Time-Resolved Electrical Scanning Probe Microscopy of Layered Perovskites Reveals Spatial Variations in Photoinduced Ionic and Electronic Carrier Motion

Abstract

We study light-induced dynamics in thin films comprising Ruddlesden–Popper phases of the layered 2D perovskite (C4H9NH3)2PbI4. We probe ionic and electronic carrier dynamics using two complementary scanning probe methods, time-resolved G-mode Kelvin probe force microscopy and fast free time-resolved electrostatic force microscopy, as a function of position, time, and illumination. We show that the average surface photovoltage sign is dominated by the band bending at the buried perovskite–substrate interface. However, the film exhibits substantial variations in the spatial and temporal response of the photovoltage. Under illumination, the photovoltage equilibrates over hundreds of microseconds, a time scale associated with ionic motion and trapped electronic carriers. Surprisingly, we observe that the surface photovoltage of the 2D grain centers evolves more rapidly in time than at the grain boundaries. We propose that the slower evolution at grain boundaries is due to a combination of ion migration occurring between PbI4 planes, as well as electronic carriers traversing grain boundary traps, thereby changing the time-dependent band unbending at grain boundaries. These results provide a model for the photoinduced dynamics in 2D perovskites and are a useful basis for interpreting photovoltage dynamics on hybrid 2D/3D structures.

Sensitive Direct Converting X‐Ray Detectors Utilizing Crystalline CsPbBr3 Perovskite Films Fabricated via Scalable Melt Processing

“…Contrary, for negative Ga potentials the dark current is instable and is continuously increasing with time. The detailed mechanism for such slow in time instable dark current is controversial discussed in the literature . But it is commonly agreed that the dark current is of dominant ionic nature and the current instabilities are caused by ion migration of the mobile interstitial halogen (i.e., Br − ) and the associated positively charged vacancy.…”

Imaging Metal Halide Perovskites Material and Properties at the Nanoscale

“…Published by The Royal Society of Chemistry. (b) Fast free time-resolved electrostatic force microscopy (FF-tr-EFM) identifies the influence of illumination ON the rate and total charging of the BAPI 3 film at grain interiors and boundaries [73]. Reproduced with permission [73].…”

Extraction of Instantaneous Frequencies and Amplitudes in Nonstationary Time-Series Data

“…Such methods are useful for extracting dynamic information in nanoscale measurements of photovoltaic or ionic conducting systems via the effect of the force on the cantilever's resonance frequency. In a typical trEFM experiment, the desired outcome is extracting an unknown characteristic time constant τ , usually describing the time-dependent change in the electrostatic force gradient [40], [44], [48], [49]. As a test case for NFMD, we apply a series of perturbation forces to the cantilever of the form (6).…”

“…Therefore, the external perturbation (namely, τ ) is not directly learned, and the calibration curve will change based on experimental variables such as the cantilever being used with a different set of physical parameters like quality factor and spring constant. For more complicated systems where the relevant timescales are more than single exponential (modern photovoltaic systems with ionic transport and dielectric relaxation in battery materials), the lack of a defined model in this calibration curve can prove limiting [40], [51]. A chief advantage of the proposed NFMD approach is that the forcing function can be detected directly from experimental data via the instantaneous mean.…”

“…The method results in a superior time-frequency analysis to the HHT for nonstationary signals, and improves both temporal and spatial resolutions compared to the STFT. The NFMD can be applied to systems with fast-changing frequencies and abrupt changes in the signal mean, such as machine health monitoring [38], seismology [39], vibration-based imaging modalities [26], [40], neurochemical and biochemical signals [41], and photonic sensing [42], [43].…”