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Quasi-1D chalcogenides have shown great promises in the development of emerging photovoltaic technologies. However, most quasi-1D semiconductors other than Sb2Se3and Sb2S3have been seldom investigated for energy generation applications. Indeed, cationic or anionic alloying strategies allow changing the bandgap of these materials, opening the door to the development of an extended range of chalcogenides with tuneable optical and electrical properties. In this work, Bi incorporation into the Sb2Se3structure has been proved as an effective approach to modulate the bandgap between 0.1. In order to better understand the underlying mechanisms leading to the formation of (Sb1-xBix)2Se3, and thus design specific strategies to enhance its properties, thin films with different annealing time and temperature have been synthesized and characterized. Interestingly, it has been observed that Sb2Se3and Bi2Se3are formed first, with Bi melting at 300°C and diffusing rapidly towards the surface of the film. At higher temperature, the binary compounds combine to form the solid solution, however as the dwell time increases, (Sb1-xBix)2Se3decomposes again into Bi2Se3and Sb. This study has shown that the material is essentially limited by compositional disorder and recombination via defects. Likewise, routes have been proposed to improve morphology and uniformity of the layer, achieving efficiencies higher than 1% for x>0.2.

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.

The relatively stagnant efficiency of Cu2ZnSnS4 (CZTS) kesterite thin film solar cells has led to the exploration of alternative materials based on the kesterite structure. The unavoidable formation of Cu-Zn disorder-related defects and Sn-related deep defects such as SnZn and its cluster in CZTS prompt various attempts to substitute Zn. However, the underlying principles behind the selection of the cation substitutes remain unclear since most studies have been performed using different synthetic strategies. In this study, CXTS (X = Zn, Mn, Mg, Ni, Fe, Co, Ba, Sr) thin films are synthesized by a facile spray pyrolysis and sulfurization method, and their physical properties and device performance are compared. It is found that a majority of the compounds form a tetragonal structure (kesterite or stannite); however, Mg + CTS and Ni + CTS are unstable in their quaternary structure and form a mixture of secondary phases, while CBaTS and CSrTS form trigonal structures. From UV-Vis spectroscopy, it is found that CMnTS, CBaTS and CSrTS exhibit steep and clear absorption edges, which make them promising solar cell absorbers. Meanwhile, high carrier concentrations (>1018 cm−3) are observed for the compounds with transition metal substitutes (Mn, Mg, Ni, Fe, Co). Promising photovoltaic responses are observed in CMnTS, Mg + CTS, CBaTS and CSrTS, with CBaTS having the highest device performance possibly due to the lower band tailing, as observed from the photoluminescence and external quantum efficiency measurements. From these findings, correlations among the suitable cation substitutes for kesterite-inspired compounds are discussed and a guide for screening different cation substitutes for Zn in alternative I2-II-IV-VI4 solar cells is provided. Ministry of Education (MOE) National Research Foundation (NRF) Published version The authors acknowledge funding support from the CREATE Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), which is supported by the National Research Foundation, Prime Minister's Office, Singapore; and Ministry of Education (MOE) Tier 2 Project (MOE2016-T2-1-030). Support by the Luxembourgish Fonds National de la Recherche FNR in the framework of the project CASK is acknowledged. The authors from IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the “Generalitat de Catalunya” (ref. 2017 SGR 862), and are grateful to the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). M. G. acknowledges nancial support from the Spanish Ministry of Science, Innovation and Universities within the Juan de la Cierva fellowship (IJC2018-038199-I).

This is the peer reviewed version of the following article: Lopez-Garcia, A. [et al.]. Ultrathin a-Si:H/Oxide transparent solar cells exhibiting UV-Blue selective-like absorption. "Solar RRL", 15 Febrer 2023, which has been published in final form at https://onlinelibrary.wiley.com/doi/10.1002/solr.202200928. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited. Herein, the fabrication of transparent solar cells based on nanometric (8 and30 nm) intrinsic hydrogenated amorphous siliconfilms (a-Si:H) and using oxidethinfilms as transparent carrier selective contacts are reported. The ultrathindevices present photovoltaic effect and high average visible transmittance (AVT).Additionally, they display a shifted spectral response toward short wavelengths.Glass/fluorine-doped tin oxide (FTO)/aluminum-doped zinc oxide (AZO)/a-Si:H/MoO3/indium tin oxide (ITO) prototypes are shown, presenting AVT=35% andphotovoltaic conversion efficiency (PCE)=2% for a device with a 30 nm a-Si:Hfilm. This yields a light utilization efficiency (LUE) of 0.7%, a record up to this datefor inorganic oxide-based transparent solar cells. For devices including an 8 nma-Si:Hfilm, the AVT reaches 66% with a PCE=0.6% (LUE=0.4%). These highAVT values are comparable or even superior in some cases to those achieved forpure oxide devices. Thesefindings confirm the potential of the proposedarchitectures for the development of highly transparent energy harvesters asfunctional components in building-integrated photovoltaics (BIPV), agrophoto-voltaics (APV), sensors and other low-power devices. In addition, these devicesare fabricated with earth-abundant materials and with up-scalable techniquesthat can allow for a feasible implementation.

