Defect-Engineered Multifunctionality in Cu-Doped Bi2Te2: Interplay of Thermoelectric, Piezoelectric, and Optoelectronic Properties from First-Principles Insights
Defect engineering can improve the linked charge, spin, and lattice behavior of thermoelectric topological insulators. Using density functional theory with spin orbit coupling, we study structural, electronic, optical, thermoelectric, piezoelectric, and charge density features of pristine and Cu doped Bi2Te3. Cu substitution slightly expands the lattice and lowers the total energy minimum, which stabilizes the structure. The density of states shows that Cu d and Te p hybridization creates sharp states near the Fermi level, raising the carrier concentration and supporting higher Seebeck coefficient and power factor. Transport calculations show an increase in the Seebeck coefficient from about 180 microvolts per kelvin in pristine Bi2Te3 to about 220 microvolts per kelvin at 300 K while keeping the electrical conductivity nearly unchanged. Optical spectra reveal strong low energy absorption and very large static dielectric constants (greater than 600), indicating tunable light matter coupling. The piezoelectric coefficient e33 rises from 0.19 C/m2 in pristine Bi2Te3 to 0.38 C/m2 at 5 percent Cu and 0.51 C/m2 at 10 percent Cu, reflecting symmetry breaking and strain driven polarization. Charge density difference maps show anisotropic redistribution, with Cu donating about 0.8 electrons mainly to Te sites, which enhances p type behavior and phonon scattering. Overall, Cu doping reshapes Bi2Te3 into a multifunctional material with coupled thermoelectric, piezoelectric, and optical responses suitable for hybrid energy harvesting, infrared detection, and spin based devices.