Role of the interplay between spinodal decomposition and crystal growth in the morphological evolution of crystalline bulk heterojunctions

TL;DR

This paper introduces a phase-field simulation framework to study how spinodal decomposition and crystal growth interact, affecting the morphology and stability of crystalline phases in organic solar cell layers.

Contribution

It presents a novel simulation approach that simultaneously models liquid-liquid demixing and polycrystalline growth in binary systems, revealing their interplay during morphological evolution.

Findings

Spinodal decomposition triggers initial phase separation.

Crystalline growth quenches phase coarsening.

Higher crystallization rates lead to more structured, percolating crystals.

Abstract

The stability of organic solar cells 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 evolution of these structures during the lifetime of the cell remains poorly understood. In this paper, a phase-field simulation framework is proposed, handling liquid-liquid demixing and polycrystalline growth at the same time in order to investigate the evolution of crystalline immiscible binary systems. We find that initially, the nuclei trigger the spinodal decomposition, while the growing crystals quench the phase coarsening in the amorphous mixture. Conversely, the separated liquid phases guide the crystal growth along the domains of high concentration. It is also demonstrated that with a higher crystallization rate, in the final morphology, single crystals are more structured and form percolating pathways for each material with smaller lateral dimensions.