The thermal stability of the Cu2ZnSnSe4 (CZTSe) absorber and CdS buffer layers in SLG/Mo/CZTSe/CdS/i-ZnO/ITO devices is explored by performing a series of soft (~200 °C) post deposition treatments (PDTs). A comprehensive analysis of a sample comprised by 56 individual devices by means of Raman and photoluminescence spectroscopies coupled with optoelectronic characterization is performed at different PDT steps. This allows isolating the effects of the PDT on the CZTSe absorber and CdS buffer layer separately and reveals clear evidences of: i) a degradation of the absorber due to Cu/Zn disorder that hinders device performance, and ii) an improvement of the buffer layer by the recrystallization of the CdS nanolayer that is the main responsible for the PDT-induced efficiency improvement. As such, it is concluded that CZTSe/CdS based PV devices present a low thermal stability under relatively low temperatures (in the 100 – 200 °C range), comparable to the temperatures employed at the final production stages of thin film PV devices or even during device operation, that leads to significant changes in solar cell performance and needs to be taken into consideration for the further development of the kesterite PV technology. These results are supported by a novel methodology for easily discerning between changes in Cu/Zn disorder and in point defects concentration in kesterites based on solely on Raman spectroscopy that is proposed in this work and developed through the analysis of a set of CZTSe powder samples with strong variation of the order parameter Q. The final version of this work can be found inSolar Energy Materials and Solar Cells249(2023)112046 (https://doi.org/10.1016/j.solmat.2022.112046)

Oxidative stress is involved in many aging-related pathological disorders and is the result of defective cellular management of redox reactions. Particularly, hydrogen peroxide (H2O2), is a major byproduct and a common oxidative stress biomarker. Monitoring its dynamics and a direct correlation to diseases remains a challenge due to the complexity of redox reactions. Sensitivity and specificity are major drawbacks for H2O2 sensors regardless of their readout. Luminiscent boronate-based probes such as 3-mercaptophenylboronic acid (3-MPBA) are emerging as the most effective quantitation tool due to their specificity and sensitivity. Problems associated with these probes are limited intracellular sensing, water solubility, selectivity, and quenching. We have synthesized a boronate-based nanosensor with a surface-enhanced Raman spectroscopy (SERS) readout to solve these challenges. Furthermore, we found out that environmental pH gradients, as found in biological samples, affect the sensitivity of boronate-based sensors. When the sensor is in an alkaline environment, the oxidation of 3-MPBA by H2O2 is more favored than in an acidic environment. This leads to different H2O2 measurements depending on pH. To solve this issue, we synthesized a multiplex nanosensor capable of concomitantly quantifying pH and H2O2. Our nanosensor first measures the local pH and based on this value, provides the amount of H2O2. It seems that this pH-dependent sensitivity effect applies to all boronic acid based probes. We tested the multiplexing ability by quantitatively measuring intra- and extracellular pH and H2O2 dynamics under physiological and pathological conditions on healthy cells and cells in which H+ and/or H2O2 homeostasis has been altered. P.R.G. acknowledges the Ministry of Science, Innovation and Universities (MICINN-AEI) (AEI-PID2019-106755RB-I00, RYC-2012-10059, CEX2018-000792-M) and the AGAUR (2017 SGR 1054 and 2021PROD00041) for financial support. C.X. and P.R.G. appreciate the financial support from China Scholarship Council (CSC) (201609110104).

As single-junction solar cells are approaching theoretical limits, multijunction solar cells are becoming increasingly relevant, and low-cost wider bandgap light harvesters in tandem with silicon are the next frontier in thin film photovoltaic research. Cu-based chalcogenide compounds have achieved great success as standard absorbers, but performance for bandgaps above 1.5¿eV is still lacking. Additionally, the use of transparent back contacts remains challenging for this class of materials. In this work, we report on the fabrication of wide bandgap CuGaSe2 absorbers by a combination of metallic sputtering and reactive thermal annealing grown on transparent fluorine-doped tin oxide-coated glass substrate. The annealing temperature is carefully tuned in regard to material and photovoltaic device properties. The introduction of an ultrathin Mo interlayer at the CuGaSe2/back interface favors a higher contact's ohmicity and results in an important improvement of all figures of merit. A record conversion efficiency of 5.4% is obtained, which is the highest value reported for this class of absorber on transparent back contact. Fundamental material characterization of the as-grown CuGaSe2 films reveals a better homogeneity in Cu distribution throughout the absorber's thickness when using a Mo interlayer, along with an enhanced crystalline quality. The sub-bandgap transparency of the final device remains perfectible, and improvement pathways are proposed using transfer matrix-based optical modeling, suggesting to use more specular interfaces to enhance optical transmission.

Herein, a detailed study of the stability of different ITO-based back-contact configurations (including bare ITO contacts and contacts functionalized with nanometric Mo, MoSe2, and MoS2 layers) under the coevaporation processes developed for the synthesis of high-efficiency Cu(In,Ga)Se2 (CIGSe) solar cells is reported. The results show that bare ITO layers can be used as efficient back contacts for coevaporation process temperatures of 480¿ºC. However, higher temperatures produce an amorphous In–Se phase at the ITO surface that reduces the contacts transparency in the visible region. This is accompanied by degradation of the solar cells’ efficiency. Inclusion of a Mo functional layer leads to the formation of a MoSe2 interfacial phase during the coevaporation process, which improves the cells’ efficiency, achieving device efficiencies similar to those obtained with reference solar cells fabricated with standard Mo back contacts. Optimization of the initial Mo layer thickness improves the contact transparency, achieving contacts with an optical transparency of 50% in the visible region. This is accompanied by a relevant decrease in back reflectivity in the CIGSe devices, confirming the potential of these contact configurations for the development of semitransparent CIGSe devices with improved optical aesthetic quality without compromising the device performance.

