Mechanochemical Synthesis, Electronic Structure, and Photovoltaic Potential of Lead-Free Hybrid Halocobaltates (CH3NH3)2CoX4 (X = Cl, Br)

Here, (CH3NH3)2CoCl4 and (CH3NH3)2CoBr4 were prepared via a solvent-free mechanochemical route and characterized by powder X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, magnetic susceptibility, and thermal analysis. Both compounds crystallize in the monoclinic P21/c space group and exhibit paramagnetic behavior consistent with isolated high-spin Co2+ tetrahedra and negligible inter-site exchange. Diffuse reflectance spectroscopy yielded optical band gaps of 1.65 and 1.60 eV for the chloride and bromide, respectively. Valence-band XPS and cyclic voltammetry provided consistent experimental band-edge positions, confirming favorable alignment with TiO2 and Spiro-OMeTAD in an n-i-p architecture. SCAPS-1D simulations using experimentally determined optical and electronic parameters predicted power conversion efficiencies of 6.63% and 4.86%, at an optimum absorber thickness of 1.28 μm. Defect density was identified as the dominant performance-limiting parameter, while the parity-forbidden Co2+ d-d transitions intrinsically constrain the attainable photocurrent. These results provide the first experimental grounded photovoltaic assessment of hybrid halocobaltates, combining measured optical and electronic parameters with SCAPS-1D device simulations, and establish design parameters for future device optimization.