Quantum Dot-Sensitized Triplet–Triplet Annihilation Photon Upconversion for Solar Energy Conversion and beyond

ConspectusThe Nobel Prize in Chemistry 2023 was awarded to Moungi G. Bawendi, Louis E. Brus, and Aleksey Ekimov for the discovery and synthesis of quantum dots (QDs). Since the discovery of QDs in 1980s, endeavors have been put to improve synthetic strategies of QDs with controllable sizes, crystal structures, and surfaces. With the development of about half a century, QDs have been used in multiple optoelectronic applications, which are based on the conversion between one photon and one exciton. In this case, photons with energies lower than the bandgap cannot be absorbed by semiconductors. For example, solar cells or photocatalysts are transparent to sub-bandgap near-infrared (NIR) light in the solar spectrum, rendering a transmission loss.Photon upconversion (PUC), the combination of multiple low-energy photons to a high-energy one, plays a critical role in reshaping the solar spectrum and mitigating the transmission energy loss. Among different strategies to achieve PUC, QD sensitized triplet–triplet annihilation PUC exhibits high response to solar irradiation, which makes it an ideal strategy for solar energy conversion.Here, we introduce QD-sensitized triplet–triplet annihilation PUC and emphasize how this strategy is applied in solar energy conversion. This Account starts with the introduction of QDs acting as active materials to directly convert light to electricity and chemical bonds due to the strong light absorptivity, the flexible and tunable surface chemistry, and the capability of accommodating and transferring multiple electrons/holes. Then we move on to QD-sensitized triplet–triplet annihilation PUC which can upconvert NIR photons under one-sun condition. We discuss the deconvolution of the total QY of PUC into the efficiencies of triplet energy transfer from QD sensitizers to organic emitters, the triplet–triplet annihilation, and the singlet emission of emitters. Next, we summarize the progress in PUC-enhanced photovoltaics and photochemical reactions. For photovoltaics, we introduce two examples that can upconvert light beyond 1100 nm, which makes it possible to couple PUC with Si-based solar cells. For photochemical reactions, the discussion is separated into visible-to-ultraviolet PUC for ultraviolet light-triggered reactions and NIR-to-visible PUC for visible light-triggered reactions. In the last section, we point out challenges and future directions in this field. For example, the low QY of solid-state PUC and strong reabsorption must be overcome for photovoltaic application, and more types of PUC-assisted photochemical reactions are to be explored. We also propose other potential applications of QD-sensitized triplet–triplet annihilation PUC such as biological imaging and phototherapy. We hope that this Account will help readers understand PUC systems and promote the application of efficient PUC in solar energy conversion and beyond.