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. (25th January 2018)

• Record Type: Journal Article
• Authors: Wang, Tonghui; Chen, Xian‐Kai; Ashokan, Ajith; Zheng, Zilong; Ravva, Mahesh Kumar; Brédas, Jean‐Luc
• Is Part Of: Advanced functional materials. Volume 28:Number 14(2018)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2018-01-25
• Subjects: blend morphology -- bulk heterojunction solar cells -- density functional theory calculations -- minor structural modifications -- molecular dynamics simulations; Materials -- Periodicals; Chemical vapor deposition -- Periodicals; 620.11
• DOI: 10.1002/adfm.201705868
• Languages: English
• ISSNs: 1616-301X

Abstract

Abstract: 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 C3 H7 side‐chain; these polymers are mixed with the phenyl‐C71 ‐butyric acid methyl ester (PC71 BM) 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 electron‐transfer features between neighboring PC71 BM molecules. Abstract : Minor structural modifications to the polymer backbone can change substantially the blend morphology and thus device performance of a bulk‐heterojunction solar cell. This computational work, based on a tight combination of molecular dynamics simulations and density functional theory calculations, elucidates the molecular‐scale impact that the structural changes to the polymer backbone have on the "local" morphology of the mixed regions.