In this research, light and heavy Bi-doped SnO 2 thin lms were prepared on glass substrates by spray pyrolysis technique. The effect of heavy doped-Bi on the structural, morphological, electrical, photothermo-electrical, optical properties of SnO 2 lms has been investigated.The Bi/Sn atomic ratios (x = [Bi/Sn]) were varied from 0 to 0.30 in the spray solution.X-ray diffraction analysis showed the formation of SnO 2 tetragonal rutile structure in low doped deposited lms and amorphous structure for heavy Bi-doped SnO 2 . Also, the SnO 2 -Bi 2 O 3 binary thin lms were formed for x= [Bi/Sn] =0.05. Scanning electron microscopy (SEM) images indicated that nanostructure of the condensed lms has a rectangular-particle growth toward particle-spherical growth. The Hall effect measurements have shown n-type conductivity in all deposited lms. The lowest sheet resistance of 39.3 MΩ/□ and highest carrier concentration of n=4.53 × 10 18 cm −3 were obtained for the thin lm deposited with x = 0.10. The maximum of the Seebeck coe cient (S)=325 μVK −1 and gure of merit (ZT) = 1.85 was obtained for the thin lm deposited with x = 0.20. The average transmittance of lms varied over the range of T=72%-84%. The band gap values of samples were obtained in the range of E g =3.52-3.88 eV for direct band gap. From the photoconductivity studies, the sample prepared with x = 0.20 exhibited the highest photoconductivity among the SnO 2 : Bi thin lms.