title: Polydiketopyrrolopyrroles : Synthesis, Doping and Applications towards Thermoelectrics and Bioelectronics
Creators: name: Krauss, Gert
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Conjugated
subject: Polymer
subject: Organic Semiconductor
subject: Diketopyrrolopyrrole
subject: DPP
subject: PDPP
subject: Bioelectronic
subject: OECT
subject: MIEC
subject: mixed conduction
subject: organic electrochemical transistor
subject: doping
subject: charge transport
subject: thermoelectric
subject: 500
subject: 540

description: Organic semiconductors (OSCs) based on polydiketopyrrolopyrrole polymers (PDPPs) are well- known for their excellent charge carrier properties and stability. They offer firm handles for chemical functionalization and many subunits of the molecule can be tuned almost deliberately. In the present thesis, this chemical versatility of PDPPs was exploited to answer specific scientific questions and deduce design principles for the use of PDPPs in bioelectronic applications. Furthermore, OSCs like PDPPs often need to be doped to adjust their electrical properties to fit desired specifications. Chemical doping is a pivotal tool to tune e.g., the charge carrier density and the electrical conductivity of OSCs, but recently considerable drawbacks of this conventional molecular doping strategy were identified. We therefore proposed a novel doping concept, proved its practicality, and studied it in detail. In the first chapter, we designed a series of four PDPPs with a systematically increasing content of ethylene glycol (EG) substituents. Thereby for the first time, the basic functionality of the thiophene flanked DPP backbone i.e., a pronounced hole-transport behavior was combined with a fine-tuned hydrophilic character. The concept was to gradually increase the hydrophilic character of the polymers to improve the compatibility with water and solvated ions, ultimately leading to different mixed ion-electron conduction properties. It was successfully shown that the compatibility with water and ions indeed follows the intended behavior and mixed conduction was observed at higher EG contents. It was shown that the polymers with higher EG content perform well in organic electrochemical transistor (OECT) devices, both under sodium chloride solution as well as when a cell-growth medium is used as the electrolyte. This was the first report of OECT operation using a cell growth medium. The transistor devices were tested under steady operation for over 1200 consecutive measurement cycles, underlining the stability of the PDPP. Cell viability tests confirmed the absence of cytotoxicity of the polymer. Ultimately, the combination of stable operation and operation in a cell-growth medium and the non-toxic character demonstrates the potential for application in bioelectronic applications. Today to increase and tune the electrical conductivity of organic semiconductors, molecular doping is the strategy of choice. Conventional molecular doping usually demands high molar doping ratios up to several tenths of percent of dopant. High molar doping ratios, however, are always accompanied by side-effects: large amounts of foreign molecules within the semiconducting polymer matrix disturb the polymer morphology and therefore, hinder charge transport. Moreover, the ionization efficiency itself suffers from high doping levels and undesired charge trapping and scattering can occur. To combat these limitations, in the second chapter we have developed a new strategy to elegantly dope polymer semiconductors by using an oxidized hole-transport material (HTM) as the active dopant. The operative point of this concept is that, after the electron transfer from the oxidized HTM to the neutral polymer has occurred, the dopant becomes a HTM which does not hamper or even assists charge transport. The doping process was studied via different methods using a PDPP as organic semiconductor and the thermoelectric behavior of the doped materials was measured. This concept was then extended in the third chapter, where we studied whether oxidized HTMs in higher oxidative state yield advantages over e.g., singly oxidized HTMs. The anticipation that the required molar doping amount can be reduced even further by using multielectron acceptors, rather than one-electron acceptors was proven. The work also compared the often-discussed role of the host polymer’s polarity on the doping process by parallelly using two PDPPs as OSC which explicitly only differ in their side chains (EG vs. alkyl). By comparing mono, di and tetra cationic HTM dopants, it was shown that the charge carrier density of systems doped with the various dopants is directly linked to the valency of the introduced dopant. Thereby, the tetra cationic HTM dopant afforded unusually high doping efficiencies of ca. 20 % at only 5 % molar doping ratio in the more polar host OSC. To conclude, my research work produced guidelines for the design of active materials for operationally stable and non-toxic MIECs to be used in bioelectronics, based on a DPP polymer structure. Furthermore, a novel highly efficient new doping method was established and assessed in detail, rendering strongly increased charge carrier densities and electrical conductivities possible, at unprecedentedly low doping levels.