The operational stability of organic-inorganic halide perovskite based solar cells is a challenge for widespread commercial adoption. The mobility of ionic species is a key contributor to perovskite instability since ion migration can lead to unfavourable changes in the crystal lattice and ultimately destabilisation of the perovskite phase. Here we study the nanoscale early-stage degradation of mixed-halide mixed-cation perovskite films under operation-like conditions using electrical scanning probe microscopy to investigate the formation of surface nanograin defects. We identify the nanograins as lead iodide and study their formation in ambient and inert environments with various optical, thermal, and electrical stress conditions in order to elucidate the different underlying degradation mechanisms. We find that the intrinsic instability is related to the polycrystalline morphology, where electrical bias stress leads to the build-up of charge at grain boundaries and lateral space charge gradients that destabilise the local perovskite lattice facilitating escape of the organic cation. This mechanism is accelerated by enhanced ionic mobility under optical excitation. Our findings highlight the importance of inhibiting the formation of local charge imbalance, either through compositions preventing ionic redistribution or local grain boundary passivation, in order to extend operational stability in perovskite photovoltaics.

An electrochemical quartz crystal microbalance (EC-QCM) is a versatile gravimetric technique that allows for parallel characterization of mass deposition and electrochemical properties. Despite its broad applicability, simultaneous characterization of two electrodes remains challenging due to practical difficulties posed by the dampening from fixture parasitics and the dissipative medium. In this study, we present a dual electrochemical QCM (dual EC-QCM) that is employed in a three-electrode configuration to enable consequent monitoring of mass deposition and viscous loading on two crystals, the working electrode (WE) and the counter electrode (CE). A novel correction approach, along with a three standard complex impedance calibration, is employed to overcome the effect of dampening while keeping high spectral sensitivity. Separation of viscous loading and rigid mass deposition is achieved by robust characterization of the complex impedance at the resonance frequency. Validation of the presented system is done by cyclic voltammetry characterization of Ag underpotential deposition on gold. The results indicate mass deposition of 412.2 ng for the WE and 345.6 ng for the CE, reflecting a difference of the initially-present Ag adhered to the surface. We also performed higher harmonic measurements that further corroborate the sensitivity and reproducibility of the dual EC-QCM. The demonstrated approach is especially intriguing for electrochemical energy storage applications where mass detection with multiple electrodes is desired.

Development of future battery generations and quality control of current lithium-ion battery (LIB) systems require reliable techniques for the characterization of the complex dynamic processes in batteries. Electrochemical impedance spectroscopy (EIS) is an established tool for the electrochemical characterization of LIBs and the related ageing processes. Nevertheless, ensuring a reliable interpretation and validation of the impedance data remains challenging. Here we show how EIS complemented by microscopy data can be successfully used to follow battery ageing. We connect data from both methods in an electrochemical finite element model (FEM) to extract changes of intrinsic battery parameters, such as double layer capacitance, film resistance, exchange current density, and particle radii. Ageing mechanisms induced by cycling and temperature are investigated by EIS on commercial cells. Postmortem analysis of the anode and cathode electrodes is carried out to determine their initial micrometric and nanometric structure and to independently investigate morphological and electrical changes induced by ageing. While the obtained initial structural dimensions reduce drastically the number of adjustable parameters in the FEM model, which enables us to follow battery ageing processes noninvasively through the EIS spectra, the microscopy results of the aged cells prove independently the validity of the here presented approach.

Nanoscale investigations by scanning probe microscopy have provided major contributions to the rapid development of organic-inorganic halide perovskites (OIHP) as optoelectronic devices. Further improvement of device level properties requires a deeper understanding of the performance-limiting mechanisms such as ion migration, phase segregation and their effects on charge extraction both at the nano- and macroscale. Here, we have studied the dynamic electrical response of Cs0.05(FA0.83-MA0.17)0.95PbI3-xBrx perovskite structures by employing conventional and microsecond time-resolved Kelvin probe force microscopy (KPFM). Our results indicate strong negative charge carrier trapping upon illumination and very slow (>1s) relaxation of charges at the grain boundaries. The fast electronic recombination and transport dynamics on the microsecond scale probed by time-resolved KPFM show diffusion of charge carriers towards grain boundaries and indicate locally higher recombination rates due to intrinsic spatial heterogeneity. The nanoscale electrostatic effects revealed are summarized in a collective model for mixed-halide CsFAMA. Results on multilayer solar cell structures draw direct relations between nanoscale ionic transport, electron accumulation, recombination properties and the final device performance. Our findings extend the current understanding of complex charge carrier dynamics in stable multi-cation OIHP structures.