Nanoscale characterization of organic-inorganic perovskites with AFM-IR

Abstract

Organic-inorganic, hybrid perovskite technology is regarded as a contender for the next generation of lighting and light harvesting technologies. Perovskite LEDs (PeLEDs) and perovskite photovoltaics (PePV) are cheap, light, flexible, efficient and easy to process and manufacture. However, currently, perovskite devices suffer from rapid degradation through various failure modes (humidity, UV aging, heat,..). Pe technology offers several avenues to overcome these issues by adjusting the chemical composition (metal cation, halogen anion, organic cation, matrix components and stabilizers) of devices. Research into PeLED/PePV synthesis und fabrication has been going on for two decades. To understand these failure modes and the effects of changes to the composition on the device performance macroscale chemical characterization is insufficient. Instead, characterization at the nanoscale is required to understand and optimize PeLEDs/PePVs. In our current research we study novel perovskite materials by combining scanning probe microscopy to achieve nanoscale resolution with mid-IR spectroscopy for chemical identification (AFM-IR). The working principle is based on leveraging a local, short-lived photo-thermal expansion by absorption of infrared light - induced by a pulsed, tunable EC-QCL source. This excitation then is measured by the cantilever probe and the oscillation amplitude is directly proportional to the absorption and thus an absorption spectrum is generated. Using this nearfield IR spectroscopy technique, we study novel perovskite materials to understand their phase composition at nanometer spatial resolution. In using AFM-IR to gather mid-IR absorption spectra from sample areas smaller than a few tens of nanometers we can see chemical changes within a single perovskite crystallite. Using hyperspectral nanoscale chemical imaging we can collect images of the distribution of perovskites and stabilizers and thus provide information required to design the next generation of lighting and light harvesting devices.