Study On Stability Of Organic-inorganic Lead Halide Perovskites

The organic-inorganic lead halide perovskite materials are used in solar cells and it's power conversion efficiency has increased from 3.8%?in 2009?to over 24.2%?in2019?.Such rapid improvement is attributed to the superior and unique photovoltaic properties of lead halide perovskites,such as high optical absorption coefficients,high electron and hole mobilities,super-long photo-generated carrier diffusion lengths and lifetimes,small exciton binding energies.These outstanding properties,along with the low-cost solution-processable techniques,make them ideal candidates for new-generation photovoltaic absorbers.However,the stability of hybrid lead iodine perovskite in humid environment has been a major obstacle to developing long-term photovoltaic devices.Thus,it is crucial to understanding the detailed degradation mechanism of lead iodine perovskite in moisture and thus improve its stability.The main topics of this thesis are listed below:i.We performed a theoretical study on the effect of water intercalation and electronic structures of the lead iodine perovskites by using first-principles calculations.we find that embedded water molecules will facilitate the decomposition of lead iodine perovskites.Alloying FAPbI₃ and CsPbI₃ to form mixed-cation lead iodine perovskites not only can optimize the tolerance factor to obtain better phase stability,but also improve the moisture stabilities of them.With the accumulation of water molecules in the perovskite lattice,the optical absorption spectra show a blue-shift and decreased intensity,and the moisture stabilities of lead iodine perovskites are further lowered.The iodine vacancy in lead iodine perovskites can facilitate the water molecule migration and thus reduce the stability of the perovskite structure and accelerate its decomposition.ii.All-inorganic perovskite has better humidity and thermal stability than organic perovskite,but its phase stability is poor.Experiments indicate that incorporation of Rubidium?Rb?and Potassium?K?into all-inorganic perovskite can improve its phase stability.On the other hand,recent research indicates two-dimensional?2D?perovskites are more stable than three-dimensional?3D?ones.Herein,on basis of first-principles calculations,we find that incorporation of Rb and K with an appropriate ratio could stabilize CsPbI₂Br perovskites considerably,in which the cooperative interactions between Rb/K,I and Br play an important role.Besides,we have investigated the free energy and electronic structures of CsPbI₃?cubic phase,yellow phase,orthorhombic phase?,MAPbI₃?cubic phase,orthorhombic phase,tetragonal phase?and FAPbI₃?cubic phase,yellow phase,tetragonal phase?.it is verified that 2D orthorhombic CsPbI₃ with a thickness of¹ nm exhibits outstanding stability even exceeding the non-perovskite,yellow structure and has a suitable band gap for solar cell applications.