Oxidative stress is involved in many aging-related pathological disorders and is the result of a defective cellular management of redox reactions. Particularly, hydrogen peroxide, H2O2 is a major byproduct and a common oxidative stress biomarker. Monitoring H2O2 dynamics and a direct correlation to diseases remain still a challenge due to the complexity of redox reactions. Sensitivity and specificity are a major drawback for H2O2 sensors regardless their readout. Luminiscent boronate-based probes such as 4-MPBA are emerging as the most effective quantitation tool due to their specificity and sensitivity. Problems associated to these probes are limited intracellular sensing, water solubility, selectivity, and quenching. We have synthesized a boronate-based nanosensor with a SERS readout to solve these challenges. Furthermore, we found out that environmental pH gradients, as found in biological samples, affect the sensitivity of boronate-based sensors. When the sensor is in an alkaline environment, the oxidation promoted by H2O2 is more favored than in an acidic environment. This leads to different H2O2 measurements depending on pH. To solve this issue, we synthesized a multiplex nanosensor able to concomitantly quantify pH and H2O2. Our nanosensor first measures the local pH and based on this value, provide the amount of H2O2. It seems that this pH-dependent sensitivity effect applies to all boronic acid-based probes. We tested the multiplexing ability by quantitatively measuring intra-and extra-cellularly pH and H2O2 dynamics under physiological and pathological conditions on healthy cells and cells which H + and/or H2O2 homeostasis has been altered.

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.

In the present study the transformation between Cu2SnS3, Cu3SnS4 and Cu4SnS4 compounds is analyzed by means of X-ray diffraction and Raman scattering spectroscopy. A detailed analysis of the X-ray diffractograms showed the dependence of formation of the indicated compositional phases from the [Cu]/[Sn] ratio, which was also correlated with the evolution of the Raman spectra. The results are complemented with the detailed analysis of microscopic Raman imaging. Finally, fingerprint Raman spectrum for each compositional polymorph is presented, which should exclude any misinterpretations during phase analysis in future works.

This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Transition-Metal Oxides for Kesterite Solar Cells Developed on Transparent Substrates, copyright © American Chemical Society after peer review. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acsami.0c06992 Fabrication on transparent soda-lime glass/fluorine-doped tin oxide (FTO) substrates opens the way to advanced applications for kesterite solar cells such as semitransparent, bifacial, and tandem devices, which are key to the future of the PV market. However, the complex behavior of the p-kesterite/n-FTO back-interface potentially limits the power conversion efficiency of such devices. Overcoming this issue requires careful interface engineering. This work empirically explores the use of transition-metal oxides (TMOs) and Mo-based nanolayers to improve the back-interface of Cu2ZnSnSe4, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4 solar cells fabricated on transparent glass/FTO substrates. Although the use of TMOs alone is found to be highly detrimental to the devices inducing complex current-blocking behaviors, the use of Mo:Na nanolayers and their combination with n-type TMOs TiO2 and V2O5 are shown to be a very promising strategy to improve the limited performance of kesterite devices fabricated on transparent substrates. The optoelectronic, morphological, structural, and in-depth compositional characterization performed on the devices suggests that the improvements observed are related to a combination of shunt insulation and recombination reduction. This way, record efficiencies of 6.1, 6.2, and 7.9% are obtained for Cu2ZnSnSe4, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4 devices, respectively, giving proof of the potential of TMOs for the development of kesterite solar cells on transparent substrates. This research was supported by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the Ministry of Science and Innovation of Spain under IGNITE project (ENE2017-87671-C3-1-R), by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013) and CERCA Programme / Generalitat de Catalunya. Authors from IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the “Generalitat de Catalunya” (Ref. 2017 SGR 862)

Cs2AgBiBr6 double perovskites are intensely investigated as lead-free, stable alternatives to organic lead halide perovskites. In this contribution, we revise the vibrational modes of Cs2AgBiBr6 and show that Raman spectroscopy is well suited for the detection of common secondary phases such as Cs3Bi2Br9. The combined Raman analysis at pre-, near- and non-resonant conditions allows us to extract the four vibrational modes of Cs2AgBiBr6 to unprecedented detail, the most intense one at 177.3 cm-1. The experimental results are in good agreement with theoretical density functional theory calculations. In addition, we monitor the decomposition of Cs2AgBiBr6 into Cs3Bi2Br9 at temperatures above 255 °C.

Lead-free, highly stable, inorganic double perovskites such as Cs2AgBiBr6 promise to solve two of the main issues that currently hold back the wide-scale application of perovskite-based opto-electr...

