Solar power production and solar energy storage are important research areas for development of technologies that can facilitate a transition to a future society independent of fossil fuel based energy sources. Devices for direct conversion of solar photons suffer from poor efficiencies due to spectrum losses, which are caused by energy mismatch between the optical absorption of the devices and the broadband irradiation provided by the sun. In this context, photon-upconversion technologies are becoming increasingly interesting since they might offer an efficient way of converting low energy solar energy photons into higher energy photons, ideal for solar power production and solar energy storage. This perspective discusses recent progress in triplet-triplet annihilation (TTA) photon-upconversion systems and devices for solar energy applications. Furthermore, challenges with evaluation of the efficiency of TTA-photon-upconversion systems are discussed and a general approach for evaluation and comparison of existing systems is suggested.

Eight 9,10-disubstituted anthracenes have been synthesized, photochemically characterized and used for triplet–triplet annihilation photon upconversion.

Photon upconversion is used to facilitate the production of a solar fuel. This is done by collocating a triplet-triplet annihilation upconversion system in a microfluidic device with a molecular solar thermal energy storage system. Incoherent truncated white light is used to drive the reaction and the green part of the spectrum is upconverted to blue light, which in turn is absorbed by the solar fuel.

An important challenge when developing materials for triplet–triplet annihilation upconversion (TTA-UC) is achieving efficient and well-functioning solid-state systems. We here explore the effect of intramolecular TTA in oligomers and dendrimers based on the 9,10-diphenylanthracene (DPA) chromophore. The macromolecules are sensitized using palladium porphyrin, both in solution and in solid poly­(methyl methacrylate) (PMMA), demonstrating a positive effect on overall upconversion in the solid state correlating with the well-controlled size of the DPA constructs. The UC kinetics is modeled and fit to steady-state and time-resolved emission data to give further insight into the intramolecular excited-state migration and annihilation in the macromolecular annihilator systems.

Triplet−triplet annihilation photon upconversion (TTA-UC) systems contain both an absorbing and an emitting molecule, the sensitizer and annihilator, respectively. Through a series of energy-transfer steps, two low frequency photons can be combined into one high frequency photon. In organic solvents, the required energy transfer steps are limited by diffusion and are relatively efficient. In solid-state systems, however, the diffusion is slower, which usually results in lower efficiencies for these systems. An interesting way around this is to connect the sensitizer and annihilator. In order to increase understanding of the TTA-UC process in supramolecular systems, we synthesized four pyridine-substituted anthracene annihilators capable of coordinating axially to a zinc octaethylporphyrin sensitizer with a maximum binding constant of 6000 M −1 in toluene. This is a first example of a sensitizer−annihilator coordination complex for TTA-UC. Both the upconversion efficiency and the parasitic quenching of excited annihilator singlets by the sensitizer through Forster resonant energy transfer (FRET) were studied. On the basis of the findings herein, possible strategies for future supramolecular TTA systems with minimized FRET quenching are discussed.