Probing thermal stability in CsPbI₃ quantum dots with coupled Pb-site doping and halide passivation

All-inorganic CsPbI₃ quantum dots (QDs) exhibit exceptional optoelectronic properties but suffer from poor thermal and structural stability, hindering their device integration. Here, we systematically investigate the temperature-dependent stability of pristine and Pb-site-substituted QDs combined with halide surface passivation, namely CsPb_0.95Co_0.05I₃ and CsPb_0.95Ag_0.05I₃, within the 20-80 °C range. Comprehensive X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), time-resolved photoluminescence (TRPL), UV-visible absorption (UV-Vis), and Fourier-transform infrared (FTIR) measurements reveal that dual cation-halide doping (CoCl₂ + CoI₂ or AgCl + AgI) enhances lattice rigidity, mitigates thermal expansion, and suppresses nonradiative recombination. While pristine CsPbI₃ QDs show α-phase distortion and emission quenching above 60 °C, doped QDs retain a cubic morphology and bright PL up to 80 °C. Lifetime analysis confirms reduced thermally activated nonradiative rates (Δk_nr ≈ 6.7 × 10^-3 ns^-1 for Co²⁺-doped and 5.6 × 10^-3 ns^-1 for Ag⁺-doped versus 1.48 × 10^-2 ns^-1 for pristine QDs), evidencing significant trap suppression. The smallest lattice dilation (Δd ≈ 0.6%) and minimal bandgap narrowing (ΔE_g ≈ 0.055 eV) observed in Ag-doped QDs demonstrate superior thermal robustness. These findings elucidate a synergistic stabilization mechanism in which B-site substitution strengthens lattice bonding and halide passivation reinforces surface anchoring, providing a practical route toward thermally durable CsPbI₃-based optoelectronic materials.