New Insights Regarding the Efficient Design of Photon Upconversion Systems

Licentiate thesis, 2021

The transition from fossil fuels to renewable energy sources is arguably the most important weapon in combating global climate change. Existing solar technologies could be significantly improved using unconventional ways of manipulating the solar spectrum. Photon upconversion by triplet-triplet annihilation (TTA-UC) is a process in which non-coherent, low-energy light can be converted into light of higher energy. This could be used to e.g. harvest photons from below the band gap of existing solar cell materials, thus increasing the efficiency of such cells, or to produce UV light that can be used to drive photochemical reactions, such as photocatalytic water splitting for hydrogen fuel production.

In this thesis, two challenges currently facing the photon upconversion community are addressed. Successful implementation of TTA-UC systems in photovoltaics will require solid-state solutions, but most systems to date work best in liquid solution. Here, we investigate a group of new dimer compounds based on 9,10-diphenylanthracene, and evaluate their performance in TTA-UC when paired with platinum octaethylporphyrin. Importantly, the dimers have the ability to perform intra-molecular TTA-UC, which in solid-state systems is a potentially important path to afford higher efficiencies. Using spectroscopic techniques and modelling, the mechanism responsible for intra-molecular TTA-UC in solution under different conditions is elucidated.

TTA-UC for the production of UV light has so far suffered from low efficiencies. In the second part of this thesis, the design of TTA-UC systems based on cadmium sulfide nanocrystals (CdS NCs) is systematically investigated. The results show that relatively high efficiencies can be reached. This was achieved through synthesis of CdS NCs with improved quality, by properly aligning the energy levels of participating compounds, and by adapting how the samples were prepared. While further improvements are still necessary, these findings constitute important steps toward the development of high-efficiency systems for UV light production using solar energy.