Surface States Mediate Triplet Energy Transfer in Nanocrystal–Acene Composite Systems

Abstract

Hybrid organic:inorganic materials composed of semiconductor nanocrystals functionalized with acene ligands have recently emerged as a promising platform for photon upconversion. Infrared light absorbed by a nanocrystal excites charge carriers that can pass to surface-bound acenes, forming triplet excitons capable of fusing to produce visible radiation. To fully realize this scheme, energy transfer between nanocrystals and acenes must occur with high efficiency, yet the mechanism of this process remains poorly understood. To improve our knowledge of the fundamental steps involved in nanoparticle:acene energy transfer, we used ultrafast transient absorption to investigate excited electronic dynamics of PbS nanocrystals chemically functionalized with 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS-pentacene) ligands. We find photoexcitation of PbS does not lead to direct triplet energy transfer to surface-bound TIPS-pentacene molecules but rather to the formation of an intermediate state within 40 ps. This intermediate persists for ∼100 ns before evolving to produce TIPS-pentacene triplet excitons. Analysis of transient absorption lineshapes suggests this intermediate corresponds to charge carriers localized at the PbS nanocrystal surface. This hypothesis is supported by constrained DFT calculations that find a large number of spin-triplet states at PbS NC surfaces. Though some of these states can facilitate triplet transfer, others serve as traps that hinder it. Our results highlight that nanocrystal surfaces play an active role in mediating energy transfer to bound acene ligands and must be considered when optimizing composite NC-based materials for photon upconversion, photocatalysis, and other optoelectronic applications.

Triplet-Sensitization by Lead Halide Perovskite Thin Films for Near-Infrared-to-Visible Upconversion

“…The observed subsequent rise and slow decay in the extracted transfer dynamics (green dots) at later times can be attributed to back transfer of singlet excitons created after diffusion-mediated TTA. 16 The extracted transfer time is slower than reported previously for quenching by hole-or electronacceptors 30,45 and may indicate the presence of a long-lived state which mediates the energy transfer process as observed previously in triplet sensitization by PbS NCs 46,47 or could stem from an energy barrier for the electron transfer process due to the reported band bending at the perovskite/rubrene interface. 41 To further investigate the upconversion mechanism, we investigate the dynamics of the upconverted emission, which correlate to a convolution of the rate of energy transfer, diffusionmediated TTA in rubrene and the emission from DBP following FRET.…”

Crystalline silicon solar cells with tetracene interlayers: the path to silicon-singlet fission heterojunction devices

“…Recent work on organic-inorganic TET focuses heavily on nanocrystalline metal chalcogenide particles functionalised with linear acene derivatives. [80][81][82][83][84] Efficient TET between these components in both directions has been demonstrated, 58,85 although no systematic study of the effect of ELA on TET has been conducted. However, an argument based on exciton binding energies points towards TET being insensitive to the ELA at donor-acceptor interfaces.…”

One‐Step Fabrication of Perovskite‐Based Upconversion Devices

“…A wide variety of materials classes for use as triplet sensitizers have been reported, ranging from metal-organic complexes with strong spin-orbit coupling, [4,19,20,22,[26][27][28][29][30][31] conventional semiconductor quantum dots (QDs)" [32][33][34][35][36][37][38][39][40] two-dimensional CdSe nanoplatelets, [41] perovskite-based QDs, [17,21,42,43] transition metal dichalcogenides, [44,45] two-dimensional perovskites [46] to bulk three-dimensional perovskites. [47][48][49][50][51] In the following, we will focus on the methylammonium formamidinium lead triiodide (MAFA) bulk perovskite sensitization of triplet states in the organic semiconductor rubrene by charge transfer.…”