Chemical and Materials Sciences: Research Findings Vol. 8

Bismuth sulfide is one of the important compounds of bismuth that has garnered research attention due to its interesting properties and numerous applications. Strong optical absorption, good chemical stability, and relatively low toxicity make Bi₂S₃ thin films suitable for photovoltaic, photodetector, sensor, and thermoelectric applications. However, despite extensive investigations, a systematic understanding of the relationship between synthesis parameters and the resulting optical and morphological properties of Bi₂S₃ thin films remains limited. This chapter presents a comprehensive overview of the synthesis, optical properties, and surface morphology of Bi₂S₃ thin films deposited by the chemical bath deposition (CBD) method. The influence of various deposition parameters, such as precursor concentration, pH, temperature, and deposition time, on film growth and properties was discussed. The optical transmittance of Bi₂S₃ thin films shows a strong dependence on both wavelength and film thickness. As the wavelength increases toward the near-infrared region (>800 nm), transmittance gradually increases, often reaching values of 60–80%, especially for thinner films. Films with thickness below 200 nm may exhibit higher transparency, while thicker films (>500 nm) become increasingly opaque.  For Bi₂S₃ thin films, the band gap generally lies in the range of 1.3–1.7 eV, with most reported values between 1.4 and 1.6 eV.  The refractive index of Bi₂S₃ thin films is relatively high, typically ranging from 2.0 to 3.5 across the visible spectrum. Morphological studies using techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM) indicated that the films are uniform, well-adherent, and composed of nanocrystalline grains with sizes ranging from 20 to 150 nm. Surface roughness and grain structure were found to depend strongly on deposition conditions. Overall, the chapter demonstrates that CBD-grown Bi₂S₃ thin films possess desirable optical and morphological characteristics for applications in photovoltaics, photodetectors, and other optoelectronic devices. Careful control of synthesis parameters is essential for optimising film quality and performance.