A thermodynamic and kinetic description of PCBM phase segregation and aggregation in P3HT:PCBM blends

• Crystallite-aggregate model of PCBM segregation in P3HT:PCBM films studied.
• Crystallite formation consistent with Flory-Huggins model.
• Aggregate shape governed by non-isotropic nanoscale PCBM crystallite flux.
• Thermodynamics controls nanocrystallite size; kinetics determines aggregate shape.

The kinetics and thermodynamics of PCBM phase segregation and aggregation in P3HT:PCBM blends has been studied. We develop a thermodynamic model for PCBM phase segregation in P3HT:PCBM blends which explains the formation of nanoscale crystallites which subsequently diffuse and coalesce into larger PCBM aggregates. We show that the formation of nanoscale crystallites during the film making process prevents spinodal decomposition of the P3HT:PCBM blends even at PCBM weight fractions above the spinodal decomposition boundary for the system. Finally, we demonstrate that the observed aggregate morphology can be understood in terms of a kinetic model based on the diffusional flux lines of PCBM crystallite which, in turn, govern the evolution of the macroscopic growth front.

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