Search

In the scenario of the new emerging photovoltaics, kesterites play a lead role in the thin-film solar cell technologies. This class of compounds, mainly represented by the pure-sulfide form Cu2ZnSnS4 (CZTS) and the sulfo-selenide form Cu2ZnSn(S,Se)4 (CZTSSe), shows unique characteristics and stands as a promising p-type absorber material thanks to its high absorption coefficient, high cost-effectiveness and low toxicity. However, CdS is commonly used as the n-type partner (buffer layer) in kesterite solar cells but, beyond its toxicity, it has a nonoptimal band alignment with kesterites and exhibits parasitic absorption at low wavelengths due to its bandgap. Several efforts have been made in the last decade, to replace CdS with a suitable, Cd-free, both environmentally and economically sustainable buffer layer, and many times with successful results allowing not only to equal, but also to overcome in few cases the performances of the corresponding CdS-based reference devices. Zn1-xSnxO for instance leads to higher efficiencies than CdS when coupled with pure-sulfide CZTS, while Zn(O,S) seems to couple better with CZTSSe. TiO2 has been also considered as suitable buffer layer for kesterites and, in the last few years, several works have been reported both theoretical and experimental, showing very promising results. In this review we summarize the efforts and the improvements recorded by the scientific community working on this topic in the last ten years, with the aim to supply a landmark useful to design future experiments in a more efficient way and to push forward the related research activities, in order to fully overcome CdS limitations and to promote thin-film kesterite devices to higher performances.

High-efficiency thin film solar cells play a key role in the production of Building Integrated Photovoltaic (BIPV) or Product Integrated Photovoltaic (PIPV). Among the most interesting emerging photovoltaic (PV) materials, kesterite absorbers such as Cu2ZnSn(SxSe1-x)4 (CZTSSe) have attracted great attention in recent years. The ambient air-processed deposition of CZTSSe films is suitable for the production of low-cost and environmentally friendly flexible devices. In this work, we report on the optimization and functionalization of the back contact to design kesterite solar cells grown by molecular ink method on Mo foil, which, thanks to its high-temperature stability, high conductivity and high purity, is one of the most interesting flexible substrates. In addition, to improve the crystal morphology and promote the grain growth of the kesterite absorber, thus achieving good device efficiencies, intrinsic and extrinsic doping of CZTSSe were also investigated and the incorporation of Ag was proved to be an effective strategy to achieve high-quality flexible solar cells. The Li and Na introduction directly into the precursor solution was also studied to overcome the lack of alkali metal elements (typically needed to promote the kesterite crystallization) in the flexible substrate. Finally, thanks to these approaches, kesterite flexible solar cells with an efficiency of over 11% have been obtained, offering promising prospects for further progress in producing highly efficient flexible kesterite solar cells.

We are all users of lithium-ion batteries (LIBs), the electrochemical energy storage devices that revolutionized our life, making the diffusion of portable and wearable devices possible and now driving the switch from a combustion engine to electric vehicles in the transportation sector. These positive trends are however leading to two whiplash effects: the issue related to management of LIBs when they reach the end of their life (EoL-LIBs) and the supply of the critical raw materials (CRMs) needed to produce the essential LIBs’ components. Recycling of EoL-LIBs is the answer to these two problems, addressing the aspects of waste management while providing a secondary source of CRMs needed to produce new LIBs, in a closed-loop circular economy scheme. The proposed laboratory activity is specifically focused on the recycling of LiCoO2, the most diffused cathode material and also the one containing the highest amount of CRMs. The students will be called to analyze the degradation procedure through acidic leaching, comparing the method today mostly exploited at industrial level (using inorganic acid) and the method recently proposed in the scientific literature (exploiting organic acids). Students will be called to consider not only quantitative chemical indicators (yields of degradation and recovery of CRMs) but also the cost, safety, and disposal of the procedure. The aim of this experience is to drive the students to critically consider all aspects related to sustainability, to present them the tools to quantitatively assess it, and to create awareness regarding a technology involved in our everyday life.

