Photo-activated Charge Transport and Unified Mechanism of Self- Trapped Excitons in Cs₂NaInCl₆ Double Perovskite

On account of their environmentally-benign composition and structural stability, lead-free halide double perovskites are known for being good substitutes for Pb-based materials. This paper includes a detailed examination of the electrical, optical and structural properties of Cs₂NaInCl₆. It also shows unified photo-physical properties and a charge-transport mechanism controlled by innate self-trapped excitons (STEs). An excellent thermal stability reaching 620°C is confirmed through thermogravimetric analysis while a highly crystalline cubic \(Fm\stackrel{-}{3}m\) phase is demonstrated by X-ray diffraction. Optical absorption, however, provides not only a low Urbach energy (0.31 eV) which confirms minimal disorder but also a wide direct bandgap (4.81 eV). A broad intrinsic blue emission which originates from STEs and a reduced electron–phonon coupling at low temperatures are revealed through temperature-dependent and steady-state photoluminescence measurements. A multi-step relaxation pathway that includes the formation of long-lived localized states, lattice relaxation and ultrafast exciton self-trapping is demonstrated using transient absorption spectroscopy and time-resolved PL. It is proven that Illumination activates persistent photocarriers, improves polarization and reduces the bulk resistance considerably by means of dielectric analysis, electric-modulus modeling and impedance spectroscopy. A strong voltage-independent photoconductive gain reaching three orders of magnitude is further proven by I–V measurements without altering the underlying ohmic transport regime. These results collectively set up a coherent microscopic picture where carrier stabilization, trap filling and STE formation together influence not only the AC/DC transport properties but also the optical emission. Hence, as a lead-free perovskite having strong potential for radiation-detection, dielectric and UV-emitting applications, Cs₂NaInCl₆ seems highly stable, robust and intrinsically-excitonic.