Optical properties of organic-inorganic lead halide perovskites

One of the most important challenges for our society is to make more efficient devices in order to mitigate the rapidly increasing energy demand. In recent years a class of materials have attracted a great deal of attention in this perspective. Thanks to unique properties, including high absorption coefficient, broad absorption spectrum, high charge-carrier mobilities, long diffusion lengths, tunable band gap and low costs of fabrication, organic-inorganic lead halide perovskites have become very promising candidates for a new generation of potentially printable and efficient optoelectronic devices. The thesis aims to investigate some of the issues of optical properties of organic-inorganic lead halide perovskites. First, the nature of the band gap. Nevertheless for a long time it was considered direct, recent reports have proposed that the presence of heavy atoms in the crystal leads to Rashba-type effects. This crucial issue is addressed thanks to a study of the radiative recombination rates and photoluminescence decay dynamics as a function of temperature, the direct nature of band gap is demonstrated. Second, the exciton binding energy. Its knowledge is crucial for optoelectronic devices advances. Different measurement methods led to quite various each other values, even on the most widely studied materials, such as methylammonium lead iodide and methylammonium lead bromide. Thanks to a f-sum rule for integrated UV-vis absorption spectra, that allows to circumvent the pitfalls of least-squares fitting procedures, more accurate values of exciton binding energy is calculated and its dependence on temperature for these materials is shown. Third, the excitons formation and dissociation. Even though its understanding is of fundamental importance, many aspects of the processes of formation of an exciton population in perovskites are still elusive and some questions are still to be answered. The differential photoluminescence technique presented is able to provide a comprehensive picture of the kinetics of excitons in hybrid perovskites and to unveil the spectroscopic sign of excitons.