Imaging Energy Harvesting and Storage Systems at the Nanoscale

Abstract

Our scientific understanding of the nanoscale world is continuously growing ever since atomic force microscopy (AFM) has enabled us to “see” materials at this length scale. Beyond morphology, functional imaging is becoming standard practice as new AFM-based techniques are continuously extending its capabilities. Resolving material properties with high spatial accuracy is now extremely critical as future next-generation energy harvesting and storage systems are comprised of complex and intricate nanoscale features. Here, we review recent research discoveries that implemented AFM methods to measure and determine how the electrical, chemical, and/or optical properties influence the overall device behavior. We dedicate a portion of this Review to perovskite solar cells, which are of primary interest to photovoltaic research, and highlight the remarkable progress made toward understanding and controlling their instabilities. We conclude with a summary and outlook anticipating the most pressing materials-related challenges associated with solar cells and batteries and how that will likely be overcome in the near future by nanoimaging through AFM.

Correlated Electrical and Chemical Nanoscale Properties in Potassium‐Passivated, Triple‐Cation Perovskite Solar Cells

“…Briefly, originally theorized by Lord Kelvin, KPFM measures the work function difference between the surface and the AFM probe (see Figure S3 in the Supporting Information). [ 2a,15 ] Upon illumination of a light‐active material, the photovoltage is measured, and by subtracting a light KPFM map from a dark one, the quasi‐Fermi level splitting remains, which is related to the V oc of the semiconductor device. [ 3g ] Figure presents KPFM under dark and light conditions on both the Reference (Figure 2a–d) and KI‐treated (Figure 2e–h) half devices (without the top Au contact).…”

“…[ 22 ] Such datasets on halide perovskite materials, coupled with other local material property experiments such as EDX, [ 10 ] nano X‐ray diffraction [ 23 ] or electron diffraction imaging, [ 24 ] would extend correlative microscopy, linking structural‐chemical‐electrical properties, a “holy‐grail” approach that would unlock a deep understanding of the performance heterogeneities that currently limit the η in halide perovskites. [ 2b ]…”

“…Halide perovskite semiconductors are complex and promising materials to the optoelectronic research community, because they offer a low‐cost solution for a wide variety of applications [ 1 ] and they exhibit excellent performance in photovoltaic (PV) devices despite demonstrating performance heterogeneity at the nanoscale. [ 2 ] Yet, prior to deployment into the PV market, various questions about the unique electrical, thermal, and structural instabilities must be resolved. [ 3 ] Thus far, the most stable perovskite materials that maintain a power conversion efficiency (η) > 20% incorporate a triple‐cation (Cs, FA = formamidinium, CH 3 (NH 2 ) 2 + , and MA = methylammonium, CH 3 NH 3 + ) in the A‐site of the perovskite ABX 3 structure, Pb 2+ in the B‐site, and a mixture of halides in the X‐site (typically I or Br).…”

“…Thus, there is a need for correlative studies to link multiple nanoscale material properties. [ 2a,3f,12 ] In this work, we use the terminology correlation to establish the effect of one material property (e.g., chemical) on another (such as electrical). For example, recently, peak force infrared microscopy was implemented to relate local mechanical and chemical properties of a variety of materials, including perovskite crystals.…”

Machine Learning for Perovskites' Reap-Rest-Recovery Cycle

“…11 Because most perovskites present inhomogeneities at the nano-and microscale, microscopic techniques must be further developed to resolve the relationship between composition, morphology, optical response, and electrical behavior at the intragrain and intergrain length scales. [34][35][36][37][38] Figure 3 displays examples of how microscopic methods have been implemented to help elucidate the dynamic response of this emerging material. Through environmentally controlled micro-PL, the effect of ambient gas and vacuum was identified, showing that the presence of O 2 can lead to an order of magnitude increase in radiative recombination; however, as shown in Figure 3A, not all grains behave identically and the phenomenon is facet dependent.…”