Effect of Oxygen Vacancy on the Crystallinity and Optical Band Gap in Tin Oxide Thin Film

Herein, we have prepared tin oxide (SnO2) nanoparticles (NPs), through a co-precipitation method, using SnCl2·2H2O dissolved in distilled water (DW) as a precursor. Then, the prepared NPs were heat treated in a muffle furnace, as a function of temperature, under an open atmosphere. The prepared SnO2 NPs were then re-dispersed in DW, followed by spray casting on a glass substrate, for preparing SnO2 thin films. The average thickness of the fabricated SnO2 thin films was 2.76 µm. We demonstrated a very clear variation in the structural, compositional, and morphological features of the different films (in particular, variation of the density of oxygen vacancies), which altered their electrical and optical properties. Raising the calcination temperature of the SnO2 thin films, from 250 °C to 650 °C, led to a monotonic reduction in the crystallite size, from 10.4 nm to 6.7 nm, and a decrease in the O/Sn ratio, from 5.60 to 4.79. A 14.5% decrease in the O/Sn ratio resulted in a decrease in the crystallite size by 3.7 nm (i.e., a 35.3% decrease in the NP size), and a decrease in the band gap of 0.11 eV. The lowering of the band gap, along with an increase in the oxygen vacancies in the films, accords well with previous studies. Besides, as the calcination temperature was raised, the refractive index and absorption coefficient values were also found to notably increase. Very interestingly, by simply altering the calcination temperature, we were able to produce SnO2 thin films with optical band gaps nearly equal to the fundamental band gap (2.96 eV), even though many earlier experimental studies had reported considerably greater values (3.36–4.24 eV). SnO2 thin films with lower oxygen vacancies exhibited relatively higher band gaps, which is likely to be favorable for the desired electron transport layer in perovskite solar cells.