Systematic Studies of Triplet Annihilating Species for Photochemical Upconversion

Doctoral thesis, 2023

With climate change already setting new temperature records and causing more extreme weather events, transitioning to clean and renewable energy sources is more urgent than ever. Solar energy harvesting arguably holds the most promise with the abundance of terrestrial sunlight available. However, significant improvements to existing solar technologies would be beneficial as large parts of the solar spectrum currently can’t be properly exploited in devices.

Triplet-triplet annihilation photochemical upconversion (TTA-UC) is a process in which the interplay between a light-absorbing sensitizer and an emissive annihilator compound renders the conversion of low-energy to high-energy light; a promising prospect for existing solar devices. Photovoltaic systems require incident photons that hold an energy above a certain threshold value called the band gap, and TTA-UC could be used to make use of photons with energies below the band gap of the photovoltaic cell. Additionally, many photochemical reactions require high-energy photons, commonly in the ultraviolet (UV) region, to proceed. TTA-UC provides a pathway towards driving such demanding reactions with visible light instead.

In this thesis, the properties of the annihilator species are investigated in a systematic fashion. TTA-UC is a process dependent on several energy transfer events which are diffusion-controlled in fluid media, but for device incorporation solid state solutions are often needed. Dimers based on diphenylanthracene (DPA) are investigated as potential candidates to perform intramolecular (i)-TTA, with the two annihilating triplets emanating from within the same molecule. It is shown that DPA dimers indeed can perform i-TTA, and important insights regarding the underlying mechanism are highlighted.

Visible-to-UV TTA-UC has long suffered from much lower efficiencies than other spectral transformations. Pairing a set of annihilator molecules with nanocrystal (NC) sensitizers based on CdS yield important insights into the energetics regarding NC-sensitized TTA-UC, as well as improved conversion efficiencies when using 2,5-diphenyloxazole as the annihilator. Further improvements are achieved when switching to the organic sensitizer 4CzBN, which when paired with several UV-emitting annihilators yield efficient visible-to-UV TTA-UC. A record TTA-UC quantum yield of 16.8% is reported for the best-performing system, which is an almost 2-fold improvement on the previous record.

Finally, it is shown that triplet excimer formation competes with TTA in annihilator species based on naphthalene, which will cause TTA-UC efficiencies to decrease. The excimer formation pathway can be modulated by switching the type of substituent used, with more bulky substituents promoting TTA. The collective insights gathered herein provide a roadmap for future annihilator design, moving us closer to the ultimate goal of harnessing TTA-UC for solar energy conversion.