Publication date: 22nd July 2026
Upconversion of near-infrared (NIR) photons via quantum dot (QD)-sensitized triplet-triplet annihilation (TTA-UC) offers a promising route toward improved night vision, photovoltaic, and bioimaging technologies. Practical deployment requires efficient solid-state architectures, and bulk heterojunction (BHJ) thin films have emerged as a leading platform. Our group recently demonstrated a ternary BHJ system comprising surface-modified PbS QDs as sensitizer, TES-ADT as annihilator, and DBP as emitter — representing the current efficiency and spectral frontier of this architecture, with anti-Stokes shifts up to 0.75 eV [1]. A central challenge, however, is the tendency of TES-ADT to crystallize in the solid state, causing rapid degradation of upconversion efficiency over time. Here, we investigate the spatiotemporal evolution of upconverted photoluminescence in BHJ thin films using NIR laser excitation coupled to a motorized microstage and optical microscope, identifying the microscopic features that initiate crystallization and tracking its propagation across the active layer. Motivated by the need for greater photostability, we investigate the incorporation of polystyrene as an amorphous host matrix, suppressing crystallization and enhancing long-term stability. Together, these advances chart a clear path toward efficient, stable, and processable solid-state upconversion devices operating under incoherent illumination.
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