  Direct observation of organic molecular nanocrystals and their evolution
using electron microscopy is extremely challenging, due to their radiation
sensitivity and complex structure. Here, we introduce 4D-scanning confocal
electron diffraction (4D-SCED), which enables direct in situ observation of
bulk heterojunction (BHJ) thin films. 4D-SCED combines confocal electron
microscopy with a pixelated detector to record focused spot-like diffraction
patterns with high angular resolution, using an order of magnitude lower dose
than previous methods. We apply it to study an active layer in organic solar
cells, namely DRCN5T:PC$_{71}$BM BHJ thin films. Structural details of DRCN5T
nano-crystallites oriented both in- and out-of-plane are imaged at ~5 nm
resolution and dose budget of ~5 e$^-$/A$^2$. We use in situ annealing to
observe the growth of the donor crystals, evolution of the crystal orientation,
and progressive enrichment of PC$_{71}$BM at interfaces. This highly
dose-efficient method opens new possibilities for studying beam sensitive soft
materials.
