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Exploring new optical thermometric materials is important to realize highly accurate temperature detecting. Herein, Yb/Er activators are successfully doped into Cs 2 Ag(In/Bi)Cl 6 lead-free double perovskite (DP) semiconductor, providing dual-modal photoluminescence originated from self-trapped exciton recombination and 4f-4f transitions of Yb/Er dopants as well as Er3+ upconversion emissions. Remarkably different temperature-dependent emissive behaviors from DP host and Yb/Er activators are experimentally demonstrated, which enables to design novel fluorescence intensity ratio (FIR) between DP and Yb/Er based thermometric technology. In addition, upconversion FIR-based temperature sensing via thermal population between the 2H 11/2 level and 4S 3/2 one of Er3+ dopants in DP host is also realized. It is believed that the present Yb/Er: Cs 2 Ag(In/Bi)Cl 6 DP with excellent long-term stability and dual-functional temperature detecting is a suitable candidate as the self-calibrated optical thermometric material. The combination of the exciton recombination of Cs 2 AgInCl 6 lead-free double perovskite with Ln3+ (Ln = Yb, Er) dopants induces dual-modal light-emitting channel to produce a novel thermometric material. [Display omitted] • Yb/Er: Cs 2 Ag(In/Bi)Cl 6 lead-free double perovskite is successfully prepared. • The product exhibits both self-trapped exciton recombination and Ln3+: 4f-4f emissions. • Different temperature-dependent emissive behaviors from host and Ln3+ activators are demonstrated. • A novel DP/Ln3+ fluorescence intensity ratio thermometric channel is developed. • The product is a promising candidate for dual-modal optical temperature sensing. [ABSTRACT FROM AUTHOR]