  Suppressing charge recombination is key for organic solar cells to become
commercial reality. However, there is still no conclusive picture of how
recombination losses are influenced by the complex nanoscale morphology. Here,
new insight is provided by revisiting the P3HT:PCBM blend, which is still one
of the best performers regarding reduced recombination. By changing small
details in the annealing procedure, two model morphologies were prepared that
vary in phase separation, molecular order and phase purity, as revealed by
electron tomography and optical spectroscopy. Both systems behave very
similarly with respect to charge generation and transport, but differ
significantly in bimolecular recombination. Only the system containing P3HT
aggregates of high crystalline quality and purity is found to achieve
exceptionally low recombination rates. The high-quality aggregates support
charge delocalization, which assists the re-dissociation of interfacial
charge-transfer states formed upon the encounter of free carriers. For devices
with the optimized morphology, an exceptional long hole diffusion length is
found, which allows them to work as Shockley-type solar cells even in thick
junctions of 300 nm. In contrast, the encounter rate and the size of the
phase-separated domains appears to be less important.
