LocalEnergy - Local Resources for Multifunctional Tetrahedrite-based Energy-Harvesting Applications

Comprehensive study of the structural and optoelectronic properties of Cu-As-S thin-films deposited via radio-frequency magnetron co-sputtering, using a commercial Cu target together with a target with material obtained from local resources, specifically from mines in the Portuguese sector of the Iberian Pyrite Belt.

One of the areas of research on materials for thin-film solar cells focuses on replacing In and Ga with more earth-abundant elements. In that respect, chalcostibite (CuSbS2) is being considered as a promising environmentally friendly and cost-effective photovoltaic absorber material. In the present work, single CuSbS2 phase was synthesized directly by a short-duration (2 h) mechanochemical-synthesis step starting from mixtures of elemental powders. X-ray diffraction analysis of the synthesized CuSbS2 powders revealed a good agreement with the orthorhombic chalcostibite phase, space group Pnma, and a crystallite size of 26 nm. Particle-size characterization revealed a multimodal distribution with a median diameter ranging from of 2.93 μm to 3.10 μm. The thermal stability of the synthesized CuSbS2 powders was evaluated by thermogravimetry and differential thermal analysis. No phase change was observed by heat-treating the mechanochemically synthesized powders at 350 °C for 24 h. By UV-VIS-NIR spectroscopy the optical band gap was determined to be 1.41 eV, suggesting that the mechanochemically synthesized CuSbS2 can be considered suitable to be used as absorber materials. Overall, the results show that the mechanochemical process is a viable route for the synthesis of materials for photovoltaic applications.

The inexorable increase of energy demand and the efficiency bottleneck of monocrystalline silicon solar cell technology is promoting the research and development of alternative photovoltaic materials. Copper-arsenic-sulfide (CAS) compounds are still rather unexplored in the literature, yet they have been regarded as promising candidates for use as p-type absorber in solar cells, owing to their broad raw material availability, suitable bandgap and high absorption coefficient. Here, a comprehensive study is presented on the structural and optoelectronic properties of CAS thin-films deposited via radio-frequency magnetron co-sputtering, using a commercial Cu target together with a Cu-As-S target with material obtained from local resources, specifically from mines in the Portuguese region of the Iberian Pyrite Belt. Raman and x-ray diffraction analysis confirm that the use of two targets results in films with pronounced stoichiometry gradients, suggesting a transition from amorphous CAS compounds to crystalline djurleite (Cu31S16), with the increasing proximity to the Cu target. Resistivity values from 4.7 mΩ.cm to 17.4 Ω.cm are obtained, being the lowest resistive films, those with pronounced sub-bandgap free-carrier absorption. The bandgap values range from 2.20 to 2.65 eV, indicating promising application as wide-bandgap semiconductors in third-generation (e.g., multi-junction) photovoltaic devices.

One of the areas of research on materials for thin-film solar cells focuses on replacing In and Ga with more earth-abundant elements. In that respect, chalcostibite (CuSbS2) is being considered as a promising environmentally friendly and cost-effective photovoltaic absorber material. In the present work, single CuSbS2 phase have been synthesized directly by a short duration (2 h) mechanochemical synthesis step starting from mixtures of elemental powders. X-ray diffraction analysis of the synthesized CuSbS2 powders revealed a good agreement with the orthorhombic chalcostibite phase, space group Pnma, and a crystallite size of 26 nm. Particle size characterization revealed a multimodal distribution with a median diameter ranging from of 2.93 m to 3.10 m. The thermal stability of the synthesized CuSbS2 powders was evaluated by differential thermal analysis. No phase change was observed by heat treating the mechanochemically synthesized powders at 350 C for 24 h. By UV-VIS-NIR spectroscopy the optical bandgap was determined to be 1.41 eV, suggesting that the mechanochemically synthesized CuSbS2 can be considered suitable to be used as absorber materials. Overall, the results show that the mechanochemical process is a viable route for the synthesis of materials for photovoltaic applications.

