Molecular design of triarylamine-based organic dyes for efficient dye-sensitized solar cells

Three donor–(π-spacer)–acceptor dyes, coded as DS-3, DS-5 and DS-6, were designed and synthesized for dye-sensitized solar cells (DSSCs). These dyes possess the same donor (triarylamine) and acceptor/anchoring group (2-cyanoacrylic acid), but the varied π-spacers consisting of a combination of thiophene and/or 3,4-ethylenedioxythiophene (EDOT) units. They were evaluated by photophysical and electrochemical measurements as well as in nanocrystalline TiO2-based DSSCs fabricated using them as light-harvesting sensitizers. The results showed that incorporation of the EDOT moiety in the dye molecules strongly affected the molar extinction coefficient and photocurrent response. Among the three dyes, the DS-5-based DSSCs afforded the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 80%, a short-circuit photocurrent density (Jsc) of 14.6 mA cm−2, an open-circuit photovoltage (Voc) of 0.59 V and a fill factor (FF) of 0.70, corresponding to an overall conversion efficiency of 6.03% under 100 mW cm−2 irradiation, which reached 77% with respect to that of an N3-based device fabricated and measured under similar conditions. Further, a method of molecular orbital calculations was employed for the insight into the geometrical, electronic and optical properties of these dyes.