultraschallbefunde retrospektiv ausgewertet. Ebenso wurde untersucht, wie oft bei einem Kalb mehrere Befunde gleichzeitig vorhanden waren. Die Häufigkeit abnormer Ultraschallbefunde variierte zwischen 4 und 88%. Kometenschweifartefakte stellten mit 88% den häufigsten abnormen Befund dar. Sie wurden in abnehmender Reihenfolge gefolgt von Luftreflexen (69%), Luftbronchogrammen (44%), oberflächlichen Alveologrammen (29%), Pleuraergüssen (26%), Hepatisation (23%), Pleuraveränderungen (18%), Flüssigkeitsbronchogrammen (14%), Lungenabszessen (6%) und Fibrin oder Fibrinspangen (4%). Im Durchschnitt wurden pro Kalb 3.3 ± 1.55 krankhafte Ultraschallbefunde ermittelt. Die ­Ultraschalluntersuchung der Lunge stellt bei Kälbern mit Bronchopneumonie eine wertvolle Ergänzung der klinischen Untersuchung dar. Sie ermöglicht es, die Art und den Schweregrad der bronchopneumonischen Veränderungen darzustellen und zu objektivieren.Dans la présente étude, la fréquence des constatations échographiques anormales recueillies sur 129 veaux souffrant de bronchopneumonie a été évaluée de manière rétrospective. Il a également été étudié combien de fois un veau présentait simultanément plusieurs découvertes. La fréquence des échographies anormales variait entre 4 et 88%. Des artefacts en queue de comète représentaient, avec 88%, les découvertes anormales les plus courantes. Ils étaient, par ordre décroissant, suivi par des réflexions de l’air (69%), des bronchogrammes aériens (44%), des alvéologrammes superficiels (29%), un épanchement pleural (26%), une hépatisation (23%), des altérations pleurales (18%), des bronchogrammes liquidiens (14%), des abcès pulmonaires (6%) et de la fibrine ou des ponts de fibrine (4%). En moyenne 3,3 ± 1,55 résultats d’échographie anormaux ont été déterminés par veau. L’examen échographique des poumons est, chez les veaux souffrant d’une bronchopneumonie, un complément précieux à l’examen clinique. Il permet de représenter la nature et la gravité des changements de bronchopneumonie et de les objectiver.In questo studio si è valutata retrospettivamente la frequenza dei risultati ecografici di 129 vitelli affetti da broncopolmonite e si è esaminato quanto spesso erano presenti contemporaneamente risultati anomali per lo stesso vitello. La frequenza dei risultati ecografici anomali si riscontrava tra il 4 e l’88%. Gli artefatti ecografici a “coda di cometa” erano nell’88% dei casi i risultati anomali più comuni. In ordine decrescente seguivano ecogenicità dell’aria (69%), broncogramma aereo (44%), alveogramma superficiale (29%), versamento pleurico (26%), epatizzazione (23%), lesioni pleuriche (18%), broncogrammi liquidi (14%), ascessi polmonari (6%) e fibrina o maglie di fibrina (4%). In media sono stati riscontrati 3.3 ± 1.55 esami ecografici anomali per ogni vitello. L’esame ecografico dei polmoni dei vitelli affetti da broncopolmonite è un valido esame complementare all’esame clinico e permette di determinare e ­evidenziare il tipo e la gravità delle lesioni broncopolmonari.

Interface quality plays a key role in solar cell applications. Interface recombination at the front and rear surfaces, which determine this quality, have significant effects on open circuit voltage and fill factor values. In this work, several surface treatments were applied on Cu(In,Ga)Se2 (CIGS) surfaces to improve the interface quality. Besides, the passivation layer implementation was investigated to reduce interface recombination between the buffer and absorber layers.

In this study, CdS chemical bath deposition is investigated to improve the performance of thin film solar cell based on Cu2ZnGeSe4/CdS heterojunction. The influence of both the bath temperature and the dipping duration on the CdS thin film properties are explored thanks to the combination of scanning electron microscopy (SEM) and Raman spectroscopy, while the photovoltaic parameters of the resulting solar cells are discussed from current-voltage (I-V) and external quantum efficiency (EQE) measurements. The highest efficiency achieved herein (without antireflection coating) is 7.6%. Although it represents 35% relative improvement compared to previous best efficiency, this champion device is still limited by interface recombination. Different strategies are finally proposed to further increase the performance of these solar cells. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 640868.

The replacement dynamics of Se by S is investigated in kesterite Cu 2 ZnSn(S,Se) 4 and Cu 2 ZnGe(S,Se) 4 . Two different post-sulphurization techniques yield the same results. In the initial stages of the replacement, the original selenium-rich phase coexists with a second mixed phase. This phase has an increased sulphur content of 30 %. In later stages of the reaction, the initial phase disappears in favor of the second phase. Thus, sulphur inclusions of less than 30% seem impossible while above 30% the sulphur amount is adjustable.

In this study, CdS chemical bath deposition is investigated to improve the performance of thin film solar cell based on Cu2ZnGeSe4/CdS heterojunction. The influence of both the bath temperature and the dipping duration on the CdS thin film properties are explored thanks to the combination of scanning electron microscopy (SEM) and Raman spectroscopy, while the photovoltaic parameters of the resulting solar cells are discussed from current–voltage (I–V) and external quantum efficiency (EQE) measurements. The highest efficiency achieved herein (without antireflection coating) is 7.6%. Although it represents 35% relative improvement compared to previous best efficiency, this champion device is still limited by interface recombination. Different strategies are finally proposed to further increase the performance of these solar cells.