In this Letter, erbium (Er 3+ ) and ytterbium (Yb 3+ ) codoped perovskite Cs 2 Ag 0.6 Na 0.4 In 0.9 Bi 0.1 Cl 6 microcrystal (MC) is synthesized and demonstrated systematically to the most prospective optical temperature sensing materials. A dual-mode thermometry based on fluorescence intensity ratio and fluorescence lifetime provides a self-reference and highly sensitive temperature measurement under dual wavelength excitation at a temperature from 300 to 470 K. Combined with the white-light emission derived from self-trapped excitons (STEs), the characteristic emission peak of Er 3+ ions can be observed under 405 nm laser excitation. The fluorescence intensity ratio (FIR) between perovskite and Er 3+ is used as temperature-dependent probe signal, of which maximum value for relative and absolute sensitivities reaches to 1.40% K −1 and 8.20 × 10 −2 K −1 . Moreover, Er 3+ luminescence becomes stronger with the feeding Yb 3+ increasing under 980 nm laser excitation. The energy transfer of Er 3+ and Yb 3+ is revealed by power-dependent photoluminescence (PL) spectroscopy, and the involved upconversion mechanism pertains to the two-photon excitation process. The results reveal that the Er 3+ /Yb 3+ codoped lead-free double perovskite MC is a good candidate for a thermometric material for the novel dual-mode design.

Here, we synthesized pure Cs 3 Bi 2 Cl 9 (CBC) and manganese (Mn)-doped crystals with different feeding ratios, leading to changes in structure and luminescence. The crystals Cs 3 Bi 2 Cl 9 −Mn (CBCM) formed by doping a minor amount of Mn 2+ (Bi/Mn = 8:1) maintain the orthorhombic phase structure of the host, but when Bi/Mn = 2:1, the crystal structure is more inclined to form Cs 4 MnBi 2 Cl 12 (CMBC) of a trigonal phase. Combined with density functional theory (DFT) calculation, the results demonstrate that a moderate amount of Mn 2+ doping can create impurity energy levels in the forbidden band. However, as the structure transitions, the type of energy band structure changes from indirect to direct, with completely different electronic orbital features. Temperature-dependent time-resolved and steady-state photoluminescence spectroscopies are used to explore the structure-related thermal properties and transitional process. Differences energy transfer routes are revealed, with CBCM relying on intersystem energy transfer and CMBC mainly depending on direct excitation of Mn 2+ to produce d−d transitions. Furthermore, since CMBC is temperature-sensitive, we perform the first photoluminescent (PL) lifetime temperature measurement using CBMC and obtain a maximum relative sensitivity of 1.7 %K −1 and an absolute sensitivity of 0.0099 K −1 . Our work provides insight into the mechanism of Mn 2+ doping-induced luminescence and offers a potentially effective doping strategy for improving the PL properties of lead-free metal halide perovskites.