Spatial Fourier transforms and power spectral density were used to analyze digital TEM images of bulk heterojunction material based on poly(3-hexylthiophene) as donor and a soluble fullerene (PCBM) as acceptor. Optimized high-temperature annealing leads to well-defined nanoscale (< 20 nm) interpenetrating networks (see figure). The power spectral density provides a detailed and quantitative description of the bulk heterojunction material and the kinetics of the phase separation.