PEDOTs–PCnBMs polymer–fullerene BHJ solar cells: Quantum mechanical calculations of photovoltaic and photophysical properties

Abstract

The band gaps, geometries, electronic structures, polarizabilities, hyperpolarizabilities, dipole moments, UV–visible, IR, 1H and 13C NMR of poly (3,4-ethylenedioxythiophene) (PEDOT) and its fluorine derivatives—Cn and [6,6] phenyl Cn butyric acid methyl esters (PCnBMs) bulk heterojunction (BHJ) solar cells are investigated by using density functional theory (DFT) and time-dependant density functional theory (TD-DFT) calculations. We aim to optimize the performance of these solar cells by altering the frontier orbital energy gaps of polymers and fullerenes. This was done by functionalizing the polymers backbones with electron withdrawing fluorines step-by step, and by adding phenyl butyric acid methyl ester to the buckminster C60 and C70 fullerenes. The theoretical data were compared with the available experimental data. Based on the strategy of reducing the bandgaps, the trifluoro derivative of PEDOT–buckminster C70 fullerene blend (out of 24 blends) was found to be the best candidate for power conversion efficiency (PCE). The addition of fluorine atoms to the polymer backbone is effective in lowering both HOMOs and LUMOs. Conformational analysis confirms that the coplanar propertyof thiophene polymers are not destroyed by the incorporation of fluorine atoms. While strong localization of HOMOs occurs on the PEDOT donor subunits, strong delocaliztion of LUMOs occurs on the bridges between the subunits, proving the flow of the electron density along the polymer backbone. The polarizability term βxxx and dipole moment μx increase with increasing the power conversion efficiency. The absorption band that corresponds to the maximum coefficient of the PEDOT in vacuum is marginally shifted under the effect of incorporating fluorines. Vibrational analysis shows that functionalizing PEDOT leads to a more conjugated polymer backbone and enhances the charge transfer to an acceptor. The power conversion efficiency of the solar cell increases with increasing the chemical shifts of the constituent C, H, and O atoms.