Triplet or Charge-Separated State? Distinguishing Long-Lived, Photogenerated Electronic States in SWCNT/MoS2 Heterojunctions

Single-walled carbon nanotubes (SWCNTs) and their interfaces with an array of materials are promising platforms for next-generation energy-harvesting and conversion devices. Yet the challenge of identifying the excited-state optical spectra of electronic states is a barrier to evaluating their potential for those applications. Here, we use photoinduced absorption-detected magnetic resonance (PADMR) spectroscopy to provide unambiguous assignments in the near-infrared (NIR) photoinduced absorption spectrum of (6,5) SWCNT/MoS2 heterojunctions. Driving the distinguishable magnetic resonance transitions of weakly magnetic dipole-coupled charge separated states versus SWCNT-localized triplets shows that the NIR-induced absorption peak that is bathochromically shifted from the first exciton bleach corresponds solely to separated charges. Triplet excited-state absorption is assigned at higher energy. Signatures for the interfacial charge-separated state and triplet states are stronger in the bilayer than in the neat nanotubes, revealing a charge recombination mechanism across the SWCNT/MoS2 interface that forms spin-polarized triplet states on the SWCNTs.