P-type and N-type Semiconducting Thiophene Copolymers for Organic Electronic Applications
Kazarinoff, Peter D.
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There has been considerable research in the pursuit of organic materials to replace inorganic materials in electronic applications because of their advantages in being light weight, flexible, and solution processable over large areas. In this examination, novel p-type and n-type thiophene based semiconducting copolymers have been analyzed. First, a series of three air stable ester-functionalized p-type polythiophenes were tested for their performance in organic thin film transistors. Increased crystallinity as measured by XRD and DSC accounts for the highest mobility polymer of this series. A HOMO level of 5.6 eV as measured by CV spectroscopy account for the air stability of the polymers over one month. Second, four highly soluble naphthalene diimide (NDI) based n-type polymers are presented - each differing only in the thiophene content comprising the material. Electron mobilities are as high as 0.076 cm2/Vs for the novel material PNDI-3Th. Polymer crystallinity and general macromolecular order was shown to effectively improve by increasing the number of thiophene units along the polymer backbone. The structure-property relationship of NDI-thiophene copolymers is presented as it pertains to organic field effect transistor (OFET) performance. Third, a soluble ladder polymer precursor based on the NDI monomer, PNDI-2Boc, was investigated. After solid state ladderization methods were shown to be unsuccessful, a solution derived cyclization method was employed. Electron mobility improves by four orders of magnitude in PNDI-2BocL using this ladderization process. In the fifth chapter, the PNDI-xTh n-type polymers are blended with P3HT and evaluated as candidate materials in all-polymer OPVs. Spectroscopic techniques including photo-induced absorption spectroscopy are employed and show that polymers PNDI-1Th and PNDI-2Th are the best candidates. The next chapter discusses a novel use for the PNDI-xTh series of polymers, as electron transport materials in inverted polymer solar cells. The polymers are crosslinked and doped in order to improve solvent resistance and electrical conductivity. Devices using this novel interfacial layer show a higher power conversion efficiency compared to devices fabricated with the traditional material, ZnO. In the final chapter, the NDI monomer is polymerized with a series of fused thiophene monomers to create a series of donor-acceptor polymers. It is shown that as thiophene content increases, the band gap of these materials decrease. OFET devices fabricated from these materials show electron mobilities as high as 1.2 × 10-2 cm2/Vs.