Sb

Sb2Se3is a quasi-one-dimensional (1D) semiconductor, which has shown great promise in photovoltaics. However, its performance is currently limited by a highVocdeficit. Therefore, it is necessary to explore new strategies to minimize the formation of intrinsic defects and thus unlock the absorber’s whole potential. It has been reported that tuning the Se/Sb relative content could enable a selective control of the defects. Furthermore, recent experimental evidence has shown that moderate Se excess enhances the photovoltaic performance; however, it is not yet clear whether this excess has been incorporated into the structure. In this work, a series of Sb2Se3thin films have been prepared imposing different nominal compositions (from Sb-rich to Se-rich) and then have been thoroughly characterized using compositional, structural, and optical analysis techniques. Hence, it is shown that Sb2Se3does not allow an extended range of nonstoichiometric conditions. Instead, any Sb or Se excesses are compensated in the form of secondary phases. Also, a correlation has been found between operating under Se-rich conditions and an improvement in the crystalline orientation, which is likely related to the formation of a MoSe2phase in the back interface. Finally, this study shows new utilities of Raman, X-ray diffraction, and photothermal deflection spectroscopy combination techniques to examine the structural properties of Sb2Se3, especially how well-oriented the material is.

The latest record efficiencies of the Cu(In,Ga)Se2 (CIGSe) photovoltaic technology have been driven by heavy alkali post-deposition treatments (PDTs). Despite their positive effect, especially in the Voc of the devices, their underlying mechanisms are still under discussion. This work sheds light on this topic by performing a high statistics analysis on 620 high efficiency CIGSe solar cells submitted to four different PDT processes (different RbF source temperature) employing a combinatorial approach based on Raman and photoluminescence (PL) spectroscopies. This reveals with statistical confidence subtle differences in the spectroscopic data that relate to the redistribution of defects between the ordered vacancy compound (OVC) and the chalcopyrite phases at the absorber surface. In particular, there is an intertwined decrease of the OVC phase and increase of the so-called “defective chalcopyrite phase”. Additionally, two industry-compatible methodologies for the assessment of the RbF-PDT process and prediction of the Voc of the final devices with a ±2% error and an efficacy of ~95% are developed based on analytical and machine learning approaches. These results show that the combined Raman and PL spectroscopic techniques represent a powerful tool for the future development of the CIGSe technology at research level and for a more efficient industrial manufacturing. Final version of this manuscript have been published in Advanced Energy Materials 12 (2022) 2103163. https://doi.org/10.1002/aenm.202103163

Cu-poor compositions are key for obtaining high efficiency Cu(In,Ga)Se2 (CIGSe) devices. These conditions lead to Cu deficit accommodation mechanisms like the formation of Cu-related defectsin the CIGSe crystal structure or of ordered vacancy compound. However, the origin of the benefits of Cu-poor compositions is still under discussion since these mechanisms are difficult to detect and characterize. In this work, a high precision Raman spectroscopy and X-ray diffraction analysis on a compositionally-graded CISe sample ([Cu]/[In] ratios from 0.48 to 1.03) allows us to shed light on this topic by reporting the detection of a “defective chalcopyrite phase” in the slightly Cu-poor compositional regime that may play a critical role, rather than other previously discussed mechanisms, on device performance. This phase is mainly characterized by the formation of a specific defect in the CISe structure which also contributes to appearance of a Raman peak at 230 cm-1. Finally, the analysis of high efficiency (>18 %) CIGSe solar cells and the extrapolation of the results on defect formation obtained for CISe, suggest a possible impact of the defective chalcopyrite phase on the open circuit voltage of the devices, and opens a possible way to further investigate and develop the chalcopyrite based technologies. Final version of this work have been published in Acta Materialia 223 (2022) 117507.https://doi.org/10.1016/j.actamat.2021.117507

Among the thin film chalcogenide photovoltaic community there is an increasing interest in the study of cationic and anionic substitution in the different absorber materials, including CdTe, chalcopyrites – Cu(In,Ga)(S,Se)2 and kesterites – Cu2ZnSn(S,Se)4. In the last case, cationic substitution has been revealed as a key factor to solve or palliate to some extent part of the fundamental problems of the kesterite technology. Among the different possibilities, the partial or total substitution of Sn by Ge is one of the most promising options, with proved excellent results from very small up to almost complete replacement. In view of the relevance of Ge in kesterite, this work presents the complete analysis of the reaction formation of the Cu2ZnGeSe4 (CZGeSe) compound using a sequential process based on the sputtering of elemental stacked layers followed by reactive annealing under Se atmosphere, by implementing a break-off experiment. An unusual solid–liquid–vapor extended growth mechanism is observed, thanks to the previous formation of a eutectic GeSe9 liquid phase that melts at temperatures above 212 °C. Driven by this liquid phase, it is demonstrated that CZGeSe formation mechanisms follow a strict sequence, starting from more simple molecules (binary compounds), then evolving to the ternary one, and finally to the quaternary alloy which is formed through the reaction of Cu2GeSe3 and ZnSe solid phases. The relevance of the study is supported by the solar cells prepared with these absorbers, demonstrating conversion efficiency at the level of the best reports in the literature. Finally, possible strategies to manage this singular formation pathway are discussed.

RbInSe2 is attracting growing interest as a secondary semiconductor compound in Cu(In,Ga)Se2-based solar cells by virtue of the recent investigations on absorber post-deposition treatments with alk...

