Band gap grading strategies for high efficiency kesterite based thin film solar cells

[eng] The main subject of this work focuses on the development of advanced technological strategies for bandgap profile engineering on Earth-abundant and eco-friendly kesterite thin film solar cells which potentially optimize and enhance the energy power conversion efficiency of solar cell devices. By exposing the contemporneuous world energy consumption hassles and its direct implication with the heating imbalance produced by the current greenhouse gas emissions; it is doubtlessly notified that renewable energy supplies, mainly based on thin film solar cells, and focused on sustainable materials such as ‘kesterite’ (CZTS), could successfully perform in a wide variety of energy application scenarios. This is due to its potential to be deposited on flexible substrates, its aesthetics and selective transparency for integrations in construction and automotive sectors. As well as its use in new concepts of energy portability like the Internet of Things, even enhanced when combined with its nanostructured form as a potential thermoelectric material. However, the actual kesterite thin film solar cell devices hinder the actual energy conversion efficiencies of a single absorber layer PN junction. This fact is primordially demonstrated with a theoretical numerical modeling simulation (SCAPS-1D) of first front graded bandgap profile attempts in CZTSSe. Consequently, the effect of the front sulfurization of CZTSe solar cell with a realistic experimental compositional profile is analyzed and discussed. In light of this, it possibly to demonstrate that the next generation of kesterite (and chalcopyrite) solar cells power energy conversion efficiency improvements could be remarkably enhanced with the development of novel and more strategic methodologies for collecting photon energy. In this way, the graded bandgap profiling in kesterites is proposed as a sustainable strategy to improve the utilization of the solar spectrum, through the generation of quasi-electric internal fields along the thin films, increasing the drift and diffusion lengths of minority charge and finally improving the power conversion efficiency. First of all, by developing a novel and disruptive chalcogenization process for the fabrication of CZTSSe solar cells enabling the generation of a superficial graded compositional profile. Hence, controlling several front-graded bandgap profiles along the CZTSSe absorber thin film layer thickness. Furthermore, by means of generating a rear bandgap graded profile strategy mainly based on the spontaneous cationic substitution during the kesterite (CZTGSe) alloy synthesis, it was possible to reduce the effect of deep defect (SnCu) formation and impose an additional drift (back surface) field within the quasi-neutral region. Additionally, this improves the crystallization quality of the absorber material by generating metallic Ge liquid phase fluxes. Thus, controlling several rear-graded bandgap profiles along the CZTGSe absorber thin layer film thickness. Finally, assembling together the abovementioned strategies in order to simultaneous generate both anionic and cationic compositional grading profiles inside the same kesterite matrix structure. In this way providing for the first time a demonstration of the joint synergy between defect passivation and interface energetics-modification, as a result of applying bandgap grading strategies in kesterite-based thin film solar cells. In the case of this work studied kesterite alloyed material (CZTGSSe), the band energy offset can be independently controlled trough the sulfur (S) and germanium (Ge) contents, which is explained by a double U-Shaped graded bandgap model. Consequently, this Thesis develops advanced material synthesis techniques and surface characterization, which, when integrated with the structural complexity of kesterite (CZTGSSe), allow Nature to reveal several new and disruptive properties of matter, deliberately manipulable when working out of thermodynamic equilibrium conditions. Last but not least, by optimizing the synthesis conditions, an absolute increase in bare energy conversion efficiency is obtained for the champion kesterite-based thin film solar cell device (> 10%) without any antireflective coating (ARC) nor metallic grid.[spa] Los suministros de energía renovable basados en celdas solares de película delgada/fina, y enfocados en materiales sostenibles, tales como la ‘kesterita’ (CZTS), podrían desenvolverse de manera muy exitosa en una amplia variedad de escenarios de aplicaciones energéticas. Esto se debe a su potencial para ser depositadas sobre substratos flexibles, su estética y transparencia selectiva para integraciones en sectores como el de construcción y la automoción. Así como su uso en nuevos conceptos de portabilidad energética, tales como el Internet de las cosas. Las celdas solares actuales de kesterita y de una sola capa absorbente con perfiles de banda prohibida no variables limitan la plenitud en la obtención de mejores eficiencias de conversión energética para una unión PN. Ante esto, esta Tesis demuestra que la eficiencia de conversión energética de la próxima generación de celdas solares de kesterita (y calcopiritas) puede verse potenciada tras desarrollar nuevas metodologías más estratégicas de recolección de energía fotónica. Por tanto, se propone el graduado del perfil el de banda prohibida como una estrategia sostenible para mejorar la utilización del espectro solar, mediante la generación de campos internos cuasi-eléctricos a lo largo de las películas finas, consiguiendo incrementar las longitudes de deriva y difusión de los portadores de carga minoritarios y finalmente aumentar la eficiencia. De esta manera, esta Tesis desarrolla técnicas avanzadas de síntesis y caracterización de superficies, que al integrarse con la complejidad estructural de la kesterita (CZTGSSe), permiten a la Naturaleza revelarnos varias novedosas y disruptivas propiedades de la materia, deliberadamente manipulables cuando se trabaja en condiciones fuera del equilibrio termodinámico. Por último, al optimizar las condiciones de síntesis, se obtiene un notable incremento en la eficiencia absoluta de conversión energética en celdas solares a película delgada basadas en kesterita mayores al 10%, esto sin depositar recubrimiento anti-reflectante (ARC), ni rejilla metálica alguna

