Solar energy conversion plays an important role in satisfying mankind's ever-increasing energy usage in an environmentally friendly way. Despite several decades of optimization Silicon solar cells still lack efficiency. On the other hand, highly efficient III-V multijunction solar cells are expensive and not sustainable [1, 2]. Recently, van der Waals (vdW) heterostructures made of different monolayer (ML) transition metal dichalcogenides (TMDs) have emerged as a promising new solar cell platform due to their strong excitonic absorption in the visible spectral range [3, 4] followed by efficient ultrafast charge separation due to type II band alignment [5–10] (see Figure 1A). Interestingly, ultrafast charge separation was also found to occur in ML WS₂/graphene heterostructures despite the type I band alignment [11–13] (see Figure 1B). These heterostructures combine the benefits of a direct gap semiconductor with strong spin-orbit coupling [14, 15] and a semimetal with high-mobility carriers and long spin lifetimes [16] with great potential for novel optoelectronic and optospintronic applications [17].

One simple way to improve the efficiency of WS₂/graphene-based light harvesting devices might be to enhance the thickness and therefore the absorption of the WS₂ layer [18–20]. It is not a priori clear, however, if ultrafast charge separation survives when direct-gap monolayer WS₂ is replaced by thicker WS₂ layers with an indirect band gap (Figure 1C). In this work we address this issue by exciting carriers across the direct gap at the K-point of BL WS₂ in an epitaxial BL WS₂/graphene heterostructure on SiC(0001) and tracing the relaxation of the photogenerated electron-hole pairs as a function of time, energy, and momentum using time- and angle-resolved photoemission spectroscopy (tr-ARPES). We find that photoexcited holes in BL WS₂ are transferred to the graphene layer within 100 fs. The photoexcited electrons are found to remain in the conduction band of BL WS₂ for 420 fs resulting in the formation of a charge separated transient state with a lifetime of 770 fs. These timescales are consistent with the microscopic charge transfer model recently proposed for ML WS₂/graphene heterostructures [13] indicating that also in the case of BL WS₂ on graphene the timescale for charge separation is determined by direct tunneling at the points in the Brillouin zone where WS₂ and graphene bands intersect.

To investigate ultrafast charge transfer in our BL WS₂/graphene heterostructure we excited carriers across the direct band gap at the K-point of BL WS₂ using 2 eV pump pulses and probed the response of the heterostructure using tr-ARPES. In detail, we investigated the time dependence of two representative energy distributions curves (EDC 1 and EDC 2 in Figure 2D) to obtain the population and band structure dynamics of the WS₂ conduction and valence band (CB and VB) and the graphene Dirac cone, respectively. The pump fluence was 6.6 mJ/cm² for EDC 1 (WS₂) and 9.1 mJ/cm² for EDC 2 (graphene).

Figures 3A,B show EDC 1 and EDC2, respectively, as a function of pump-probe delay. The corresponding pump-induced changes obtained by subtracting the respective EDC at negative pump-probe delay from the transient EDCs are shown in Figures 3C,D. Upon arrival of the pump pulse the WS₂ VB is found to shift toward the Fermi level in Figure 3A. This up-shift is responsible for the strong gain (red) and loss (blue) signal around −2 eV in Figure 3C. Figure 3C also reveals a transient gain of photoelectrons in the CB of WS₂ around +1 eV following photoexcitation. The graphene π-band in Figure 3B is found to broaden and to shift down which—together with the up-shift of the WS₂ VB at lower energy—produces the gain-loss-gain signal in the energy range between +1 and −2 eV in Figure 3D.

To analyze the transient population dynamics of the individual bands we integrated the EDCs in Figure 3 over the energy range indicated by the colored brackets yielding the pump-probe traces in Figure 4. Panel A shows the transient population of the WS₂ VB and CB in green and orange, respectively. The population of the WS₂ VB is found to be unaffected by the photoexcitation within the experimental signal-to-noise ratio. The WS₂ CB on the other hand exhibits a clear gain of electrons. An exponential fit to the data yields a lifetime of the electrons in the WS₂ CB of τ = 420 ± 20 fs. Figure 4B shows the gain above and the loss below the Fermi level in graphene. We find a short-lived gain (τ = 97 ± 3 fs) and a long-lived loss (τ = 840 ± 30 fs). The transient up-shift of the WS₂ VB gives rise to a gain of photoelectrons above its equilibrium position that is plotted as a function of pump-probe delay in Figure 4C. An exponential fit to the data yields a lifetime of τ = 770 ± 30 fs.

In agreement with Aeschlimann et al. [11] and Krause et al. [13], we interpret these timescales as follows: The absence of holes in the WS₂ VB together with the short-lived gain in graphene indicates that the photogenerated holes in the WS₂ VB are rapidly (within ~ 100 fs) refilled by electrons from the Dirac cone. The photoexcited electrons are found to remain in the WS₂ CB for ~ 400 fs, indicating the formation of a charge-separated transient state where the holes reside in the graphene layer and the electrons reside in the WS₂ layer. This charge-separated state is expected to decrease the binding energy of the WS₂ states and to increase the binding energy of the graphene states [11, 13].

