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 filmsa feature that cannot be estimated
via macroscopic current–voltage measurements.