The Effects Of Compositional Engineering On The Structure And Properties Of Perovskite Materials

In recent years,the rapid improvement of photovoltaic performance and stability of perovskite solar cells has been achieved based on the compositional engineering of the material itself,especially the compositional engineering of mixed cations and mixed halides.Therefore,it is crucial to understand the effects of mixed cations and mixed halides on the structure and properties of perovskite materials.Through high-throughput synthesis and characterization,this thesis mainly reveals the effects of compositional engineering on the structure and properties of perovskite,providing vital insights for the design of durable perovskite materials.（1）The crystallographic and optical properties of the formamidinium-（FA⁺）and cesium-based lead halide perovskites(i.e.Cs_yFA_1-yPb(Br_xI_1-x)₃)were systematically explored.Using grazing-incidence wide-angle X-ray scattering（GIWAXS）technique,the crystal structures and phases of 49 perovskites in the Cs_yFA_1-yPb(Br_xI_1-x)₃ compositional space were identified qualitatively.Higher tolerance factors lead to a cubic or tetragonal structure,whereas lower tolerance factors result in an orthorhombic structure with low symmetry.While some correlation exists between the tolerance factor and structure,the tolerance factor does not provide a holistic understanding of whether or not a perovskite structure will fully form.An empirical equation for the optical bandgap of Cs_yFA_1-yPb(Br_xI_1-x)₃as a function of the composition is quantitatively determined,which contributes to tailoring bandgaps that are suitable for various optoelectronic applications.By screening I-rich compositions,our results show that Cs_1/6FA_5/6PbI₃ and Cs_1/6FA_5/6Pb Br_1/2I_5/2 deliver the highest efficiency and long-term stability.（2）The performance of perovskite solar cell based on Cs_1/6FA_5/6PbI₃ absorber yields 20.29%efficiency due to its suitable bandgap,whereas its PCE significantly reduces upon exposure to humidity.Cs_1/6FA_5/6Pb Br_1/2I_5/2introduced Br^-exhibits better humidity stability than Cs_1/6FA_5/6PbI₃.The internal mechanism of the enhanced humidity stability in Cs_1/6FA_5/6Pb Br_1/2I_5/2 is explored by in-situ GIWAXS.The results show that Cs_1/6FA_5/6Pb Br_1/2I_5/2with the incorporation of Br^-mainly degrades into the hexagonal 4H phase under humidity,which is the reason for the slowdown of its degradation kinetics upon humidity.Furthermore,heterogeneous Cs⁺and FA⁺distributions are found under humidity,and the Cs-rich clusters are photoinactive and current-blocking,which is the main cause for the loss of photovoltaic performance of both upon humidity.Our findings provide a microscopic perspective on the performance loss of devices under humidity and highlight the crucial role of bromine in improving durability of perovskite.（3）Preferred crystal orientation has been shown to be correlated with charge transport and ion migration in perovskite materials.It is paramount to explore the role of ionic size-effects of cations and halides on the crystal orientation of perovskite films for the design and optimization of perovskite materials.The GIWAXS results reveal that small ions（such as Br^-and MA⁺）drive to highly oriented thin films.Solar cells show an enhancement of charge carrier transport and mobilities for a higher orientation.However,the highly ordered channels also facilitate ionic movement,which is detrimental to the device performance.In addition,the larger the cation,a re-orientation occurs upon humidity.For the smaller Br^-,intrinsically ordered films present high structural stability,and the orientation is reorganized towards a higher order under humidity.Nevertheless,for the larger ion I^-,humidity does not change the disordered crystals but does destabilize the perovskite structure leading to degradation.Br-based perovskites showed a higher preferred crystal orientation and proved to be more robust and stable to humidity.