Formation of crystalline bulk heterojunctions in organic solar cells: insights from phase-field simulations

TL;DR

This paper uses advanced phase-field simulations to understand how various physical processes during drying influence the morphology of organic solar cell layers, aiming to optimize their performance.

Contribution

It introduces a comprehensive simulation framework that integrates evaporation, crystallization, and phase separation to study BHJ formation in organic solar cells.

Findings

Simulation results align with experimental in-situ data

Morphology depends on process parameters and material properties

Final BHJ structure is a mix of crystalline and amorphous domains

Abstract

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.