Quantification of the solid-state charge mobility in a model radical polymer

Abstract

We establish that an oft-used radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), has a solid-state hole mobility value on the order of 10−4 cm2 V−1 s−1 in a space charge-limited device geometry. Despite being completely amorphous and lacking any π-conjugation, these results demonstrate that the hole mobility of PTMA is comparable to many well-studied conjugated polymers [e.g., poly(3-hexylthiophene)]. Furthermore, we show that the space charge-limited charge carrier mobility of these macromolecules is only a weak function of temperature, in contrast to many thermally-activated models of charge transport in polymeric materials. This key result demonstrates that the charge transport in radical polymers is inherently different than that in semicrystalline, conjugated polymers. These results establish the mechanism of solid-state charge transport in radical polymers and provide macromolecular design principles for this emerging class of organic electronic materials.

On the Environmental and Electrical Bias Stability of Radical Polymer Conductors in the Solid State

“…Therefore, electrolyte‐supported radical polymer systems have extremely useful transport properties, as the liquid‐based electrolytes provide ample mobility to the radical polymer pendant groups. On the other hand, only recently was it established that these types of reactions can also occur in a rather rapid manner in solid‐state organic electronic applications . Specifically, the model radical polymer, poly(2,2,6,6‐tetramethylpiperidinyloxy methacrylate) (PTMA), is an extraordinarily transparent material and can be easily synthesized in large batches .…”

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

“…For instance, a PTMA-co-PVS macromolecule containing 12% PVS (on a molar basis) was sandwiched between ITO and Al contacts to form the ITO/PTMA-co-PVS/Al space charge-limited device geometry. Previously, we have demonstrated that the singularly occupied molecular orbital (SOMO) energy level that is used to transport charge in PTMA forms a nearly ohmic contact with ITO and that the work function of Al is so far-removed from this transport level that the Al contact serves as an excellent blocking electrode [27]. Because the hole and electron diffusion are negligible in this system and the hole mobility is fieldindependent over the voltages employed, there is a region of the current density versus voltage curve that can be modeled according to the Mott-Gurney relationship (Eq.…”

“…The dielectric constant was assumed to be 3.4 because this is the experimentally-measured dielectric constant of PTMA, and representative current voltage versus voltage curves are depicted in Fig. 7a [27].…”

“…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].…”

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

“…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.…”