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Many efforts have been made word-wide to obtain earth-abundant variants of Cu2(In,Ga)(S,Se)4 for photovoltaic application. With the most famous Cu2ZnSnS4, Cu2MnSnS4 (CMTS) has been lately investigated thanks to the manganese’s Earth abundancy higher than zinc. [1-3] Here, we present CMTS produced by sputtering the metal’s precursors and subsequent sulphurization of the layer in argon atmosphere. A thin layer of cadmium sulphide was deposited by chemical bath deposition to obtain a p-n junction. The device was then finalized with a capping layer of intrinsic zinc oxide, a window layer of aluminum-doped zinc oxide, and an aluminum grid to collect charge effectively. Designed heat treatment on the entire device has been employed in the air atmosphere to enhance the photovoltaic performances. X-ray diffraction and µ-Raman spectroscopy have shown CMTS as the primary phase. Scanning electron microscopy images showed compact and uniform material with a thickness of about 900 nm, and through the energy dispersive X-ray analysis, we confirmed the composition of the final material. J-V measurements on the optimized devices recorded a maximum efficiency of 0.9%. The photoluminescence and external quantum efficiency data analysis identified the detrimental defects acting in working conditions, suggesting paths to improve the photovoltaic properties. [1] Le Donne et al. Frontiers in Chemistry 7 (2019) 297. [2] Marchionna et al. Journal of Alloys and Compounds 693 (2017) 95. [3] Le Donne et al. Solar Energy 149 (2017) 125.

L'obiettivo di questa tesi di dottorato è la convalida di un processo di produzione ad alto rendimento per tre diversi calcogenuri terrestri abbondanti applicati nella tecnologia TF PV: Cu2ZnSnS4 (CZTS), Cu2FeSnS4 (CFTS) e Cu2MnSnS4 (CMTS). Lo strato adsorbente è stato prodotto con un processo in due fasi: la deposizione dei precursori metallici sul substrato mediante magnetron sputtering seguita da solforazione in forno con zolfo elementare. Dopo la caratterizzazione del materiale cresciuto, sono state realizzate e caratterizzate celle solari prototipali. I risultati sul CZTS hanno dimostrato le buone prestazioni dei processi produttivi: il materiale ottenuto ha mostrato alta qualità, buona omogeneità e assenza di fasi secondarie. Gli SC ottenuti con questi film sottili hanno mostrato prestazioni fotovoltaiche in linea con la letteratura. L'applicazione di due diversi trattamenti post deposizione ha aumentato l'efficienza delle celle solari, portando a un dispositivo record con η=4,5%, JSC=15,1mA/cm2, VOC=561mV e FF=54%. Studi di difettosità hanno mostrato la presenza di coppie antisito CuZn+ZnCu. Il lavoro su CFTS è stato più esplorativo: è stata esplorata una regione compositiva più ampia, per correlare i rapporti stechiometrici dei campioni CFTS con la presenza di fasi secondarie. Sono state rilevate tre diverse fasi secondarie: rodostannite, calcopirite e solfuro di ferro. L'applicazione di due diversi approcci nella deposizione del metallo ha dimostrato la forte correlazione tra l'uniformità del materiale e la stabilità del rate di deposizione. Lo studio dello stato di ossidazione degli elementi nel film sottile ha mostrato che gli atomi di Fe sono presenti sia come Fe2+ che come Fe3+. Come per CFTS, il lavoro su CMTS ha testato la correlazione tra la composizione del film sottile e la presenza di fasi secondarie. Sono state osservate due diverse fasi secondarie: tiospinello e solfuro di manganese. Con campioni CMTS puri, abbiamo ottenuto celle solari con efficienze discrete. L'applicazione dei trattamenti post deposizione studiati per CZTS ha aumentato le prestazioni finali di questi dispositivi, portando ad un dispositivo record di η=0.89% VOC=430mV, JSC=6.14mA/cm2 e FF=33,69%. Il valore VOC è stato, per quanto a nostra conoscenza, il più alto riportato in letteratura. Gli studi sui difetti hanno mostrato la presenza di difetti VCu e l'assenza di difetti antisito CuZn+ZnCu, ma gli studi sugli stati di ossidazione hanno mostrato la presenza di MnO all'interno del film sottile ottenuto, che influisce fortemente sulle proprietà di conduzione del materiale. Focus of this PhD thesis is the validation of a high-throughput production process for three different Earth-abundant chalcogenides applied in TF PV technology: Cu2ZnSnS4 (CZTS), Cu2FeSnS4 (CFTS) and Cu2MnSnS4 (CMTS). The adsorber layer has been produced with a two steps process: the deposition of the metallic precursors on the substrate by magnetron sputtering followed by sulphurisation in oven with elemental sulphur. After the characterization of the grown material, prototypal solar cells (SCs) have been realized and characterized. The results on the CZTS TF demonstrated the good performances of the production processes: the CZTS material obtained showed high quality, good homogeneity, and absence of secondary phases. The SCs obtained with this TFs exhibited PV performances in line with literature. The application of two different PDTs raised the efficiencies of the SCs, leading to a record SC with η=4.5%, JSC=15.1mA/cm2, VOC=561mV and FF=54%. Defectivity studies showed the presence of antisite couples CuZn+ZnCu. The work on CFTS has been more explorative: a wider compositional condition has been explored, to correlate the compositional ratios of CFTS samples with the presence of secondary phases. Three different secondary phases have been detected: rhodostannite, chalcopyrite, and iron sulphide. The application of two different approaches in the metal deposition demonstrated the strong correlation between uniformity of the material and the steadiness of the deposition rate. The study of oxidation state of the elements in the TF showed that Fe atoms are present both as Fe2+ and as Fe3+. As for CFTS, the work on CMTS tested the correlation between TF composition and the presence of secondary phases. Two different secondary phases have been observed: thiospinel and manganese sulphide. With pure CMTS samples, we obtained SC with discrete efficiencies. The application of the PDT studied for CZTS increased the final performances of these devices, leading to a record device of η=0.89% VOC=430mV, JSC=6.14mA/cm2 and FF=33,69%. The VOC value was, to the best of our knowledge, the highest reported in literature. Defect studies showed the presence of VCu defects and the absence of antisite defects CuZn+ZnCu, but oxidation states studies showed the presence of MnO inside the obtained TF, that strongly affect the material conduction properties.

