Band Gap Engineering in MASnBr₃ and CsSnBr₃ Perovskites: Mechanistic Insights through the Application of Pressure

Abstract

Here we report on the first structural and optical high-pressure investigation of MASnBr3 (MA = [CH3NH3]+) and CsSnBr3 halide perovskites. A massive red shift of 0.4 eV for MASnBr3 and 0.2 eV for CsSnBr3 is observed within 1.3 to 1.5 GPa from absorption spectroscopy, followed by a huge blue shift of 0.3 and 0.5 eV, respectively. Synchrotron powder diffraction allowed us to correlate the upturn in the optical properties trend (onset of blue shift) with structural phase transitions from cubic to orthorhombic in MASnBr3 and from tetragonal to monoclinic for CsSnBr3. Density functional theory calculations indicate a different underlying mechanism affecting the band gap evolution with pressure, a key role of metal-halide bond lengths for CsSnBr3 and cation orientation for MASnBr3, thus showing the impact of a different A-cation on the pressure response. Finally, the investigated phases, differently from the analogous Pb-based counterparts, are robust against amorphization showing defined diffraction up to the maximum pressure used in the experiments.

Origin of pressure-induced band gap tuning in tin halide perovskites

“…These results highlight the impact of different A-cations on the pressure response. 20 Triggered by these results, clearly correlating the observed trends to local lattice properties, we extended the HP study to FASnBr 3 by virtue of its peculiar structural features, and also to confirm the mechanistic picture we obtained on MASnBr 3 and CsSnBr 3 . 20,21 Previously, we reported that FASnBr 3 is characterized by a distorted structure with partly uncoupled SnBr 3 units, and that its 3D structure originates from a dynamical averaging of quasi-0D structures.…”

“…20 Triggered by these results, clearly correlating the observed trends to local lattice properties, we extended the HP study to FASnBr 3 by virtue of its peculiar structural features, and also to confirm the mechanistic picture we obtained on MASnBr 3 and CsSnBr 3 . 20,21 Previously, we reported that FASnBr 3 is characterized by a distorted structure with partly uncoupled SnBr 3 units, and that its 3D structure originates from a dynamical averaging of quasi-0D structures. 21 Such structural characteristics make FASnBr 3 a useful system to probe the extent of pressure effects on the electronic structure and to correlate these effects to local metal-halide bond distances.…”

“…Other than that, the structural high-pressure behaviour of FA and MA compounds is very similar. 20 Another peculiarity of the ASnBr 3 (A = MA, FA) system is the important splitting of the a and c axes, leading to a much more pronounced orthorhombic distortion than in the Pb and I counterparts, where the distortion is generally within 0.1 Å. 18,22 The unit cell volume trend was determined from XRD pattern refinements, both during compression and pressure release, as shown in Fig.…”

“…No evidence of amorphization was found in the XRD patterns, in analogy with other Sn-based MHPs, differing from the behavior of all the lead-containing phases. 3,4,20 After pressure release, the XRD pattern of FASnBr 3 is cubic and clearly crystalline (see Fig. S5 and S6, ESI † for raw data and refinement).…”

“…We recently conducted HP investigations of Sn-based systems, which have been much less extensively investigated with respect to their lead counterpart, despite the huge interest toward lead-free perovskites. 19,20 In particular, we reported the structural and optical properties of MASnBr 3 and CsSnBr 3 , where large red shifts of 0.4 eV for MASnBr 3 and 0.2 eV for CsSnBr 3 were observed by B1.5 GPa, followed by large blue shifts of 0.3 and 0.5 eV, respectively. 20 Using density functional theory (DFT) calculations, it was then possible to determine the underlying mechanism affecting the band gap evolution with pressure, the key role of metal-halide bond lengths for CsSnBr 3 and of cation orientation for MASnBr 3 .…”