A defining property of hybrid organic inorganic perovskites (HOIP) is the ability to conduct both ionic and electronic charge carriers. This interplay between conduction channels offers a rich physical and chemical landscape, with potential opportunities for many existing and emerging technologies from optoelectronic to ionic memory and energy storage devices.  Like all aspects of nanoscience and nanotechnology, electrochemical functionalities often emerge on the scale of individual micron and nanometer scale defects and defect assemblies, functional interfaces and artificial elements. The characteristic length scale and relevant energy scales of these phenomena would seem to benefit from a scanning probe microscopy (SPM) approach.

In the first part of my talk I will discuss investigations of ferroelectricity in HOIP using interferometric detection sensing-(IDS) piezoresponse force microscopy (PFM). IDS-PFM enables intrinsically calibrated measurements of a materials ferro/piezo-response, as well as the ability to decouple (unwanted) cantilever motion from tip-sample motion of interest. Using IDS-PFM, combined with multimodal chemical imaging, we demonstrate that twin domains in MAPbI₃ are associated with chemical segregation.[1] We see no detectable (<0.4 pm/V) evidence of piezo or ferroelectric response in these films and describe how the observed twin domain structures are due to changes in viscoelastic properties. I will describe how traditional PFM can be misinterpreted in the presence of cantilever crosstalk. I will further demonstrate how the commonly used switching spectroscopy PFM technique is ill equipped to study ferroelectric behavior in the presence of ionic motion and cantilever coupling.[2] I hope this aspect of my talk acts as a cautionary tale for scientists who intend to use PFM as a tool to study ferroic properties in mixed ionic electronic conductors.

In the second part of my talk I will discuss opportunities to uncouple multiscale charge dynamics by time resolved Kelvin probe force microscopy (KPFM). With this aim in mind we develop and implement a multiscale multimodal mapping approach (M³) combining multitemporal (10²-10^-6 S) macroscopic and nanoscale time resolved KPFM measurements on a single device. I will use this approach to successfully decouple hysteretic device behavior in a series of ternary blended HOIP (i.e. (FA)_x(MA)_1-x(Cs)_0.5 PbI₃ where x=0,15,75%) devices. Backed by MD simulations, our results demonstrate that insertion of the larger FA cation leads to a more dynamic lattice and a greater propensity for dipole reorientation and ion migration. Overall, we demonstrate that ion migration plays only a small role in the underlying hysteresis, and that the inclusion of the larger FA cation into the organic lattice results in the activation of an ionic subsystem which is not observed in the parent MAPbI₃ composition. In this aspect of my talk I hope to demonstrate that the M³ approach enables the ability to (near-) simultaneously link device characteristics with local charge dynamics, which may begin to alleviate controversies regarding the exact nature of different phenomena including hysteresis, ion migration, ferroelectricity, charge carrier trapping, redox processes, etc.

[1] Liu, Yongtao, et al. "Chemical nature of ferroelastic twin domains in CH 3 NH 3 PbI 3 perovskite." Nature materials 17.11 (2018): 1013.

[2] Collins, Liam, et al. "Quantitative Electromechanical Atomic Force Microscopy." arXiv preprint arXiv:1904.06776 (2019). (just accepted ACS Nano)