Triplet Dynamics in Crystalline Perylene Diimides

Conjugated organic materials are interesting for application in opto-electronic devices where they can act as a light absorbing layer, for instance in solar cells or as a light emitting layer in light-emitting diodes. In addition, they can also be used as the semiconducting materials in for instance field-effect transistors. Conjugated organic materials have certain desirable properties that are generally found in organic materials such as light weight, flexibility and cheap processing from solution. A particularly attractive aspects of such materials is that their solid-state properties can be tuned by making changes in the molecular structure by organic synthesis techniques. This also makes it possible to modify the materials so that they exhibit more uncommon processes that may be beneficial for solar cells. Two of such processes, singlet exciton fission (SF) and triplettriplet annihilation upconversion (TTA-UC) are the main subjects of this thesis. The first (SF) is a process in which a singlet excited state, formed by absorption of light, is transformed into a combination of two triplet excited states each with half of the energy. This can, in principle, double the number of electrons that are injected in a solar cell device, and hence double the current from the device. The second (TTA-UC) is the reverse process of SF in which two triplet states with low energy can be combined into a single higher-energy singlet excited state from which an electron can be injected in a solar cells. Exploiting these two processes can in principle lead to considerable improvements in the efficiency of solar cells based on these devices. SF can be exploited to use the excess energy in photons with more than twice the bandgap energy to excite an additional electron. In this way, the excess energy that is otherwise lost as heat, is used to increase the current and therefore the overall energy efficiency of the device. TTA-UC addresses another energy-loss in solar cells, that of photons in the solar spectrum that have a lower energy than the bandgap of the active material of the solar cell. TTA-UC can be used to combined the energy of two of these photons, that are normally not absorbed by the solar cells, to generate a single higher-energy excited state that has sufficient energy to charge separate at an interface. Together, these two processes can address two of the factors that cause major energy losses in solar cells. In order to efficiently exploit these processes, a detailed fundamental understanding of these processes is required, with particular emphasis on the effect of molecular packing in the solid-state as this is the state where they are to be used in devices.