Exploring the Mystery of “Negative Thermal Quenching” in Cs3Cu2I5 Single‐Crystal

Abstract As a high‐interest emerging effect, negative thermal quenching (NTQ) may bring revolutionary advances in luminescence. However, the reason for NTQ is still unclear, making it challenging to target design materials with such unique properties. Interestingly, it is found that the Cs 3 Cu 2 I 5 single‐crystals grown using the Bridgman and antisolvent methods exhibit the conventional thermal quenching, while the single‐crystal grown by the aqueous solution method yet has the NTQ. This suggests that a specific structural change in the single crystals can be induced to produce NTQ, harboring the secrets of NTQs. It is found that the Cs 3 Cu 2 I 5 single‐crystal from the aqueous solution method has a more compact crystal structure, smaller Huang–Rhys factor, and a more considerable exciton binding energy than other methods. In this case, the structural distortion of Cs 3 Cu 2 I 5 single‐crystal after photoexcitation is limited at low temperatures, and consequently, the self‐trapped exciton (STE) energy levels are incompletely formed. As the temperature increases, the STE energy levels gradually become fully formed, and their ability to trap electrons improves, resulting in the marvelous phenomenon of NTQ. This work provides a plausible mechanism for the mysterious NTQ and will guide the future design of NTQ materials.