(178c) Molecular Simulations of Blends of Conjugated Polymers and Fullerene Derivatives for Bulk Heterojunction Organic Solar Cells

Organic solar cells consist of an active layer made of an electron donating species (e.g. conjugated polymer) and an electron accepting species (e.g. fullerene derivative). Despite their lower cost, lighter weight, and higher flexibility compared to conventional silicon solar cells, organic solar cells are not commercially viable because of their relatively low efficiencies. The efficiency of a solar cell is strongly dependent on the morphology of the donor and acceptor materials within the active layer. For higher efficiency, domains of donor and acceptor materials must have high interfacial area to facilitate charge separation, while being small enough that charge carriers can diffuse to the donor-acceptor interface before their energy is dissipated. Continuous domains through the active layer are also important so that charge carriers can reach their respective electrodes. The donor-acceptor morphology in the active layer is a function of the chemistry and architecture of the donor material (conjugated polymer) and its interactions with the fullerene derivatives. Commonly used conjugated polymers  are poly(3-hexylthiophene) (P3HT), poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT), poly(3,3’’’-didodecylquaterthiophene) (PQT-12), and poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophen-2-yl)thiophen-2-ylthiazolo[5,4-d]thiazole (PTzQT). Commonly used acceptors are fullerene derivatives, such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM).  We use coarse-grained Langevin dynamics simulations of blends of these conjugated polymers with varying chemistry and architecture, and fullerene derivatives with varying chemistry to understand phase separation and equilibrium morphology. A fundamental understanding of how conjugated polymer architecture and chemistry, fullerene derivative chemistry, and blend processing conditions affect the morphology of conjugated polymer/fullerene derivative blends will lead to valuable guidelines for designing higher efficiency organic solar cells.