The surface photovoltage (SPV) effect is key to the development of opto-electronic devices such as solar-cells and photo-detectors. For the prototypical transition metal dichalcogenide WSe2, core level and valence band photoemission measurements show that the surface band bending of pristine cleaved surfaces can be readily modified by adsorption with K (an electron donor) or C60 (an electron acceptor). Time-resolved pump-probe photoemission measurements reveal that the SPV for pristine cleaved surfaces is enhanced by K adsorption, but suppressed by C60 adsorption, and yet the SPV relaxation time is substantially shortened in both cases. Evidently, adsorbate-induced electronic states act as electron-hole recombination centers that shorten the carrier lifetime.

Crystals of transition metal dichalcogenides (TMDCs) are composed of two-dimensional (2D) layers that are bonded with neighbors by the weak van der Waals (vdW) interaction. Such 2D layered materials have been identified as promising components for vdW heterostructure devices.1 The concept of the vdW heterostructure has been further extended from TMDC layers to metal/TMDC or organic molecules/TMDC layers.2 WSe2, one of the TMDC semiconductors, is known to exhibit a large surface photovoltage (SPV) effect,3 and it has been reported that an even higher SPV is induced by deposition of metals (Rb and In).4,5 On the other hand, an organic/TMDC heterojunction (C60/MoS2) has been predicted to show a high quantum yield and a high photovoltaic efficiency for solar cell applications.2 Concerning opto-electronic devices, the most fundamental optical response is the generation of photovoltage by the spatial separation of the photo-excited electrons and holes that are created at the surface or the interface region. To make further progress, it is important to perform a detailed study of the surface/interface electronic states and its carrier dynamics.

In this study, time-resolved x-ray photoemission spectroscopy (TRXPS) experiments were performed on WSe2 to investigate carrier dynamics in relation to the SPV effect. The same measurements were carried out for the surfaces after depositions of potassium (K) atoms or fullerene (C60) molecules. The resulting photoemission band diagrams allow us to draw conclusions about the electronic states and the carrier dynamics at the pristine and modified surfaces, and the derived basic knowledge will be useful for developing TMDC heterojunctions. Figures 1(a) and 1(b) show schematic drawings of the atomic structure. The clean surface of WSe2 was obtained by cleavage in an ultrahigh vacuum chamber. A metal/WSe2 surface was prepared by depositing ∼1 ML potassium on the clean surface with a SAES Getters dispenser at room temperature (RT) while an organic molecule/WSe2 surface was assembled by depositing ∼0.8 ML fullerene on it with a K-cell evaporator at RT. The coverage calibration is explained in the supplementary material.

FIG. 1.

(a) Top and (b) side views of the WSe2 atomic structure. (c) Schematic diagram of the surface band bending. (d) and (g) W 4f, (e) and (h) Se 3d, (f) K 3p, and (i) C 1s core-level spectra of the pristine WSe2 surface (blue) and the surfaces covered with K (orange) or C60 (pink). The spectra were taken at room temperature at the photon energy of hv = 253 eV.

FIG. 1.

(a) Top and (b) side views of the WSe2 atomic structure. (c) Schematic diagram of the surface band bending. (d) and (g) W 4f, (e) and (h) Se 3d, (f) K 3p, and (i) C 1s core-level spectra of the pristine WSe2 surface (blue) and the surfaces covered with K (orange) or C60 (pink). The spectra were taken at room temperature at the photon energy of hv = 253 eV.

Close modal

Photoemission band mapping was performed by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation (SR) at beamline 21B of Taiwan Light Source, National Synchrotron Radiation Research Center. The spectra were recorded using a hemispherical analyzer with an energy resolution of 50 meV at photon energies  = 28 and 42 eV. TRXPS measurements were carried out at BL07LSU at SPring-8 by the pump-probe method.6,7 The pump light was a laser pulse of  = 1.55 eV with a pulse duration of 60 fs and a pulse interval of 4.79 μs. The probe light was provided by SR pulses of  = 253 eV that were generated by F-mode operation of the storage ring with a pulse duration of 50 ps. TRXPS data were obtained using a time-of-flight analyzer. All the photoemission measurements were performed at RT.

Figures 1(d)–1(i) show the changes of core level spectra after surface modifications. After K deposition, both W 4f and Se 3d spectra indicate an energy shift of 0.18 eV toward higher binding energies as shown in Figs. 1(d) and 1(e), respectively. As shown in Fig. 1(f), the appearance of the K 3p peak confirms the presence of the K overlayer. Figures 1(g)–1(i) show the results for C60 deposition. The W 4f and Se 3d core levels show an energy shift of 0.08 eV to the lower binding energies, while the C 1s core-level confirms the presence of C60 adsorbates. Since photoemission spectroscopy probes the near-surface region (hν = 253 eV), the energy shifts of the core levels correspond to variations of the band-bending effect by adsorbates2,10,11 and indicate the opposite roles of the two overlayers in modifying bulk states near the surface region or the space-charge layer (SCL).