Cu2MnSnS4 (CMTS) is a promising candidate for application in thin film for photovoltaics (PV), thanks to sustainability and low cost of its components. Nevertheless, reported efficiencies are still poor, with a current world-record of 1.13% for a solar cell with CMTS absorber layer made by sputtering and sulfurization [1]. This work reports about a simple and low-cost synthesis procedure, starting from a solution containing all the precursors, without the need of hazardous external sulfurizing agents. CMTS thin films were made by blade coating of the solution on molybdenum substrate and followed by an annealing at 550°C under argon atmosphere, without the aid of external sulphurating agents. After annealing, samples were quenched at RT, but some selected samples underwent a slow cooling inside the furnace. Some selected CMTS thin films were subjected to HCl etching (3% HCl, at 75°C for 10 minutes). Prototype devices were made with the architecture Mo/CMTS/CdS/i-ZnO/AZO/Al grid: CdS was deposited by chemical bath deposition, i-ZnO and AZO by sputtering and Al grid was thermally evaporated. Some selected devices were subjected to a thermal treatment at 260°C for 10 minutes on titanium hotplate. After measurements of photovoltaic performances, upper layers were removed and CMTS thin films were extensively studied employing the GI-XRD technique to determine the lattice constants as well as microstructural properties like microstrain and size of coherent scattering domains. These results of the evaluation of GI-XRD data and photovoltaic performances were correlated to precursor composition, cooling conditions after annealing and post deposition treatments (i.e. HCl etching of the absorber before CdS deposition and thermal treatment of the complete device). All CMTS thin films analyzed by GI-XRD exhibited stannite structure, as expected [2], with a very good crystalline quality. The impact of metal ratio in the solution of precursors was studied, keeping copper poor condi

Heat is an inexhaustible source of energy, and it can be exploited by thermoelectronics to produce electrical power or electrical responses. The search for a low-cost thermoelectric material that could achieve high efficiencies and can also be straightforwardly scalable has turned significant attention to the halide perovskite family. Here, we report the thermal voltage response of bismuth-based perovskite derivates and suggest a path to increase the electrical conductivity by applying chalcogenide doping. The films were produced by drop-casting or spin coating, and sulfur was introduced in the precursor solution using bismuth triethylxanthate. The physical-chemical analysis confirms the substitution. The sulfur introduction caused resistivity reduction by 2 orders of magnitude, and the thermal voltage exceeded 40 mV K-1 near 300 K in doped and undoped bismuth-based perovskite derivates. X-ray diffraction, Raman spectroscopy, and grazing-incidence wide-angle X-ray scattering were employed to confirm the structure. X-ray photoelectron spectroscopy, elemental analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were employed to study the composition and morphology of the produced thin films. UV-visible absorbance, photoluminescence, inverse photoemission, and ultraviolet photoelectron spectroscopies have been used to investigate the energy band gap.

Cu2MnSnS4 (CMTS) is regarded as a promising material for thin film solar cells thanks to the abundance, low cost and low toxicity of its constituents and its good optical properties. The record efficiency (1.13%) is still low due to the presence of manganese oxides in the absorber [1]. In this work, CMTS was synthesized by a cheap sol-gel methodology. The thin film was made by blade coating of a solution containing metal precursors and thiourea, acting as the unique source of sulfur, followed by annealing at 550°C. The commonly used CdS was added by chemical bath deposition to act as buffer layer. Solution composition was fine-tuned and effect of deposition under controlled atmosphere and of post-deposition treatments (HCl etching of the absorber and thermal treatment of complete device at 260°) were studied. XRD and Raman spectroscopy revealed the formation of CMTS with stannite structure while optical bandgap was found to be comprised between 1.5 and 1.6 eV. X-ray photoelectron spectroscopy (XPS) was used to determine manganese oxidation state finding only Mn (II), which is the correct state in stannite structure. No oxygen was detected in the material bulk, but deposition under a controlled atmosphere was found to be important to avoid the formation of oxidized species on the samples’ surface. Besides, post-deposition treatments were found to be beneficial for photovoltaic performance, as well as device ageing. The champion device displayed 0.92% efficiency, which is the current record for wet-synthesized CMTS in substrate configuration. The still poor efficiency could be linked to CdS used as a buffer layer. XPS and energy dispersive X-rays (EDX) revealed a manganese loss from the samples’ surface due to ammonia presence in CdS chemical bath deposition; moreover, ultraviolet photoelectron spectroscopy (UPS) found a detrimental cliff-like band alignment between CMTS and CdS. [1] V. Trifiletti et al., Solar Energy Materials & Solar Cells 254 (2023) 112247

