A computational proof toward correlation between the theoretical chemical concept of electrophilicity index for the acceptors of C60 and C70 fullerene derivatives with the open-circuit voltage of polymer-fullerene solar cells

A quantum chemical computational study has been performed at B3LYP/6-311G(d, p) level to investigate electronic structures of the C60 and C70 fullerene derivatives, as electron acceptor in bulk heterojunction solar cells, seeking ways to improve their efficiency in corresponding photovoltaic devices. The key point of the present survey is to achieve a significant linear correlation between the electrophilicity index calculated for the fullerene derivatives and the corresponding open-circuit voltage of the photovoltaic device. This index is a prominent feature in determining tendency of the fullerene derivative towards acquiring an additional electron from the adjacent electron-rich donor molecule, and also its resistance to give the electron back transfer. The results reveal that replacing butyric acid ester by propionic acid ester in the aliphatic part or increasing the donating strength of the substituent in the esteric part will cause an increase in electrophilicity. Considering the derivatives of C70 generally yields higher electrophilicity than those of C60 derivatives. Also for indene-C60 derivatives a significant increase in the electrophilicity is observed by increasing the number of adducts, leading to raise in the VOC values.

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► Linear correlation between the electrophilicity index and VOC.
► Replacing butyric acid ester by propionic one in the aliphatic part causes a higher electrophilicity.
► Our C70 derivatives appear higher electrophilicity than those of C60 derivatives.
► Increasing the number of adducts causes a higher electrophilicity.
► Linear correlation between the electron affinity and VOC.