Silicon (Si) solar cells are the most widespread solar energy conversion material. In the last decade Si cell efficiencies have plateaued at approximately 25% efficiency, effectively reaching the Schockley-Queisser limit of 30 percent. This is due in part to the energy mismatch between molecules in an overlayer and the Si, along with thermalization loss due to internal conversion in these materials. Therefore, an additive material that could increase the detailed balance limit of Si solar devices would be a significant improvement, preferably one that circumvents the one photon to one electron limit. Materials such as PDIs and -acenes produce triplets through singlet fission, meaning that for every one photon there are two electrons excited. However, the process by which fission-generated triplets can migrate across semiconductor interfaces is poorly understood and parameterized, meaning, it would be ideal and experimentally more viable to study a simplified system in which photo-generated triplets can be directly accessed<i>.</i> Naphthalenediimides (NDIs) readily generate triplets and this work explores the synthesis and photoharvesting properties of NDIs with the goal of attachment to a Si(111) surface. Varying the symmetry of the NDIs will influence the ability to attach them to a Si surface and the mobility of the excitons through the material. Several different synthetic pathways were explored to achieve asymmetric NDIs and depending on the initial amine addition, the asymmetric product is more likely to be formed over the symmetric derivative. Electron withdrawing groups such as bromophenylethylamine encourage asymmetric additions whereas electron donating groups such as (trifluoromethyl)benzylamine form mainly the symmetric products. Once these design NDIs are attached to the surface, a thin film of NDIs will be deposited on top. Varying the type of NDI in the thin film will also lead to an understanding of how the packing affects the triplet generation, mobility, and chemical passivation of the Si surface. Further characterizations of the materials and the surface will include NMR, mass spectrometry, X-ray photoelectron spectroscopy, atomic force microscopy, photoluminescence, and transient reflective spectroscopy.

Design of triplet generating naphthalenediimides for attachment to Si(111) surfaces

ACS Spring 2022

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