Optoelectronic Properties of (CH₃NH₃)₃Sb₂I₉ Thin Films for Photovoltaic Applications

We present solution-based fabrication and characterization of the lead-free perovskite-related methylammonium antimony iodide (CH₃NH₃)₃Sb₂I₉ compound. By photothermal deflection spectroscopy (PDS), we determined a peak absorption coefficient α ≈ 10⁵ cm^–1 and an optical band gap of 2.14 eV for amorphous films of (CH₃NH₃)₃Sb₂I₉. Compared to the related Bi compound, the Sb-perovskite shows no exciton peak in its absorption spectrum. The photoluminescence emission (PL) is observed at 1.58 eV, and the Urbach tail energy of this amorphous compound is E_u = 62 meV, indicating a substantial amount of energetic disorder. We fabricate a planar heterojunction solar cell with a (CH₃NH₃)₃Sb₂I₉ absorber layer that yields a power conversion efficiency of η ≈ 0.5%, already featuring a decent fill factor (FF) of 55% and open-circuit voltage of 890 mV but low photocurrent densities. The result of this basic study on (CH₃NH₃)₃Sb₂I₉ shows that this compound is a possible starting point for further research into Sb-based lead-free perovskite solar cells.