This work presents the development of a novel chalcogenization process for the fabrication of Cu2ZnSn(S,Se)4 (CZTSSe or kesterite)-based solar cells that enable the generation of sharp graded anionic compositional profiles with high S content at the top and low S content at the bottom. This is achieved through the optimization of the annealing parameters including the study of several sulfur sources with different predicted reactivities (elemental S, thiourea, SnS, and SeS2). As a result, depending on the sulfur source employed, devices with superficially localized maximum sulfur content between 50 and 20% within the charge depletion zone and between 10 and 30% toward the bulk material are obtained. This complex graded structure is confirmed and characterized by combining multiwavelength depth-resolved Raman spectroscopy measurements together with in-depth Auger electron spectroscopy and X-ray fluorescence. In addition, the devices fabricated with such graded band gap absorbers are shown to be fully functional with conversion efficiencies around 9% and with improved VOC deficit values that correlate with the presence of a gradient. These results represent one step forward toward anionic band gap grading in kesterite solar cells.

Presented (oral presentation) at the Virtual Workshop on Transparent PV: Challenges and Opportunities that was co-organised on June 11th 2021 by the Impressive and Tech4Win European projects. The presentation reviewed the main results achieved in Tech4Win (at June 2021) on the development of inorganic UV multifunctional coatings combining UV filtering and PV functionalities

Using a controlled bromine etching on kesterite absorbers, two major results are obtained. We establish the first defect depth profiling and secondary phases depth profiling of a state of the art Cu2ZnSnSe4 (CZTSe) film by using surface sensitive characterization methods (XPS and Raman spectroscopy) on successively etched samples, obtaining a direct insight on the factors hampering the performance of this class of absorber. In a second step, we demonstrate the possibility of significant improvement to the p-n interface in Cu2ZnGeSe4 (CZGSe)/CdS solar cells when a short bromine etching of the absorber is performed, with the Fill Factor improving by more than 7 points. This method offers a simple improvement pathway for state of the art kesterite devices, with a potentially broader application to thin film solar cells where the p-n interface is limiting.

This work combines experimental results and modelling assessment of high Ga, wide bandgap CIGSe solar cells fabricated on a transparent glass/TCO substrate yielding efficiencies above 10% without AR coating. An alkali pre-deposition treatment is performed, and the material analysis of the devices by GDOES, XRD and Raman spectroscopy reveals a significant improvement of the Ga incorporation to the matrix in the presence of an alkali element. The optoelectronic characterization of the solar cells reveals an absolute increase by more than two efficiency points for the alkali-doped samples, overcoming the 10% threshold and being, at the time of this work, the highest reported efficiency for a wide bandgap CIGSe solar cell on transparent substrate. The use of the device developed in this study in a full chalcogenide tandem configuration is assessed by numerical modeling, and different improvement pathways for tandem devices are proposed. Specifically, the replacement of the MoSe2 back interlayer by a more transparent MoO3 is deemed critical in tandem configuration.

Solar cells based on quaternary kesterite compounds like Cu2ZnGeSe4 are complex systems where the variation of one parameter can result in changes in the whole system, and, as consequence, in the global performance of the devices. In this way, analyses that take into account this complexity are necessary in order to overcome the existing limitations of this promising Earth-abundant photovoltaic technology. This study presents a combinatorial approach for the analysis of Cu2ZnGeSe4 based solar cells. A compositional graded sample containing almost 200 solar cells with different [Zn]/[Ge] compositions is analyzed by means of X-ray fluorescence and Raman spectroscopy, and the results are correlated with the optoelectronic parameters of the different cells. The analysis results in a deep understanding of the stoichiometric limits and point defects formation in the Cu2ZnGeSe4 compound, and shows the influence of these parameters on the performance of the devices. Then, intertwined connections between the compositional, vibrational and optoelectronic properties of the cells are revealed using a complex analytical approach. This is further extended using a machine learning algorithm. The latter confirms the correlation between the properties of the Cu2ZnGeSe4 compound and the optoelectronic parameters, and also allows proposing a methodology for device performance prediction that is compatible with both research and industrial process monitoring environments. As such, this work not only provides valuable insights for understanding and further developing the Cu2ZnGeSe4 photovoltaic technology, but also gives a practical example of the potential of combinatorial analysis and machine learning for the study of complex systems in materials research.

An effective encapsulation solution for flexible CIGS is urgently needed to ensure a competitive market entry of the technology. In this work, we demonstrate the feasibility to effectively encapsulate module-level (10 × 10 cm2) CIGS/glass solar cells by employing a thin Al2O3 barrier layer grown by atomic layer deposition (ALD). As determined by a direct methodology, 10 nm ALD-Al2O3 is proved to be sufficient in preventing electrical degradation of the Al:ZnO (AZO) window layer upon exposure to damp heat test (DHT) and equally effective to encapsulate 10 × 10 cm2 CIGS/glass mini-modules by efficient blockage of moisture ingress. CIGS mini-modules encapsulated by ALD-Al2O3 barrier layer retain an average of 80% and 72% of initial efficiency after 1000 and 2000 h of DHT, respectively. Whereas unencapsulated modules drop to an average of 67% (1000 h DHT) and 22% (2000 h DHT) of initial efficiency. Thanks to the presence of ALD-Al2O3 barrier layer, less electrical degradation occurred in AZO window layer and P3 interconnection; also less shunting paths appeared – both led to a lower FF drop in encapsulated CIGS mini-modules. However, an issue of Na migration out of the CIGS layer is observed, which negatively impacts the module stability during DHT.

