Low-cost synthesis and comprehensive characterization of Cr3+ and Sn2+ co-doped lead sulfide thin films for optoelectronic applications

Abstract

This study investigates the microstructural, compositional inhomogeneity and optoelectronic characteristics of Cr³⁺ and Sn²⁺ co-doped lead sulfide (PbS) films. X-ray diffraction reveals that all films are highly crystalline and possess a cubic structure. FTIR spectra show absorption peaks of Pb–S stretching with trace impurities like Pb–O, C–H and C–O–H. Raman spectra reveal vibrational modes of Pb–S with extra peaks and/or modes showing minor impurity content of PbO, PbSO₄, Cr₂(SO₄)₃ and SnO. All films exhibit uniform surface morphology. The Cr and Sn-doped PbS films possess different degrees of microstructural features and grain size distribution (in the range 120–325 nm). EDX analysis shows near-stoichiometric composition of Pb and S. EDX examination reveals that the Cr and Sn-doped PbS films exhibit 4.7 at% Cr and 4.3 at% Sn content, which is close to the intended (5.0 at%) doping level set during chemical synthesis. The un-doped PbS, 5% Cr-doped PbS and PbS film co-doped with 5% Cr + 5% Sn show hydrophobic surfaces unlike the substrate, whereas the 5% Sn-doped PbS film exhibits hydrophilic/hydrophobic borderline surface wettability character. The doping of Cr³⁺ and Sn²⁺ ions in PbS tunes the energy band gap, which is observed to be varied between 1.60 eV and 1.98 eV. Electrical measurements reveal the improved electrical characteristics of the Cr, Sn-doped films and among the investigated films, the 5% Sn-doped film possesses the lowest electrical resistivity (1.48 Ω cm). The experimental results show that the Cr, Sn-doped lead sulfide films possess a suitable combination of structural, chemical and physical properties suitable for optoelectronic applications.