High-efficiency thin film solar cells play a key role in the production of Building Integrated Photovoltaic (BIPV) or Product Integrated Photovoltaic (PIPV). Among the most interesting emerging photovoltaic (PV) materials, kesterite absorbers such as Cu2ZnSn(SxSe1-x)4 (CZTSSe) have attracted great attention in recent years. The ambient air-processed deposition of CZTSSe films is suitable for the production of low-cost and environmentally friendly flexible devices. In this work, we report on the optimization and functionalization of the back contact to design kesterite solar cells grown by molecular ink method on Mo foil, which, thanks to its high-temperature stability, high conductivity and high purity, is one of the most interesting flexible substrates. In addition, to improve the crystal morphology and promote the grain growth of the kesterite absorber, thus achieving good device efficiencies, intrinsic and extrinsic doping of CZTSSe were also investigated and the incorporation of Ag was proved to be an effective strategy to achieve high-quality flexible solar cells. The Li and Na introduction directly into the precursor solution was also studied to overcome the lack of alkali metal elements (typically needed to promote the kesterite crystallization) in the flexible substrate. Finally, thanks to these approaches, kesterite flexible solar cells with an efficiency of over 11% have been obtained, offering promising prospects for further progress in producing highly efficient flexible kesterite solar cells.

Pure sulfide kesterite (Cu2ZnSnS4) is one of the most promising emerging photovoltaic technologies thanks to its excellent absorption coefficient, cost-effectiveness, and environmental sustainability. However, record efficiencies are not exceeding 11% due to several issues, such as absorber defects or a nonoptimal band alignment with the toxic but conventionally used CdS buffer layer. To get rid of it, several efforts have been made in the past few years. Among recent theoretical works, TiO2 has been suggested as a suitable buffer layer due to its optical and electrical properties, giving extremely promising results in device simulation. However, there are few experimental examples combining TiO2 with kesterite, 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.

Thin film photovoltaic devices based on CdTe and Cu(In,Ga)Se2 find in Cu2ZnSn(S,Se)4-based technology a more eco-friendly alternative. 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. 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.

We investigate samples of compositionally disordered potassium-based perovskite single crystals with different composition and stoichiometry. The dielectric and Raman response is inspected over the nominal cubic-to-tetragonal long-range phase transition. The comparison between results shows that the occurrence of the phase transition is controlled by the temperature evolution of polar nanoregions (PNRs). We are able to correlate PNR order, formation, percolation, and freezing to the characteristic temperatures of each macroscopic crystal. The onset of the phase transition is found to occur when PNRs undergo a percolative ordering process, in correlation to the sudden arising of a specific high-frequency Raman mode.

Cu2ZnSnS4 (CZTS) is a promising absorber material to produce thin film solar cells thanks to its high absorption coefficient, low cost and low toxicity. CdS is commonly used as a buffer layer for CZTS solar cells but, beyond its toxicity, it has a nonoptimal band alignment with CZTS. ZnxSn1−xO (ZTO), based on earth-abundant and nontoxic elements and with a large and tunable band gap, is a suitable alternative buffer layer. In this paper, the atomic layer deposition (ALD) of ZTO was employed by testing different compositions and thicknesses. ALD not only leads to very compact and homogenous ZTO layers (enabling tuning the stoichiometry of the ZTO so prepared) but also makes the i-ZnO layer (usually sandwiched between the buffer layer and the transparent contact) redundant and detrimental. Through SCAPS simulation and impedance measurements, the ZnSnO/AZO interface impact on the Cd-free kesterite solar cells’ performances has been investigated, highlighting its leading role in achieving an effective charge extraction and the detrimental effect of the i-ZnO layer. With this approach, a solar cell based on an architecture simpler and more eco-friendly than the conventional one has been produced with comparable efficiencies.

