Ultrafast Spectroscopic Signature of Charge Transfer between Single-Walled Carbon Nanotubes and C₆₀

Abstract

The time scales for interfacial charge separation and recombination play crucial roles in determining efficiencies of excitonic photovoltaics. Near-infrared photons are harvested efficiently by semiconducting single-walled carbon nanotubes (SWCNTs) paired with appropriate electron acceptors, such as fullerenes (e.g., C60). However, little is known about crucial photochemical events that occur on femtosecond to nanosecond time scales at such heterojunctions. Here, we present transient absorbance measurements that utilize a distinct spectroscopic signature of charges within SWCNTs, the absorbance of a trion quasiparticle, to measure both the ultrafast photoinduced electron transfer time (τpet) and yield (ϕpet) in photoexcited SWCNT–C60 bilayer films. The rise time of the trion-induced absorbance enables the determination of the photoinduced electron transfer (PET) time of τpet ≤ 120 fs, while an experimentally determined trion absorbance cross section reveals the yield of charge transfer (ϕpet ≈ 38 ± 3%). The extremely fast electron transfer times observed here are on par with some of the best donor:acceptor pairs in excitonic photovoltaics and underscore the potential for efficient energy harvesting in SWCNT-based devices.

Energy transfer pathways in semiconducting carbon nanotubes revealed using two-dimensional white-light spectroscopy

“…Photogenerated free carriers on (8,6/7) nanotubes could perturb the S 2 of nearby (7,5/6) nanotubes, Stark shifting the electronic states to yield a photoabsorption and photobleach, as previously assigned using transient absorption spectroscopy 12,29 . The free carrier signals are much weaker than the exciton peaks, which is consistent with prior work that showed charge generation is a minority process 12,30 . Unlike pump-probe measurements, these 2D-WL cross peaks have no other overlapping features and so are background free, allowing the free carrier dynamics to be resolved at very early time delays.…”

“…Unlike pump-probe measurements, these 2D-WL cross peaks have no other overlapping features and so are background free, allowing the free carrier dynamics to be resolved at very early time delays. Auger ionization has also been shown to produce free carriers 31 , however based on our signal intensity, we believe that we are in a regime where most nanotubes have a single exciton initially excited (see Supplementary discussion) 30 . Figure 3 | 2D-WL spectra and energy levels for the S 2 /S 2 and S 2 /S 1 quadrants.…”

Trap-limited carrier recombination in single-walled carbon nanotube heterojunctions with fullerene acceptor layers

“…The principles of operation for such devices echo those of other bilayer organic PV devices: photons absorbed in s-SWCNTs produce correlated electron-hole pairs (excitons), which diffuse to the interface with the "acceptor" (C 60 ) layer, an interfacial free energy driving force induces exciton dissociation via fast electron transfer (≤ 120 fs) from s-SWCNTs to fullerenes [1,10], and charges then migrate through each phase to the appropriate contacts where they are collected. Although significant improvements have been made in SWCNT-based PV devices, balancing this complex series of steps can make device optimization arduous and significant improvements remain to realize the full potential of SWCNT-based PV.…”

“…In both polymer-SWCNT and SWCNT-fullerene systems, TRMC reveals the presence of long-lived charges following excitation of the electron donor [14][15][16]. These long-lived charges result from exciton dissociation across the interface [10], such that electrons and holes separated into the two different phases experience a reduced Coulombic attraction and a correspondingly reduced rate of recombination. In the SWCNT-C 60 system, wavelength-dependent excitation of different SWCNT bandgaps revealed that the efficiency of charge generation depends sensitively on the free energy driving force for charge separation (ΔG CS ), which trends toward zero at a diameter of ~1.1 nm [16].…”

“…For example, residual charges on (7,5) sSWCNTs (e.g. from purification in strong acids) [23] could induce absorption at ~1230 nm that is associated with charged excitons (trions) [10,24,25]. The TRMC technique employed here enables direct measurement of the photoinduced free carrier population by monitoring the resulting microwave absorption by the sample ΔP, which is proportional to the high-frequency (~9 GHz) photoconductance (ΔG).…”