(621a) Coarse-Grained Computer Simulations of Polymer/Fullerene Bulk Heterojunctions for Photovoltaic Applications

Organic photovoltaics (OPV), which includes polymer-based solar cells (PSCs), shows promise as a cheap alternative to current silicon-based photovoltaics, but efficiencies must improve if OPV is to become commercially viable. Typical PSCs use a mixture of a light-absorbing conducting polymer as the electron donor and a fullerene derivative as the electron acceptor in the solar cell's photo-active layer. The donor and acceptor are generally mixed together to produce a bicontinuous percolating network called a bulk heterojunction (BHJ), thereby allowing optimization of both light absorption, which favors thicker devices (>100 nm), and charge-carrier generation and transport, which requires that photogenerated excitons be no further than an exciton diffusion length (~10 nm) from a donor-acceptor interface. The delicate balance between maximizing interfacial area and maintaining percolating pathways for charge transport to the electrodes means that PSC performance is sensitive to the BHJ morphology. But prediction of the active-layer microstructure based on the constituent electron-donor and electron-acceptor phases and the processing conditions remains challenging. Nano-scale morphological information is also often difficult to obtain experimentally. On the other hand, atomistic computer simulations are only feasible for studying systems not much larger than an exciton diffusion length. We overcome this hurdle by developing a coarse-grained (CG) simulation model, in which collections of atoms from an atomistic model are mapped onto a smaller number of "superatoms", of mixtures of the widely used conducting polymer poly(3-hexylthiophene) (P3HT) and fullerene C₆₀. By comparing the results of atomistic and CG simulations, we demonstrate that the model, parametrized at one temperature and two mixture compositions, accurately reproduces the system structure at other points of the phase diagram. We then use the CG model to characterize the structure and dynamic evolution of the BHJ microstructure as a function of polymer:fullerene mole fraction and polymer chain length for systems approaching the scale of photovoltaic devices.