Title: Coarse-Grained Molecular Dynamics Simulations of Thermal Annealing of P3HT:PCBM Bulk Heterojunctions for Organic Photovoltaic Applications

Presenter: Jan Michael Carrillo

Affiliation: Oak Ridge National Laboratory

Abstract: We performed coarse-grained molecular dynamics simulations of the thermal annealing of blends of poly(3-hexylthiophene) (P3HT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM) at different weight fractions fm and degrees of polymerization N of the P3HT polymer chains at 423 K based on the force field by Lee, Pao and Chu Lee et al., Energy Environ. Sci., 2011, 4, 4124-4132. The simulations were performed up to 400 ns in terms of “simulation time” which translates to 0.27 ms in “real time” and one order of magnitude longer than previous MD simulations. The simulation sizes are comparable to the thickness of the active layer of a bulk heterojunction device which is around 100 nm. We observed that the thermal annealing process proceeds in two stages which is characterized by the fast evolution of morphological quantities such as the interfacial area to volume ratio ? and the location of the scattering peak qmax and then followed by the slowing down of the evolution of these quantities. The simulation shows two phases which are the loosely packed PCBM domains and the P3HT matrix with dissolved PCBM and is reflective of the nature of the force field used which implies that PCBM is immiscible to P3HT. We characterized microscopic properties such as the packing fraction of the P3HT and PCBM beads ?, mean square end-to-end distance of the P3HT chain and the polymer bond orientational order parameter ?Si?; and morphological properties of the P3HT:PCBM blends such as the interface area to volume ratio ?, pore size of the P3HT network ?DPSD?, domain size d-spacing d and Debye correlation function correlation length ?. For the case of P3HT:PCBM blend systems with fm = 0.5, it is more advantageous for the performance of the device as the degree of polymerization of the P3HT chain N is increased because this translated into a decrease in the volume fraction of dissolve PCBM fv,PCMB, increase in the surface interface area to volume ratio ? , decrease in the d-spacing d and correlation length ? . For the systems with different fm and N = 100, there are several quantities that are non-monotonic with respect to fm. These include the average local polymeric bond orientational order parameter ?Si?, P3HT chain mean square end-to-end distance , characteristic time tR of the location of the peak of S(q) or qmax and the correlation length ? . This is a clear indication that there is an optimal value of fm and for N = 100 that translates into outstanding device performance and it is in the range between fm equal to 0.5 to 0.67.