Theoretical investigations of electronic and optical properties of double perovskite Cs$_2$Tl$BX_6$ ($B=$ Bi, In; $X=$ Cl, Br, I) for photovoltaic application (2505.09982v1)

Abstract: 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$BX_6$ ($B=$ 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 $5\times10^5$ 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 $8\times10^6$ S/m, and high electron mobility of 120 cm$^2/$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.