Figures 2(a)–2(f) show ARPES maps of the valence bands of (a) the pristine WSe2 surface, (b) the K-covered WSe2 surface (K/WSe2), and (c) the C60-covered WSe2 surface (C60/WSe2). In Fig. 2(a), dispersion curves of the two bulk valence bands, labeled as VB1 and VB2, reach K¯ at EEF of −1.26 eV and −1.75 eV, respectively. The valence band maximum (VBM) of the clean WSe2 surface is thus located at EEF = −1.26 eV. Since the bulk bandgap of WSe2 is 1.3 eV,15 the Fermi level is located very close to the conduction band minimum (CBM), and the surface has an n-type band structure. On the other hand, the VBM within the interior of the sample, VBMbulk, is evaluated to be EEF = –0.22 eV (p-type, see supplementary material for further details) for the current sample; thus, there is a built-in surface band bending of Vs = 1.04 eV in the sample that gives rise to a space-charge layer or a natural p-n junction [see Fig. 1(c)]. With K deposition, VB1 and VB2 shift to higher binding energies by 0.18 eV, as evaluated from the results in Figs. 2(a) and 2(b). This change is consistent with the shift seen in Figs. 1(d) and 1(e). On the other hand, deposition of C60 causes the VBM to move toward a lower binding energy, as seen in Figs. 1(g) and 1(h). The spectral features of VB1 and VB2 appeared faint in Fig. 2(c) due to the C60 overlayer, which strongly attenuates the photoemission signal from the substrate at the photon energy of 28 eV used for the measurement.

FIG. 2.

Photoemission band diagrams of surfaces of (a) WSe2 (b) K/WSe2, and (c) C60/WSe2 taken along the Γ¯ − K¯ axis at (a) hν = 42 eV, (b) hν = 42 eV, and (c) hν = 28 eV. (d) Measured kǁ range in SBZ. (e) Selected region for K/WSe2. (f) Angle-integrated spectra of C60/WSe2 taken at hν =28 eV. In the figure, peaks assigned to the molecular orbitals are labeled.

FIG. 2.

Photoemission band diagrams of surfaces of (a) WSe2 (b) K/WSe2, and (c) C60/WSe2 taken along the Γ¯ − K¯ axis at (a) hν = 42 eV, (b) hν = 42 eV, and (c) hν = 28 eV. (d) Measured kǁ range in SBZ. (e) Selected region for K/WSe2. (f) Angle-integrated spectra of C60/WSe2 taken at hν =28 eV. In the figure, peaks assigned to the molecular orbitals are labeled.

Close modal

The observed p-type band bending of the pristine WSe2 surface counters our expectation that a pure crystal of WSe2 should exhibit flat bands due to the absence of intrinsic surface states on the prefect vdW crystal surface.11,12 However, previous ARPES studies have also revealed similar surface band bending effects. It has been argued that the actual surfaces might have various types of defects, such as atomic steps or dislocations.3 Buck et al.4 reported that these factors induce the large band-bending effect and the SCL width of WSe2 extends to ∼120 nm. The WSe2 SCL has now attracted much interest as a carrier reservoir for photovoltaics.

Focusing on the photoemission map of K/WSe2 near the Fermi level [Fig. 2(e)], an apparent spot at kǁ = 0.65 Å−1 and a broad dispersive feature are observed in the spectra. The former can be assigned to the CBM located at T¯ [Fig. 2(d)] of WSe2, while the latter likely arises from a K-derived metallic band. The K-derived electronic state can be regarded as a quantum-well state (QWS)13 in the overlayer and gives rise to a parabolic dispersion near the Brillouin zone center [Fig. 2(e)]. This assignment is consistent with previous studies13 and the fact that no K intercalation appeared between the WSe2 layers8 [see Fig. 1(f)]. The K overlayer behaves as an electron donor to the WSe2 layer.

