Spin–Mediated Effects in Photochemical Upconversion

In photochemical upconversion, the energy of two (or more) photons is pooled as a molecular excitation and emitted as one higher energy photon. Upconversion is a promising means of accessing solar irradiance below the silicon bandgap and surpassing the detailed balance efficiency limit for conventional crystalline silicon solar cells. This thesis focuses on triplet--triplet annihilation upconversion in solution, where the absorbed photons produce excited molecular triplet states, which as they diffuse through the fluid may collide and annihilate to produce one molecular singlet state that fluoresces. As triplet--triplet annihilation is a spin-selective process, it exhibits a magnetic field response, which has traditionally been described and modelled in the context of Atkins and Evans’ theory. Herein, the theory is reformulated using a modern open quantum systems approach, corrected, and extended to describe not only the fluorescent singlet state, but also the optically dark triplet and quintet spin manifolds. Power dependence is then quantitatively incorporated into the model, and used to extract characteristic physical parameters of the triplet--triplet annihilation process. Finally the influence of oxygen on TTA upconversion in solution is investigated. A kinetic model is developed and applied to explain the different photoluminescence profiles of oxygenated versus deoxygenated systems. The conditions required to maximize upconversion photoluminescence intensity in oxygenated solution are determined, providing a set of design principles to guide molecule choices for robust and air-stable upconversion systems in the future.