Qiannan Zhang1 Ankur Solanki1 Mingjie Li1 David Giovanni1 Kaushik Parida2 Thomas Jansen3 Maxim Pshenichnikov3 Tze Chien Sum1

1, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, , Singapore
2, School of Material Science and Engineering, Nanyang Technological University, Singapore, , Singapore
3, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh, , Netherlands

Ruddlesden-Popper (RP) halide perovskites are promising optoelectronic materials due to their high-performance in PV applications and excellent ambient stability. Their ferroelectric properties provide an exciting opportunity to further improve PV performance based on the nature of their layered structure. Polar domains in ferroelectric materials play an important role in separating electrons and holes. However, the relation between structure and function which lead to the RP ferroelectrics remains unknown. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP halide perovskite (PEA)2(MA)n-1PbnI3n+1 by varying the number of inorganic layers and tuning the correlation length of the ferroelectric order. Firstly, the non-centrosymmetric nature of RP thin films is confirmed by nonlinear optics. Secondly, switchable polarity leading to ferroelectric properties is validated by piezo force microscopy and polarization-electric field measurements microscopically and macroscopically. Then, the origin of ferroelectricity in the RP halide perovskites is investigated by MD simulations. Finally, dark current-voltage hysteresis phenomenon implies potential issues for light harvesting and light emitting applications. Importantly, our findings reveal an exciting approach to engineer tunable RP ferroelectrics, which could pave the way to new functionalities for perovskite optoelectronics.