To further optimize the solar cell devices based on hybrid perovskites materials, understanding the major contributions of organic cations to ion migrauion and surface degradation is urgently needed. In this presentation, I show the tremendous impact of the structure of organic cations on halogen migration, vacancies, and interstitials, carrier lifetimes as well as surface degradation of perovskites using a combination of experimental and theoretical investigations. We found that Br- vacancies and interstitials have much lower formation energies and much higher density in MAPbBr3 compared to FAPbBr3 counterpart. The results also demonstrated clearly that the transition energy barrier for Br migration through vacancies within the bulk phase is much lower in MAPbBr3 than in FAPbBr3. Finally, we found that the rotation barrier of the organic cation is much higher in the case of FAPbBr3 than for MAPbBr3, which points to a much stronger H-bonding with Br- in the former case. Our results imply that incorporating organic cations with stronger H-bonding capacity, appropriate structure and more restricted motion inside the inorganic framework, is beneficial for suppressing ion migration and thus improving the performance of hybrid perovskite-based optoelectronic devices.