Unraveling the Effect of Stacking Configurations on Charge Transfer in WS2 and Organic Semiconductor Heterojunctions

Abstract

Photoinduced interfacial charge transfer plays a critical role in energy conversion involving van der Waals (vdW) heterostructures constructed of inorganic nanostructures and organic materials. However, the effect of molecular stacking configurations on charge transfer dynamics is less understood. In this study, we demonstrated the tunability of interfacial charge separation in a type-II heterojunction between monolayer (ML) WS₂ and an organic semiconducting molecule [2-(3″',4'-dimethyl-[2,2':5',2':5″,2″'-quaterthiophen]-5-yl)ethan-1-ammonium halide (4Tm)] by rational design of relative stacking configurations. The assembly between ML-WS₂ and the 4Tm molecule forms a face-to-face stacking when 4Tm molecules are in a self-aggregation state. In contrast, a face-to-edge stacking is observed when 4Tm molecule is incorporated into a 2D organic-inorganic hybrid perovskite lattice. The face-to-face stacking was proved to be more favorable for hole transfer from WS₂ to 4Tm and led to interlayer excitons (IEs) emission. Transient absorption measurements show that the hole transfer occurs on a time scale of 150 fs. On the other hand, the face-to-edge stacking resulted in much slower hole transfer without formation of IEs. This inefficient hole transfer occurs on a similar time scale as A exciton recombination in WS₂, leading to the formation of negative trions. These investigations offer important fundamental insights into the charge transfer processes at organic-inorganic interfaces.

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Face-to-face stacking of monolayer (ML) WS₂/4Tm film heterojunctions. (a) Scheme showing the face-to-face vertical stacking of ML-WS₂/4Tm film heterostructures and a laser with the wavelength of 447 nm (2.77 eV) used for optical measurements. (b) Band alignment between ML-WS₂ and 4Tm molecules, indicating a type-II junction and the possibility of forming interlayer excitons. Dash line indicates the exciton level, and ε _A represents the binding energy. (c) Optical image and corresponding photoluminescence (PL) image of ML-WS₂/4Tm film heterojunction, indicating a severe PL quench for ML-WS₂. (d) Steady-state PL spectra of 4Tm thin film and ML-WS₂/4Tm film heterostructure at the temperature of 120 K. The new emission band at 1.51 eV indicates the formation of interlayer charge transfer (CT) excitons. *The asterisk mark represents the signal from the laser.

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Hole transfer from ML-WS₂ to 4Tm molecule in a face-to-face stacking. (a) Band alignment showing the hole transfer process in the ML-WS₂/4Tm film heterostructure under excitation with a pump energy of 2.25 eV. (b) Transient absorption contour and a specific transient absorption spectrum at a pump–probe delay time of 272 fs after excitation at 550 nm (2.25 eV) for the ML-WS₂/4Tm heterojunction. (c) Transient absorption spectra of the ML-WS₂/4Tm heterojunction at three different time delays to investigate the hole transfer from WS₂ to 4Tm molecule. The pump excitation beam was tuned to 550 nm (2.25 eV). (d) Transient absorption dynamics of bare ML-WS₂ and ML-WS₂/4Tm pumped at 2.25 eV and probed at 2.0 eV. The dynamics of isolated WS₂ and ML-WS₂/4Tm film were fitted using exponential and biexponential decay functions, accordingly. Both of these functions were convoluted with the experimental response function.

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Face-to-edge vertical stacking of ML-WS₂/4Tm molecule heterojunction through incorporating 4Tm into 2D halide perovskite. (a) X-ray diffraction profile of the 2D perovskite using 4Tm as bulky ligands, simplified as 4Tm₂PbBr₄. (b) Scheme showing the vertical stacking of the ML-WS₂ and 4Tm₂PbBr₄ heterojunction and a laser with the wavelength of 447 nm used for optical measurements. (c) Optical image (left) and PL image (right) of a specific heterojunction fabricated by ML-WS₂ (top) and 4Tm₂PbBr₄ (bottom). PL image indicates a slight PL quench for ML-WS₂. The scale bars are both 10 μm. (d) Steady-state PL spectra comparison among isolated WS₂, the ML-WS₂/4Tm junction, and the ML-WS₂/4Tm₂PbBr₄ junction at room temperature to quantify the difference led by stacking configurations. (e) Normalized PL spectra with peak fitting to indicate the formation of trions at the WS₂/4Tm₂PbBr₄ heterojunction. X⁰ and X^T represent A excitons and trions, respectively. (f) Temperature-dependent PL spectra of ML-WS₂/4Tm₂PbBr₄ heterojunction at temperatures from 10 to 295 K with the excitation energy of 2.77 eV (447 nm).

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Temperature-dependent PL emission spectra of ML-WS₂/4Tm₂PbBr₄ heterojunction under selective excitation with a lower energy. (a) Specific temperature-dependent PL spectra of the ML-WS₂/4Tm₂PbBr₄ heterojunction excited at 550 nm (2.25 eV), which can only excite WS₂. *The asterisk mark represents the signal from laser with a wavelength of 550 nm. (b) Room temperature and low temperature PL spectra comparison under different optical excitation conditions for the ML-WS₂/4Tm₂PbBr₄ heterojunction. Top: collected PL results under the excitation of only WS₂ in the heterojunction. Bottom: collected PL results under the excitation of WS₂ and the 4Tm molecule in the heterojunction. The dashed lines are the fitting for the emission peak acquired under the excitation energy of 2.25 eV (550 nm) at 10 K. The fitting shows the coexistence of the A exciton and trion.