Improved polarization retention in epitaxial BiFeO3 thin films induced by strain relaxation

•This study focuses on investigating the impact of lattice mismatch strain (LMS) on the polarization retention of ferroelectric BiFeO3 thin films.•The research findings reveal that LMS plays a crucial role in driving the formation of preferred polarization.•The study suggests that the mechanism behind the reduced polarization retention is intimately connected with the built-in field induced by oxygen vacancies-relevant space charge alignment.•The results propose a straightforward approach to enhance the retention of switched polarization by charge injection of electrons to fully screen the built-in field, paving the way for the development of reliable ferroelectric memories with excellent retention properties.