In the last few decades, the attention of scientific community has been driven toward the research on renewable energies. In particular, the photovoltaic (PV) thin-film technology has been widely explored to provide suitable candidates as top cells for tandem architectures, with the purpose of enhancing current PV efficiencies. One of the most studied absorbers, made of earth-abundant elements, is kesterite Cu2ZnSnS4(CZTS), showing a high absorption coefficient and a band gap around 1.4-1.5 eV. In particular, thanks to the ease of band-gap tuning by partial/total substitution of one or more of its elements, the high-band-gap kesterite derivatives have drawn a lot of attention aiming to find the perfect partner as a top absorber to couple with silicon in tandem solar cells (especially in a four-terminal architecture). In this work, we report the effects of the substitution of tin with different amounts of germanium in CZTS-based solar cells produced with an extremely simple sol-gel process, demonstrating how it is possible to fine-tune the band gap of the absorber and change its chemical-physical properties in this way. The precursor solution was directly drop-cast onto the substrate and spread with the aid of a film applicator, followed by a few minutes of gelation and annealing in an inert atmosphere. The desired crystalline phase was obtained without the aid of external sulfur sources as the precursor solution contained thiourea as well as metal acetates responsible for the in situ coordination and thus the correct networking of the metal centers. The addition of KCl in dopant amounts to the precursor solution allowed the formation of well-grown compact grains and enhanced the material quality. The materials obtained with the optimized procedure were characterized in depth through different techniques, and they showed very good properties in terms of purity, compactness, and grain size. Moreover, solar-cell prototypes were produced and measured, exhibiting poor charge extraction due to heavy back-contact sulfurization as studied in depth and experimentally demonstrated through Kelvin probe force microscopy.

Thin film photovoltaic devices based on CdTe and Cu(In,Ga)Se2 rely on critical raw materials, but they find an eco-friendlier alternative in Cu2ZnSn(S,Se)4-based technology. To further reduce production costs and improve sustainability, other abundant metals, such as manganese, can be tested as a potential alternative to zinc. Mn is a safe and Earth-abundant element, and it can be used in light absorber materials when it is part of quaternary chalcogenides with copper and tin. This work reports on the growth and characterization of Cu2MnSnS4 thin films produced by a two-step deposition process, to improve the fundamental understanding of this compound and to promote its application in photovoltaic. The metallic precursors have been deposited by sputtering and the stack annealed at high temperatures in sulphur atmosphere. The layers, obtained in Cu-poor Mn-poor compositional regime, have been tested in solar devices with a record efficiency of 1.13% and a high open-circuit voltage of about 445 mV delivered by the champion device after over one year from the first PV measurement. X-ray diffraction and X-ray photoelectron, Raman, photoluminescence, and admittance spectroscopies have been used to investigate the Cu2MnSnS4 defectivity, and a scenario of high defects has emerged. Therefore, to promote the development of Mn-based photovoltaics the synthesis methodology should be optimized, and the device architecture should be specifically designed for the compound.

