This study examines the influence of lead (Pb²⁺) ions on the optical properties of cad mium–lead–selenium (CdPbSe) alloyed thin films synthesized in an alkaline medium. By systematically varying Pb concentration and deposition time, we demonstrate that Pb is not a passive impurity but actively modulates the bandgap energy. A Pb concentration of 0.03 mol significantly enhances infrared optical transitions, while prolonged deposition reduces the bandgap consistent with quantum confinement effects. Surprisingly, increased deposition time also led to higher Pb incorporation, challenging the assumption that band gap tuning is governed solely by particle size. Transmission electron microscopy revealed no clear nanoparticle growth, suggesting complex structural dynamics. To avoid model dependent assumptions, bandgap energies were determined using the Derivative of the Inverse Transmission Method (DITM), a robust alternative to the Tauc plot. These findings highlight the critical role of ionic composition particularly Pb in tailoring the optoelectron ic properties of alloyed quantum dots, offering new design strategies beyond size control.
