Trap state mediated triplet energy transfer from CdSe quantum dots to molecular acceptors

Abstract

Triplet energy transfer (TET) from quantum dots (QDs) to molecular acceptors has received intense research interest because of its promising application as triplet sensitizers in photon up-conversion. Compared to QD band edge excitons, the role and mechanism of trap state mediated TET in QD-acceptor complexes have not been well understood despite the prevalence of trap states in many QDs. Herein, TET from trap states in CdSe QDs to adsorbed 9-anthracene carboxylic acid (ACA) is studied with steady state photoluminescence, transient absorption spectroscopy, and time-resolved photoluminescence. We show that both band edge and trap excitons undergo direct Dexter energy transfer to form the triplet excited state of ACA. The rate of TET decreases from (0.340 ± 0.002) ns−1 to (0.124 ± 0.004) ns−1 for trap excitons with decreasing energy from 2.25 eV to 1.57 eV, while the TET rate from band edge excitons is 13–37 times faster than trapped excitons. Despite slightly higher TET quantum efficiency from band edge excitons (∼100%) than trapped excitons (∼95%), the overall TET process from CdSe to ACA is dominated by trapped excitons because of their larger relative populations. This result demonstrates the important role of trap state mediated TET in nanocrystal sensitized triplet generation.

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

“…67 Additionally, while we discuss our results in the context of (correlated) excitonic energy transfer, we cannot rule out that transfer also occurs via a sequential mechanism following single-carrier trapping-a model that is consistent with experiments on larger QDs paired with triplet acceptors. 64,71,76,77 Exothermic carrier trapping on the QD would slow TET (relative to transfer from band-edge states) by increasing the effective endothermicity of the key QD / molecule transfer, but our experiments do not discern between these hypotheses. 42,78 However such trapping is not unique to PbS, 76,79 nor it is exclusively observed in ultra-small particles, 64 so we do not consider that its presence is the primary origin of the slow ($5.4 ms) TET dynamics that we observe.…”

“…42,78 However such trapping is not unique to PbS, 76,79 nor it is exclusively observed in ultra-small particles, 64 so we do not consider that its presence is the primary origin of the slow ($5.4 ms) TET dynamics that we observe. Still, ongoing 77,80 and future work to reveal the chemical nature of electronic trap states in these materials may establish whether a sequential mechanism would have stronger coupling or an overall efficiency benet.…”

Interfacial Trap‐Assisted Triplet Generation in Lead Halide Perovskite Sensitized Solid‐State Upconversion

“…Trap‐state‐mediated triplet transfer has been previously demonstrated in several excitonic systems with NC sensitizers. [ 36–38 ] For example, CuInS 2 NCs, in which the photogenerated excitons are self‐trapped due to hole localization to intragap Cu states, can efficiently transfer triplets to surface‐anchored anthracene. [ 38 ] Formation of surface intermediate states is also reported in the triplet transfer from PbS NC to TIPS‐pentacene molecules.…”

Shallow distance-dependent triplet energy migration mediated by endothermic charge-transfer

“…On the other hand, the PL quantum yield is considerably improved from <10% for the core to 58-94% for the core/shell samples, indicating that the carrier traps on the core surface are effectively passivated by the shell. Because of the low PL quantum yield of the CdSe core-only QDs, TET from them involves not only bandedge excitons but also poorly-understood surface-trapped excitons 34 . For this reason, the core sample will not be evaluated here.…”