Towards low cost and sustainable thin film thermoelectric devices based on quaternary chalcogenides

This is the peer reviewed version of the following article: Isotta, E. [et al.]. Towards low cost and sustainable thin film thermoelectric devices based on quaternary chalcogenides. "ADVANCED FUNCTIONAL MATERIALS", 20 Maig 2022, núm. 2202157, which has been published in final form at https://onlinelibrary.wiley.com/doi/10.1002/adfm.202202157. 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.A major challenge in thermoelectrics (TEs) is developing devices made of sustainable, abundant, and non-toxic materials. Furthermore, the technological drive toward low sizes makes crucial the study of nano and micro configurations. In this work, thin film TE devices based on p-type Cu2+xZn1-xSnS4 and Cu2+xZn1-xSnSe4, and n-type AlyZn1-yO are fabricated by physical vapor deposition. The kesterite phases show good purity and promising TE power factor, likely enhanced by the copper–zinc order–disorder transition. Thin film generators in planar configuration are assembled by a sequential deposition of the p-type, n-type, and contact materials. The power per unit planar area reaches 153 and 279 nW cm-2 for the sulphur- and selenium-based generators, respectively. These values significantly outperform any other literature attempt based on sustainable and low-cost thin films. Furthermore, if compared with traditional TEs often made of scarce and toxic materials, these devices offer a cost reduction above 80%. This allows reaching comparable values of power density per unit material cost, representing a first real step toward the development of sustainable and non-toxic thin film TE devices. These can find applications in micro energy harvesters, microelectronics coolers, and temperature controllers for wearables, medical appliances, and sensors for the internet of things.A.J. thanks the European Social Fund+ for the FI fellowship. The authors would like to acknowledge the help of Dr. Mirco D’Incau, Dr. Narges Ataollahi, and Prof. Della Volpe for the design of the measuring setup, as well as useful discussion with Prof. Dario Narducci. This research has received funding from the Spanish Ministry of Science, Innovation and Universities under the MATER-ONE projects (PID2020-116719RB-C41). 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 European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). M.G. acknowledges the financial support from Spanish Ministry of Science, Innovation and Universities within the Juan de la Cierva fellowship (IJC2018-038199-I). E.S. acknowledges the ICREA Academia Program. Open Access Funding provided by Universita degli Studi di Trento within the CRUI-CARE Agreement.Peer ReviewedPostprint (published version

Study and optimization of alternative MBE‐deposited metallic precursors for highly efficient kesterite CZTSe:Ge solar cells

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Insights into interface and bulk defects in a high efficiency kesterite-based device

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.Peer ReviewedPostprint (author's final draft

Sputtered ZnSnO Buffer Layers for Kesterite Solar Cells

International audienceReplacing the CdS buffer layer with a ZnSnO one in Cu$_2$ ZnSnS$_4$-based solar cells allows both to improve the device performances and to avoid using toxic Cd. Additionally, using a sputtered buffer layer is a major asset for solar cells fabricated by physical vapor deposition processes. In this study, ZnSnO layers are deposited by sputtering of a single metal oxide target. Structural and optical properties of the layers deposited on Si or glass are first described. The possibility of modifying the ZnSnO metallic composition by adjusting the deposition power is demonstrated. Attempts to improve the optoelectronic properties of the ZnSnO layers with Ar:O$_2$ or Ar:SF$_6$ reactive sputtering are shown as well. These ZnSnO buffer layers are transferred in Mo/CZTS/ZnSnO/ZnO:Al solar cells. After postdeposition thermal treatment and optimization of the deposition condition (notably with the use of Ar:O$_2$ or Ar:SF$_6$ reactive sputtering), a solar cell with a power conversion efficiency of 5.2% is demonstrated. It is 0.6% absolute higher than the reference solar cell with a CdS buffer layer. To avoid absorber damaging and achieve high performances, deposition power must be as low as possible. A two-stage sputtering process is used to conciliate both the absorber surface preservation and a reasonable deposition time. Last, photovoltaic properties of optimized CZTS-and CZTSe-based solar cells with ZnSnO buffer layers are compared

