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  • p/n-Polarity of thiophene oligomers in photovoltaic cells: Role of molecular vs supramolecular properties

p/n-Polarity of thiophene oligomers in photovoltaic cells: Role of molecular vs supramolecular properties

TanwisthaGhosh, AneshGopal, Akinori Saeki,* Shu Seki and Vijayakumar C. Nair*

Molecular and supramolecular properties play key roles on the optoelectronic properties and photovoltaic performances of organic materials. In this work, we show how a small change in the molecular structure affects such properties, which in turn control the intrinsic and fundamental property such as the p/n-polarity of organic semiconductors in bulk-heterojunction solar cells. Herein, we designed and synthesized two acceptor-donor-acceptor type semiconducting thiophene oligomers end-functionalized with oxazolone/isoxazolone derivatives (OT1 and OT2 respectively). The HOMO – LUMO energy levels of both derivatives were found to be positioned in such a way that they can act as electron acceptors to P3HT and electron donors to PCBM. However, OT1 functions as a donor (with PCBM) and OT2 as an acceptor (with P3HT) in BHJ photovoltaic cells, and the reverse roles either results no or poor performance of the cell. Detailed studies using UV-vis absorption and fluorescence spectroscopy, time-correlated single photon counting, UV-photoelectron spectroscopy, density functional theory calculations, X-ray diffraction, and thermal gravimetric analysis proved that both molecular and supramolecular properties were contributing equally but in a contrasting manner to the above mentioned observation. The obtained results were further validated by flash-photolysis time-resolved microwave conductivity studies which showed excellent correlation between the structure, property, and device performances of the materials. Physical Chemistry Chemical Physics, 2015, 17, 10630 – 10639.