This work explores the thermal stability of antimony-based photovoltaic (PV) technologies investigating the effect of low-temperature (50—350¿°C) postdeposition annealings (PDAs) on bare Sb2Se3 absorbers and complete SLG/Mo/Sb2Se3/CdS/i-ZnO/ITO devices (5.7% power conversion efficiency with no anti-reflecting coating or metallic grid). A comprehensive structural analysis by means of X-ray diffraction and Raman spectroscopy, coupled with optoelectronic characterization, reveals clear evidences of a degradation process dominated by selenium diffusion. The degradation process is observed to start at low PDA temperatures as a shrinkage of the Sb2Se3 unit cell. Further increasing the PDA temperature above 200¿°C leads to the formation of Sb oxides and Se secondary phases in bare absorbers, and of CdS1—x Se x in complete devices at the Sb2Se3 front interface which completely degrade the heterojunction and kill device performance. Furthermore, a clear correlation is found between PV performance decrease and Sb2Se3 layer lattice shrinkage with the increasing PDA temperature (T¿>¿50¿°C). This is the first time that thermal instability is reported for the Sb2Se3 compound at temperatures commonly used during PV module fabrication processes such as emitter/transparent conducting oxide deposition or encapsulation.

Electrodeposition is one of the most attractive techniques for the synthesis of thin films chalcogenides for photovoltaic applications, in particular for earth abundant kesterite. In this work, the influence of core parameters of this technique, such as the applied potential and deposition time, on the chemical composition of co-deposited Cu–Zn–Sn precursors is studied. Indeed, how the precursor composition affects the final composition of Cu2ZnSnSe4 after reactive annealing in Se atmosphere is also investigated, not only at the film level but also in solar cell devices. X-ray based techniques (XRF and XRD) reveal that the chemical and phase composition of electrodeposited precursors is highly sensitive to small modifications (25 mV) of the applied potential during electrodeposition. Only when the applied potential is −1.2 V (vs. Ag/AgCl) the precursor composition matches to what is required for operating devices. Multi-wavelength Raman spectroscopy of selenized precursors confirms that lower potential than −1.2 V favor the formation of Sn-rich secondary and ternary compounds after selenization, while a higher overpotential led to the formation of ZnSe in the film. On the other hand, the use of longer deposition times enhance the incorporation of zinc in the film, modifies the morphology of the precursor and promotes the formation ZnSe after selenization. Photovoltaic devices were only obtained at an electrodeposition potential of −1.2 V, and reaching a maximum efficiency close to 5% for a 10 min electrodeposited CZT precursor. Fil: Valdes, Matias Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina Fil: Sanchez , Y.. L’Institut de Recerca en Energia de Catalunya; España Fil: Perelstein, Gonzalo Martín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina Fil: Oliva, F.. L’Institut de Recerca en Energia de Catalunya; España Fil: Izquierdo Roca , V.. L’Institut de Recerca en Energia de Catalunya; España Fil: Perez Rodriguez, A.. L’Institut de Recerca en Energia de Catalunya; España Fil: Saucedo, E.. L’Institut de Recerca en Energia de Catalunya; España

With the recent rise of new photovoltaic applications, it has become necessary to develop specific optoelectronic properties for thin-film technologies such as Cu(In,Ga)Se2 and to take advantage of their high degree of tunability. The feasibility of efficient wide bandgap absorbers on transparent conductive oxide substrates is, in that context, of critical importance. Using an original approach based on a predeposition sodium treatment, Cu(In,Ga)Se2 absorbers fabricated by sputtering and reactive annealing with a Ga to (Ga þ In) content over 0.7 and an optical bandgap above 1.4 eV are deposited on transparent fluorine-doped tin oxide films, with the insertion of an ultrathin MoSe2 layer preserving the contact’s ohmicity. Different material characterizations are carried out, and a thorough Raman analysis of the absorber reveals that the sodium pretreatment significantly enhances the Ga incorporation into the chalcopyrite matrix, along with markedly improving the film’s morphology and crystalline quality. This translates to a spectacular boost of the photovoltaic performance for the resulting solar cell as compared with a reference device without Na, specifically in the voltage and fill factor. Eventually, an efficiency exceeding 10% is obtained without antireflection coating, a record value bridging the gap with the state of the art on nontransparent substrates., This research was supported by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the MasterPV project from the SOLARERANET International program (subproject ref. PCI2018-092945Spanish "Agencia Estatal de Investigacion") by the Spanish Ministry of Science, Innovation and Universities under the WINCOST (ENE2016-80788-C5-1-R) project, and by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). Authors from IREC and the University of Barcelona belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the "Generalitat de Catalunya" (Ref. 2017 SGR 862). ENEA researchers gratefully acknowledge the Italian Ministry of Economic Development for the support received in the frame- work of the Operating Agreement with ENEA for the Research on the Electric System. M.P. thanks the Government of Spain for the Ramon y Cajal Fellowship (RYC-2017-23758). M.O.S. acknowledges the Islamic Development Bank for the financial support. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No 712949 (TECNIOspring PLUS) and the Government of Catalonia's Agency for Business Competitiveness (ACCIÓ).

In this work, the effect of different alkali doping strategies in kesterite Cu 2 ZnGeSe 4 absorbers is studied, including the impact on the structural and morphological characteristics of the semiconductor material, and the optoelectronic properties of the solar cell devices. The studied alkali elements include Li, Na, and K, and are introduced by two main approaches: post-deposition treatment (PDT), and pre-absorber synthesis (PAS). This study shows that PDT strategies are more interesting than PAS, and Li as the best alkali candidate for the further improvement of Cu 2 ZnGeSe 4 devices performance.