Figures 5A,B illustrate the fitting procedure used to determine the transient peak positions of the WS₂ VB and CB and the graphene Dirac cone, respectively. Details are provided in the figure caption. The transient positions of the WS₂ CB and VB are shown in Figures 5C,D, respectively. We find that the WS₂ VB shifts up by ~ 110 meV with a lifetime of τ = 600 ± 20 fs (Figure 5D). The transient WS₂ band gap at k ≈ 1.1 Å⁻¹ obtained by subtracting the transient position of the VB from the transient position of the CB is displayed in Figure 5E. An exponential fit yields a transient band gap reduction of ~230 meV with a lifetime of τ = 140 ± 40 fs. Note that the equilibrium gap size of 2.7 eV at k ≈ 1.1 Å⁻¹ is bigger than the direct band gap at the K-point [13, 30]. In good agreement with Chernikov et al. [31], Liu et al. [32], Ulstrup et al. [33], and Pogna et al. [34], we attribute this band gap renormalization to the presence of photoexcited carriers that screen the Coulomb interaction.

The transient band gap renormalization results in shifts of the WS₂ VB and CB that are symmetric with respect to the center of the WS₂ band gap (see inset of Figure 5E). By subtracting |ΔE_gap|/2 from the transient position of the WS₂ VB we obtain the transient VB shift shown in green in Figure 5F that shows a remaining up-shift of ~ 90 meV with a lifetime of τ = 560 ± 30 fs. At the same time the π-bands of graphene shown in red in Figure 5F shift down by ~ 110 meV with a lifetime of τ = 900 ± 50 fs. As discussed previously, these shifts are a direct consequence of the transient charge-separated state where excess negative charge on the WS₂ layer decreases the binding energy of the WS₂ states and the corresponding excess positive charge on the graphene layer increases the binding energy of the graphene states. These charging shifts are sketched in the inset of Figure 5F. At this point we are able to attribute the pump-probe signal in Figure 4C to the transient up-shift of the WS₂ layer the lifetime of which is linked to the lifetime of the charge-separated state [13]. Also, the dynamics of the WS₂ CB shown in Figure 5C can now be explained by the combined effect of band gap renormalization that increases the binding energy of the WS₂ CB on the timescale of the photoexcitation and charging that decreases the binding energy of the WS₂ CB on the timescale of the hole transfer.

All things considered, our data reveals a detailed picture of the ultrafast charge separation following photoexcitation of the WS₂/graphene heterostructure. We find that (1) hole transfer from WS₂ to graphene occurs within ~ 100 fs, (2) photoexcited electrons remain inside the WS₂ CB for ~ 400 fs, and (3) the charge-separated state decays within ~ 800 fs (This number corresponds to the average of the lifetimes of the graphene loss in Figure 4B, the WS₂ VB gain in Figure 4C and the charging shifts in Figure 4F).

The observed lifetimes for electron and hole transfer are in good agreement with previous tr-ARPES [11, 13] and time-resolved optical techniques [12] on similar WS₂/graphene heterostructures. Next we will discuss if the microscopic model for ultrafast charge separation across the interface between monolayer WS₂ and monolayer graphene [13] also applies to heterostructures made of bilayer WS₂ and monolayer graphene. In this model [13], the timescale for ultrafast charge separation in the heterostructure is determined by direct tunneling of hot carriers from WS₂ to graphene at those points in the Brillouin zone where the respective bands intersect. The associated energy barrier is smaller for holes than for electrons which, combined with a larger tunneling matrix element and a larger scattering phase space, results in hole transfer being faster than electron transfer. The lifetime of the charge separated state, on the other hand, is determined by defect-assisted tunneling via in-gap states originating from S vacancies inside the WS₂ layer. This decay channel is extremely sensitive to the number of S vacancies in the sample, resulting in lifetimes of the transient charge-separated state between ~ 1 ps in high-quality epitaxial samples [11, 13] and > 1 ns in commercial manually assembled heterostructures [12]. In this model ultrafast charge transfer occurs close to the K-point of WS₂ where the WS₂ VB maximum and CB minimum are located.

It is not a priori clear whether this model also applies to heterostructures made of bilayer WS₂ and monolayer graphene because, in this case, the maximum of the WS₂ VB and the minimum of the WS₂ CB are located at Γ and Σ (in between Γ and K), respectively. The observed timescales for hole transfer from WS₂ to graphene within ~ 100 fs and a lifetime of the electrons inside the WS₂ CB at K of ~ 400 fs are perfectly consistent with direct tunneling via band intersections close to the K-point. The lifetime of the charge-separated transient state of ~ 800 fs observed in the present heterostructure is consistent with defect-assisted tunneling via S vacancies in similar high-quality epitaxial samples [11, 13]. Therefore, we conclude that, despite the band structure differences, the microscopic model for ultrafast charge transfer developed for the interface between monolayer WS₂ and monolayer graphene [13] also applies for heterostructures made of bilayer WS₂ and monolayer graphene (see sketch in Figure 6).

In summary, we have shown that ultrafast charge separation also occurs at the interface between bilayer WS₂ and monolayer graphene. Together with the enhanced absorption in the visible spectral range compared to monolayer WS₂ our findings provide important insights that will guide the design of novel optoelectronic applications.

The raw data supporting the conclusions of this article will be made available by the authors upon request.

IG, CCa, ES, and CCo organized the project. SF, FF, AR, and CCo prepared the sample. RK, MC-C, SA, SF, FF, AR, YZ, PM, RC, CCa, and IG prepared and conducted the tr-ARPES experiments. RK analyzed the data. RK and IG wrote the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.