Multiscale Modeling of Small Molecule Organic Photovoltaic Cells

Currently the power conversion efficiency of the polymer-based, solution-processed BHJ OPVs can already reach over 10[IMAGE png]; however, despite of these recent successes, polymer solar cells suffer from problems such as batch-to-batch variations in solubility, molecular weight, and polydispersity, which lead to inconsistency in both processing conditions and performance and hinders their commercialization progresses. An alternative electron donor for OPVs is small molecule. Solution-processed small molecule (SM) BHJ solar cells comprise well-defined molecules with much higher molecular precision than synthetic polymers, thereby minimizing the batch-to-batch variations. Recently, the small molecule BHJ OPVs can reach efficiencies beyond 8[IMAGE png]; however, the correlations of device fabrication protocols, BHJ nanomorphologies, and resultant charge transport properties remain elusive. In this project,we construct a multiscale molecular simulation platform to fill the gap between device fabrication conditions and performance.

In this project, we presented a coarse-grained (CG) ellipsoid molecular simulation model to systematically study the nanomorphology of SMDPPEH/PCBM/solvent ternary blend during solution-processing and blade-coating processes. By coarse-graining each SMDPPEH molecule (containing 140 atoms) into one ellipsoid bead, we were able to mimic the strong pi-pi interaction between neighboring SMDPPEH molecules, while retaining the computational efficiency for large-scale simulations. The interaction potential between ellipsoid beads were constructed based on the Gay-Berne type ellipsoidal potential, and were fitted based on atomistic trajectories from all-atom molecular simulations of smaller system of interests. With the significantly reduced overall system degrees of freedom, and the optimized molecular simulation codes on GPU workstation, we were able to go well beyond the limitation of conventional all-atom molecular simulations with system size reaching almost 100 nm with molecular details. This CG molecular simulation platform is versatile, and can be readily extended to other novel small molecule electron donor/acceptor materials to investigate the BHJ nanomorphologies during solution processing, blade coating, and even vacuum co-deposition processes, thereby providing insights helping experimental groups optimize device fabricating protocols for superior device performance.