Theoretical investigations of electronic and optical properties of double perovskite Cs${}_2$Tl$B{X}_6$ ($B=$ Bi, In; $X=$ Cl, Br, I) for photovoltaic application

Lead-free double perovskites are gaining attention for photovoltaic (PV) applications due to their long carrier lifetimes, tunable bandgaps, and low toxicity. Using first-principles calculations, we studied the structural, electronic and optical properties of Cs${}_2$Tl$B{X}_6$ ($B=$ Bi, In; $X=$ Cl, Br, I). The cubic phase (space group Fm3m) was analyzed within the projector-augmented wave (PAW) method. Our calculations predict direct bandgaps of 1.9-1.2 eV for Cs${}_2$TlBi$X_6$ and indirect bandgaps of 2.4–0.8 eV for Cs${}_2$TlIn$X_6$. Notably, the bandgap energy decreases with anion substitution from Cl to I, making these materials highly active in the near-infrared to visible light range. We reveal that Cs${}_2$TlBi$X_6$ exhibits the highest optical absorption, with a peak value of $5\times {10}^5$ cm${}^{-1}$ at an incident photon energy of 3 eV. Additionally, we evaluated the transport properties using the Boltzmann transport equations. The results indicate that Cs${}_2$TlBi$X_6$ exhibit high electrical conductivity, reaching $8\times {10}^6$ S/m, and high electron mobility of 120 cm${}^2\mathrm{/}$V.s. PV performance analysis further reveals promising power conversion efficiencies (PCE) of up to 42%, with Cs${}_2$TlBi$X_6$ showing significantly higher PCE than Cs${}_2$TlIn$X_6$. These reports highlight the potential of Cs${}_2$TlBi$X_6$ for advanced photovoltaic devices.