"Modifying Quantum Dots to Boost Near Infrared-To-Visible Upconversion"
The conversion of low-energy near-infrared (NIR) photons into higher-energy visible photons holds transformative potential for technologies including silicon-based photovoltaics, NIR imaging, night vision, and anti-counterfeiting. Triplet-triplet annihilation upconversion (TTA-UC) is an appealing technique to combine the energy of two NIR photons to generate one visible photon under low-intensity, incoherent illumination. Colloidal quantum dots (QDs) have emerged as appealing sensitizers for TTA-UC because their size-tunable bandgaps and high absorption cross-sections enable broadband NIR sensitization well beyond the reach of traditional organometallic complexes. However, thin-film QD-based TTA-UC technology has been plagued by two fundamental bottlenecks: (1) inefficient exciton extraction from QDs to annihilator molecules due to insulating ligand shells, and (2) parasitic energy back transfer of upconverted singlet excitons from the annihilator back to the QD sensitizer. Together, these loss channels have constrained external quantum efficiencies (EQEs) and created a paradoxical trade-off between NIR absorption and upconversion efficiency. This dissertation presents a systematic investigation of QD surface/composition manipulation and device architecture strategies to overcome these limitations, advancing the performance and spectral reach of solid-state NIR-to-visible TTA-UC.
The first part of this dissertation addresses the long-standing challenge of parasitic back transfer in the benchmark PbS QD/rubrene/DBP bilayer UC system. In this system, increasing the PbS QD loading to improve NIR absorption simultaneously exacerbates FRET-mediated BT from the rubrene singlet to the PbS QDs, causing EQE to deteriorate with higher QD concentrations. To decouple NIR absorption from back transfer losses, we introduce a novel multilayer device architecture in which 5-tetracene carboxylic acid (TCA) is incorporated as an organic interlayer between the PbS QD film and the rubrene/DBP layer. With a singlet energy higher than rubrene and a triplet energy between the PbS bandgap and the rubrene triplet, TCA serves the dual role of a Dexter energy extractor and a Förster back-transfer blocker. The optimized architecture achieveda 5-fold improvement in UC EQE and demonstrated that strongly absorbing films (up to 6.2% at 808 nm) can be realized without sacrificing internal quantum efficiency.
The second part of this dissertation tackles the spectral limitation of the rubrene-based system, which cannot efficiently upconvert photons beyond ~1100 nm due to the energetic ceiling imposed by rubrene's triplet level (~1.14 eV). By replacing rubrene with TES-ADT (5,11-bis(triethylsilylethynyl)anthradithiophene), an annihilator with a lower triplet energy (~1.08 eV), and incorporating PbS QDs directly into a single-layer bulk heterojunction (BHJ) film alongside TES-ADT and DBP, the accessible UC window is extended 1200 nm. Quantitative NMR and UV-vis spectroscopy confirm the replacement of native oleic acid ligands with ~1 TCA ligand/nm². Transient absorption and time-resolved photoluminescence studies were used to prove accelerated PbS exciton decay and TES-ADT triplet population rise upon TCA incorporation.
The third part of this dissertation broadens the materials landscape for QD-based TTA-UC by exploring alternative QD compositions beyond PbS. HgTe QDs are examined as sensitizers in TES-ADT/DBP matrices based on the hypothesis that their exceptionally large Bohr exciton radius (~40 nm for HgTe versus ~18 nm for PbS) enables greater wavefunction leakage beyond the physical QD boundary, enhancing Dexter-type TET to the annihilator without requiring mediator ligands. Comparative UC measurements with 1020 nm and 1120 nm HgTe and PbS QDs demonstrate that HgTe-based UC films achieve approximately an order-of-magnitude higher EQE and IQE under 1130 nm excitation without TCA ligands, consistent with the wavefunction leakage theory. The chapter also hypothesizes silver-based QDs as non-toxic alternative sensitizers for TTA-UC. Together, these results establish Bohr exciton radius engineering as a generalizable design principle for enhancing TET in QD-sensitized TTA-UC.