Improvement of the optoelectronic properties of tin oxide transparent conductive thin films through lanthanum doping

• La-doped SnO2 has been synthesized by the facile spray pyrolysis method.
• Influences of doping on the electrical and optical properties of the films were investigated.
• La doped SnO2 films exhibit high transparency in the visible range and low sheet resistance.
• The calculated values of Haacke's figure of merit show that La doping improves the optoelectronic properties of SnO2.
• A new figure of merit has been introduced to qualify the photo-thermal conversion applications.

This work highlights some physical investigations on tin oxide thin films doped with different lanthanum content (ratio La–to-Sn = 0–3%). Such doped thin films have been successfully grown by spray pyrolysis onto glass substrates at 450 °C. X-ray diffraction (XRD) patterns showed that SnO2:La thin films were polycrystalline with tetragonal crystal structure. The preferred orientation of crystallites for undoped SnO2 thin film was along (110) plane, whereas La-doped ones have rather preferential orientations along (200) direction. Although the grain size values exhibited a decreasing tendency with increasing doping content confirming the role of La as a grain growth inhibitor, dislocation density and microstrain values showed an increasing tendency. Also, Raman spectroscopy shows the bands corresponding to the tetragonal structure for the entire range of La doping. The same technique confirms the presence of La2O3 as secondary phase. Moreover, SEM images showed a porous architecture with presence of big clusters with different sizes and shapes resulting from the agglomeration of small grains round shaped. Photoluminescence spectra of SnO2:La thin films exhibit a decrease in the emission intensity with La concentration due to the decrease in grain size. Optical transmittance spectra of the films showed high transparency (∼80%) in the visible region. The dispersion of the refractive index is discussed using both Cauchy model and Wemple–Di-Domenico method. The optical band gap values vary slightly with La doping and were found to be around 3.8 eV. It has been found that La doping causes a pronounced decrease in the sheet resistance by up to two orders of magnitude and allows improving the Haacke's figure of merit (Φ) of the sprayed thin films. Moreover, we have introduced for a first time a new figure of merit for qualifying photo-thermal conversion applications. The obtained high conducting and transparent SnO2:La thin films are promising to be useful in various optoelectronic applications.

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