Pseudo‐Solid‐State Polymer Materials for QD‐Sensitized NIR‐I and NIR‐II Upconversion Beyond the Silicon Bandgap

Quantum-dot (QD) sensitized triplet-triplet annihilation upconversion (TTA-UC) offers a promising strategy for harvesting sub-bandgap photons in silicon photovoltaics and infrared photodetectors. However, translating solution-based upconversion to the solid state remains a major challenge due to severe efficiency losses. Here, a hybrid approach is reported that encapsulates QD-sensitized upconversion mixtures into mesoscale droplets within a rigid acrylate matrix, preserving liquid-like dynamics in a pseudo-solid form. Using a system of PbS sensitizer, carboxytetracene mediator, and TES-ADT annihilator, a normalized UC emission efficiency (η_UC) of 0.72% and upconverted singlet state generation efficiency ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>η</mml:mi> <mml:mrow><mml:mi>U</mml:mi> <mml:msub><mml:mi>C</mml:mi> <mml:mi>s</mml:mi></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${\eta _{U{C_s}}}$</mml:annotation></mml:semantics> </mml:math> ) of 18% in the NIR-I regime (785 nm excitation) is achieved, record-high for solid-state QD-sensitized systems. A key advance lies in the rational design of a chemically compatible polymer system that enables spontaneous phase separation of QDs into nanodroplets while preserving QD surface chemistry, preventing an over 1000-fold drop in upconversion efficiency observed in chemically incompatible control systems. Transient absorption spectroscopy confirms that PbS surface chemistry and dynamics are maintained within nanodroplets. Extending this approach to the NIR-II regime, the first quantifiable solid-state TTA-UC under 1064 nm excitation is demonstrated, achieving η_UC of 0.022% ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>η</mml:mi> <mml:mrow><mml:mi>U</mml:mi> <mml:msub><mml:mi>C</mml:mi> <mml:mi>s</mml:mi></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${\eta _{U{C_s}}}$</mml:annotation></mml:semantics> </mml:math> of 0.55%). These findings represent a substantial step toward integrating QD-sensitized TTA-UC into Si-based optoelectronics.