Effect of Solvent Additives On the Self-Aggregating Properties of Phenyl-C61-Butyric Acid-Methyl-Ester (PCBM)

In organic photovoltaics, the morphology at the interface between the electron-rich (donor) and electron-deficient (acceptor) materials in a bulk hetero-junction (BHJ) film directly impacts the critical processes of exciton dissociation and charge separation; on the other hand, aggregates of the individual components influence the charge-carrier mobilities in the bulk-like areas of the active layer.  Solvent additives with high boiling points have been found to improve the efficiency of organic solar cells^{[1, 2]} by aiding in the formation of improved morphological or topological^[3-5] features within the active layer of the BHJ. Here, we aim to understand how solvent additives induce self-aggregation in solution, prior to spin-casting, through molecular dynamics simulations with force fields previously validated within our group. We evaluate properties like aggregation numbers, energies of aggregates of different sizes, cluster-size distributions in various solvents, and cluster shapes. By understanding the aggregation of PCBM clusters in chlorobenzene with and without solvent additives (e.g., 1-chloronaphthalene and 1-8-diiodooctane), we determine the role of solvent additives in the self-aggregation processes and PCBM-PCBM orientations. We also evaluate the energetic landscape of a PCBM dimer in multiple solvent / solvent additive environments to investigate how local molecular packing affects the ordering of small-to-large clusters. These studies shed light on the morphological nature of the PCBM domains, an important feature that affects the power conversion efficiency of organic solar cells.

 

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