Radical Polymers As Transparent Conductors in Organic Photovoltaic and Thermoelectric Applications

Conducting polymers have been studied extensively for their use in a range of energy conversion devices. Previously, π-conjugated polymers have dominated the research focus due to the high degree of electronic delocalization associated with their molecular structure; however many challenges regarding synthetic routes and the control of nanoscale structure continue to prevent their viability in widespread applications. To this end, we will discuss an emerging class of non-conjugated, electronically-active macromolecules, radical polymers, which have shown immense potential to transport charge despite being completely amorphous. These redox-active macromolecules have been used previously in electrolyte-supported applications (e.g., flexible batteries). However, quantifying the ability of these non-conjugated macromolecules to conduct charge in the solid state has not been studied to the same degree.

Here, a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), was synthesized using the reversible addition-fragmentation chain transfer (RAFT) controlled polymerization mechanism, which produced polymers with readily-tunable molecular weights (5 kg mol^-1 < M_n < 100 kg mol^-1) and narrow molecular weight distributions (Ð ≤ 1.2). Importantly, we have demonstrated, for the first time, that the solid-state hole mobility (µ_h ~10^-4 cm² V^-1 s^-1) and conductivity (σ ~10^-5 S cm^-1) values of these radical polymers are of the same order of many common conjugated polymers [e.g., poly(3-hexylthiophene) (P3HT)]. Furthermore, because the polymer backbone is non-conjugated, these macromolecules are extremely transparent. Because of these promising optoelectronic properties, PTMA lends itself readily to serve as the hole-transporting layer in both regular and inverted architectures of organic photovoltaic (OPV) devices. We have shown that the addition of this radical polymer interlayer facilitated charge transport in both OPV geometries, and it allowed for relatively high power conversion efficiencies to be realized. Additionally, we have established that PTMA has a rather high thermopower value (S > 700 µV K^-1), which allowed it to perform at high levels in thermoelectric devices. Therefore, the combination of a well-controlled synthetic methodology with high electronic performance allows these radical polymers to be of great utility when they are incorporated into polymer-based electronic conversion devices.