For C60/WSe2, the highest occupied molecular orbital (HOMO) and the next highest occupied molecular orbital (NHOMO) of the fullerene are observed at EEF = −2.62 and −4.06 eV, respectively, in the ARPES spectrum [Fig. 2(f)]. The LUMO position of the C60 molecule is estimated to be around −1.02 eV.14 The spectral feature of LUMO can be specified from the gentle slope, as shown in the inset of Fig. 2(f). Comparing the clean surface data shown in Fig. 2(a) with the C60 deposited surface data shown in Fig. 2(c), the LUMO level is located within the bandgap of WSe2 and the energy position of HOMO is lower than the VBM of WSe2. It is likely that the LUMO orbital is responsible for the acceptor-type9 character of the C60 overlayer on WSe2. The LUMO position is above the VBM but below EF, and this is supported by a previous model suggested by a photoluminescence study.9 

To probe the dynamical behavior of the WSe2, K/WSe2, and C60/WSe2 surfaces, we carried out TRXPS measurements of the W 4f core level by the pump-probe method. In the experiment, electron carriers are photo-excited by the ultrafast laser pulses of hν = 1.55 eV that is larger than the indirect bandgap (1.3 eV).10,15 Figure 3(a) shows the energy shifts of the core-level peaks by the SPV effect with respect to the power density of the pump pulses. At a pump power density of 1 mJ/cm2/pulse, the measured photovoltage is about 0.15 V for both K/WSe2 and WSe2 and about 0.05 V for C60/WSe2. Figure 3(b) shows the W4f7/2 XPS spectra of K/WSe2 at different delay times. The time evolution of the SPV is obtained from the energy difference between the peak position as a function of the delay time and the peak position measured at equilibrium (without pump laser). Figure 3(c) displays SPV as a function of delay times for WSe2, K/WSe2, and C60/WSe2. Evidently, the relaxation time for the SPV is different for the three cases, suggesting differences in the dynamical behavior.16,17

FIG. 3.

(a) Energy shifts in the W 4f core-level in spectra of WSe2 (blue), K/WSe2 (orange), and C60/WSe2 (pink), as functions of the pumping laser power. The data were recorded at the delay time of 0.1 ns. The power-dependence of the individual surfaces is curve-fitted as blue curves by Eq. (1). (b) A collection of time-resolved W 4f7/2 core-level spectra of the K/WSe2 surface taken by the pump (hν = 1.55 eV) and probe (hν = 253 eV) method. The pulse energy of the pump was 0.34 mJ/cm2 per pulse. The spectra (circles) are fitted by Voigt functions (solid lines). (c) Temporal variations of the surface photovoltage (SPV) for WSe2 (blue), K/WSe2 (orange), and C60/WSe2 (pink). Blue curves are the fitting results of Eq. (2).

FIG. 3.

(a) Energy shifts in the W 4f core-level in spectra of WSe2 (blue), K/WSe2 (orange), and C60/WSe2 (pink), as functions of the pumping laser power. The data were recorded at the delay time of 0.1 ns. The power-dependence of the individual surfaces is curve-fitted as blue curves by Eq. (1). (b) A collection of time-resolved W 4f7/2 core-level spectra of the K/WSe2 surface taken by the pump (hν = 1.55 eV) and probe (hν = 253 eV) method. The pulse energy of the pump was 0.34 mJ/cm2 per pulse. The spectra (circles) are fitted by Voigt functions (solid lines). (c) Temporal variations of the surface photovoltage (SPV) for WSe2 (blue), K/WSe2 (orange), and C60/WSe2 (pink). Blue curves are the fitting results of Eq. (2).

Close modal

Conventionally, SPV relaxation can be described by the thermionic emission model18–20 if thermionic emission dominates the carrier recombination process. The illumination power dependence and the temporal relaxation derived from the rate equation of hole (electron) charge density and the relation between SPV and hole (electron) density are as follows:

(1)

where kB, T, and V0 are the Boltzmann constant, the sample temperature, and the SPV shift at t =0. I and t are the fluence of the pump laser and the delay time between pump and probe pulses. Parameters η, γ, and τ, are the ideality factor, the efficiency of the optical excitation, and the relaxation time of SPV. The data points in Figs. 3(a) and 3(c) are well fitted by Eqs. (1) and (2). Table I shows a summary of the fitting parameters for WSe2, K/WSe2, and C60/WSe2. The pre-factor of η was around 2 for the three surfaces. The quantity η is related to a ratio of thermal cross-section for electrons and holes, and it typically ranges between 0.5 and 2, indicating appropriateness of the model.19 The relaxation time, τ, of WSe2 was longer than those of K/WSe2 and C60/WSe2, suggesting a faster electron-hole pair recombination rate caused by the overlayers.

TABLE I.

SPV shift at t0 (V0), ideality factor (η), efficiency of the optical excitation (γ), SPV relaxation times (τ), surface potential barrier heights (Vs), widths of the space charge layer (W), and surface recombination velocity (S).

