Controlled Radical Polymerization and Quantification of Solid State Electrical Conductivities of Macromolecules Bearing Pendant Stable Radical Groups

Abstract

Macromolecules with aliphatic backbones that bear pendant stable radical groups (i.e., radical polymers) have attracted much attention in applications where a supporting electrolyte is capable of aiding charge transport in solution; however, the utilization of these materials in solid state applications has been limited. Here, we synthesize a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), through a controlled reversible addition-fragmentation chain transfer (RAFT) mediated polymerization mechanism to generate well-defined and easily-tunable functional polymers. These completely amorphous, electronically-active polymers demonstrate relatively high glass transition temperatures (Tg ∼170 °C) and, because of the aliphatic nature of the backbone of the radical polymers, are almost completely transparent in the visible region of the electromagnetic spectrum. Additionally, we quantify the conductivity of PTMA (∼1×10(-6) S cm(-1)) and find it to be on par with pristine π-conjugated polymers such as poly(phenylene vinylenes) (PPVs) and poly(3-alkylthiophenes) (P3ATs). Furthermore, we demonstrate that the addition of small molecules bearing stable radical groups provides for more solid state charge hopping sites without altering the chemical nature of radical polymers; this, in turn, allows for an increase in the conductivity of PTMA relative to neat PTMA thin films while still retaining the same high degree of optical transparency and device stability. Because of the synthetic flexibility and easily-controlled doping mechanisms (that do not alter the PTMA chemistry), radical polymers present themselves as promising and tunable materials for transparent solid-state plastic electronic applications.

Tuning the Thermoelectric Properties of a Conducting Polymer through Blending with Open-Shell Molecular Dopants

“…Previously, both the highly robust nature of this stabilized radical site and its ability to undergo oxidation have been well-characterized. 13,14 In addition to its high potential for one electron chemistry, the highly-polar nature of the Small molecules bearing stable radical groups have been of immense utility in a number of organic electronic applications (e.g., magnetic devices, 15 spin-controlled valves 16 ); however, the implementation of these functional materials in thermoelectric devices has not been established. This is despite the fact that having a very well-defined, low dispersity transport level has been predicted to yield materials with remarkably high thermopower values in single molecule systems.…”

Suppressing the environmental dependence of the open‐circuit voltage in inverted polymer solar cells through a radical polymer anodic modifier

“…37,38 This high degree of optical transparency of PTMA as a thin film has been observed previously. 26 Furthermore, the consistent reproduction of the P3HT:PCBM absorption spectrum across all PTMA coating conditions speaks well to the fact that the coating treatment used to deposit the PTMA layer does not impact the nanostructure of the underlying photovoltaic active layer. And, the topological maps imaged through atomic force microscopy (AFM) indicate that there was little change in surface topology and surface roughness when PTMA was cast on top of the P3HT:PCBM blend (Supporting Information Fig.…”

“…Radical polymers (i.e., macromolecules with nonconjugated backbones but with stable radical sites attached as pendant groups on the polymer chain) have emerged as potentially revolutionary materials in a host of organic electronic devices; [26][27][28][29][30][31] however, they have not been utilized as charge-conducting layers in OPV devices. Here we fundamentally alter this archetype, and we demonstrate that the addition of a PTMA layer, which can be deposited from solution, causes these inverted OPV devices to have a reproducible open-circuit voltage (V oc ) of 0.6 V. Additionally, the V oc does not vary with respect to device exposure to ambient conditions over the course of multiple days, even when the easilyoxidized thin film of silver (Ag) is used as the anodic contact.…”

“…Previously, we have established that PTMA is a charge-conducting material in the solid state, and due to its amorphous nature, the charge transport in thin films is isotropic. 26,31 Moreover, the singularly occupied molecular orbital (SOMO) transport energy level of PTMA resides at 5.2 eV removed from free vacuum (inset of Fig. 1), 27,31 which causes the transport level of PTMA to align well with the highest occupied molecular orbital (HOMO) energy levels of many common (e.g., the P3HT that was used in this work) and emerging electron-donating (hole-transporting) semiconductors implemented in high-performance OPV devices.…”

Effect of intrachain sulfonic acid dopants on the solid-state charge mobility of a model radical polymer

“…c o m / l o c a t e / t s f exposure conditions seen commonly in ambient and operational conditions [24][25][26]. And, although radical polymers lack the molecular properties (e.g., a conjugated backbone and semicrystalline regions in the thin film state) that are highly preferred for high charge transport in conjugated polymers, the redox charge transport mechanism allows PTMA to possess solid-state electrical transport properties that are comparable to common, pristine (i.e., not doped) conjugated polymers [e.g., poly(3-hexylthiophene) and poly(2-methoxy-5-(2-ethylhexyloxyl)-1,4-phenylenevinylene)] [27,28]. One strategy utilized frequently in the conjugated polymer regime to increase the charge transport ability of the materials is the introduction of small molecule and macromolecular dopants to optoelectronicallyactive materials [e.g., the addition of poly(styrene sulfonate) to poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) produces a material with a high electrical conductivity] [29][30][31][32][33][34].…”