A Sensitizer of Purpose: Generating Triplet Excitons with Semiconductor Nanocrystals

Abstract

The process of photon upconversion promises importance for many optoelectronic applications, as it can result in higher efficiencies and more effective photon management. Upconversion via triplet-triplet annihilation (TTA) occurs at low incident powers and at high efficiencies, requirements for integration into existing optoelectronic devices. Semiconductor nanocrystals are a diverse class of triplet sensitizers with advantages over traditional molecular sensitizers such as energetic tunability and minimal energy loss during the triplet sensitization process. In this Perspective, we review current progress in semiconductor nanocrystal triplet sensitization, specifically focusing on the nanocrystal, the ligand shell which surrounds the nanocrystal, and progress in solid-state sensitization. Finally, we discuss potential areas of improvement which could result in more efficient upconversion systems sensitized by semiconductor nanocrystals. Specifically, we focus on the development of solid-state TTA upconversion systems, elucidation of the energy transfer mechanisms from nanocrystal to transmitter ligand which underpin the upconversion process and propose novel configurations of nanocrystal-sensitized systems.

Figure 1

Schematic of NC-sensitized TTA-UC. (a) Direct TTA: Triplet excitons are transferred directly from the NC to an annihilator via triplet energy transfer (ET). TTA yields the high energy singlet state. (b) Ligand-mediated TTA: Triplet excitons are transferred from the NC to a surface-bound transmitter ligand. A second ET step populates the annihilator, enabling TTA. (c) Illustration demonstrating the two triplet sensitization pathways. Direct ET to the annihilator occurs (left) and ligand-mediated population of the annihilator triplet state (right).

Figure 2

Schematic summarizing the approaches of NC-centered modifications that have been implemented toward improving TTA-UC. Top left: emphasis on development of nontoxic NCs. Top middle: the energy of a NC band gap is size dependent. Top right: passivation of the NC surface is often achieved by ligand addition or coating the surface with another material. Bottom left: the number of dimensions in which NCs are confined can be altered. Bottom middle: the Bohr exciton diameter, and therefore the possible extent of quantum confinement, varies between NC compositions. Bottom right: at the surface of the NCs, vacancies can be filled by the binding group of the ligand or by the shell material.

Figure 3

Schematics of energy transfer in various UC systems. (a) ET from NCs to freely diffusing annihilators impeded by presence of long, 18-C oleic acid passivating ligands. (b) Direct ET from NCs to freely diffusing annihilators occurring when 8-C octanoic acid passivating ligands are used. (c) ET1 from NC sensitizer to 9-ACA transmitter ligand followed by ET2 from 9-ACA to DPA annihilator. The 2-ACA transmitter ligand is shown on the left of the NC; there is less wave function overlap between the 2-ACA and the NC as compared to the 9-ACA and the NC.

Figure 4

Schematic detailing the difficulty of solid-state NC-sensitized TTA-UC, where single-monolayers of nanocrystals (a) outperform thicker sensitizer films (b) as exciton diffusion cannot occur through thicker films.

Figure 5

Schematic detailing potential improvements of NC-sensitized TTA-UC systems.