Bulk Heterojunction Solar Cells: Impact of Minor Structural Modifications to the Polymer Backbone on the Polymer- Fullerene Mixing and Packing and on the Fullerene- Fullerene Connecting Network

摘要

The morphology of the active layer of a bulk heterojunction solar cell, made of a blend of an electron-donating polymer and an electron-accepting fullerene derivative, is known to play a determining role in device performance. Here, a combination of molecular dynamics simulations and long-range corrected density functional theory calculations is used to elucidate the molecular-scale effects that even minor structural changes to the polymer backbone can have on the "local" morphology; this study focuses on the extent of polymer-fullerene mixing, on their packing, and on the characteristics of the fullerene-fullerene connecting network in the mixed regions, aspects that are difficult to access experimentally. Three representative polymer donors are investigated: (i) poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'''-di(2-octyldodecyl)-2,2';5',2 '';5 '',2'''-quaterthiophen-5,5'''-diyl)] (PffBT4T-2OD); (ii) poly-[(2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'-di(2-octyldodecyl)2,2';5',2 '';5 '',2'''-quaterthiophen-5,5'''-diyl)] (PBT4T-2OD), where the fluorine atoms in the benzothiadiazole moieties of PffBT4T-2OD are replaced with hydrogen atoms; and (iii) poly[(2,2'-bithiophene)-alt-(4,7-bis((2-decyltetradecyl)-thiophen-2-yl)-5,6-difluoro-2-propyl-2H-benzo[d][1,2,3]triazole)] (PT2-FTAZ), where the sulfur atoms in the benzothiadiazole moieties of PffBT4T-2OD are replaced with nitrogen atoms carrying a linear C3H7 side-chain; these polymers are mixed with the phenyl-C-71-butyric acid methyl ester (PC71BM) acceptor. This study also discusses the nature of the charge-transfer electronic states appearing at the donor-acceptor interfaces, the electronic couplings relevant for the charge-recombination process, and the electrontransfer features between neighboring PC71 BM molecules.