A series of novel metal-free organic dyes containing the thiazolo[5,4-d]thiazole moiety were designed and synthesized for quasi-solid-state dye-sensitized solar cells (DSSCs). Different alkoxy chains were introduced into the electron donor part of the dye molecules for comparison. The optical, electrochemical, and photovoltaic properties for all sensitizers were systematically investigated. It was found that the sensitizers with the different alkoxy groups have similar photophysical and electrochemical properties, such as absorbance and energy levels, owing to their close chemical structures. However, the quasi-solid-state DSSCs based on the resulting sensitizers exhibit different performance parameters. The quasi-solid-state DSSC based on sensitizer with two octyloxy chains possessed the highest solar energy conversion efficiency of 5.10 % under standard AM 1.5G sunlight illumination without the use of coadsorbant agents.