Investigation on limiting factors affecting Cu2ZnGeSe4 efficiency: Effect of annealing conditions and surface treatment

This work aims to unveil the optimal annealing conditions and surface treatments of CZGeSe absorbers, synthesized using vacuum-based deposition technique, with an eye to optimizing the main parameters allowing better control of secondary phases formation and improving crystalline quality of this absorber. Firstly, a comparative study is given of one and two-step annealing profiles, where, for each thermal treatment, the optimal temperature is probed. The second section of this study underlines the evaluation of the surface treatment effect on the as-annealed absorber using different etching agents. Finally, the effect of different post-annealing treatment temperatures on the overall performance of the fabricated devices is evaluated. For the studied optimizations, a deep understanding of the cell behavior is provided through structural, morphological and electrical characterizations. Preliminary results have given an efficiency up to 5.6% with higher Voc = 572 mV and FF = 65% compared to the reported record cell using similar absorber (Voc = 558 mV, FF = 59%). This performance is linked to the implementation of a two-step annealing process with lower temperatures (330 °C/480 °C) as it showed the best crystallinity-efficiency trade-off along with the smallest amount of ZnSe secondary phase among all the thermal routines studied. In addition, after the evaluation of several etching agents, the implementation of a KCN etching has shown to be the most effective leading to a remarkable improvement of the PN junction through a surface passivation.Peer ReviewedPostprint (author's final draft

High efficiency Cu2ZnSnS4 solar cells over FTO substrates and their CZTS/CdS interface passivation via thermal evaporation of Al2O3

The fabrication of soda lime glass/fluorine-doped tin oxide (SLG/FTO) transparent substrates represents an important advance in the fabrication of tandem solar cells. In addition, possible passivation using Al2O3, which can be deposited using cheap systems, can open up the development of Cu2ZnSnS4 (CZTS) solar cells. In this work, we report the simultaneous use of FTO/Mo (20 nm) as a transparent substrate and Al2O3 as apassivation layer at the p–n interface in CZTS solar cells. The best devices show efficiencies of 7.7% (Voc = 677 mV) and 7.3% (Voc = 700 mV). Complete analysis, in terms of morphology, structure and electrical properties, is discussed.Peer ReviewedPostprint (author's final draft

Controlling the anionic ratio and gradient in kesterite technology

Accurate anionic control during the formation of chalcogenide solid solutions is fundamental for tuning the physicochemical properties of this class of materials. Compositional grading is the key aspect of band gap engineering and is especially valuable at the device interfaces for an optimum band alignment, for controlling interface defects and recombination and for optimizing the formation of carrier-selective contacts. However, a simple and reliable technique that allows standardizing anionic compositional profiles is currently missing for kesterites and the feasibility of achieving a compositional gradient remains a challenging task. This work aims at addressing these issues by a simple and innovative technique. It basically consists of first preparing a pure sulfide absorber with a specific thickness followed by the synthesis of a pure selenide part of complementary thickness on top of it. Specifically, the technique is applied to the synthesis of Cu2ZnSn(S,Se)4 and Cu2ZnGe(S,Se)4 kesterite absorbers, and a series of characterizations are performed to understand the anionic redistribution within the absorbers. For identical processing conditions, different Se incorporation dynamics is identified for Sn- and Ge-based kesterites, leading to a homogeneous or graded composition in depth. It is first demonstrated that for Sn-based kesterite the anionic composition can be perfectly controlled through the thicknesses ratio of the sulfide and selenide absorber parts. Then, it is demonstrated that for Ge-based kesterite an anionic (Se–S) gradient is obtained and that by adjusting the processing conditions the composition at the back side can be finely tuned. This technique represents an innovative approach that will help to improve the compositional reproducibility and determine a band gap grading strategy pathway for kesterites. Furthermore, due to its simplicity and reliability, the proposed methodology could be extended to other chalcogenide materials.Peer ReviewedPostprint (published version

Transition-Metal Oxides for Kesterite Solar Cells Developed on Transparent Substrates

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.0c06992Fabrication 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)Peer ReviewedPostprint (author's final draft