Heat is an inexhaustible source of energy and stimuli. Thermoelectric generators and thermal sensors can produce power or an electrical signal, harvesting the waste heat. The search for a material that could be low cost, guarantee high efficiencies and be directly scalable has drawn attention to the family of halide perovskites. Here, we report bismuth-based perovskite-derivates to be used in thermal harvesting applications and suggest a path to increase the electrical conductivity by applying the mixed-anion approach, where the halide was partially substituted with sulphur to reduce the electrical resistivity. The layers were produced by drop-casting or spin coating; bismuth tri-ethylxanthate has been successfully used as a sulphur source. The Seebeck coefficients measured were higher than 40 mV K-1 in doped and undoped bismuth-based perovskite-derivates. Here, we present an extensive result discussion, based on Raman spectroscopy, X-ray diffraction, UV–visible characterization, ultra-violet, X-ray and inverse photoemission spectroscopies, SEM and EDS analysis. The significant Seebeck coefficient and the micrometre-sized sample dimensions make these compounds highly attractive for heat-flow sensing applications and, above all, for those technologies requiring miniaturised and flexible devices.

Thin film low-cost and eco-friendly photovoltaic devices are based on Cu2ZnSn(S,Se)4. To reduce production costs and improve sustainability, manganese can substitute zinc since Mn is safe and Earth-abundant, and it can be used as a light absorber1) Here, we report on the growth and characterization of Cu2MnSnS4 thin films, produced both by a two-step deposition process and by a sol-gel-based method. The metallic precursors have been deposited by sputtering, and the stack annealed at high temperatures in sulphur atmosphere. The layers, obtained in a Cu-poor and Mn-poor compositional regime, have been tested in solar devices with a record efficiency of 1.13%, with an open-circuit voltage of about 445 mV, delivered after over one year from the first PV measurement. X-ray diffraction and photoelectron, Raman, photoluminescence, and admittance spectroscopies have been used to characterize Cu2MnSnS4, and a scenario of high defectivity has emerged.2) Therefore, we moved to a sol-gel-based method, to improve the quality of the material reducing defectivity. An ink, prepared by dissolving in dimethylsulfoxide metal salts and thiourea, used as the only source of sulphur, was deposited by blade coating to obtain thin films. Sol-gel transition took place in air and then samples were annealed at 550°C under argon atmosphere. The so obtained thin films were fully characterized showing promising preliminary results. References: 1)Le Donne A., Trifiletti V., Binetti S., Front. Chem, 2019, 7:297. 2)Trifiletti V., Frioni L., Tseberlidis G., Vitiello E., Danilson M., Grossberg M., Acciarri M., Binetti S., Marchionna S., Solar Energy Materials & Solar Cells, 2023, 254 112247

In the present study, an ethylene glycol-based electrolyte is used to electrodeposit copper onto zinc for the synthesis of the Zn/Cu/Sn stack, precursor of Cu2ZnSnS4. The employed solution contains diethanolamine (DEA) which supports the formation of amine-complexes, whose features are studied by UV–vis absorption, ESI-MS, and FTIR measurements, providing insight on the optimal concentration of DEA for copper complexation. Cyclic voltammetries (CVs) are carried out at increasing [DEA]/[Cu2+] ratios, highlighting the amine effect on the reduction potential of the copper species. The displacement reaction between copper and zinc is investigated by immersion potential measurements, showing a direct correlation to the voltammetric data. The optimized copper solution resulted in a negligible displacement contribution, allowing the electrochemical synthesis of Mo(sub)/Zn/Cu/Sn stack on a relatively large area (4 cm2). The metallic precursor is analyzed by SEM/EDS, AFM, XRD, and GDOES techniques. Finally, the kesterite CZTS phase is obtained through reactive annealing with elemental sulfur and it is characterized by SEM/EDS, XRD, and Raman spectroscopy. Photoelectrochemical water splitting on CZTS/CdS/Pt heterostructure is carried out in an aqueous solution at pH 6.85, showing a photocurrent of -5.05 mA/cm2 at 0 V vs RHE. A maximum applied bias photon-to-current efficiency (ABPE) value of 2.15% was found at 0.84 V bias.