Among the thin film chalcogenide photovoltaic community there is an increasing interest in the study of cationic and anionic substitution in the different absorber materials, including CdTe, chalcopyrites – Cu(In,Ga)(S,Se)2and kesterites – Cu2ZnSn(S,Se)4. In the last case, cationic substitution has been revealed as a key factor to solve or palliate to some extent part of the fundamental problems of the kesterite technology. Among the different possibilities, the partial or total substitution of Sn by Ge is one of the most promising options, with proved excellent results from very small up to almost complete replace- ment. In view of the relevance of Ge in kesterite, this work presents the complete analysis of the reac- tion formation of the Cu2ZnGeSe4(CZGeSe) compound using a sequential process based on the sputtering of elemental stacked layers followed by reactive annealing under Se atmosphere, by imple- menting a break-off experiment. An unusual solid–liquid–vapor extended growth mechanism is observed, thanks to the previous formation of a eutectic GeSe9liquid phase that melts at temperatures above 2121C. Driven by this liquid phase, it is demonstrated that CZGeSe formation mechanisms follow a strict sequence, starting from more simple molecules (binary compounds), then evolving to the ternary one, and finally to the quaternary alloy which is formed through the reaction of Cu2GeSe3and ZnSe solid phases. The relevance of the study is supported by the solar cells prepared with these absorbers, demonstrating conversion efficiency at the level of the best reports in the literature. Finally, possible stra- tegies to manage this singular formation pathway are discussed.

We report the effect of an ultra-thin Al2O3 layer (down to 3 nm) as interface passivation strategy for the improvement of the performance of Cu2ZnSnS4/CdS based solar cells. After an initial optimization, the Al2O3 deposited by thermal evaporation is proved to improve the properties of the p-n junction. The fabricated devices showed an increment in Voc depending on the composition of the absorber, and an improvement in fill factor (FF) apparently related to the insulation of possible shunt-paths. Also, the impact on other optoelectronic parameters is discussed., This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and the Government of Catalonia's Agency for Business Competitiveness (ACCIÓ). This research has also been supported by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the Spanish Ministry of Science, Innovation and Universities under the IGNITE project (ENE2017-87671-C3-1-R), and by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). Authors from IREC and the University of Barcelona belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the "Generalitat de Catalunya" (Ref.2017 SGR 862). M.P. thanks the Government of Spain for the Ramon y Cajal Fellowship (RYC-2017-23758). This research was also supported by Consejo Nacional de Ciencia y Tecnología (CONACYT) with the scholarship No. 433146 and with the project CB-CONACYT-255062. Finally, this research was also supported by Programa de Fortalecimiento de la Calidad Educativa (PFCE 2019) and by Project VIEP 2019.

Up to date, the absorber low grain growth and the secondary compound formation remain the main factors causing the poor performance of kesterite solar cells based on spray pyrolysis derived Cu2ZnSnS4 (pure sulfide CZTS = PS-CZTS) compared with other deposition techniques. As the sodium (Na) inclusion is known to play a crucial role in affecting the grain growth, we evaluated in this report a thorough investigation in depth of the effect of Na doping content on crystallinity and phase segregation. The PS-CZTS was deposited by ultrasonic spray pyrolysis while the Na incorporation was carried out by soaking the pre-heated sprayed films in a solution of an environmentally-friendly NaCl salt. The change in the investigated parameters was followed throughout the absorber by scanning electron microscopy, Raman spectroscopy with different excitation wavelengths, grazing incidence X-ray diffraction (GIXRD) with various incidence angles, energy dispersive X-ray spectroscopy (EDS), secondary ion mass spectrometry (SIMS) and UV–Vis spectrometry. Increasing Na content significantly improves the crystallinity and the compactness simultaneously, resulting in smoothest surfaces with grains up to 2 µm in size. The Na was uniformly diffused leading to similar grain growth throughout the films. Whatever the Na content, the deposited materials are mostly consisting of ketserite-type PS-CZTS, whereas the use of different Raman excitations reveal the presence of Cu2S, ZnS and ZnO as secondary compounds in trace forms. The depth profile analysis with GIXRD showed that no further phase segregation has occurred while the EDS and SIMS results showed uniform distribution of PS-CZTS′ constituents along the surface and within the deposited films. By combining Raman, GIXRD and EDS analysis, the secondary compounds were found to be mostly gathered in the upper absorber layer. The absorption coefficient and the band gap range around 104 cm−1 and 1.55 eV respectively, the optimal values required for photovoltaic energy conversion. The Na doping developed in this study has a promising potential to yield high quality sprayed absorbers and opens up access to opportunities in enhancing of sprayed PS-CZTS thin film solar cell performances.

