Triplet exciton harvesting via TADF in hafnium chlorides array scintillator screen enables ultrahigh-resolution X-ray imaging

Indirect X-ray imaging is an indispensable non-destructive testing technology in the medical and industrial fields. However, its quality is restricted by the light output and optical crosstalk of the scintillation screens. Herein, we report a series of hafnium-based organic-inorganic metal halides (OIMHs) with regulated organic cation chain lengths. The thermally activated delayed fluorescence (TADF) is demonstrated in these materials by their anti-thermal quenching luminescence characteristics, fitting of temperature-dependent photoluminescence decay lifetime, and a series of theoretical calculations. These represent non-luminescent triplet excitons that can be emitted by reverse intersystem crossing (RISC), thus improving the utilization rate of excitons and increasing the light output of scintillators. The highest performance of our reported hafnium-based OIMHS shows a light yield of 56563.31±1250 photons/MeV and detection limit of 23.86 nGy_air/s. Moreover, the optical crosstalk is suppressed by developing silicon array scintillation screens, and an ultra-high spatial resolution of 31.41 lp/mm is achieved. These results offer insights into the luminescent mechanism of hafnium-based OIMHs and initiate a new paradigm for exciton-optical co-management for high-quality X-ray imaging.