Understanding structural and electronic properties of dithienyl benzothiadiazole and its complex with C70

• Conformational and electronic properties of DTBT and DTBT/C70 are investigated.
• Trans-planar conformation is found to have the lowest energy.
• The lowest-energy conformation is found to be stabilized by hydrogen bonding.
• Equilibrium separation between DTBT and C70 is found to be ∼3.2 Å.

Understanding the bulk heterojunction (BHJ) morphology of dithienyl benzothiadiazole (DTBT)-based conjugated polymers, the most widely used third-generation electron-donors in BHJ-based Organic photovoltaic (OPV) devices, is the current focus of the OPV community. However, there are still debates [J. Am. Chem. Soc. 2013, 135, 1806–1815 and J. Am. Chem. Soc. 2012, 134, 3498–3507] on the most stable conformation of DTBT and is the main focus of the present study. Herein, we report the conformational and electronic properties of DTBT performing detailed first-principle calculations at the molecular level. We found that the energy difference between the two debated DTBT conformations is about 1.3 kT, regardless of methyl or hexyl substituted thiophenes. This energy difference is mainly due to the extent of intramolecular hydrogen bonding. We further report that the low-energy DTBT conformation has a low energy gap, low equilibrium separation (∼3.2 Å) with C70 and proper orbital energy offset, thereby suggesting DTBT-based polymers to be efficient electron donors for OPV devices.

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