Pure sulfide kesterite (Cu2ZnSnS4) is one of themost promising emerging photovoltaic technologies thanks toits excellent absorption coefficient, cost-effectiveness, andenvironmental sustainability. However, record efficiencies arenot exceeding 11% due to several issues, such as absorberdefects or a nonoptimal band alignment with the toxic butconventionally used CdS buffer layer. To get rid of it, severalefforts have been made in the past few years. Among recenttheoretical works, TiO2 has been suggested as a suitable bufferlayer due to its optical and electrical properties, givingextremely promising results in device simulation. However, there are few experimental examples combining TiO2 withkesterite, and they generally show very modest performances. In this Letter, we report on the preliminary and promising results of our experimental procedure for the production of Cd-free kesterite photovoltaic devices featuring ALD-TiO2 as a buffer layer, leading to efficiencies comparable with our CZTS/CdS reference devices.

In current days, the research in the photovoltaic field aims to find and investigate cheap thin-film materials based on earth-abundant element suitable for terawatt era. To this purpose Cu2ZnSnS4 (CZTS) has been widely studied in the last decade, being structurally analogous to the well-documented Cu(In,Ga)Se2 but featuring inexpensive and abundant components. Recently we have reported a simple and straightforward sol-gel process to produce CZTS-based solar cells demonstrating how it is possible to provide a working device with an easily transferable on an industrial scale and low cost process. In this work we report important improvements to the described above methodology by growing CZTS thin films on a transparent substrate aiming to produce bifacial solar cells suitable for an incorporation into a tandem solar cell architecture. The direct droplet deposition of the precursor ink onto the FTO substrate, leads to the kesterite phase. Gelation and annealing steps, without the addition of further sulphur sources make this procedure suitable for industrial scale-up saving a considerable amount of raw materials. UV–Vis absorption, μ-Raman, Photoluminescence, X-ray diffraction and Energy Dispersive Spectroscopy measurements have been performed on the so-produced thin films. The morphology of the samples has been investigated by scanning electron microscopy, proving the extremely good quality of the material. PV devices with a FTO/Mo/CZTS/CdS/ZnO/AZO architecture were obtained showing η = 1.11% in a bifacial set up. A lower η = 0.99% has been measured by illuminating the front side of the cell only, demonstrating that the bifacial setup allows a +11% efficiency enhancement.

The PV research is nowadays focused on finding low cost and easily processable thin-film materials, trying to move towards lighter and more versatile technologies. Among them, Cu2ZnSnS4 (CZTS) drew the attention of the scientific community thanks to its structural similarity to the already reported and well-studied Cu(In,Ga)Se2 but with the great advantage to use cheap and earth-abundant elements. In this work, we report CZTS-based solar cells produced with a simple and straightforward sol–gel process, demonstrating how further expensive and industrially non-scalable instruments are not needed in order to provide a working device. Direct drop-casting of a precursor solution onto the substrate, followed by gelation and final annealing in Ar atmosphere, generates the desired phase of the material without any further source of sulphur. The precursor solution contains metal acetate salts which play a primary role in networking the metals in the gel matrix, together with thiourea and dimethylsulfoxide. With this methodology, only the strictly necessary amount of reagents is used saving until 2–3 orders of magnitude of starting materials compared to other wet deposition techniques and working devices were obtained showing η = 1.1%. The addition of KCl as a dopant into the precursor solution was proven to be beneficial for the grain growth and material quality, so the performance of the corresponding devices rises it up to η = 1.75%. The synthesised thin films have been fully characterised by UV–Vis absorption, μ-Raman, X-ray diffraction and Energy Dispersive Spectroscopy measurements. Their morphology has been investigated by Scanning Electron Microscopy, proving the high quality of the material. The so-produced PV devices with a standard SLG/Mo/CZTS/CdS/ZnO/AZO architecture show interesting and promising results, especially in terms of short circuit current density.