Extended abstract and presentation at the Virtual Conference on Thermoelectrics, VCT 2020, International Thermoelectrics Society (ITS), July 21-23, 2020

ABSTRACT The development and implementation of energy efficient technologies with low global warming potential requires a continuous innovation in the field of materials for energy. This strategy includes research on the processing of thermoelectric materials based on tetrahedrite. In the present work, it is evaluated the feasibility of the direct incorporation, without any pretreatment, of minerals from the tennantite-tetrahedrite series in the processing of tetrahedrite-based materials by mechanochemical synthesis. For this purpose, several powder mixtures were prepared containing different ratios of synthetic tetrahedrite samples, also obtained by mechanochemical synthesis, and of ore samples, collected at the Neves Corvo mine and at the abandoned Barrigão mine dumps. From the structural and microstructural characterization of the materials processed for 1 h and 2 h it was observed the formation of a tetrahedrite-tennantite-(Fe) compound, regardless of the origin of the ore sample. These results are an indication of the success of the approach followed. KEYWORDS: Iberian Pyrite Belt, Tetrahedrite, Mechanochemical synthesis, Thermoelectric

This work describes the main lines of investigation that are underway within LocalEnergy project (http://localenergy.lneg.pt). LocalEnergy is a multidisciplinary Research, Development and Innovation project, involving activities in the field of materials science, geology and renewable energies. These activities are oriented towards the industry and are based on a disruptive approach aimed at the development of thermoelectric materials, through the use of materials based on natural and synthetic tetrahedrites, and the development of new absorbers for thin film solar cells, using materials based on synthetic tetrahedrites. Considering that Portugal is one of the European countries with the highest level of irradiation of solar energy and that tetrahedrite is a local mineral resource, present in the Portuguese zone of the Iberian Pyrite Belt, LocalEnergy project represents an opportunity for the development of sustainable energy systems based on the maximization and exploration of two important endogenous resources (solar energy and mineral resources).

Considering that the synthesis of tetrahedrite-based materials usually uses high purity elements, the evaluation of the direct application of ore samples as raw materials for their synthesis is a pertinent issue. In the present study, multiphase synthetic tetrahedrite samples were mixed with tetrahedrite-tennantite ore samples (in weight ratios of 0.8/0.2, 0.5/0.5 and 0.2/0.8) to produce tetrahedrite-based materials by solid-state mechanochemical synthesis. The ore samples were obtained from the abandoned Barrigão mine dumps and from the Neves Corvo mine, both located in the Portuguese zone of the Iberian Pyrite Belt, whose main constituents were found to be As-rich tetrahedrite and tennantite-(Fe), respectively. Depending on the ore sample and on the mixture ratios, the displacement reactions occurring during the synthesis process gave rise to different phase transformation paths. For the ratios of 0.5/0.5 and 0.2/0.8, the mixtures with the Barrigão ore were found to consist of a single sulfide phase, tetrahedrite-tennantite-(Fe), plus quartz, while famatinite was also observed in the 0.8/0.2 mixture. Tetrahedrite-tennantite-(Fe) phase was the main constituent of all mixtures with the Neves Corvo ore, but in all of them other sulfide phases, famatinite-luzonite and pyrite, were also present in addition to quartz. Despite being dissimilar, these results are very promising and encouraging, by confirming the possible direct usage of ore samples and of dump material for the synthesis of tetrahedrite-based materials with all the potential environmental-economic gains that can be obtained.

In this study, we propose the direct synthesis of nanocrystalline and single-phase Cu12-x-yFexZnySb4S13-z tetrahedrites, with 0 ≤ x, y ≤ 1 and 0 ≤ z ≤ 0.3, using an powder metallurgical route consisting in a short duration (2 h) high energy milling step followed by a subsequent densification step through hot pressing. Elemental powders of Cu, Fe, Zn and Sb and S pieces were used as starting materials. The influence of the chemical composition and of the doping elements will be presented in relation to the structural, thermal stability and thermoelectrical properties.

The Barrigão re-mobilised copper vein deposit is revisited in the light of tetrahedrite-tennantite importance for EU Critical Raw Materials (CRM) knowledge as well as material research for development of thermoelectric (TE) and solar energy harvesting technologies. The transition to decarbonised economies (+ e-mobility) and full circular economy implementation implies continuous innovation in materials and in low-C energy technologies. The continued strategic importance of raw materials for the EU manufacturing industry, antimony, amongst another 25 mineral raw materials, was added to the latest list of CRM. Barrigão is structurally associated with an inferred late Variscan NE–SW striking fault zone. The ore, dominated by chalcopyrite + tennantite-tetrahedrite, with minor arsenopyrite, pyrite, and löllingite, is a breccia-type ore, characterised by up to four ore-forming stages. Hand selected composite mine dump samples were crushed (1-2 and 2-4 mm) from where tetrahedrite-tennantite grains were handpicked and chemically and mineralogically characterised for application in future tests to produce TE materials.