Effect of heat treatment temperature on the spin coated fluorine doped tin oxide thin films for solar cell applications

Flourine doped tin oxide thin films have been prepared on glass substrate by spin coating method via sol-gel route. The rheological behavior of the prepared sol solution has been studied by viscosity measurement and from this, the optimum coating period has been investigated and reported. The films have been developed with fixed dopant concentration (F/Sn ratio 7.5 at %), under optimized operating condition viz., turntable spin rate (3000 rpm), spin time (10 Seconds) and number of coatings (10). The spin coated film has been heat treated at temperatures 325 °C, 350 °C, 375 °C, 400 °C, 425 °C and 450 °C. The X-ray diffraction studies of FTO films indicate the tetragonal rutile structure of tin oxide. All films show polycrystalline nature with preferred orientation along the (110) plane and the lattice constant a and c have been evaluated as 4.7324 A ° and 3.1924 A. The variation of average grain size with different heat treatment temperature have been investigated and presented. All the FTO films show good transmittance in the visible region and the fundamental absorption edge lies in the UV region. The direct band gap energy evaluated from the optical absorption data lies between 3.25 eV and 3.85 eV. The minimum value of sheet resistance and electrical resistivity come to 40 Ω/ and 7.4 x 10 Ω-cm for the FTO film heat treated at 375 °C. The effect of heat treatment temperature on resistivity, carrier concentration and mobility of the as coated FTO films have been investigated and the results are discussed. The AFM micrographs of the FTO film shows uniform surface pattern, at the optimized heat treatment temperature 400 °C. Studies indicate that the sol gel spin coated FTO films prepared under the optimized operating conditions could be the potential candidates for the solar cell technology.