DFT studies of the role of anion variation in physical properties of Cs2NaTlBr6-xClx (x = 0, 1, 2, 3, 4, 5, and 6) mixed halide double perovskites for optoelectronics

Halide-double perovskites have various benefits over lead-based
perovskites due to their suitable optical absorption efficiency, higher
stability, tunable bandgap, large carrier mobility, easy availability, and
low cost. The structural, electrical, optical, and mechanical
characteristics of the lead-free halide double perovskites Cs2NaTlBr6-xClx
(x = 0, 1, 2, 3, 4, 5, and 6) are investigated by utilizing the
first-principles density functional theory (DFT). The structural
properties were computed at equilibrium, revealing that the crystals
undergo structural phase transitions as the doping concentration varies.
However, crystal stability was confirmed through the evaluation of the
tolerance factor. The HSE06 functional is used to correct the bandgap
underestimation by GGA-PBE. The band edge profile and electron density of
states confirm the direct-bandgap semiconducting nature of the compounds.
The bandgap increases approximately linearly with Cl incorporation,
sharply tuned from 0.80–1.75 eV (GGA-PBE) and 1.78–2.98 eV (HSE06), making
them highly suitable for photovoltaic and optoelectronic applications. The
effective mass of electron ranges from 0.283m0 to 0.449m0, and the carrier
mobility is from 83.913 cm2v-1s-1 to 305.485 cm2v-1s-1, respectively,
suggesting excellent charge transport characteristics, essential for
high-performance solar cells and photodetectors. The considered NaTL-based
double perovskites have strong optical absorption in the visible and UV
spectrum, with enhanced conductivity as Cl content increases. Binding
energy analysis confirms strong lattice stability, with values decreasing
from -3.193 eV (Br-rich) to -3.559 eV (Cl-rich), indicating improved
thermodynamic robustness as Cl concentration increases. Mechanical
analysis shows the ductile behavior of the considered perovskites,
supported by Poisson’s ratio and Pugh’s modulus. The rising bulk modulus
and Debye temperature with Cl incorporation enhance rigidity and thermal
stability. These insights advance the anion-engineered design of stable,
high-efficiency, lead-free NaTl-based perovskites for next-generation
optoelectronic applications.