D-A-π-A Featured Sensitizers Bearing Phthalimide and Benzotriazole as Auxiliary Acceptor: Effect on Absorption and Charge Recombination Dynamics in Dye-Sensitized Solar Cells

Two organic D-A-pi-A sensitizers LS-2 and WS-5 containing N-octyl substituted phthalimide and benzotriazole as auxiliary electron withdrawing units with similar dimension and structure architecture were systematically studied, focusing on photophysical and electrochemical as well as photovoltaic properties in nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs). Interestingly, with similar five-member benzo-heterocycles, the two auxiliary acceptors of phthalimide and benzotriazole play exactly different roles in absorption and intramolecular charge transfer: (i) in contrast with WS-5 delocalized throughout the entire chromophore, the HOMO orbital of LS-2 is mainly located at the donor part due to the twist conformation with the existence of two carbonyl groups in phthalimide; (ii) the dihedral angles of D-A plane and A-pi plane in LS-2 further suggest that the incorporation of phthalimide moiety results in curvature of electron delocalization over the whole molecule, in agreement with its blue-shifted, relatively narrow absorption spectra and low photocurrent density; (iii) in contrast with the beneficial charge transfer of benzotriazole in WS-5, the phthalimide unit in LS-2 plays an oppositely negative contribution to the charge transfer, that is, blocking intramolecular electron transfer (ICT) from donor to acceptor to some extent; and (iv) in electrochemical impedance spectroscopy, the incorporated benzotriazole unit enhances electron lifetime by 18.6-fold, the phthalimide only increases electron lifetime by 5.0-fold. Without coadsorption of chenodeoxylic acid (CDCA), the DSSCs based on WS-5 exhibited a promising maximum conversion efficiency (eta) of 8.38% with significant enhancement in all photovoltaic parameters (J(SC) = 15.79 mA cm(-2), V-OC = 791 mV, ff = 0.67). In contrast, with the very similar D-A-pi-A feature changing the additional acceptor from benzotriazole to phthalimide unit, the photovoltaic efficiency based on LS-2 was only 5.11%, decreased by 39%, with less efficient photovoltaic parameters (J(SC) = 10.06 mA cm(-2), V-OC = 748 mV, ff = 0.68). Therefore, our results demonstrate that it is essential to choose proper subsidiary withdrawing unit in D-A-pi-A sensitizer configuration for DSSCs.