Optoelectronic devices based on hybrid organic-inorganic perovskites (HOIP) have exhibited a meteoric rise in the last years. Current large area solar cells (156 mm × 156 mm) with power conversion efficiencies surpassing 20% in combination with their low production costs make them suitable for the next generation of photovoltaic devices [1]. So far, the photo-physical parameters such as carrier mobility, charge diffusion and recombination dynamics of perovskites have been subjected to an intense scrutiny using several methods [2]. However, a consensus concerning the underlying photophysical properties and charge transport in these materials has still not been reached as all measurements have been performed around temperatures relevant for applications (mostly room temperature) where the thermal energy is high hence making it practically impossible to distinguish between different scattering mechanisms.

To address these issues, we investigated single crystals of different perovskite groups using temperature-dependent photoluminescence, magneto-transport under steady-state illumination [3] and thermal expansion between 4.2 K and room temperature [4]. The latter techniques are beyond conventionally employed characterization techniques. We demonstrate that magneto-transport under steady-state illumination is a powerful tool to unravel the fundamental charge carrier dynamics and underlying carrier scattering mechanisms at different temperatures and show that in contrast to MAPbI₃ and MAPbBr₃ in which acoustic phonon scattering is the dominant scattering mechanism, optical phonon scattering plays a significant role in CsPbBr₃.

By means of a high-resolution capacitive dilatometer, we scrutinize the phase transitions in a temperature range between 4.2 and 280 K. Depending on the material, we observe abrupt changes in the linear thermal expansion ΔL(T)/L that we associate with structural phase transitions. The overall background in linear thermal expansion is positive and varies smoothly within the specific phases. The linear thermal expansion observed for FAPbI₃ and FAPbBr₃ is by far more complex showing several features attributed to several orientation of the organic group within different symmetry groups where is under investigation via single crystal XRD.

While FAPbBr₃ exhibits several sharp so far unidentified but most likely tetragonal phases separated by sharp transitions between 120 and 160 K, FAPbI₃ is distinct from all other materials due to two wide regions of negative thermal expansion below 173 and 50 K, i.e. the material expands upon decreasing the temperature. We also show that thermal expansion is a technique to determine the crystal quality [4].

Furthermore, these phase transitions also appear in transport under steady-state illumination. We present the results on MAPbBr₃ (see figure 1 as an example), FAPbBr₃ and MAPbI₃ [4]. The latter exhibits strong increase in resistance of the overall temperature dependence. Results are corroborated with temperature-dependent photoluminescence to unravel the electron-phonon coupling constants and the dominant scattering mechanisms for each of the above mentioned crystals.

These combined techniques can be applied to novel HOIP materials and devices with a different architecture such as the double or triple perovskites where the structure is by far more complicated.

Figure 1. Structural phases of cubic, the tetragonal phases and the low temperature orthorhombic for MAPbBr₃ in agreement with transport (R_xx) and normalized photoluminescence as a function of the temperature. The abrupt changes indicate the phase transitions (n is the carrier concentration).