  Optimizing the properties of the mosaic morphology of bulk heterojunction
(BHJ) organic photovoltaics (OPV) is not only challenging technologically but
also intriguing from the mechanistic point of view. Among the recent
breakthroughs is the identification and utilization of a three-phase
(donor/mixed/acceptor) BHJ, where the (intermediate) mixed-phase can inhibit
morphological changes, such as phase separation. Using a mean-field approach,
we reveal and distinguish, between generic mechanisms that alter through
transverse instabilities the evolution of stripes: the bending (zigzag mode)
and the pinching (cross-roll mode) of the donor/acceptor domains. The results
are summarized in a parameter plane spanned by the mixing energy and
illumination, and show that donor-acceptor mixtures with higher mixing energy
are more likely to develop pinching under charge-flux boundary conditions. The
latter is notorious as it leads to the formation of disconnected domains and
hence to loss of charge flux. We believe that these results provide a
qualitative road-map for BHJ optimization, using mixed-phase composition and
therefore, an essential step toward long-lasting OPV. More broadly, the results
are also of relevance to study the coexistence of multiple-phase domains in
material science, such as in ion-intercalated rechargeable batteries.
