Strategies for High‐Performance Solid‐State Triplet–Triplet‐Annihilation‐Based Photon Upconversion

Abstract

Photon upconversion via triplet-triplet annihilation (TTA) has achieved high efficiencies in solution and within polymer matrices that support molecular migration systems. It has diverse potential applications including bioimaging, optical sensors, and photovoltaics. To date, however, the reported performance of TTA in rigid solid-state systems is substantially inferior, which may complicate the integration of TTA in other solid-state devices. Here, solid-state loss mechanisms in a green-to-blue upconversion system are investigated, and three specific losses are identified: energy back transfer, sensitizer aggregation, and triplet-charge annihilation (TCA). Strategies are demonstrated to mitigate energy back transfer and sensitizer aggregation, and a completely dry-processed solid-state TTA upconversion system having an upconversion efficiency of ~2.5% (by the convention of maximum efficiency being 100%) at a relatively low excitation intensity of 238 mW/cm 2 is reported. This device is the first demonstration of dry-processed solid-state TTA comparable to solution-processed solid-state systems. The strategies reported here can be generalized to other upconversion systems and offer a route to achieving higher performance solid-state TTA upconversion devices that are compatible with applications sensitive to solvent damage.

Red-to-Blue Photon Upconversion Enabled by One Dimensional CdTe Nanorods

“…21 The welldefined facets and unique anisotropic shape of 1D nanomaterials also allow for self-assembly into macrostructures, 22 which could be useful for photon UC, especially in the solid state. 23,24 In-depth studies of the unique properties of 1D materials endowed by their aspect ratio and anisotropic nature are required pertaining to their application as triplet sensitizers in photon UC via TTA.…”

Efficient Solid-State Photon Upconversion Enabled by Spin Inversion at Organic Semiconductor Interface

“…TTA UC has been studied intensively to date; however, the greatest challenge remaining is that solid-state UC, which is necessary for real device applications, is inefficient, 3,4 with an external quantum efficiency (EQE) of less than 0.1%. 5 Moreover, other problems such as the necessity of strong laser excitation and the use of minor metals, rare-earth or toxic elements must be overcome simultaneously. 2 In this study, we realized efficient solid-state UC with 100 times higher EQE than a conventional system by discovering a novel phenomenon in which UC could be observed in bilayer organic semiconductor heterojunctions.…”

“…In particular, the EQE of the UC in the rubrene + 0.5% DBP/ITIC-Cl film was very high, namely 2.30% at 152 mW/cm 2 , which was more than 100 times more efficient under a smaller excitation intensity than that of recently reported solid-state UC systems. 5 This was because of the large absorbance of the NFA layer as well as the high IQE in the novel system.…”

“…The solid-state UCs reported to date have been inefficient as a result of non-radiative triplet annihilation occurring in the sensitizer layer during inefficient triplet diffusion. 5 To avoid these loss pathways, in most cases, the sensitizer is dispersed in matrix materials. 3,4 However, the dispersion decreases the absorbance of the sensitizer layer significantly.…”

“…As a result, about 90% of incident photons were absorbed in the Rub/ITIC-Cl bilayer, whereas only a small percentage can be utilized in dispersed films in conventional UC systems. 5 This difference as well as the high IQE resulted in a 100 times increase in the EQE in the novel UC system compared to conventional systems.…”