Synthesis and Development of “Green” Chalcogenides Inks via High-Energy Ball Milling for the Realization of Ultra-Low-Cost Thin Film Solar Cells

New technological solutions for green energy and optoelectronics are needed to surpass current methodologies and performance benchmarks, demanding an extra effort from researchers in the field of material science for the synthesis, characterization, and prototyping of more performant materials and devices. This PhD thesis demonstrates how mechanochemistry (MC) can be smartly applied in this field, particularly exploiting high energy ball milling methodologies. This simple and cost-effective technique allows direct control of materials reactivity and the synthesis of stable/metastable inorganic solids through solid-state reactions of the constituent elements or compounds. MC leverages the non-equilibrium thermodynamic regime, enabling the process to occur around ambient conditions while exchanging extreme local energy to the reactants. Moreover, it is possible to obtain large amounts of pure products without using toxic reactants and solvents. Specifically, this thesis presents the application of MC to Cu(In,Ga)(S,Se) (CIGSSe) chalcogenides, well-known as efficient absorbers (α > 10^4 cm^(-1)) for thin film solar cells. Through MC, pure phases can be obtained by finely varying the cationic (In/Ga) and anionic (S/Se) ratios of the solid solution, allowing the tuning of the optical bandgap from 1.02 eV (CuInSe2) to 2.5 eV (CuGaS2) to match the solar spectrum. A bottom-top process is presented that stabilizes a PV ink from mechanochemically synthesized low bandgap chalcogenides (CuInS2 and CuInSe2) for liquid-phase deposition. The process was optimized to obtain a stable suspension in n-butanol with the correct crystallographic structure and composition. A high temperature selenization treatment demonstrated effectiveness in improving the crystallinity and conductivity of the CuInSe2 films. The addition of ethanolamine as a complexing agent inside the CuInSe2 ink further enhanced these properties, achieving a resistivity of kΩ⋅cm. The photovoltaic cell demonstrated promising performance with a VOC of 331 mV and a JSC of 1.8 mA/cm². For high bandgap chalcogenides (CuInGaS2 and CuGaSSe2), mechanochemical synthesis and refinement were successful in obtaining a pure phase. The refinement conditions allowed fine control of the particle size of CIGS, resulting in a semi-transparent ink with over 70% transparency in the spectral region between 750 and 1300 nm. A prototype device on a transparent FTO substrate showed a photovoltaic response. Additionally, the use of high energy ball milling processed CuInSe2 as a hole transport layer in carbon-based Formamidinium Lead Iodide hybrid-perovskite solar cells was explored. Devices assembled with different concentrations and layers of CISe ink maintained efficiency over 24 hours, with some cells outperforming reference cells after aging, suggesting a possible stabilizing effect on perovskite cells. The second part of the thesis focused on the mechanochemical synthesis of the FE-PV compound SbSI and its post-deposition treatments using Pulsed Electron Deposition (PED). The solid-state reaction of SbSI was successfully applied using mechanochemical synthesis from its constituent elements. A collaboration with the Universitat Politècnica de Catalunya, Barcelona, studied oriented recrystallization using a high-pressure furnace. XRD and SEM analyses confirmed notable recrystallization and preferential orientation of crystallites, addressing sublimation issues and promoting crystallization with a specific orientation. This research represents a significant milestone in developing functional SbSI FE-PV devices.