Electron Transport in Thin Films of Polymer and Small-Molecule Acceptors Visualized by Conductive Atomic Force Microscopy

In this study, spatial variations in electron transport in thin films of perylene diimide (PDI)- and naphthalene diimide (NDI)-based acceptors and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) were characterized by conductive atomic force microscopy. Electron flow images at the nanometer scale revealed that electron transport was spatially inhomogeneous in the PDI- and NDI-based polymer acceptor films, with the formation of good conducting regions with sizes of several tens of nanometers to 200 nm. In contrast, it was relatively uniform in the PDI-based small-molecule acceptor and PCBM films. The local electron currents flowing through the good conducting region of the NDI-based polymer film were higher in magnitude than those flowing in the PCBM film as a consequence of the local ordering of the polymer chains. This suggests that the electron mobility can be further improved via morphological optimizations. Conversely, electron transport was inherently inhibited in the PDI-based polymer and small-molecule acceptor films because of a loose interchain (intermolecular) π–π stacking because of the twisted intrachain (intramolecular) structures. Nanoscale observation of the local electron currents provides insights into the electron transport-related performance limitations of the nonfullerene acceptor filmsa feature that cannot be estimated via macroscopic current–voltage measurements.