Intrinsic Formamidinium Tin Iodide Nanocrystals by Suppressing the Sn(IV) Impurities

The long search for nontoxic alternatives to lead halide perovskites (LHPs) has shown that some compelling properties of LHPs, such as low effective masses of carriers, can only be attained in their closest Sn(II) and Ge(II) analogues, despite their tendency toward oxidation. Judicious choice of chemistry allowed formamidinium tin iodide (FASnI3) to reach a power conversion efficiency of 14.81% in photovoltaic devices. This progress motivated us to develop a synthesis of colloidal FASnI3 NCs with a concentration of Sn(IV) reduced to an insignificant level and to probe their intrinsic structural and optical properties. Intrinsic FASnI3 NCs exhibit unusually low absorption coefficients of 4 x 103 cm-1 at the first excitonic transition, a 190 meV increase of the band gap as compared to the bulk material, and a lack of excitonic resonances. These features are attributed to a highly disordered lattice, distinct from the bulk FASnI3 as supported by structural characterizations and first-principles calculations.

Automated summary

The search for nontoxic alternatives to lead halide perovskites (LHPs) has revealed that the properties of LHPs can only be achieved in their closest Sn(II) and Ge(II) analogues, despite their tendency to oxidize. A judicious choice of chemistry led to the development of formamidinium tin iodide (FASnI3) with a power conversion efficiency of 14.81% in photovoltaic devices. The synthesis of colloidal FASnI3 NCs with reduced Sn(IV) concentration allowed for the investigation of their intrinsic structural and optical properties, showing characteristics distinct from bulk FASnI3.