In this work, a fast two-step process combining co-electrodeposition of a CuZnSn precursor plus a reactive selenization step with a rapid thermal annealing to synthesize Cu2ZnSnSe4 thin films is presented. By tuning the electrochemical procedure is feasible to obtain with a short electrodeposition time (15 min) the precursor composition required for high efficiency solar cell devices. XRD characterization reveals that the precursor is mainly composed of binary Cu-Zn and Cu-Sn alloys that after thermal treatment completely react to form a kesterite thin film. Raman spectra and Raman mappings demonstrate that best quality kesterite films are obtained after selective chemical etching necessary to remove secondary phases, like ZnSe and SnSe, which are detrimental for solar cell performance. Best solar cells prototypes achieve efficiencies of 5% with a current density of 30.9 mA/cm2, an open circuit potential of 327 mV and a fill factor of 51.5%. The low value of open circuit voltage is attributed to the presence of voids and partial delamination observed in the Mo/Cu2ZnSnSe4 interface. Fil: Valdes, Matias Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina Fil: Hernández Martinez, A.. Instituto de Investigación en Recursos Cinegéticos; España Fil: Sánchez, Y.. Instituto de Investigación en Recursos Cinegéticos; España Fil: Oliva, F.. Instituto de Investigación en Recursos Cinegéticos; España Fil: Izquierdo Roca, V.. Instituto de Investigación en Recursos Cinegéticos; España Fil: Perez Rodriguez, A.. Instituto de Investigación en Recursos Cinegéticos; España Fil: Saucedo, Edgardo. Instituto de Investigación en Recursos Cinegéticos; España

A variety of approaches is being used to fabricate kesterite absorbers, such as sputtering, co-evaporation and solution based techniques. Annealing at high temperatures is a common processing step to stimulate elemental mixing and grain growth. This study investigates the effect of pre-annealing of metal stack precursors at 150 °C, 200 °C, 300 °C and 450 °C on kesterite absorber and solar cell properties. Structural, morphological and vibrational properties of the thin films were investigated. It was found that pre-annealing at 450 °C exhibited a structural mixture distinct from precursors annealed at lower temperatures. The absorber showed improved thickness uniformity, compositional surface homogeneity and absence of Sn-S phases. Lower temperatures resulted in Sn-S compounds in the absorber. In situ XRD is used to monitor phase transitions during pre-annealing. It is shown that implementing a pre-annealing step can improve the absorber properties and the device efficiency. The best performing solar cell had a 4.02% efficiency and was achieved by pre-annealing the precursor layer at 450 °C.

Cu-poor compositions are key for obtaining high efficiency Cu(In,Ga)Se2 (CIGSe) devices. These conditions lead to Cu deficit accommodation mechanisms like the formation of Cu-related defects in the CIGSe crystal structure or of ordered vacancy compound. However, the origin of the benefits of Cu-poor compositions is still under discussion since these mechanisms are difficult to detect and characterize. In this work, a high precision Raman spectroscopy and X-ray diffraction analysis on a compositionally-graded CISe sample ([Cu]/[In] ratios from 0.48 to 1.03) allows us to shed light on this topic by reporting the detection of a “defective chalcopyrite phase” in the slightly Cu-poor compositional regime that may play a critical role, rather than other previously discussed mechanisms, on device performance. This phase is mainly characterized by the formation of a specific defect in the CISe structure which also contributes to appearance of a Raman peak at 230 cm−1. Finally, the analysis of high efficiency (>18 %) CIGSe solar cells and the extrapolation of the results on defect formation obtained for CISe, suggest a possible impact of the defective chalcopyrite phase on the open circuit voltage of the devices, and opens a possible way to further investigate and develop the chalcopyrite based technologies.

This is the peer reviewed version of the following article: Lopez-Garcia, A. [et al.]. Ultrathin wide-bandgap a-Si:H-based solar cells for transparent photovoltaic applications. "Solar RRL", 1 Gener 2022, vol. 6, núm. 1, p. 2100909:1-2100909:8. , which has been published in final form at https://onlinelibrary.wiley.com/doi/full/10.1002/solr.202100909. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. Herein, the fabrication of UV-blue selective transparent solar cells based on ultrathin (

Wide band gap Cu(In,Ga)Se2-ZnSe (CIGZSe) thin films have been synthesized using a sequential process, with the objective to demonstrate the possibility to tune the band gap and introduce double gradients playing with the content of Ga at the back and Zn at the front. In a first approach, we start by varying the 3-valent cationic composition of the system, and then we modify the reactive annealing conditions in order to understand and control the elemental gradients. Structural, compositional, and morphological properties of the corresponding absorbers were analysed by X-ray diffraction, Raman spectroscopy, energy dispersive spectrometer, Auger spectroscopy, and scanning electron microscopy. Solar cells were fabricated and characterized, focusing on the identification of the most promising cationic composition. The compounds from the complex Cu(In,Ga)Se2-ZnSe system can adopt either the chalcopyrite or sphalerite phases depending on the [Zn]/[metals] and [Ga]/([Ga] + [In]) ratios. We demonstrate that Ga naturally diffuses towards the back region forming a Ga-rich, wide band gap chalcopyrite phase at the rear contact, as is commonly observed for a Cu(In,Ga)Se2 synthesized via selenization process. On the contrary, Zn is preferably accumulated at the surface, forming wide band gap sphalerite Cu(In,Ga)ZnSe3 phases with high Zn and very low Ga contents at the surface. This opens an additional way to control the surface's band gap. With this approach, the formation of a doubly graded band gap profile with Ga-rich layers at the back and Zn-rich layers at the front is demonstrated in a single selenization step, showing promising efficiency and open circuit voltage values (up to 6.7% and 709 mV, respectively).