V0 (meV)ηγ (cm2/mJ)τ (ns)Vs (eV)W (nm)S (m/s)
WSe2 60 ± 5 1.7 ± 0.9 53 ± 22 16 ± 6 1.04 120 7.5 
1 ML K/WSe2 147 ± 57 1.81 ± 0.43 31.0 ± 20.5 1.2 ± 0.3 1.22 128 107 
0.8 ML C60/WSe2 55 ± 6 1.9 ± 2.2 3.1 ± 6.2 5.5 ± 1.0 0.96 114 21 
V0 (meV)ηγ (cm2/mJ)τ (ns)Vs (eV)W (nm)S (m/s)
WSe2 60 ± 5 1.7 ± 0.9 53 ± 22 16 ± 6 1.04 120 7.5 
1 ML K/WSe2 147 ± 57 1.81 ± 0.43 31.0 ± 20.5 1.2 ± 0.3 1.22 128 107 
0.8 ML C60/WSe2 55 ± 6 1.9 ± 2.2 3.1 ± 6.2 5.5 ± 1.0 0.96 114 21 

Electronic states at surfaces play various roles in controlling the performance of photovoltaic materials. One is to change the degree of band bending that is directly linked to the spatial separation of the photo-excited carriers or generation of photovoltage. The other is to regulate the recombination process of the photo-excited electron-hole pairs. A heterojunction of WSe2 with the donor-type (K) overlayer enhances the surface potential such that SPV also increases relative to that of the pristine WSe2 surface at lower pumping laser power density. This may be attributed to the fact that the space charge layer near the surface now includes a region with electron occupation in the conduction band. The electrostatic potential becomes constant within this region, and there is no drift force within to drive the photo-exited carriers. Concerning the relaxation time, τ was suppressed after K deposition. When the WSe2 surface makes contact with the acceptor-type (C60) overlayer, the surface potential and the SPV are both reduced, as shown in Figs. 1(g), 1(h), and 3. Furthermore, the relaxation time becomes short, compared to the pristine WSe2 surface.

In order to describe the different relaxation times by the surface modification, we recall the surface recombination model that approximates the SPV relaxation time as19,20τ=τsWS, where τs, W, and S are the surface recombination time, width, and surface recombination velocity, respectively. The SCL widths for our WSe2, K/WSe2, and C60/WSe2 are evaluated to be about 120 nm from the value of the band bending. Thus, variations in W are not the key factor of the significant changes in relaxation time. On the other hand, the surface recombination velocity can vary with the trapped states at a surface having the 2D density. The surface recombination velocity can be written as21,22S=NtvthσpeeVs/ηkBT, where Nt, vth, σp, and Vs are the trapped carrier density near surfaces, the thermal carrier velocity, the hole carrier capture cross-section, and the surface potential, respectively. The parameters Nt, σp, and Vs can vary with surface modifications, but vth should remain as a constant. In this study, Vs changes within ±20% in three of the surfaces (see Table I); thus, the key factors are the density of trapped states and the hole carrier capture cross-section. It is inferred that the QWS and the occupied bulk state near CBM at the K/WSe2 surface, as observed in Fig. 2(e), may become recombination centers, leading to an increase in Nt or σp. In the K/WSe2 case, one may conclude that the increase in charges (states) at a surface enhances the band-bending effect but it may also enhance the electron-hole pair recombination rate. Concerning the case of C60/WSe2, the system also has shorter relaxation time than that of the pristine WSe2 surface. It can be explained by the decrease in Vs [Fig. 1(g)] and the increase in Nt or σp [Fig. 2(f)]. The LUMO states located within the bulk bandgap of WSe2 likely dominate the recombination of the photo-excited electron-hole pairs.

Photoemission band mapping and time-resolved photoelectron spectroscopy were systematically performed on surfaces of a p-type WSe2 crystal and the modified surfaces with two types of adsorbates, K and C60. For the K/WSe2 surface, the potassium layer acts as a donor and it enhances the band-bending effect, while the fullerene layer at C60/WSe2 behaves as an acceptor and it reduces the effect. Transient surface photovoltage effects were observed for the three surfaces. The larger SPV is observed on K/WSe2 due to the larger initial band bending. The shorter SPV relaxation time after the formation of the overlayers indicates that adsorbate-induced electronic states likely become recombination centers of photo-excited carriers. This system offers a good model to illustrate a trade-off between the magnitude of the SPV and the SPV relaxation rate and the multi-parameter dependencies of SPV generation and relaxation. These observations should provide critical insights into designing TMDC-based optoelectronic devices in the future.

See supplementary material for the coverage estimation of C60 and K at the WSe2 surface and the band bending profile and the space charge layer of p-type WSe2.

This work was supported by the Grant-in-Aid for Scientific Research (KAKENHI) (Grant Nos. 16H03867 and 16H06027) and the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering (Grant No. DE-FG02-07ER46383 for T.-C.C.). The experiment was carried out as a joint research in the Synchrotron Radiation Research Organization and the Institute for Solid State Physics, the University of Tokyo (Proposal Nos. 2013B7454, 2014B7480, 2016A7503, and 2016B7523). T.-C.C. received a visiting professor appointment at the University of Tokyo during the experiment.

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Supplementary Material