High-sensitivity optical thermometry with Cr ³⁺ -doped hybrid formate perovskites: comparative analysis of ratiometric and lifetime-based approaches

Optical thermometry, due to its properties such as rapid response, usability, high sensitivity, and remote measurements, is gaining increasing interest among researchers. Particularly promising materials for low-temperature thermometry applications are metal-organic frameworks (MOFs) containing chromium(III) ions. In this work, we present a method for optimising data for the development of a temperature sensor, with particular focus on the relative sensitivity parameter (<i>S</i>_r). As part of the optimisation process, we distinguish two approaches using different data analysis techniques and observe the impact of individual parameters on the temperature characteristics of the materials. Additionally, we present the structural and spectroscopic properties of a series of [EA]Mg_<i>x</i>Cr_1-<i>x</i>(HCOO)₃, where EA = ethylammonium cation, and <i>x</i> = 0, 0.01, 0.03, and 0.05. These materials are characterised by a perovskite structure and exhibit a strong dependence of spectroscopic properties on the concentration of the dopant and temperature. The luminescence properties of chromium(III) ions change due to variations in the crystal field strength (<i>Dq</i>/<i>B</i>), which is measured using diffuse reflectance spectroscopy and photoluminescence techniques. The investigated materials exhibit particular temperature-dependent luminescence, which can be implemented as a basis for the thermometric model determination. Moreover, the influence of temperature on luminescence lifetime provides a secondary thermometric pathway, which has not been reported for hybrid formate perovskites before. The obtained optical ratiometric sensors are characterized by high relative sensitivity (up to 3.07% K^-1 at 130 K) and a wide operating range (up to 80-170 K interval). The thermometric analysis of the lifetime-based model shows high relative sensitivity up to 2.97% K^-1 (140 K). In this work, we investigated the influence of the chemical composition on the observed thermometric performance within a series of EA-based hybrid compounds and compared two thermometric approaches - ratiometric and lifetime-based. What is more, we provide a comprehensive comparison of the spectroscopic properties, especially concerning the thermometric potential, with those of other reported hybrid formates, differing in the type of organic cation. The presented results show a particular potential of hybrid formate perovskites doped with Cr³⁺ ions, especially within the context of highly sensitive luminescence thermometry.