Designed organic and hybrid organic-inorganic systems for optoelectronic applications

Passarelli, James

doi:https://doi.org/10.21985/n2-g0qa-n128

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Designed organic and hybrid organic-inorganic systems for optoelectronic applications

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As the pursuit for higher performance and lower cost photovoltaics, and for new applications of optoelectronic devices moves beyond crystalline silicon, there are many unique opportunities for materials research into hybrid organic-inorganic, and organic semiconductors. This dissertation focuses on both hybrid organic-inorganic materials and organic materials for optoelectronic applications. In the first two studies, layered perovskites which consist of alternating organic and inorganic layers are investigated. First, the typically insulating organic layers were systematically changed to organic semiconductors through the synthesis of custom ammonium iodide salts. The energy level of the organic semiconductor is varied in order to better match that of the inorganic semiconductor layer by using the aromatic moieties, naphthalene, pyrene, and perylene. In this investigation, it was found that better matching of the energy levels of the organic and inorganic layers facilitates better charge transfer. Compared to aliphatic layered perovskites, the pyrene containing layered perovskites achieved greater than two orders of magnitude improvement in out of plane conductivity as measured on single crystals (from 10-7 for aliphatic control samples to 10-5 S/m for pyrene containing layered perovskites). The perylene containing layered perovskite showed an improvement of more than an order of magnitude compared to the pyrene containing layered perovskite crystals (10-4 S/m). Photovoltaic devices fabricated from one of the pyrene containing layered perovskites demonstrated a peak photovoltaic efficiency of 1.38% which at the time of publication was a record for this material type. In addition, structural stability was observed in these systems when immersed in water for up to five minutes. In the second investigation, a series of layered perovskites were synthesized with naphthalene-based organic layers as electron donors. When fabricating thin films of these perovskites the electron accepting molecule 1,2-chloranil was incorporated in the precursor solution. This led to incorporation of the small molecule acceptor into the final layered perovskite through donor acceptor complexation. The incorporation of 1,2-chloranil resulted in minimal layer expansion (1-2%) without any structural distortion to the in plane inorganic crystalline lattice. The resultant increased polarizability of the organic layer, owing to donor-acceptor complexation, resulted in a progressive blue shifting of the 1s excitonic transition as more 1,2-chloranil was incorporated within the organic lattice. However, fitting of variable temperature absorption spectra to a modified Elliot model revealed that the bandgap of the layered perovskite is invariant with 1,2-chloranil incorporation. Therefore, it is concluded that the blue shifting of the 1s exitonic transition upon 1,2-chlornail incorporation results from a decreased exciton binding energy. In one of these systems, this reduction was measured from 400 meV in the unaltered state to 230 meV where the corresponding 1,2-chloranil: naphthalene cation ratio was approximately 1: 10. The final project of this dissertation investigated self-assembling donor-acceptor chromophore-peptide amphiphiles. In these systems electron donating, electron accepting, and aqueous solubilizing amino acid sequences were covalently linked. Upon self-assembly in aqueous media, crystalline nanoscale ribbons were observed. By appending amino acids with their innate chirality, the crystal structure of the parent donor-acceptor chromophore-amphiphile was altered. In one system, selected area electron diffraction reveals that the two-dimensional crystalline lattice is halved in one dimension from 16Å by 7Å in the donor-acceptor-carboxylate to 8Å by 7Å donor-acceptor-valine-lysine dipeptide. The resultant assemblies of donor-acceptor peptide-amphiphiles were found to be second harmonic active with induced chirality in the donor and acceptor chromophores, and these novel crystalline nanostructures were found to be ferroelectric. Taken together, these investigations demonstrate the importance of organic-synthetic chemistry in tuning the optoelectronic properties of hybrid organic-inorganic materials and self-assembled organic materials. As optoelectronic devices creep away from the silicon dominated paradigm of the past half century, new materials will be needed that either improve upon the performance of silicon or enable novel functions. This dissertation explored strategies to design and evaluate materials towards these goals.

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