Lead halide perovskites show slow (from seconds to minutes) and fast (milliseconds to submicroseconds) charge dynamics. We use scanning Kelvin probe microscopy and dissipation microscopy to probe these charge dynamics in a thin film of CsPbBr3. We demonstrate the existence of a light-intensity-dependent τfast in CsPbBr3 that exhibits a slow, activated, intensity-independent recovery in the dark. The observed τfast, while highly light-dependent, remained essentially unchanged when the light was turned off, taking 10 ± 2 s to relax at room temperature. The data presented here show direct evidence that the slow and fast charge dynamics ubiquitously seen in lead-halide perovskites have a common origin related to a highly activated charge redistribution.

A new way to measure the rate of light-induced charge generation in organic solar cell films with a nanometer-scale probe.

Films of (FA 0.79 MA 0.16 Cs 0.05 ) 0.97 Pb(I 0.84 Br 0.16 ) 2.97 were grown over TiO 2 , SnO 2 , ITO, and NiO. Film conductivity was interrogated by measuring the inphase and out-of-phase forces acting between the film and a charged microcantilever. We followed the films' conductivity vs. time, frequency, light intensity, and temperature (233 to 312 K). Perovskite conductivity was high and light-independent over ITO and NiO. Over TiO 2 and SnO 2 , the conductivity was low in the dark, increased with light intensity, and persisted for 10's of seconds after the light was removed. At elevated temperature over TiO 2 , the rate of conductivity recovery in the dark showed an activated temperature dependence (E a = 0.58 eV). Surprisingly, the light-induced conductivity over TiO 2 and SnO 2 relaxed essentially instantaneously at low temperature. We use a transmission-line model for mixed ionic-electronic conductors to show that the measurements presented are sensitive to the sum of electronic and ionic conductivities. We rationalize the seemingly incongruous observations using the idea that holes, introduced either by equilibration with the substrate or via optical irradiation, create iodide vacancies.

Scanning probe microscopy is often extended beyond simple topographic imaging to study electrical forces and sample properties, with the most widely used experiment being frequency-modulated Kelvin probe force microscopy. The equations commonly used to interpret this frequency-modulated experiment, however, rely on two hidden assumptions. The first assumption is that the tip charge oscillates in phase with the cantilever motion to keep the tip voltage constant. The second assumption is that any changes in the tip-sample interaction happen slowly. Starting from an electro-mechanical model of the cantilever-sample interaction, we use Lagrangian mechanics to derive coupled equations of motion for the cantilever position and charge. We solve these equations analytically using perturbation theory, and, for verification, numerically. This general approach rigorously describes scanned probe experiments even in the case when the usual assumptions of fast tip charging and slowly changing samples properties are violated. We develop a Magnus-expansion approximation to illustrate how abrupt changes in the tip-sample interaction cause abrupt changes in the cantilever amplitude and phase. We show that feedback-free time-resolved electric force microscopy cannot uniquely determine sub-cycle photocapacitance dynamics. We then use first-order perturbation theory to relate cantilever frequency shift and dissipation to the sample impedance even when the tip charge oscillates out of phase with the cantilever motion. Analogous to the treatment of impedance spectroscopy in electrochemistry, we apply this approximation to determine the cantilever frequency shift and dissipation for an arbitrary sample impedance in both local dielectric spectroscopy and broadband local dielectric spectroscopy experiments. The general approaches we develop provide a path forward for rigorously modeling the coupled motion of the cantilever position and charge in the wide range of electrical scanned probe microscopy experiments where the hidden assumptions of the conventional equations are violated or inapplicable. arXiv:1807.01219v2 [cond-mat.mes-hall]