Evolution of ferroelectric domains in methylammonium lead iodide and correlation with the performance of perovskite solar cells

While more and more experimental evidence for the ferroelectricity of methylammonium lead iodide (MAPbI(3)) is being reported in the literature, the scientific community still controversially discusses whether or not the ferroelectric polarization has any influence on the performance of perovskite solar cells. In this work, the evolution of ferroelectric domains and their polarization orientation in MAPbI(3) thin films during thermal annealing are investigated using piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). Right after deposition and annealing for 5 s, small grains with non-uniform crystal orientation and polarization are formed. During the next 30-60 s, these small grains increase in diameter and large ferroelectric domains with out-of-plane polarization appear. In the annealing regime of several minutes to one hour, these large grains produce uniform domains with alternating in-plane polarization and (110) texture. The corresponding MAPbI(3) solar cells show a distinct performance enhancement and improved operational stability if the ferroelectric polarization is oriented in-plane. In contrast, solar cells with out-of-plane-polarized MAPbI(3) exhibit only moderate fill factors and reduced open-circuit voltages.