Chemical tuning of hybrid perovskites for solar-driven clean energy technologies

The 29th of July 2019 was the Earth overshoot day, day in which we ended the available resources of the year and started consuming more than the planet can produce. From that day on, we are using the resources allocated to future generations. The use of fossil fuels, exercised by man since the First Industrial Revolution, still plays a key role in satisfying the growing energy demand. However, the limited availability of exhaustible primary energy sources and the long-term negative effects on the environment due to the greenhouse effect, make the development of new strategies urgent, so that other primary energy sources can be used effectively: renewable ones. Among all the renewable energy sources available, solar energy is the easiest candidate to provide all the energy required by our growing population. Scientific research on this subject has produced very interesting results and hybrid perovskite solar cells are the proof of this. Hybrid perovskites are one of the most promising photovoltaic material: their use in photovoltaic cells increased up to 25% in a decade, making them the fastest growing photovoltaic technology in history. According to the Golden Triangle rule, in addition to efficiency, stability and cost are the two parameters that must be considered to evaluate the possible commercialization. Efficiency and costs are now competitive with those of traditional silicon cells, so the interest has turned to the stability and toxicity of lead. To meet these two major challenges, the best solution is to concentrate research efforts on both the development of new materials and their characterization. In sight of this, in this thesis the attention focused on the development and characterization of new materials with the main goal of deeply investigate how the perovskites’ physical-chemical properties can be modulated by “ad hoc” substitutions in order gain stability. To do this, different materials chemistry strategies have been tackled. The first part of the work deals with a new system in which the most common methyl ammonium (MA) cation has been partially replaced with formammidinium (FA), which leads to more symmetrical perovskites with lower bandgap and better stability. Aware of the high toxicity of lead, in the second part of the work, attention has turned to its replacement with less harmful elements. We first took care of the metallic substitution: the FAPb1−xSnxBr3 system showed an unusual evolution of the band gap values, which proved to be the result of a dynamic average of a strongly distorted structure. The study then continued on two organic cation substitutions in tin-based systems: FA1-xMAxSnBr3 e MA1-xDMAxPbBr3. In addition to the research on bulk perovskites, in the third and last part, the study of chemical properties tuning by chemical substitution, in particular anionic substitution, was extended to nano crystalline and two-dimensional perovskites. In both cases, the most evident result is certainly the remarkable modulation of the optical properties. The synthesis methods used are also noteworthy: in both cases, was employed a completely new approach, fast and easy, which does not require precursors difficult to synthesize or post-synthetic treatments. The results of this work show that the greater stability of tin-based perovskites, combined with an extremely low toxicity, makes them the most interesting topic for the scientific community to devote the greatest efforts in the near future.