Photon upconversion is the process by which low energy photons combine to form a higher energy photon. This phenomenon is useful for application in solar energy and photocatalysis. Traditional methods for upconversion, like second harmonic generation in inorganic crystals, require high-intensity, coherent light for efficient upconversion. In contrast, triplet-triplet annihilation upconversion (TTA-UC) can use incoherent, low-intensity light such as that emitted from the sun. TTA-UC begins with the creation of triplets in a donor material through intersystem crossing (ISC) from photoexcited singlet states, followed by the transfer of the triplet excitation energy to acceptor molecules. When two triplet excitons on separate acceptor molecules meet each other via exciton diffusion this can result in triplet-triplet annihilation, a process whereby one triplet exciton is promoted to a higher energy single state (from which a photon can be emitted) at the expense of the other triplet exciton returning to the ground state. In this study, we investigate ways of improving TTA-UC in organic semiconductors. One improvement lies in minimizing the energy loss typically associated with intersystem crossing by employing thermally activated delayed fluorescence (TADF) donor materials, which are known for having small singlet-triplet splitting energies. Furthermore, we improved the efficiency of intersystem crossing in the TADF donor by incorporating heavy atoms, in this case bromine, thereby promoting spin-orbit coupling. Finally, we studied a variety of acceptor molecules in an attempt to understand how chemical structure affects the overall efficiency of the TTA-UC process.