5, University of California, Berkeley, Berkeley, California, United States
2, University College Dublin, Dublin, , Ireland
6, Institut Photovoltaique d'Ile de France, Palaiseau, , France
3, Colorado School of Mines, Golden, Colorado, United States
4, Colorado State University, Fort Collins, Colorado, United States
7, Oak Ridge National Laboratory, Knoxville, Tennessee, United States
8, The Pennsylvania State University, State College, Pennsylvania, United States
Methylammonium lead iodide exhibits spectacular photovoltaic performance with solar cell efficiencies over 19%. However, there remains significant controversy over the existence of ferroelectricity and its impact on the photovoltaic response in these materials. In this work, we definitively confirm ferroelectricity in methylammonium lead iodide using single crystals. This study employs d33 Berlincourt piezoelectric measurements, band excitation piezoresponse force microscopy with concurrent contact Kelvin probe force microscopy, and temperature dependent Rayleigh analysis. Large signal poling greater than 16 V/cm induced permanent macroscopic ferroelectric domains (up to 40 µm wide, and mm in length), which show preferential stabilization over a period of weeks and a distinguishable domain specific electronic response. The impact of the ferroelectric domains on the opto-electronic response was characterized through X-ray photoemission spectroscopy, scanning microwave impedance microscopy and electric force microscopy. The ability to control the ferroelectric response provides routes to increase both device stability and improve photovoltaic performance through domain engineering.