This paper demonstrates a straightforward strategy to improve the TiO2-based solar cell performances employing highly qualified copper-indium-sulfide (CIS) ternary colloids. The CIS colloids in different In/Cu ratios of 1-4 capped with thioglycolic acid (TGA) were prepared in "green" water media. And, subsequently the CIS colloidal solutions were treated with microwave (MW)-irradiation at different temperatures (80-200 degrees C) for 5 min. The preferable solar cell employing CIS colloids in In/Cu ratio of 2 treated at 160 degrees C showed high photo-conversion efficiency (PCE) yielding with 6.12% (short-circuit current: 14.0 mA/cm(2), open-circuit voltage: 0.91 V and fill factor: 48.0%). To our best knowledge, this value is the highest among the solar cells employing CIS colloids prepared in water media. It was also confirmed that the MW-irradiation improved the light absorption of the CIS colloids as well as induced photo-electrochemical cathodic shift of the Fermi levels for the CIS-TiO2 photoelectrode due to the reduction of defective sites involved in the CIS colloids.