Enhanced Near-Infrared-to-Visible Upconversion by a Singlet Sink Approach in a Quantum-Dot-Sensitized Triplet-Triplet Annihilation System

Photon upconversion (UC) from near-infrared (NIR)-to-visible in the solid state via the triplet-triplet annihilation (TTA) process is important for numerous applications. Inorganic quantum dot (QD)-sensitized TTA-UC is capable of utilizing a broad range of NIR photons. However, achieving a high UC efficiency in solid systems remains challenging due to weak energy transfer between the QD and emitter and the low fluorescence quantum yield (FL-QY) of emitter molecules in the solid state. Previously, we had addressed the energy transfer issue between QD and emitter in the solid state by using a directly attaching emitter, i.e., 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT); however, the low FL-QY of TES-ADT in the solid state (similar to 3%) remains a bottleneck to realize an efficient NIR-to-visible UC system in the solid state. Here, we adopted a singlet sink approach using tetraphenyldibenzoperiflanthene (DBP) as an auxiliary fluorescent dopant in a TES-ADT matrix. A small amount of DBP inclusion in the TES-ADT solid resulted in similar to 10-fold enhancement in the fluorescence intensity, and FL-QY was maximized to similar to 24% in TES-ADT:DBP solid films. Using an optimized ratio of TES-ADT:DBP mixed with PbS QD, a UC quantum efficiency (QE) eta(UC) of (1.5 +/- 0.5)% (at 100% scale) was obtained in solid films under 980 nm excitation. To the best of our knowledge, this is the first report on the application of the fluorescent dopant approach in a PbS-QD:TES-ADT system for realizing the efficient TTA-UC in the solid state, which opens a way forward to design an efficient NIR-to-visible UC system at long wavelength excitation.