Number of Surface-Attached Acceptors on a Quantum Dot Impacts Energy Transfer and Photon Upconversion Efficiencies

Abstract

Composites of organic molecules and inorganic quantum dots (QDs) have emerged as attractive photon upconversion systems that use triplet–triplet annihilation upconversion (TTA-UC). However, the upconversion efficiency of such systems is still far from reaching their theoretical potential. The number of acceptor molecules directly coordinated on a QD (n) should determine triplet–triplet energy transfer (TTET) efficiency (ΦTTET), which consequently affects the efficiency of TTA-UC, but the research focusing on the n value has been limited. In the present report, the effect of n on TTET from CdSe or CdTe QDs to perylene-3-carboxylic acid (Pe; i.e., acceptor) were systematically investigated. The TTET and TTA-UC efficiencies increase with increasing n. The regulation of n on a QD could provide a straightforward means to realize high-performance TTA-UC. For the molecule/QDs systems, small QDs with a wide band gap are favorable for intrinsic TTET (i.e., TTET in a one-to-one QD-Pe composite system), because intrinsic TTET efficiency is detemined by the triplet energy of QDs. On the other hand, the small QDs limit the n due to the small surface area. Therefore, the proper choices of QDs and acceptors providing both sufficient free energy change for TTET and large n are important to achieve efficient TTA-UC.

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…Improved QD passivation may also permit a greater density of triplet-extracting ligands to be achieved, accelerating forward TET, as well as providing additional acceptor microstates to push the population equilibrium forward. 83,89 Lastly, because the timescale of TET is slow in this system (as discussed above), it is only the comparatively long lifetime of PbS that permits meaningful transfer. All else equal, our model predicts that only $0.1% of excitations on hypothetical absorbers with a lifetime matching CdSe QDs (i.e.…”

“…Here, recent work has begun to address the role of entropy from the multiplicity of ligand loading. 83,89 Then, while the band-edge degeneracy of PbS NCs has been established, 62 there is a wide variation in the expected number of surfaceoriented states. 64 Further experiments are warranted to explore the origin of the anomalously broad, strongly Stokes-shied photoluminescence from ultra-small colloidal quantum dots.…”

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…Improved QD passivation may also permit a greater density of tripletextracting ligands to be achieved, accelerating forward TET, as well as providing additional acceptor microstates to push the population equilibrium forward. 81,87 Lastly, because the timescale of TET is slow in this system (as discussed above), it is only the comparatively long lifetime of PbS that permits meaningful transfer. All else equal, our model predicts that only ~0.1% of excitations on hypothetical absorbers with a lifetime matching CdSe QDs (i.e.…”

“…Here, recent work has begun to address the role of entropy from the multiplicity of ligand loading. 81,87 Then, while the band-edge degeneracy of PbS NCs has been established, 60 there is a wide variation in the expected number of surfaceoriented states. 62 Further experiments are warranted to explore the origin of the anomalously broad, strongly Stokes-shifted photoluminescence from ultra-small colloidal quantum dots.…”

Efficient Visible‐to‐UV Photon Upconversion Systems Based on CdS Nanocrystals Modified with Triplet Energy Mediators

“…In general, ΔE 1 ≥ 0.2 eV is required from our observations, and a larger driving force provides a higher Φ TET1 . Previous studies demonstrated that the number of mediator molecules (m) attached to the surface of the NCs plays a crucial role in governing the energy transfer and UC efficiency, i.e., Φ TET1 and Φ UCg increases with higher values of m. [32] The calculation from Piland et al indicates that m = 16 could achieve Φ TET1 = 0.95. [16] However, using ligand exchange methods, it is challenging to load enough mediators onto the surface of NCs to achieve efficient Φ TET1 (>90%).…”