Molecular modeling study of benzo dithiophene based polymers and organic nanoparticles for organic photovoltaic solar cells

摘要

Organic photovoltaic (OPV) solar cells, which utilize organic nanoparticles in conjugated polymers as photoactive layers for energy conversion, have generated significant scientific interest owing to the inexpensive fabrication process, light weight, structural flexibility and low operating temperature.1 However, the power conversion efficiency (PCE) of OPV solar cells is still limited to 10-12% and can be attributed to the high band gap of polymers, recombination of excitons before they can reach polymer-nanoparticle interfaces and low charge carrier mobility. Some of these factors that cause low PCE in turn depend on the specific morphology of the photoactive layer. The morphology, which is determined by the molecular arrangements of nanoparticles and polymers, is influenced by the molecular interactions between nanoparticles and polymers in the photoactive layer. Benzo dithiophene based polymers can potentially enhance the PCE of OPVs, since they have some advantages over other conventional polymers. These include their tunable band gap for sufficient photon absorption and high charge carrier mobility.2 Therefore, in recent times, poly[[4,8-bis[(2-ethylhexyl) oxy]benzo [l,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbo-nyl]thieno[3,4-b]thiophenediyl]] (PTB7) based OPVs have generated great scientific interest.3'4 However, to the best of our knowledge, the morphology of the photoactive layer containing nanoparticles and PTB7 is not well characterized. In this study, we performed molecular dynamics simulations of systems comprising the organic nanoparticle, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and PTB7 that mimic the structure of realistic photoactive layers after spin coating. We analyzed the agglomeration behavior of PCBM by determining sizes of the largest agglomerates. The agglomerate structure was analyzed by obtaining the radial distribution functions (RDFs), which describe the association of one group of atoms or molecules, such as PCBMs or PTB7s, with the same or other group of atoms/molecules in agglomerate. We also compared the agglomeration behavior of PCBM in PTB7 with that of PCBM in widely used polymer, P3HT. The presented results shed light into the molecular mechanisms that determine the specific morphologies of the photoactive layer for enhanced PCE.