Organic cation dynamics in formamidinium and methylbenzylammonium based metal halide perovskites: A quasielastic neutron scattering investigation

Licentiate thesis, 2024

Metal halide perovskites (MHPs) are currently accumulating considerable attention because of their photoluminescent properties and concomitant interest for application in both solar cells and light emitting diodes. However, the underlying physics and chemistry that give rise to these photoluminescent properties remain unclear. In recent years, there has been an increasing body of work indicating that the photoluminescent properties correlate with the dynamics of the organic cations, but the nature of dynamics is also not clear. Accordingly, this thesis focuses on investigations of the organic cation dynamics in a class of promising MHPs, namely the three-dimensional (3D) formamidinium tin halide perovskites FASnX3 [FA = HC(NH2)+, X = Cl, Br] and the vacancy-ordered variant FA2SnI6, and the zero- dimensional (0D) methylbenzylammonium manganese chloride perovskite MBAMnCl3 (MBA = C6H5CHCH3NH+). The main techniques used are quasielastic neutron scattering (QENS) and X-ray diffraction (XRD).

For FASnX3 (X = Cl, Br) and FA2SnI6, the results unravel the onset of localized rotational dynamics of FA cations present at around 150 K for FASnBr3 and FA2SnI6, and 250 K for FASnCl3. For all the materials, the dynamics show a strong temperature dependence. For FASnBr3 and FA2SnI6 at relatively low temperatures (T ≤200 K) the dynamics can be generally described as 2-fold and 4-fold jump-diffusion dynamics of the FA cations, whereas at higher temperatures (230–350 K), the dynamics of the FA cations can be considered as isotropic reorientation of the FA cation. The activation energy of the isotropic reorientation is found to be around 66 and 50 meV for FASnBr3 and FA2SnI6, respectively. The timescale and activation energy of the dynamics of the FA cation vary in the MHPs with different halide ions. A comparison to the QENS data on similar system FAPbX3 (X = Cl, Br, I) indicates a minor effect of the type of metal cation (Pb or Sn), but a stronger effect of the type of halide ion (Cl, Br, I) on the organic cation dynamics.

For the chiral perovskite MBAMnCl3, the results unravel the onset of uniaxial rotational diffusion of the methyl and ammonium, groups at 270 K, and the phenyl group at 300 K. Furthermore, the onset of multiaxial rotational diffusion of the MBA cations are found at 350 K. Additionally, the structure properties of MBAMnCl3·2H2O have been investig- ated through XRD. The single crystal XRD studies reveal a hitherto unknown phase of MBAMnCl3·C2H5OH, but powder XRD results show that the dominant structure in the sample is the MBAMnCl3·2H2O.

It is expected that these new results will be valuable for the development and optimization of design criteria for new MHP materials with desirable properties.