Exploration of novel concepts for organic photovoltaics

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In organic solar cells, the incoming photons generate localized excited states in the photoactive materials the excitons that can be seen as quasiparticle. In order to be converted into photocurrent, these excitons need to migrate to an interface formed between two materials where they are dissociated. Often, the exciton diffusion length a measure how far an exciton can travel within the material cannot match the optical absorption length needed to fully harvest the solar power. As a consequence, the active materials typically are thin and not completely absorptive layers.As a result of different spin configurations of the incorporated electrons, excited molecular states can either be singlet or triplet states. Conventional organic materials are fluorescent materials, where singlet excitons are excited by photons. Because interconversion to the triplet state typically happens with small rates, exciton migration in solar cells is typically mediated via singlet exciton diffusion. On the other hand, triplet states are much longer-lived because their deactivation is quantum mechanically forbidden. To date, materials have been investigated that efficiently channel singlets to triplets, however the energetic splitting between the states is large and detrimental for application in organic solar cells.This application focuses on the exploration of novel photoactive materials, which are able to efficiently convert singlets into triplets. As a consequence, these triplets will be able to migrate further within the layers, enabling higher external quantum efficiencies. Equally important, in order to overcome the energy loss during the intersystem crossing step, materials will be investigated that show a high intersystem crossing as a result of small energy differences between singlet and triplet states. Solar cells comprising the proposed materials are expected to show a noticeable increase in power conversion efficiency.