Conducting polymers for n-type semiconductors, molecular actuators, and organic photovoltaics

Abstract

The majority of conjugated polymers are more stable as p-doped materials than n-doped materials. Stable n-doped polymers are still desirable and for all polymer OPVs, pLEDS, n-channel FETs, and other polymeric electronic devices. The use of donor-acceptor architectures has led to improvements in n-type polymer performance. The approach taken here has been to include a metal-coordination site within a donor-acceptor polymer backbone in order to explore the effect of redox matching between the conjugated polymer backbone and the transition metal center. Conducting polymers have shown promise as polymeric actuators for prosthetics, robotics, and dynamic braille displays. For the majority of conducting polymers, the actuation mechanism is a bulk phenomenon related to the uptake and expulsion of counterions. This performance may be improved by incorporating monomers which display geometry changes as a function of oxidation state into the polymer backbone. The molecular-level actuation should additively yield a macroscopic actuation that would surpass as well as compliment the bulk mechanism discussed above. We have synthesized a conjugated polymer which incorporates the sym-dibenzocyclooctatetraene moiety, which is known to undergo a change in geometry from a tub-shaped neutral structure to a planar radical anion, into the polymer backbone. The solution processability of conjugated polymers promises large-scale roll-to-roll processing for organic photovoltaics. However, the use of thin active layers in the majority of high efficiency devices reported to date prohibits this. The recently reported donor-acceptor copolymer KP115 shows high efficiencies in polymer-fullerene blend bulk heterojunction devices even with very thick active layers. This has been reported to be unrelated to the morphology of the blends. By further characterizing this material and preparing derivatives of this polymer, we aim to relate the unique performance of these devices to a structural feature of the polymer. It is proposed that the low recombination rates observed for these blends may be due to the presence of discrete donor and acceptor units in the polymer backbone. In order to further explore this idea, we have a prepared a derivative of KP115 in which a conjugation-breaking meta-phenyl linkage has been introduced between the silolodithiophene unit and the dithienylthiazolo[5,4-d]thiazole unit.