Long-Lived Free Charge Carriers at Heterojunctions between Semiconducting Single-Walled Carbon Nanotubes and Perylene Diimide Electron Acceptors

Abstract

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been thoroughly investigated as the components of various photovoltaic cells due to several advantages such as spectral tunability, absence of charge-transfer (CT) states, giant aspect ratio, chemical robustness, and hydrophobicity. In the previous study, it was reported that the heterojunctions between s-SWCNTs and perylene diimide (PDI)-based electron acceptors yield long-lived charge separated states whose lifetimes are more than 1.5 µs. Besides the potential of PDI-based electron acceptors to substitute fullerene-based electron acceptors, this work noted the significance of the molecular geometries of PDI-based electron acceptors which result in molecular aggregation and the associated charge delocalization in the acceptor phase. However, the true characteristics of the long-lived charge carriers for these heterojunctions have not been revealed yet. For instance, the degree to which the charge carriers generated at these heterojunctions are free or trapped was not yet clear. Moreover, it was not determined whether the charge recombination process from these heterojunctions is monomolecular-like or bimolecular-like. In this study, we explored the nature of charge carriers for the heterojunctions between (6,5) s-SWCNTs and two different PDI-based electron acceptors by combining two effective spectroscopic techniques: transient absorption (TA) and time-resolved microwave conductivity (TRMC). Two PDI-based electron acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, were synthesized and coated on (6,5) s-SWCNT films to form donor-acceptor heterojunctions. TA and TRMC studies reveal that the dynamics of the charge-separated states across the (6,5) s-SWCNT/PDI-based acceptor heterojunctions remain similar over three orders of magnitude in absorbed photon flux. This fluence independence of the heterojunctions indicates that the charge carriers recombine ‘pseudo’-monomolecularly. Moreover, the charge recombination kinetics from TA and TRMC studies are well-matched, indicating that most of the generated charge carriers are free, not trapped. The unconventionally strong suppression of bimolecular charge recombination from these heterojunctions, supported by fluence independence of charge recombination dynamics, may be attributed to the high carrier mobility and good charge delocalization in both (6,5) s-SWCNTs and PDI-based acceptors. These factors can also be regarded as the origin of high free charge carrier generation in these heterojunctions. These photophysical studies provide the fundamental understandings of the charge generation process in s-SWCNT-based heterojunctions and how different electron acceptor materials can impact the nature of charge generation with respect to the heterojunction energetics and molecular orientations. The results can inform rational design strategies for s-SWCNT-based optoelectronic applications.