Defect engineering of inorganic sensitizers for efficient triplet–triplet annihilation upconversion

Sensitized triplet–triplet annihilation upconversion (TTA-UC) using inorganic–organic hybrids has great potential for applications in energy, biomedicine, advanced light source, and manufacturing. These applications require highly efficient and low-threshold TTA-UC. Here, we review the recent progress and critical role of defect engineering of inorganic sensitizer on triplet energy transfer (TET) and TTA-UC. The formation mechanism and influence of various kinds of defects in sensitizers are discussed. In particular, we focus on comparing the intrinsic, extrinsic, and surface defect engineering strategies for controlling triplet population and lifetime, band alignment, carrier mobility, and TET rate. We explore the challenges of controllable defect engineering and the nature of defects, highlighting future opportunities for fundamental photophysics and related applications.