Effect of thickness on structural and optical properties of spin-coated nanocrystalline PbS thin films

This paper reports the influence of thickness on structural and optical properties of spin-coated NC PbS films. NC PbS films were deposited on glass by spin-coating precursor layers from molecular ink and then heating in air at 100 degrees C. Thicknesses of films investigated were 250-500 nm deposited by layer-by-layer coating. The films were shiny, smooth, uniform and continuous. X-ray diffraction (XRD) studies show that the films are nanocrystalline with crystallite size of about 10 nm is-respective of thickness. Atomic force microscopy (AFM) show that as thickness increases, roughness increases from 1.4 to 5.1 nm and grain sizes increases from 9 to 30 nm. Optical parameters of PbS films were determined by measuring transmittance and reflectance spectra from 400 to 2400 nm. The absorption coefficient of films were better than 10(5) cm(-1). The band gap of films decreased from 2.02 to 1.64 eV and Urbach energy decreased from 0.32 to 0.205 eV as the thickness increased. The refractive indices (at 900 nm) of NC PbS films increased from 1.44 to 2.17 while extinction coefficient (at 500 nm) also increased from 0.54 to 1.04 as the film thickness increased from 250 to 500 nm. Interestingly, the refractive indices of NC PbS films is much below the bulk value of 4.2. The real and imaginary dielectric constants of films also increased with increase in thickness. Thickness variation of dispersion parameters of refractive indices were evaluated with Wemple and DiDomenico single oscillator model. Oscillator energy decreased from 3.9 to 2.7 eV while dispersion energy was conststant at 3.8 eV.