Bismuth drives the morphology and piezoresistivity of lead-free (TMSO)3Sn3xBi2(1−x)I9 halide perovskite thin films

The design of lead-free perovskite thin films with tunable optoelectronic properties is a hot topic in materials science, since it might enable the discovery of interesting properties outside the extensively explored field of photovoltaic solar cells. Herein, we report on the bending strain sensitivity of a lead-free hybrid organic–inorganic monodimensional iodide (TMSO)3Sn3xBi2(1−x)I9 (0 ≤ x ≤ 1) showing complete miscibility of Bi3+ and Sn2+, with TMSO being trimethylsulfoxonium. As previously shown in monodimensional haloplumbates, vacant cation sites are formed upon Bi3+ insertion, which in turn lead to the tunable shrinkage of the unit cell along the a axis. The effective substitution of Bi3+ and Sn2+ and their chemical state are investigated with EXAFS/XANES. Notably, Bi3+ also lowers the bandgap from 2.75 eV to 1.96 eV. Thin films prepared by spin coating on flexible ITO/PET supports are then used to assess the bending strain sensitivity as a function of composition, reaching an optimal value of the gauge factor (about 110 at 0.6% strain) at the highest Bi3+ concentration. The observed features are explained in terms of the surface morphology of the films as probed by AFM, highlighting the role of Bi3+. The effects on microstructural and electrical features after strain are further investigated by SEM and EIS, underpinning the key role of microcracks and delamination for triggering the observed responsivity to bending strain.