Visible Imaging of Incoherent 1200-nm Light via Triplet--Triplet Annihilation Upconversion

Upconversion of low-energy photons to higher-energy photons provides an opportunity to surpass traditional limitations in fields such as 3D printing, photovoltaics, and photocatalysis. Triplet–triplet annihilation upconversion (TTA-UC) is particularly appealing for such applications as it can efficiently upconvert low-intensity, incoherent light. However, previously demonstrated thin-film TTA systems are simultaneously constrained by modest efficiencies and limited reach into the near infrared (NIR). Here, we design a single-layer thin-film bulk heterojunction that integrates PbS quantum dots (QDs) as tunable NIR absorbers within an organic semiconductor matrix of TES-ADT, achieving large anti-Stokes shifts up to 500 nm and high internal quantum efficiencies across the NIR-I and NIR-II windows (800-1200 nm). Through the incorporation of 5-tetracene carboxylic acid ligands on the PbS QD surface, the yield of sensitized triplets was boosted, as confirmed by transient absorption and time-resolved photoluminescence measurements. The resulting films demonstrated a 15-fold improvement in UC efficiency. Furthermore, we demonstrate visible imaging of incoherent 1200 nm light via thin-film TTA-UC at incident intensities at the imaging mask as low as 20 mWcm$^2$, marking a significant advance toward practical implementation of solid-state NIR-to-visible upconversion.

💡 Research Summary

This paper presents a high‑performance thin‑film platform for upconverting incoherent near‑infrared (NIR) light (800–1200 nm) into visible photons (~700 nm) via triplet‑triplet annihilation (TTA) upconversion (UC). The authors address two fundamental bottlenecks that have limited previous TTA‑UC systems: (1) inefficient triplet energy transfer from the sensitizer to the annihilator, and (2) insufficient anti‑Stokes shift to reach the NIR‑II window.

Materials and Design
Lead‑sulfide quantum dots (PbS QDs) are used as tunable NIR sensitizers. Their native oleic‑acid ligands are replaced post‑synthetically with 5‑tetracene carboxylic acid (TCA). NMR spectroscopy confirms the ligand exchange: oleic‑acid proton signals (≈5.3 ppm) disappear while aromatic TCA signals (7–9.5 ppm) appear, yielding a surface coverage of ~1 ligand nm⁻² at the optimal 4 vol % TCA addition. TCA provides a triplet‑energy level that matches the lowest triplet of the organic matrix, thereby acting as a molecular “bridge” for Dexter energy transfer.

The organic matrix consists of TES‑ADT (5,11‑bis(triethylsilylethynyl)anthradithiophene) as the annihilator (triplet energy 1.08 eV, ≈1150 nm) and DBP (tetraphenyldibenzoperiflanthene) as a high‑fluorescence dopant that harvests the singlet excitons generated by TTA. By spin‑coating a single solution containing TCA‑functionalized PbS QDs, TES‑ADT, and DBP, the authors obtain a uniform bulk‑heterojunction (BHJ) film ≈3.3 µm thick. The film morphology is markedly improved relative to unmodified QD blends, which previously suffered from aggregation and poor dispersion.

Spectroscopic Evidence of Enhanced Triplet Transfer
Transient absorption (TA) measurements on films excited at 1064 nm reveal a shortened ground‑state bleach (GSB) decay for PbS‑TCA QDs compared with bare QDs, indicating rapid depopulation of the QD exciton via triplet extraction. Time‑resolved photoluminescence (TRPL) at 700 nm shows a 51 ns rise component that mirrors the GSB decay, confirming that the upconverted emission originates from TCA‑mediated triplet transfer followed by TTA in TES‑ADT and subsequent Förster energy transfer to DBP. Control films lacking TCA display negligible rise, underscoring the pivotal role of the ligand.

Efficiency Metrics
Using a relative method with an integrating sphere, the authors determine external quantum efficiencies (EQE) and internal quantum efficiencies (IQE). Under 808 nm excitation (850 nm QDs), the optimized film reaches an EQE of 0.61 % (out of a 50 % theoretical maximum for a two‑to‑one TTA process) and an IQE of 9.8 %. This represents a ~15‑fold increase over the same composition without TCA and a ~7‑fold improvement over the best reported PbS‑rubrene systems (≈1.4 % IQE). The system remains functional across the NIR‑II range: with 1150 nm QDs, the authors achieve a champion IQE of 0.58 % at 1208 nm excitation, despite a lower EQE (0.002 %) caused by reduced absorption.

A systematic study of QD loading shows that while higher QD concentrations increase absorption (and thus EQE), they also introduce parasitic Förster back‑transfer of singlet excitons from the TES‑ADT/DBP matrix to the QDs, which reduces IQE. Therefore, an optimal balance between NIR absorption and back‑transfer suppression is essential.

The authors note a “potential source of error” in the efficiency measurements and indicate that revised data will be uploaded after further investigation. This caveat should be kept in mind when interpreting the absolute values.

Imaging Demonstration
A key proof‑of‑concept is the visible imaging of incoherent 1200 nm light. Using a broadband LED (FWHM = 65 nm) at an incident intensity as low as ~20 mW cm⁻² (≈0.4 W cm⁻² on the mask), the film produces a clear upconverted image of a Stanford logo at 700 nm. The image quality is limited by residual crystallization in the film, but the demonstration confirms that low‑power, continuous‑wave NIR sources can be converted to visible patterns without the need for high‑peak‑power pulsed lasers.

Impact and Outlook
The work establishes a practical route to solid‑state NIR‑II → visible upconversion by (i) engineering the QD surface with a triplet‑mediating ligand, (ii) employing a low‑energy organic annihilator (TES‑ADT), and (iii) integrating all components in a single‑layer BHJ architecture. The reported anti‑Stokes shift of ~500 nm and the ability to image 1200 nm light at modest intensities open avenues for night‑vision displays, sub‑bandgap solar‑cell harvesting, and low‑light bio‑imaging. Future improvements could involve coupling the BHJ film to plasmonic nanostructures or optical cavities to boost absorption, as well as refining film uniformity to suppress back‑transfer losses. If the pending efficiency‑measurement issue is resolved, the platform could surpass the 1 % EQE threshold that is often cited as a benchmark for commercial relevance.

Overall, this study delivers a compelling combination of materials chemistry, device engineering, and spectroscopic validation that pushes TTA‑UC toward real‑world applications in the challenging NIR‑II spectral region.