The efficiency and stability of organic solar cells (OSC) is strongly affected by the morphology of the photoactive layers, whose separated crystalline and/or amorphous phases are kinetically quenched far from their thermodynamic equilibrium during the production process. The formation of these structures and their evolution during the lifetime of the cell remains poorly understood.

In this talk, we show how the bulk-heterojunction (BHJ) morphology formation upon drying and its evolution under thermal loading of OSC can be simulated, using a recently developed phase-field (PF) model.[1] For the first time, this allows to investigate the interplay between all the potentially relevant physical processes (nucleation, growth, grain coarsening, amorphous phase separation, composition-dependent kinetic properties), within a single coherent framework. It is shown how the morphology evolution depends on the thermodynamic and kinetic properties of the donor acceptor blend as well as on the processing conditions. [2-4]^.The possible phase separation pathways and associated morphologies are discussed in detail. The approach is applied to several real material system. In all cases, the simulation results are in very nice agreement with the experimental findings.

Moreover, an original approach allowing to calculate morphology-dependent JV-curves of OSC is presented. It is based on a morphological analysis of the BHJ nanostructure, which allows to calculate morphology-aware descriptors for light absorption, exciton dissociation, charge recombination and mobilities. These descriptors are fed into a standard 1D drift-diffusion model to calculate the JV-curve. Opposite to state-of-the-art approaches based on computationally demanding Master equation, Monte-Carlo or 2D/3D drift-diffusion simulations, morphology-dependent JV-curves for a known morphology can be calculated within less than 1 minute. Using morphology evolution under thermal loading obtained from PF simulations, the performance increase of as-cast films upon thermal annealing, and the performance drop during cell lifetime related to intrinsic stability is calculated. Finally, it is shown how the relationship between BHJ evolution and performance evolution can be fully elucidated.

Overall, this contribution illustrates how advanced simulations can help understanding OSC efficiency and intrinsic stability, thus accelerating the development of 3^rd generation photovoltaics and contribute to the energy transition.