Self-Doped Conjugated Polymers and Copolymers - Feasibility and Advantages in Devices

Mixed ionic-electronic conductors are widely studied for accumulation mode (undoped systems) or depletion mode (doped systems) devices. PEDOT:PSS has been established as the working horse for depletion mode organic electrochemical transistors (OECTs) as it consists of the conjugated polymer PEDOT⁺ which is intrinsically doped and stabilized by PSS^- counterions. It is neither soluble in water nor organic solvents, wherefore thin films are processed from aqueous dispersion. This results in a nonuniform film morphology consisting of doped PEDOT-rich domains surrounded by an insulating PSS matrix. This particular structure leads to several drawbacks such as long-term instability. Furthermore, its complex structure hampers the ability for chemical modification.<br/><br/>In contrast to PEDOT:PSS, most conjugated polymers are intrinsic (pristine or undoped) semiconductors and hence show very low electrical conductivities. Therefore, doping is necessary to increase the charge carrier density and thus the electrical conductivity. Upon molecular doping, active redox species are added to the OSC which can donate (n-type) or accept electrons (p-type) generating free charge carriers. For p-type doping, this means a transfer of electrons from the highest occupied molecular orbital (HOMO) of the OSC to the lowest unoccupied molecular orbital (LUMO) of the dopant. Issues here are the requirement of deep-lying dopant LUMO, thus stability against air/water, and the need for high amounts of dopants for charge carrier formation which ultimately disturbs the film morphology.<br/><br/>To address these challenges, our group took high efforts in the field of molecular doping of conjugated polymers. The first concept published takes advantage of HOMO-HOMO doping by mixing two donor materials (OSC-1 and OSC-2). OSC-2 is a chemically oxidized small molecule semiconductor namely Spiro-OMETAD(TFSI)₂ which is capable to oxidize the OSC PDPP[T₂]-EDOT (OSC-1). Thereby, the doping takes place by an electron transfer from the HOMO_OSC-1 to the partly occupied HOMO_OSC-2.¹ A second work extends this concept through the use of multielectron acceptor salts. For this, oxidized triphenylamines were linked to form multielectron acceptor dopants, which can accept up to four electrons per dopant. This strategy allows for significantly reducing the number of dopant molecules compared to common single electron acceptor dopants.²<br/><br/>Here, we present a novel synthetic strategy of a conjugated polyelectrolyte (CPE) based on a polythiophene-derivative toward a stable self-doped system which can be solution-processed from water as well as organic solvents. Our new CPE is intrinsically doped after synthesis, which is promoted by the stabilization of free charge carriers by its ionic side chains. We present self-doping as an elegant way to avoid the use of molecular dopants fully, while still maintaining high and stable conductivity. This was investigated by time-resolved spectroelectrochemistry in aqueous electrolytes, UV-vis spectroscopy in thin films in the dry state as well as conductivity measurements. We were furthermore able to employ our self-doped CPE as a mixed conductor and showed its application in a depletion mode OECT device with high transconductance and currents. By copolymerization with a comonomer, which is not self-dopable, we were able to tune the self-dopability and therefore to switch the transistor's mode of operation towards a well-performing accumulation mode device with a low threshold voltage.³ These findings could help to study the self-doping processes and to develop new CPEs with low threshold voltages towards innovative device applications.<br/><br/>(1) Goel, M.; Siegert, M.; Kraus, G.; Mohanraj, J.; Hochgesang A.; Heinrich, C. D.; Fried, M.; Pflaum, J.; Thelakkat, M. <i>Adv. </i><i>Mater.</i> <b>2020</b>, 32, 2003596<br/>(2) Kraus, G.; Hochgesang A.; Mohanraj, J.; Thelakkat, M. <i>Macromol. </i><i>Rapid Commun.</i> <b>2021</b>, 42, 2100443<br/>(3) Hungenberg, J.; Hochgesang, A.; Meichsner, F.; Thelakkat, M. <i>To be submitted.</i>