Structure - Property Correlation in MA1-yFAyPbX3 (X = I, Br) Perovskites

Abstract

Hybrid Organic-Inorganic Lead Halide Perovskites (HOIP) are represented by APbX3, where A is the methylammonium cation (MA: (CH3NH3)+) or the formamidinium cation (FA: (CH(NH2)2)+), X is halide anion. HOIP have emerged as efficient photovoltaic materials and are promising candidates for developing solar cells with high photo-conversion efficiencies. Dielectric properties of the perovskite absorber layer play very important role, controlling several fundamental properties, such as the excitonic binding energy, defect screening and charge carrier separation, with important consequences on its use as a solar cell material. Keeping this in mind, we have investigated dielectric properties of the solid solution series (in Chapter-3), MA1-yFAyPbI3 for various compositions with 1  y  0 in chapter-3. Through temperature dependent single-crystal and powder XRD, we could establish a structural phase diagram for these solid solutions. Most interesting finding in this system is that, structure and dielectric property are strongly correlated. In Chapter-4, we have investigated the effect of FA-doping on surface defects and intrinsic defects. Generally, MAPbX3 samples show a sharp fall in PL intensity under vacuum, which arises due to removal of surface passivating ligands and the surface exposed defects quench the PL. Also, upon lowering the temperature, a new defect state emission appears below tetragonal-orthorhombic phase transition (162 K). By appropriate amount of FA-inclusion at MA-site, it was found that these two kinds of defects are suppressed effectively. In Chapter-5, we have successfully prepared MA1-yFAyPbBr3 series in their pure phase and from temperature dependent powder XRD and dielectric measurement, we could understand the relation between structure and dielectric property. It was observed that the intermediate compositions show dipolar glassiness approximately below 130 K, which could be ascribed to originating from the appearance of large-cubic phase (Im-3) at low temperatures. Cations are freezed in random orientations in large-cubic phase, which gives glassiness to the systems. MAPbBr3 and FAPbBr3 show different dielectric behaviours even though they have similar sequence of structural phase transitions. To understand the dynamical nature of these two cations (in Chapter-6), namely MA+ and FA+, we have performed quasi elastic neutron scattering (QENS) experiments, which is a suitable probe to detect the exact geometry of motions for light elements e.g. H, C and N. We could establish that MA+ exhibits 3-fold rotation around C-N axis and 4-fold jump rotation about C-N axis itself, in low-temperature orthorhombic and high-temperature tetragonal/cubic phase, respectively. In contrast, FA+ exhibits an isotropic motion throughout all the temperature range covering orthorhombic-tetragonal-cubic phases. Thus, through QENS experiments, we could realise the contrasting behaviours of these two cations. In Chapter-7, we report results of our structural investigation of the impact of high pressure on MA1-yFAyPbI3 with y = 0, 0.075, 0.125, 0.15, 0.2, 0.6 and 0.9. It is to be noted that, there is no consensus on structural phase transitions of MAPbI3 and FAPbI3 with pressure, mainly because of use of different pressure transmitting mediums and refining the XRD data with inappropriate space groups and crystallographic parameters. Hence, the aim of the chapter is to evaluate the proper crystallographic phases of these hybrid perovskites with pressure and correlate it with their physical properties. It was observed for all the compositions, the ambient phases e.g. cubic (Pm-3m) and/or tetragonal (I4/mcm) structures transform into the large-cubic (I-3m) in the pressure range 0.2-0.4 GPa and partial amorphization sets in above 3 and 3.5 GPa for low FA-content and high FA-content compositions, respectively. Much of the pressure phase space is dominated by tetragonal structure with P4/mbm space group.