The performance of a bulk-heterojunction organic solar cell critically depends on the morphology of the active layer. The solution temperature-dependent aggregation characteristics of a series of polymer donors have been recently exploited as an effective protocol for morphology control in high-efficiency devices. Here, we use an approach combining molecular dynamics simulations and long-range corrected density functional theory calculations to investigate the impact of solution temperature-dependent aggregation on the polymer solid-state packing and electronic properties. We consider two representative polymer systems: (i) PffBT4T-2OD (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)]), and (ii) PBT4T-2OD (poly[(2,1,3-benzothiadiazole-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl)-2.2′;5′,2′′;5′′,2′′′-quarterthiophen-5,5′′′-diyl)]), where the fluorine atoms on the benzothiadiazole moieties of PffBT4T-2OD are replaced with hydrogen atoms. We find that both temperature-dependent aggregation and the presence of fluorine atoms are important in determining the nature of the solid-state packing and the electronic properties in the polymer phases. Our results are consistent with the experimental data that show that PffBT4T-2OD aggregates at lower temperatures and leads to higher OPV efficiency.
Ashokan A, Wang T, Ravva MK, Brédas J-L (2018) Impact of solution temperature-dependent aggregation on the solid-state packing and electronic properties of polymers for organic photovoltaics. Journal of Materials Chemistry C 6: 13162–13170. Available: http://dx.doi.org/10.1039/c8tc05378b.
This work was supported by the Office of Naval Research in the framework of Award No. N-00014-17-1-2208, as well as by the Georgia Institute of Technology. The work at KAUST was supported internally in the framework of the KAUST Collaborative Research Grant program. The authors acknowledge the Supercomputing Laboratory at KAUST and the PACE team at the Georgia Institute of Technology for providing computational and storage resources. The authors thank Dr Veaceslav Coropceanu and Dr Simil Thomas for stimulating discussions. This article is dedicated to Professor Martin Bryce, an outstanding synthetic organic chemist and a pioneer of the field of organic functional materials.