We present solution-based fabrication and characterization of the lead-free perovskite-related methylammonium antimony iodide (CH 3 NH 3 ) 3 Sb 2 I 9 compound. By photothermal deflection spectroscopy (PDS), we determined a peak absorption coefficient α ≈ 10 5 cm −1 and an optical band gap of 2.14 eV for amorphous films of (CH 3 NH 3 ) 3 Sb 2 I 9 . 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 (CH 3 NH 3 ) 3 Sb 2 I 9 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 (CH 3 NH 3 ) 3 Sb 2 I 9 shows that this compound is a possible starting point for further research into Sb-based lead-free perovskite solar cells.L ead-based organic−inorganic hybrid semiconductors like CH 3 NH 3 PbI 3 (MAPI) have shown high potential as efficient absorber materials for single-junction solar cells due to their outstanding optoelectronic properties 1 and the easy and cheap fabrication methods. The power conversion efficiencies of these three-dimensional (3D) perovskite solar cells have increased rapidly up to more than 22% in 2016 2−5 after only 6 years of research starting from the first reported perovskite solar cell with an efficiency of 3.8%. 6 However, the low chemical stability of this lead-based perovskite under ambient air and the toxicity of the heavy metal lead could be obstacles for commercialization. 6−11 The tin analogous perovskite compound CH 3 NH 3 SnI 3 has exhibited moderate conversion efficiencies of up to 6% but is even more unstable under air and moisture because of the rapid oxidation of the Sn 2+ state to the Sn 4+ state. 12−14 Another promising group of materials for lead-free alternatives are the zero-dimensional (0D) Biperovskites A 3 Bi 2 X 9 (e.g., A = Cs + , MA + ; X = I − , Br − ). 15−17 These materials show high band gaps of E g > 1.8 eV, 18,19 which are suitable for tandem or triple solar cells. 20,21 Bismuth is a nontoxic element, and the Bi-based perovskites showed a better chemical stability under ambient atmosphere than the MAPI perovskite. 22 However, the Bi-perovskites also show the signature of excitons in their absorption spectrum that have binding energies in the range of 400 meV. 19,22 This leads to relatively low short-circuit current densities due to insufficient exciton splitting and charge carrier extraction. Bi-perovskitebased solar cells exhibit conversion efficiencies of 1% 22 with a TiO x electron transport layer (ETL) and 0.1% 23 in planar devices with pin-architectures where the exciton splitting at the interfaces is even more critical.In this work, we present the exchange of bismuth by antimony, which is less toxic than P...