Two new D−A−π-spacer−A organic dyes, KM-10 and KM-11, containing a benzothiadiazole unit in a π-spacer and a cyanoacrylic acid as an acceptor have been synthesized and tested as sensitizers in dye-sensitized solar cells. Structural variations of the donor moiety, i.e., π-extension of the diphenylamine electron-donating groups, gave rise to different photovoltaic efficiencies –7.1% for KM-10 and 8% for KM-11– despite having comparable absorption properties. A detailed investigation, including transient photo- current and photovoltage decay measurement, transient absorption spectroscopy, and quantum chemical methods, provided important conclusions about the nature of the substitution on the photovoltaic properties of dyes.