  The performance of organic solar cells strongly depends on the bulk
heterojunction (BHJ) morphology of the photoactive layer. This BHJ forms during
the drying of the wet-deposited solution, because of physical processes such as
crystallization and/or liquid liquid phase separation (LLPS). However, the
process-structure relationship remains insufficiently understood. In this work,
a recently developed, coupled phase field fluid mechanics framework is used to
simulate the BHJ formation upon drying. For the first time, this allows to
investigate the interplay between all the relevant physical processes
(evaporation, crystal nucleation and growth, liquid demixing,
composition-dependent kinetic properties), within a single coherent theoretical
framework. Simulations for the model system P3HT-PCBM are presented. The
comparison with previously reported in-situ characterization of the drying
structure is very convincing: the morphology formation pathways,
crystallization kinetics, and final morphology are in line with experimental
results. The final BHJ morphology is a subtle mixture of pure crystalline donor
and acceptor phases, pure and mixed amorphous domains, which depends on the
process parameters and material properties. The expected benefit of such an
approach is to identify physical design rules for ink formulation and
processing conditions to optimize the cell performance. It could be applied to
recent organic material systems in the future.
