Singlet Exciton Fission and Photochemical Upconversion

The Sun is the main source of renewable energy on Earth. Our planet receives about 174 PW of solar power. At the same time, global energy consumption from all energy sources is orders of magnitude lower and is equal to approximately 16 TW. Clearly, solar energy has a tremendous potential, as well as numerous advantages over conventional sources of energy. In recent years the photovoltaic industry exhibited significant growth, however the main obstacles that are preventing widespread introduction of solar cells are their still low efficiency and high cost. The major factors contributing to the low efficiency are non-efficient conversion and lack of absorption of sunlight by the active layer material. Sunlight has a broad spectrum spanning over the UV, visible, and NIR optical ranges. Commonly, solar cells are constructed using a single junction, which leads to two fundamental loss mechanisms: i) for photons with a higher energy than the band gap the energy surplus is lost as heat and only the band gap energy harvested, ii) photons with an energy lower than the band gap are not harvested at all. Together, these losses constitute a limitation of the efficiency of a silicon solar cell by half. In the work described in this thesis we have explored ways to decrease these losses by using advanced materials that allow to efficiently down-convert high energy photons, and up-convert low energy photons to the energies close to the band gap energy. Both of these approaches can be achieved in certain classes of organic materials and are known as singlet exciton fission and photochemical upconversion. This thesis combines the results of experimental research of both singlet exciton fission and photochemical upconversion in organic materials by means of laser spectroscopy tools: transient absorption and time resolved luminescence. Singlet exciton fission is a process by which a singlet excited state is converted into a combination two triplet excited states with half the energy. The two triplet together constitute an overall singlet state and hence it is a spin-allowed process that can in principle be very efficient. In this thesis, singlet fission has been studied in a range of perylenediimide (PDI) derivatives in the crystalline state. Substitution at imide nitrogen position allows to obtain different crystal structures in the solid state. In this way the electronic coupling between neighboring molecules in a crystal can be varied without significantly changing the energetics of their singlet and triplet levels. Singlet exciton fission was experimentally detected for a variety of different crystal structures of PDI. The formation of triplet excited states was found to occur on a sub-picosecond time scale. The experimentally detected fission rates and triplet yields were significantly higher than predicted by earlier theoretical calculations and were found to depend only very weakly on the crystal structure. The latter can be explained by intermolecular vibrational modes that could significantly speed up fission in perylenediimides. Photochemical upconversion can be seen as the reverse process of singlet fission. In this case, two triplet excited states, formed through a triplet sensitizer, are combined into a single higher lying singet excited state by triplet-triplet annihilation. Using this approach it is possible to convert low-energy photons that are normally not absorbed in a solar cell into higher energy photons that can be converted efficiently. In this thesis, photochemical upconversion was studied in bi-component mixtures of triplet sensitizer and triplet acceptor in solutions. Metal based porphyrins were used as triplet sensizers, and diphenylanthracene was used as triplet acceptor. The triplet sensitizer produces triplets by fast intersystem crossing due to spin-orbit coupling. The triplets are subsequently transferred to the triplet acceptor by Dexter energy transfer. When two acceptor molecules in the triplet state encounter each other, triplet-triplet annihilation occurs which results in emission from a singlet state. The overall process can be described as conversion of two low energy photons into one high energy photon. In this way, photons that are normally not absorbed by an active material in a solar cell can be converted into photons that can be absorbed, leading to a significant potential enhancement of the overall efficiency. In this work the dependence of the photochemical upconversion process on the metal in triplet sensitizer (porphyrin) was studied. Efficient upconversion was observed for platinum and palladium based porphyrins ( 25-30%) while for the zinc porphyrin the efficiency was considerably less ( 12%). For free base porphyrin, no upconversion was detected. The upconversion process is the result of a series of individual steps outlined above where most steps are the same for all the combinations. The differences in efficiency are traced back to the dependence of energy transfer efficiency on core metal of the porphyrin. Finally, we have made an attempt to achieve upconversion from the near infrared region. For this purpose, we have used porphyrin oligomers consisting of two or four porphyrin rings that have a red-shifted absorption beyond 700 nm. It is shown that using the two-ring oligomer as a sensitizer for a perylene bisimid, it is possible to convert 700 nm light to the 500-600 nm range. For the four-ring oligomer, no noticeable upconversion was observed, possibly because the triplet of the porphyrin oligomer is just below that of the perylene bisimid in this case.