(Invited) Photoexcited Electron Transfer from Semiconducting Carbon Nanotubes to Non-Fullerene Acceptors

Abstract

We have previously demonstrated efficient electron transfer from photoexcited small diameter (< 1 nm) semiconducting carbon nanotubes to C60 fullerene electron acceptors in planar, bilayer heterojunction diode devices, with internal quantum efficiency of 90%. However, the out-of-plane exciton diffusion length in planar nanotube thin films is poor (~ 5 nm), limiting the optical density of nanotube films that can be implemented in these devices, which in turn has limited external quantum efficiency to 50% at the nanotube bandgap and to << 50% off-resonance. Polymer solar cells also face a similar limitation (poor exciton diffusion length) and overcome this problem by blending the polymer and acceptor together into a bulk heterojunction. These bulk heterojunctions are typically formed by co-casting the polymer with a solubilized fullerene derivative, phenyl-C61 (or 71) -butyric acid methyl ester (PBCM) to create the blend. However, this strategy has not yet worked well to create nanotube-based bulk heterojunction devices that are more efficient that bilayered devices. One problem is that the electron affinity of PCBM is shallower than the electron affinity of C60, which detrimentally impacts the nanotube to acceptor charge transfer efficiency. To address this shortcoming, new electron acceptors are needed. We have surveyed several new non-fullerene electron acceptors, including both small molecule and polymers, as potential C60 and PCBM replacements. In this talk, the results of this survey will be presented. Moreover, the use of the most promising new non-fullerene acceptors in bulk heterojunctions with semiconducting nanotubes will be discussed.