**4. Conclusions**

Recent detailed studies of the phase behavior of conjugated polymer:fullerene blends have begun to provide important information for understanding the property-processing rela‐ tionships in organic solar cell blends. Until these studies conventional wisdom suggested that the P3HT:PCBM system was a simple two phase bulk heterojunction with well-defined interfaces between regions of pure P3HT and pure PCBM. The latest results clearly demon‐ strate that this assumption is not correct, but instead unequivocally show that BHJ materials are more complex systems incorporating regions of crystalline P3HT, PCBM, and inter‐ mixed regions of amorphous P3HT and PCBM. Upon annealing there is considerable inter‐ diffusion of PCBM into the amorphous P3HT. It has also been shown that in the case of thin BHJ films (~100 nm) such as those used in OPVs, the substrate's surface energy can have a crucial role on the final morphology.

Furthermore, the kinetics as manifested in the processing routes and conditions can also play a dominant role in the observed structure/morphology. For all these reasons it is very difficult to predict the structure of the BHJ since apparently the same process used by differ‐ ent groups can end to with different results.

Whilst significant insight has been derived in understanding the behavior of a limited num‐ ber of systems, with P3HT:PCBM the most widely studied system by far, there is still no predictive understanding in even this system. If technologically meaningful device efficiencies are to be achieved significant additional work must be undertaken to derive predictive capability for these complex BHJ systems. This will ultimately require a global effort of complementary experimental measurements combined with theoretical and computational modeling.
