Triplet–triplet annihilation based photon up-conversion in hybrid molecule–semiconductor nanocrystal systems

Abstract

Photon up-conversion based on triplet–triplet annihilation (TTA) in a hybrid system exploits the annihilation of optically dark triplets of an organic emitter, sensitized by a semiconductor nanocrystal, to produce high-energy singlets that generate high energy emission.

High Photon Upconversion Efficiency with Hybrid Triplet Sensitizers by Ultrafast Hole‐Routing in Electronic‐Doped Nanocrystals

“…Specifically, upon replacing OA with 9‐ACA, the undoped NCs show a dramatic drop of the zero‐delay PL intensity ( = with η = 0.02) accompanied by the substantial acceleration of their PL lifetime (estimated as the time after which the PL intensity has dropped by a factor e ). Consistent with previous reports, [ 35 ] the zero‐delay drop is ascribed to ultrafast exciton dissociation by hole‐transfer to 9‐ACA—with some minor contribution by hole trapping in surface defects due to incomplete surface coverage by 9‐ACA molecules—occurring on a timescale faster than our experimental resolution and affecting the vast majority (>95%) of the NC population. The accelerated PL decay, on the other hand, is caused by ET′ to 9‐ACA in the complementary minor fraction (<5%) of the NC ensemble, in which hole‐transfer is inefficient.…”

“…[ 15 ] As a result, as shown in Figure 1C for the (CdSe NC)–(9‐ACA) hybrid sensitizer, fast transfer (≈hundreds of picoseconds) [ 31,34 ] of the photohole from the NC's VB to the HOMO of 9‐ACA can result in efficient exciton dissociation that outcompetes ET′ and thereby hinders the upconversion process. [ 32,35 ] We highlight that, because of the heavier hole effective mass with respect to the electron in many chalcogenide semiconductors, the VB energy is less sensitive to quantum confinement with respect to the CB. As a result, in such systems size control is ineffective to prevent hole‐transfer, because the limited upward energy shift of the VB at increasing particle size does not allow its energy to rise above the triplet acceptor HOMO.…”

“…The excitation intensity at which QY uc is half of its saturation value is typically referred to as the excitation threshold for TTA, and is a relevant figure of merit that marks the beginning of the high excitation regime. [ 35,44 ] It is worth noting that, as detailed in the Supporting Information, both QY uc and the excitation threshold depend on the efficiency of the ET′ step by which the NC sensitizes the 9‐ACA triplet: the first grows linearly with whereas the latter scales with . As a result, owing to the effective suppression of nonradiative losses due to hole‐transfer that boosts from the Au:CdSe NCs to their 9‐ACA ligands, the upconversion efficiency saturates at the unprecedented value of QY uc = 12 ± 1% (24 ± 2% in the normalized convention), which is nearly 40 times higher than for the standard undoped NCs (QY uc = 0.3%) in good agreement with the observed enhancement of .…”

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

“…Overall, both spin statistics and kinetic considerations should favor the formation of QD-3 ACA* over ground-state QD-ACA. We note that a few studies proposed a competition between hole transfer and TET from QDs to molecules [52][53][54] , or in another word, recombination of the hole-transfer CT state generates ground-state QD-molecule rather than molecular triplets. However, no direct spectroscopic evidence supporting this competition has been reported.…”

Triplet energy migration pathways from PbS quantum dots to surface-anchored polyacenes controlled by charge transfer

“…Colloidal semiconductor nanocrystals or quantum dots (QDs) have recently been developed as efficient and versatile sensitizers for molecular triplets that are important for many applications. [1][2][3][4][5][6][7][8][9][10] One of these applications takes advantage of long-lived molecular triplets for low-threshold triplet-fusion photon upconversion, 2,3,[11][12][13][14][15][16][17][18][19][20][21][22][23] a process useful in fields ranging from solar energy conversion and photoredox catalysis to bioimaging. [24][25][26][27] Compared to traditional organic and organo-metallic sensitizers, QD sensitizers have negligible intersystem crossing energy loss, in principle allowing for large anti-Stokes shifts in photon upconversion.…”