Charge Transfer from Photoexcited Semiconducting Single-Walled Carbon Nanotubes to Wide-Bandgap Wrapping Polymer

Abstract

As narrow optical bandgap materials, semiconducting single-walled carbon nanotubes (SWCNTs) are rarely regarded as charge donors in photoinduced charge-transfer (PCT) reactions. However, the unique band structure and unusual exciton dynamics of SWCNTs add more possibilities to the classical PCT mechanism. In this work, we demonstrate PCT from photoexcited semiconducting (6,5) SWCNTs to a wide-bandgap wrapping poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(6,6')-(2,2'-bipyridine)] (PFO-BPy) via femtosecond transient absorption spectroscopy. By monitoring the spectral dynamics of the SWCNT polaron, we show that charge transfer from photoexcited SWCNTs to PFO-BPy can be driven not only by the energetically favorable E₃₃ transition but also by the energetically unfavorable E₂₂ excitation under high pump fluence. This unusual PCT from narrow-bandgap SWCNTs toward a wide-bandgap polymer originates from the up-converted high-energy excitonic state (E₃₃ or higher) that is promoted by the Auger recombination of excitons and charge carriers in SWCNTs. These insights provide new pathways for charge separation in SWCNT-based photodetectors and photovoltaic cells.

Figure 1

Schematic energy level alignment of (6,5) SWCNT and PFO–BPy. The density of states of (6,5) SWCNT with characteristic van Hove singularities of the valence (v₁, v₂, v₃) and conduction (c₁, c₂, c₃) band was based on ref (28) and shifted by the reported ionization potential. The HOMO and LUMO energies of PFO–BPy indicated by red horizontal lines were reported by Jang et al. The gray arrows are simplified representations for observed excitonic absorption bands E₁₁, E₂₂, and E₃₃. The inset shows the molecular structure of PFO–BPy.

Figure 2

Stationary absorption spectra of surfactant-dispersed (6,5) SWCNT in water, PFO–BPy-wrapped (6,5) SWCNT Hybrid in THF, and PFO–BPy in THF. The positions of absorption peaks are marked with corresponding colors.

Figure 3

Selected TA spectra of SWCNT in water upon the (a) E₁₁, (b) E₂₂, and (c) E₃₃ excitations. Experimental conditions: (a) λ_ex = 1000 nm, (b) λ_ex = 576 nm, and (c) λ_ex = 350 nm; pump energy: 100 nJ·pulse^–1. Dotted lines highlight major transition manifolds.

Figure 4

Selected TA spectra for the Hybrid in THF upon the (a) E₁₁, (b) E₂₂, and (c) E₃₃ excitations. Experimental conditions: (a) λ_ex = 1000 nm, (b) λ_ex = 576 nm, and (c) λ_ex = 350 nm; pump energy: 100 nJ·pulse^–1. Dotted lines highlight major transition manifolds. The shaded shapes indicate the absorption signature of the suspected SWCNT polaron. The asterisks (*) denote the wavelength of 1050 nm.

Figure 5

Pump-energy-dependent peak-shifting dynamics of the E₀₀ → E₁₁ bleaching in TA spectra of the SWCNT (a, b, c) and the Hybrid (d, e, f) in the time window of 0.1–500 ps. Excitation wavelength and corresponding pump energy per pulse are given in legends. Due to dispersion instability under high pump fluences, TA spectra of SWCNT are unavailable at higher fluences in b and c.

Figure 6

(a) NIR stationary absorption spectra monitor the oxidative titration of the Hybrid with NOBF₄ in toluene:CH₂Cl₂ (ratio 1:1) mixed solution. Experimental conditions: [(6,5) SWCNT] ∼ 2.74 nM; SWCNT length ∼1000 nm; optical path length = 10 mm. (b) Selected TA spectra for a heavily hole-doped ([NOBF₄] ∼ 128 μM) Hybrid in toluene:CH₂Cl₂ (ratio 1:1) mixed solution. Experimental conditions: λ_ex = 1000 nm, i.e., in resonance with E₁₁; pump energy = 50 nJ·pulse^–1. Scaled steady-state absorption spectrum (inverted shaded shape) is shown for comparison.

Figure 7

Normalized pump-energy-dependent TA traces at 1050 nm for the Hybrid in THF upon (a) E₁₁, (b) E₂₂, and (c) E₃₃ excitation. Note that traces were normalized by the ΔA amplitude at 0.1–0.2 ps considering the instrumental response. Normalized pump-energy-dependent TA spectra for the Hybrid in THF at a time delay of ∼3 ps upon the (d) E₁₁, (e) E₂₂, and (f) E₃₃ excitations. Note that spectra were normalized at the E₀₀ → E₁₁ bleaching maximum. The blue lines represent the stationary absorption feature of the (6,5) SWCNT hole-polaron obtained by redox-chemical doping ([NOBF₄] ∼ 128.4 μM, shown in Figure 6a).

Figure 8

UV–vis TA spectra for the Hybrid in THF, SWCNT in water, and PFO–BPy in THF at a time delay of 0.2 ps. Excitation wavelength: 350 nm. Pump energy: 100 nJ·pulse^–1.

Figure 9

(a) Schematic description of the Auger recombination of charge carriers (left) and excitons (right). ET denotes energy transfer. (b) Kinetics of the integral E₀₀ → E₁₁ bleaching in the TA spectra of the Hybrid in THF upon the E₁₁, E₂₂, and E₃₃ excitations, plotted as {[ΔA(0)/ΔA(t)]² – 1} (red dots, left axis) and {[ΔA(0)/ΔA(t)] – 1} (blue squares, right axis). Traces are shifted by different offset on the vertical axis for a better comparison. Solid black lines represent the results of the linear fitting. Adjusted R-squared (Adj. R²) and fitting residuals are shown with corresponding colors. Note that the fitting of the E₁₁-excited TA trace started from 1 ps to exclude the nondiffusion-controlled rapid annihilation. Pump energy: 100 nJ·pulse^–1.