Quaternary colloidal CuIn0.7Ga0.3S₂ nanocrystals were synthesized by hot injection method using oleylamine as single solvent. The structure, morphology and optical absorption properties of assynthesized CuIn0.7Ga0.3S₂ nanocrystals were characterized in detail using X-ray powder diffraction, transmission electron microscopy and UV-vis spectrometry. The as-fabricated CIGSSe thin film was obtained by spin-casting as-synthesized colloidal CuIn0.7Ga0.3S₂ nanocrystals after selenization. The classical pn heterojunction CIGSSe/CdS thin film solar cells displayed the 6.18% power conversion efficiency under AM1.5G illumination.