New organic dyes with varied arylamine donors as effective co-sensitizers for ruthenium complex N719 in dye sensitized solar cells

Three new organic dyes (TD1-TD3) featured with donor-pi-acceptor architecture are synthesized and applied to dye-sensitized solar cells (DSSCs). In these sensitizers, an alkyl oligothiophene pi-spacer is used to link the 2-cyanoacrylic acid acceptor with varied arylamines donors, i.e. carbazole (TD1), phenothiazine (TD2) or triphenylamine (TD3). The effects of different donor units on the photophysical, electrochemical, and photovoltaic properties of sensitizers are investigated in detail. They all show intensive UV-vis absorption around 400 nm with rod-like molecular structures. The co-sensitizations of TD1-TD3 with ruthenium complex N719 are further evaluated. Consequently, the co-sensitized DSSCs demonstrate superior photovoltaic performances with power conversion efficiencies (PCEs) ranging from 7.60% to 8.02% in relation to the cell with N719 (PCE = 7.29%) alone, which is mainly ascribed to the higher short-circuit photocurrents (J(sc)) in combination with larger open-circuit voltages (V-oc). Moreover, the origins of these improvements are examined by incident photon to current efficiency (IPCE) spectra and electrochemical impedance spectroscopy (EIS) investigations. The J(sc) is improved because TD1-TD3 could compensate for the photocurrent loss induced by I-/I-3(-); whereas, the V-oc is enhanced owing to the fact that the electron recombination process is retarded upon co-sensitization.