Efficient Structural Modification of Triphenylamine-Based Organic Dyes for Dye-Sensitized Solar Cells

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A series of new organic D−π−A dyes, coded as DS-1, DS-2, DS-3, and DS-4, was designed, synthesized, and characterized by 1 H NMR, 13 C NMR, infrared spectroscopy, mass spectrometry, and elemental analysis. These dyes consist of a di( p -tolyl)phenylamine moiety as an electron donor, a cyanoacetic acid moiety as an electron acceptor/anchoring group, and different types of conducting thiophene units as electron spacers to bridge the donor and acceptor. It was found that both the use of di( p -tolyl)phenylamine donor and the variation of electron spacers in the D−π−A dyes played an essential role in modifying and/or tuning physical properties of organic dyes. These dyes were developed as sensitizers for the application in dye-sensitized TiO 2 nanocrystalline solar cells (DSSCs), and their photophysical and electrochemical properties were investigated. The DSSCs based on the dyes gave good performance in terms of incident photon-to-current conversion efficiency (IPCE) in the range of 400−700 nm. A solar-energy-to-electricity conversion efficiency (η) of 7.00% was obtained with the DSSC based on 5-[[2-[ p -(di- p -tolylamino)]styryl]thiophene-yl]thiophene-2-cyanoacrylic acid (DS-2) under simulated AM 1.5 G irradiation (100 mW/cm 2 ): short-circuit current density ( J sc ) of 15.3 mA cm −2; open-circuit voltage ( V oc ) of 0.633 V; fill factor (FF) of 0.725. The density functional theory (DFT) calculation suggests that the electron-transfer distribution moves from the donor unit to the acceptor under light irradiation, which means efficient intramolecular charge transfer.