Facile synthesis, microstructural, phase composition, wettability behavior and optical properties of Cu:PbS films for optoelectronic applications

Abstract

This paper reports the experimental results from a comprehensive study investigating the microstructural, phase composition, wettability behavior and optical properties of Cu-doped PbS (Cu:PbS) films. Thin Cu:PbS films with different volume concentrations of Cu content (ranging from 0.0 to 10 ml) are deposited on glass substrates by chemical bath deposition. X-ray diffraction (XRD) spectra reveals polycrystalline nature of all Cu:PbS films with face-centered cubic (fcc) structure. No peak corresponding to metal-oxide phase (such as PbO and CuO) or any other impurity or secondary phase (such as PbS₂ or CuS) is observed from XRD. Fourier transform infrared (FTIR) spectroscopy analysis reveals a significant shift of absorption peaks related to Pb–S bond due to doping of different Cu concentrations in PbS films. FTIR spectra show the presence of absorption peaks related to metal sulfides (PbS₂ and CuS) and metal oxides (CuO and PbO) as secondary or minor traces of impurity phases. Raman spectroscopy also confirms the formation of PbS as main phase in all Cu:PbS films along with minor impurity phase of metal-oxides (CuO or PbO) and lead sulfate (PbSO₄). Surface SEM micrographs show uniform and granular surface morphology with decrease of average grain size (320 ± 10 nm to 98 ± 7 nm) on increasing Cu concentration from 0.0 to 10 ml. Wettability analysis show that all Cu:PbS films exhibit hydrophobic nature (with a contact angle greater than 90°) in contrast to hydrophilic nature of bare glass substrate. Calculated surface free energy increases with increasing Cu concentration and attains its maximum value 28.49 mJ/m² at Cu-doping concentration of 10 ml in PbS. All Cu:PbS films exhibit high absorption coefficient (∼10⁴ cm⁻¹) in the UV–visible region. All films exhibit direct energy band gap (E_g) which decreases from 1.58 eV to 1.40 eV with increase of Cu concentration from 0.0 to 10 ml. A decrease of E_g with increasing Cu concentration makes Cu:PbS films suitable for optoelectronic devices such as infrared (IR) detectors and LEDs.