Impact of potassium doping on the physical properties of cadmium sulfide thin films prepared by sol-gel screen printing technique

Abstract

Cadmium sulfide (CdS) thin films, both undoped and doped with various concentrations of potassium (K) (0.1, 0.2, 0.3, 0.4, and 0.5 M), were synthesized on borosilicate glass substrates using the sol-gel screen-printing technique. The results showed that K-doping at concentrations above 0.3 M promoted grain agglomeration, resulting in a compact morphology with no significant voids or porosity across all thin film conditions, indicating strong adherence and coverage between grains and nanoparticles (NPs). X-ray diffraction (XRD) analysis confirmed a hexagonal structure for the undoped and 0.1 M K-doped films; however, this phase disappeared at higher K concentrations. A low-intensity cubic phase was observed across all the samples. These structural observations were further validated using Raman spectroscopy. The evolution of the photoluminescence (PL) peaks with increasing K-doping concentration indicates structural distortions, suggesting that the incorporation of K+ions may introduce new defects or degrade the crystal quality. The presence of Cd-S bonds within the films was validated by Fourier-transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Optical analysis showed low transmittance (0.45 – 1.75%) and reflectance (0.07 – 0.70%) values in the 550 – 1000 nm wavelength range, indicating distinct absorptive behavior. The energy band gap (Eg) decreased from 2.21 eV for the undoped film to 1.92 eV as the K concentration increased up to 0.4 M, then rose slightly to 2.18 eV at 0.5 M doping. Mott-Schottky analysis demonstrated a shift in the conduction band edge from -1.15 V (undoped) to between -0.68 and - 0.99 V (K-doped), along with an increase in donor density from 5.329 × 1020 to 27.156 × 1020 cm3 for the 0.2 M to 0.5 M K-doped films. According to the EIS results, the 0.4 – 0.5 M K-doped CdS thin films induce the generated electrons and exhibit a lower impedance with electron lifetimes due to the higher electrocatalytic activity and faster exciton recombination, causing subsequently degraded more rapidly in electrolyte region. Therefore, the optimal K-doping condition presents a highly promising active electrode material for application in electrochemical systems and other optoelectronic devices.