Currently, the PV research focuses on finding low cost and easily processable materials. Here, we propose a simple chemical procedure for synthesizing Earth-abundant chalcogenide thin-films with tunable bandgap, leading to well-defined phases of Cu2XYS4 (with X = Zn, Fe; Y = Sn, Ge). The deposition process is straightforward and very cheap, based on the sol-gel technique, where the thin films are produced thanks to a direct drop-casting of the precursor solution, followed by a gelation process and heat treatment in argon atmosphere to generate the desired crystalline phase of the quaternary alloy. Metal acetate precursors were proved to have a primary role in creating a network, during the sol-gel transition, by coordinating and pre-organizing the metals in solution, together with thiourea (as the only source of Sulphur) and DMSO as solvent. Moreover, the addition of dopant amounts of KCl into the precursor solution was experimentally demonstrated to be beneficial for the grain growth and material quality, both crucial for the final solar device performance. XRD, μ-Raman, UV-Vis, and EDX spectroscopy measurements have supported the characterization of the so-synthesized layers (CZTS, Fe-substituted CZTS, Ge-substituted CZTS); their morphology was studied by SEM imaging, confirming the thin-film good quality. The bandgap, obtained from transmittance measurements, ranges from 1.4 to 2.1 eV depending on the combination of the chosen metal precursors, suggesting this class of materials as a suitable candidate as a top absorber in a tandem device architecture and promising single-junction prototypes of working solar-devices have been produced, even on semi-transparent substrate. [1 - 4] [1] V. Trifiletti et al. Chemistry Select 4 (2019) 4905. [2] V. Trifiletti et al. Materials 13 (2020) 1471. [3] G. Tseberlidis et al. Solar Energy 208 (2020) 532. [4] G. Tseberlidis et al. Solar Energy 224 (2021) 134.

Identifying new low cost and low environmental impact energy sources is becoming an imperative need of civil society. Photovoltaic (PV) is the most mature alternative renewable energy technology, led by silicon and whereas chalcogenide-based thin-film solar cells have gain market section… commercialization. Terawatt generation Here we present the synthesis and the characterization of Earth-abundant chalcogenide thin-films with tunable bandgap, leading to well-defined phases of Cu2XYS4 (with X = Zn, Fe, Mn; Y = Sn, Ge). As an alternative to the physical deposition of the metal precursor, here, we propose a straightforward and very cheap synthesis method based on the sol-gel technique. The Cu2XYS4 (with X = Zn, Fe; Y = Sn, Ge) thin films are produced thanks to a direct drop-casting of the precursor solution, followed by a gelation process and heat treatment in argon atmosphere to generate the desired crystalline phase of the quaternary alloy.3 UV-Vis, μ-Raman and XRD have characterized the so-synthesized layers, and EDX spectroscopy measurements and their morphology was studied by SEM imaging, proving the excellent quality of the material. The optical bandgaps of the presented compounds range from 1.4 to 2.1 eV, depending on the combination of the chosen metal precursors, making them suitable to be employed as stand-alone devices or as top solar cells in tandem architecture with various bottom cells. Promising single-junction prototypes of working devices have been produced, even employing semi-transparent substrates.4-6 An exciting alternative to the most studied Cu2ZnSnS4 (CZTS) is Cu2MnSnS4 (CMTS), a p-type semiconductor with Earth-abundant and low-cost elements. Manganese is cheaper (2$/kg vs 2,7$/kg) and has a higher occurrence in the Earth's crust than zinc (440ppm mean vs 65ppm mean), leading to the more economical and environmental sustainability of CMTS in respect of CZTS.1,2 Here we report the CMTS thin films, grown by a two-step deposition process (sputtering, followed by high-temperature annealing in sulphur vapours), produced in collaboration with RSE S.p.A., the industrial partner in the research project. The CMTS-based solar cells reached a record efficiency of about 1% and an open-circuit voltage exceeding 400 mV. Photoluminescence measurements allowed us to identify the detrimental defects acting in working conditions, suggesting paths to improve the photovoltaic properties. 1. S. Marchionna, A. Le Donne, M. Merlini, S. Binetti, M. Acciarri and F. Cernuschi, Journal of Alloys and Compounds, 2017, 693, 95-102. 2. A. Le Donne, S. Marchionna, M. Acciarri, F. Cernuschi and S. Binetti, Solar Energy, 2017, 149, 125-131. 3. V. Trifiletti, S. Mostoni, F. Butrichi, M. Acciarri, S. Binetti and R. Scotti., ChemistrySelect, 2019, 4, 4905-4912.