Highly mobile charge-transfer excitons in two-dimensional WS2/tetracene heterostructures

Abstract

Charge-transfer (CT) excitons at heterointerfaces play a critical role in light to electricity conversion using organic and nanostructured materials. However, how CT excitons migrate at these interfaces is poorly understood. We investigate the formation and transport of CT excitons in two-dimensional WS₂/tetracene van der Waals heterostructures. Electron and hole transfer occurs on the time scale of a few picoseconds, and emission of interlayer CT excitons with a binding energy of ~0.3 eV has been observed. Transport of the CT excitons is directly measured by transient absorption microscopy, revealing coexistence of delocalized and localized states. Trapping-detrapping dynamics between the delocalized and localized states leads to stretched-exponential photoluminescence decay with an average lifetime of ~2 ns. The delocalized CT excitons are remarkably mobile with a diffusion constant of ~1 cm² s^-1. These highly mobile CT excitons could have important implications in achieving efficient charge separation.

Fig. 1. Construction of a 1L-WS₂/Tc heterostructure.

(A) Optical image of WS₂ flakes exfoliated on a Si/SiO₂ substrate. The 1L-WS₂ is indicated by the dashed line. The square shows the area imaged by AFM in (B). Scale bar, 10 μm. (B) AFM image of the same 1L-WS₂ flake in (A) with a Tc thin film deposited on top. Scale bar, 5 μm. (C) Schematic of the formation of CT excitons and the band alignment of the 1L-WS₂/Tc heterostructure, showing the formation of a type II heterojunction. (D) Steady-state PL spectra of a 1L-WS₂, Tc thin film, and a 1L-WS₂/Tc heterostructure. The new emission band at 1.7 eV indicates the formation of interlayer CT excitons.

Fig. 2. Interlayer CT exciton emission and hole transfer from WS₂ to Tc.

(A) PL spectra of a 1L-WS₂/Tc heterostructure, a 3L-WS₂/Tc heterostructure, and a Tc thin film with excitation energy of 2.1 eV and 700-nm long-pass filter selectively exciting WS₂ in the heterostructures and detecting only interlayer CT exciton emission. (B) Time-resolved PL measurements on the interlayer CT exciton in a 1L-WS₂/Tc heterostructure, a Tc film, and a 1L-WS₂ flake. The CT exciton PL decay is fitted with a stretched exponential function, as described in the main text. (C) Transient absorption dynamics probed at the A exciton bleach of the 1L-WS₂ before and after Tc deposition with a pump energy of 2.1 eV (pump fluence, 50 μJ cm⁻²). Red solid lines are fittings with a biexponential function convoluted with an experimental response function. Inset: Band alignment shows the hole transfer process of the 1L-WS₂/Tc heterostructure. (D) Transient absorption dynamics probed at the A exciton bleach of 2L-WS₂ before and after Tc deposition with a pump energy of 2.1 eV, showing no hole transfer in the 2L-WS₂/Tc heterostructure.

Fig. 3. Electron and energy transfer from Tc to WS₂.

(A) 1L-WS₂ dynamics before and after Tc deposition. (B) 2L-WS₂ dynamics before and after Tc deposition. Pump = 3.1 eV (pump fluence, 2.2 μJ cm⁻²) and probe = 2.0 eV. (C) Subtraction of 1L-WS₂ dynamics from 1L-WS₂/Tc dynamics fitted with a exponential growth function with a time constant of 2.1 ± 0.2 ps and subtraction of 2L-WS₂ dynamics from 2L-WS₂/Tc dynamics yielding a rise time constant of 44 ± 5 ps. (D) Schematic illustration of electron and energy transfer processes. In the heterostructures constructed from 2L-WS₂ or thicker, type I heterojunctions are formed, and only exciton energy transfer is possible. (E) Energy transfer rate dependence on numbers of WS₂ layers fitted to the electromagnetic model developed by Raja et al. (39), as described in the main text.

Fig. 4. Transport of interlayer CT excitons.

(A) Exciton population profiles fitted with Gaussian functions at different delay times with the maximum ΔT signal normalized (Norm) to unity for the control 1L-WS₂. The pump photon energy is 3.1 eV (pump fluence, 4.4 μJ cm⁻²), and the probe energy is 2.0 eV. (B) ${\mathrm{\sigma }}_{\mathit{t}}^2-{\mathrm{\sigma }}_0^2$ as a function of pump-probe delay time, with a linear fit to Eq. 3 (line) for the control 1L-WS₂. Error bars of ${\mathrm{\sigma }}_{\mathit{t}}^2-{\mathrm{\sigma }}_0^2$ are the SEs estimated from Gaussian fitting to the spatial intensity distributions. (C) TAM image of the same 1L-WS₂/Tc heterostructure shown in Fig. 1 taken with spatially overlapped pump and probe beams at 0 ps. Scale bar, 2 μm. The pump energy is 3.1 eV (pump fluence, 4.4 μJ cm⁻²), and the probe energy is 2.0 eV. (D) Exciton population profiles fitted with a sum of two Gaussian functions as described in the text at different delay times with the maximum ΔR signal normalized to unity for the 1L-WS₂/Tc heterostructure along the line indicated in (C). (E) ${\mathrm{\sigma }}_{1,\mathit{t}}^2-{\mathrm{\sigma }}_0^2$ and ${\mathrm{\sigma }}_{2,\mathit{t}}^2-{\mathrm{\sigma }}_0^2$ as a function of pump-probe delay time, with a linear fit to Eq. 3 (line) for the 1L-WS₂/Tc heterostructure.