Plasmons Enable Ultralow Threshold Solid-State Triplet Fusion Upconversion with a 2D Sensitizer.

Solid-state triplet-triplet annihilation (TTA) upconversion has significant potential for application in light harvesting, optoelectronic devices, and bioimaging. However, the high optical powers required to achieve efficient upconversion have inhibited its adoption. In this work, we demonstrate plasmon-enhanced near-infrared (NIR)-to-blue TTA upconversion in a monolayer WSe/organic heterojunction. Under far-field excitation, the device reaches a threshold of 19 mW/cm and an external quantum efficiency (EQE) of 0.17% with an anti-Stokes shift of 1.1 eV. Plasmon excitation lowers the threshold to 0.9 mW/cm and improves the EQE to 3.6%. We attribute the plasmon enhancement to surface plasmon polariton (SPP) near-field enhancement and dark-exciton absorption. Optimization of the WSe transfer process is identified as a key factor for the device performance. This work demonstrates that plasmon excitation overcomes the low far-field absorption of 2D transition-metal dichalcogenide (TMD) sensitizers. Consequently, monolayer TMDs can achieve solid-state upconversion with a performance among the best reported.