Percolation Pathway Formation and Annihilation Mechanism in Next-Generation Photovoltaics and Memory Devices Probed by In Situ Conductive Atomic Force Microscope

Recently, significant advancements have been witnessed in the field of thin-film optoelectronic devices, attributed to the development of novel active layer materials including metal-halide perovskite quantum dots (PQDs) and small molecule electron acceptors with fused-ring structures. In device active layers, the presence of percolation pathways (PPs) can either positively or negatively impact device performances, depending on their compositions, sizes, and locations, which must be thoroughly probed and carefully controlled. Using in-situ conductive atomic force microscopy (C-AFM), we directly observe the formation, growth, and annihilation of PPs within the CsPbI₃ perovskite quantum dot (PQD) thin films induced by the bias and thermal-driven migration of ionic iodine vacancies. PPs are initially formed at the grain boundary, which gradually extend towards grain interiors at elevated bias due to different local activation energy for ion migrations. This allows us to fabricate novel CsPbI₃ PQD-based write-once-read-many-times memory devices with intrinsic ternary states.[1] In organic photovoltaics (OPVs), shunt pathways negatively impact fill factor and open-circuit voltage of devices. Using ex-situ C-AFM, we directly visualize the presence of those morphologically unfavorable PPs in shunted devices, allowing us to develop effective post-treatment strategies to passivate shunt pathways. This significantly improves the reproducibility of OPVs, especially for low-light applications. Additionally, the energy level offset between donor and acceptor materials in OPVs allows C-AFM to map the phase-separated structure in active layers at the nanometer scale. Through hole current mappings, we directly observe the transition of Y6 crystalline domains in PM6:Y6 blend films from spherically-like to network-like induced by different additives. These results correlate perfectly with the fractal dimension analysis using grazing-incidence small-angle scattering (GISAXS) measurements, allowing a thorough understanding of active layer morphology in both real and imaginary space.[2] Our works demonstrate the versatile applications of C-AFM for PP analysis in perovskite memory devices and organic photovoltaics, allowing effective optimization of their electrical properties.
[1] Xu, L. et al. ACS Applied Materials & Interfaces 2024, 16 (30), 39827-39834.
[2] Fu, Y.; Xu, L.; Li, Y. et al. Energy & Environmental Science 2024, 10.1039/D4EE02961E.