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Surface modification of nanomaterials for electronically tunable polymer composites
Levin, Jacob
Levin, Jacob
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2022
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Abstract
Developing a sustainable energy future has recently come to the forefront of many research endeavors in academia, industry, and government laboratories. One approach that has been presented as a viable, long-term option is the conversion of solar radiation to power, using perovskite solar cells. These devices have garnered a great deal of attention because of their ability to absorb large amounts of light, superior charge-carrier mobility, scalability, and high efficiencies. For these devices to be produced on an industrial scale, further optimization, and development of layers such as the hole transport layer (HTM) will require significant advancements.
Current commercially used HTM are polymers such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and poly(3-hexylthiophene-2,5-diyl) (P3HT), which require multistep syntheses with complex purifications leading to high cost materials. Until recently, these were the challenges that researchers have been attempting to address through new synthetic approaches. Fortunately, a library of new triarylamine based polymers with tunable properties have been developed, addressing many of these constraints while producing efficiencies and stabilities exceeding PTAA. Although based on the hydrophobic nature of these polymers, they do have desired interfacial interactions with perovskite ink during the solution deposition process. This unfavorable condition often requires the addition of an interfacial wetting layer which costs upwards of $2,000/g. This thesis describes the enhancement of wetting properties within triarylamine based polymers, while retaining their ability to behave as excellent HTMs.
The approach was to maintain the backbone of these polymers while adjusting the long alkyl chain moieties to increase hydrophilicity, ensuring that the optical and electronic properties would not be affected. By introducing methoxyethoxy(ethane) chains, the inherent affinity for polar solvents increased significantly. With this in mind, small equivalence of these wettable monomers were introduced into the polymerization resulting in random terpolymers with improved wettability. Different incorporation ratios were explored for both carbazole and fluorene-based polymers, resulting in HTMs with enhanced perovskite ink affinity that did not require additional interfacial wetting layers.
The second portion of this thesis focuses on tuning the work function and conductivity of these wettable polymers through functionalized nanomaterial doping. By surface functionalizing nanoparticles with electron withdrawing/donating ligands, the work function and conductivity of those particles can be adjusted. The approach focused on three main nanomaterials: graphene, carbon nanodiamonds and MXenes. Synthetic routes were developed for the surface functionalization of each material, followed by extensive characterization and eventual doping into the wettable polymer termed CzMee10. Doing so allowed for tunability of the polymer/nanomaterial composite’s work function over a range of 0.51 eV depending on the nanomaterial and ligand used. The conductivity at the time of writing this thesis is still being evaluated. This analysis and characterization encompassed much of the final part of the thesis and is followed by a short introduction to future projects as well as a recap of the interdisciplinary skills utilized to achieve these advancements.
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