Counting Defects is the Pathway to Understand Degradation in Lead Halide Perovskites

Stability in perovskites is often connected to the presence of electronic defects, but rarely their chemical nature, reactivity and mobility are considered. Only the development of a powerful, quality and durability evaluation tool that allows direct probing of these reactive defect densities under <i>operando</i> conditions (i.e. away from equilibrium condictions) is able to correlate their chemical, electronic and physical makeup to irreversible degration processes in lead halide perovskite films.<br/>In this work, an electrochemical methodology based on a simple half-cell device stack using a "peel and stick" electrolyte top contact that is easily combined with various existing spectroscopic and surface characterization techniques introduces a new measurement approach to study stability across atomic-to-molecular-to device length scales. Systematic modulation of the potential and the introduction of specific redox probes to support electron and hole injection enables us to draw the most complete electronic band structure of a perovskite and by extension any semiconductor. By reducing charging currents and improving energy resolution, unprecedented detection limits are reached when probing surface defects close to both valence and conduction band with the additional benefit of identifying their (electro)chemical reactivity. Combined with X-ray scattering and photoelectron techniques, we have recently discovered that controlled polarization allows the collection of bias-dependent structural data to gain insight into interfacial and bulk degradation mechanisms during real-time operation.<br/>Overall, this probe has demonstrated to overcome challenges for defect quantification in printable electronic materials and to promote <i>operando </i>characterization of perovskites, organic semiconductors, quantum dots, material blends and device stacks, where the removable solid electrolyte functions as the “top contact”. This type of advanced electrochemical characterization platform will prove to be crucial for the quality control of low cost (opto)electronic materials and device stacks as well as the improvement of their stability and durability in order to realize industry-scalable technologies in the field of photovoltaics, charge storage systems, photoelectrochemical cells, etc.