Enhancing the thermal properties of polymers presents a significant problem across multiple applications. This chapter explores the impact of ZnSe and CdSe nanoparticles on the thermal conductivity and diffusivity of Poly(3-hexylthiophene) (P3HT) thin films. Using the photothermal deflection technique, PTD called the “Mirage effect,” thermal conductivity and diffusivity were assessed. As nanoparticle concentration in CdSe-based composites increased, thermal conductivity rose from 1.78 to 3.18 W/m·K, while thermal diffusivity decreased from 3.52 to 0.23 m2/s. A poly (3-hexylthiophene) with 40% CdSe nanoparticles (NPs) concentration produced the highest conductivity and lowest diffusivity. Similarly, ZnSe-doped P3HT films exhibited a redshift in sub-bandgap energy and decreased thermal properties at higher nanoparticle concentrations. These variations in opto-thermal parameters are attributed to nanoparticle concentration and their interaction with the polymer matrix. The findings emphasize the role of nanoparticle concentration in tailoring polymer nanocomposite properties, offering the potential for advanced therapeutic technologies, including pain management and controlled heat therapy for biomedical applications.