Aditya G. Baradwaj

Controlled radical polymerization and quantification of solid state electrical conductivities of macromolecules bearing pendant stable radical groups.

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.

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

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.

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

Significance We demonstrate that, instead of the charge generation mechanism, charge collection can readily explain the bottleneck toward higher efficiency single organic semiconductor based OPVs (SS-OPVs). This change in archetype has the potential to transform the design rules for materials used in OPV devices and would inspire searches for a completely different set of polymers for OPV cells. Furthermore, we believe that our findings will broaden the understanding of the physics of charge transport and the impact of charged defect states in organic electronic devices. Hence, this work should have deep and immediate impact on the chemists, materials scientists, and device physicists in the field and would be of broad interest to the organic electronics community. The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials.