New metal-free sensitizers (IK 3-6), based on the thieno[2,3-b]indole ring system, bearing various aliphatic substituents at the nitrogen atom (electron-donating part), several thiophene units (π-bridge linker) and 2-cyanoacrylic acid (the electron-accepting and anchoring group) have been synthesized for application in dye-sensitized solar cells (DSSCs). The relationship between the IK dye structure and efficiency of the corresponding DSSC has been elucidated. Power conversion efficiency (PCE) up to 6.3% (short-circuit photocurrent density (JSC) 19.0 mA cm-2, open-circuit voltage (VOC) 0.59 V, and fill factor (FF) 56.4%) were obtained for the DSSC, based on 2-cyano-3-{5-[8-(2-ethylhexyl)-8H-thieno[2,3-b]indol-2-yl]thiophen-2-yl}acrylic acid (IK 3), which proved to be a highly synthetic available compound, under simulated AM 1.5 G irradiation (100 mW cm-2), thus indicating that thieno[2,3-b]indole-based organic dyes are perspective candidates for DSSCs.