**4. Schematic structures of bulk-heterojunction film morphology**

The morphological studies discussed above highlight the importance of phase separation between donor and acceptor, and reveal a schematic film structures for polymer-based bulk-heterojunction solar cells, as shown in Fig. 14..(Hoppe et al., 2006; Huang et al., 2010; Peumans et al., 2003)

In the top Fig. 14 (a), the percolated pathways for electrons and holes is created allowing them to reach the respective electrodes. In Fig. 14 b the situation for an enclosed PCBM cluster is shown: here electrons and holes will recombine, since percolation is insufficient.

The center Fig. 14 show that the lower surface energy of P3HT, relative to PCBM, provides the driving force for the interconcentration gradient observed in both the rapidly (a) and slowly (b) grown films. The film prepared through a rapidly grown process leads to an extremely homogeneous blends. A greater number of percolating pathways are formed in slow grown films.

Furthermore, the effect of annealing on the interface morphology of a mixed-layer device was modeled using a cellular model, as shown in Fig. 14 (bottom) for different temperatures. Annealing temperatures has been shown to crucially influence the morphology of the mixed-layer device, while the modeled morphology resemble experimentally measured devices.