Earth-abundant and non-toxic Kesterite-based Cu2ZnSnS4 (CZTS) thin film solar cells are successfully fabricated on flexible Molybdenum (Mo) foil substrates by an electrodeposition-annealing route. A well-adherent, densely packed, homogeneous, compact, and mirror-like CZT precursor is initially produced through electrodeposition by using a rotating working electrode. Subsequently, the co-electrodeposited CuZnSn (CZT) precursor is sulfurized in quartz tube furnace at 550 °C for 2 h in N2 atmosphere with the presence of elemental sulfur in order to form CZTS. Different characterization techniques like XRD, SEM, HR-TEM, Raman, and Photoluminescence demonstrate that almost phase-pure CZTS formed after sulfurization. A flexible Al/Al-ZnO/i-ZnO/CdS/CZTS/Mo foil solar cell is produced, where CdS is deposited by chemical bath deposition and transparent conducting oxide (TCO) is deposited by DC sputtering. The CZTS solar device shows a 0.55% power conversion efficiency on flexible Mo foil substrate and it constitutes the first prototype of this kind of solar cell produced by electrodeposition-annealing route without any surface modification of the Mo substrate. Graphic abstract: [Figure not available: see fulltext.].

In this work, Cu-Zn-Sn (CZT) is co-electrodeposited onto a flexible Mo substrate exploiting an alkaline bath (pH 10). The plating solution is studied by cyclic voltammetry, highlighting the effects of potassium pyrophosphate (K4P2O7) and EDTA-Na2 on the electrochemical behavior and stability of the metallic ionic species. The optimized synthesis results in a homogeneous precursor layer, with composition Cu 44 ± 2 at. %, Zn 28 ± 1 at. %, and Sn 28 ± 2 at. %. Soft and reactive annealing are employed respectively to promote intermetallic phase formation and sulfurization of the precursor to obtain CZTS. Microstructural (XRD, Raman), morphological (SEM), and compositional (EDX, XRF) characterization is carried out on CZT and CZTS films, showing a minor presence of secondary phases. Finally, photo-assisted water splitting tests are performed considering a CZTS/CdS/Pt photoelectrode, showing a photocurrent density of 1.01 mA cm−2 at 0 V vs. RHE under 1 sun illumination. Graphical abstract: [Figure not available: see fulltext.]

Germanium quantum dots are very interesting for applications such as solar cells, photodetectors, and light emitters because their small bandgap can be tuned over a wide energy range by changing the particle size. One obstacle to applications is the presence of defects, both in the interior and at the surface of the nanoparticles. The defects function as nonradiative recombination centers or trap charge carriers, which will hinder further optical performance. Introducing hydrogen, as employed in a-Si:H solar cells, has proven to be a good method to counter such detrimental defect effects. In this work, germanium quantum dots were fabricated by an ultraclean, vacuum-based nanoparticle reactor in which hydrogen was supplied during growth. Optical spectroscopy of the a-Ge:H quantum dots, together with Raman and X-ray photoelectron spectroscopy, revealed a direct bandgap and that the presence of hydrogen resulted in amorphous Ge:H quantum dots. These a-Ge:H quantum dots are a step forward toward reducing charge carrier recombination in quantum dot solar cells.

Lead halide perovskites have been revolutionary in the last decade in many optoelectronic sectors. Their bismuth-based counterparts have been considered a good alternative thanks to their composition of earth-abundant elements, good chemical stability, and low toxicity. Moreover, their electronic structure is in a quasi-zero-dimensional (0D) configuration, and they have recently been explored for use beyond optoelectronics. A significant limitation in applying thin-film technology is represented by the difficulty of synthesizing compact layers with easily scalable methods. Here, the engineering of a two-step synthesis in an air of methylammonium bismuth iodide compact thin films is reported. The critical steps of the process have been highlighted so that the procedure can be adapted to different substrates and application areas.

Photovoltaics is a promising technology to produce sustainable energy, thanks to the high amount of energy emitted by the sun. One way of having solar cells with low production costs is to apply thin-film technology and with earth-abundant raw materials. A keen interest is arising in kesterite compounds, which are chalcogenides composed of abundant and non-toxic elements. They have already achieved excellent performance at the laboratory level. Here, we report the synthesis and characterization of mixed chalcogenides based on copper, zinc, iron, and tin. Solutions have been studied with different zinc and iron ratios. The distortion of the elementary cell of kesterite increases with the addition of iron until a phase transition to stannite occurs. The process of synthesis and deposition proposed herein is cheap and straightforward, based on the sol-gel technique. These thin films are particularly attractive for use in cheap and easily processable solar cells. The synthesized layers have been characterized by X-ray diffraction, UV-Vis absorption, and Raman, X-ray photoelectron, and energy-dispersive X-ray spectroscopy measurements.

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 ch