Article pubs.acs.org/JPCC
Charge Transport and Separation Dynamics at the C60/GaAs(001) Interface Jeong Won Kim,*,† Heungman Park,‡ and Xiaoyang Zhu*,‡ †
Korea Research Institute of Standards and Science, Yuseong, Daejeon 305-340, Korea Department of Chemistry, Columbia University, New York, New York 10027, United States
‡
ABSTRACT: We probe charge separation dynamics at a model hybrid organic/ inorganic semiconductor heterojunction, C60/GaAs(001), using time-resolved twophoton photoemission spectroscopy (TR-2PPE). For a p-type GaAs(001) surface with downward band bending, TR-2PPE allows us to directly follow the subpicosecond drift of photoexcited electrons toward the surface by the space charge field. Upon C60 adsorption, we find that electron transfer from the GaAs conduction band to C60 occurs on the faster time scale of ≤0.1 ps. We discuss the role of the space charge field in assisting electron transfer at the hybrid organic/inorganic semiconductor interface.
1. INTRODUCTION Hybrid organic−inorganic solar cells1−6 take advantage of complementary characters of both materials. Organic materials are strong absorbers with easily tunable optical characteristics, while their inorganic counterparts possess good charge conduction properties. How charge separates across the interface between these two distinctively different materials is a question of significance to the fundamental understanding of hybrid solar cells. Charge carriers in crystalline inorganic semiconductors move almost freely in delocalized bands, and they are separated in space charge fields generated by surface states or p−n junctions. In organic semiconductor materials, charge carriers are localized polarons. At the hybrid interface, the space charge field is expected to affect/control not only separation of free carriers in the inorganic semiconductor, but also charge transfer at the delocalized/localized inorganic/ organic interface. We have recently addressed the role of the space charge field in controlling charge separation at a model organic/inorganic semiconductor interface of copper phthalocyanine (CuPc) and gallium arsenide (GaAs) by time-resolved second harmonic generation.7 This technique follows the transient electric field generated by charge separation in the inorganic semiconductor and at the organic/inorganic interface, but is insensitive to the energetics of the charge carriers. Knowing the energetics of the charge carriers is particularly important to unraveling the dynamic competition between hot carrier relaxation/transport in the delocalized inorganic band structure and transfer across the delocalized/localized interface. To address the energetics problem, we choose the model interface between p-type GaAs and n-type fullerene (C60). While the n-type GaAs/C60 shows a strongly rectifying interface with an energy barrier of 0.58 eV,8 the energy level alignment at p-type GaAs/C60 interface is expected to be favorable for electron injection from the GaAs conduction band to C60 molecular orbitals.9 We use the experimental technique of timeresolved two-photon photoelectron (TR-2PPE) spectroscopy, © 2014 American Chemical Society
which measures the energetics of excited electrons on femtosecond time scales.10−13 In a TR-2PPE experiment, a pump photon (hν1) excites an electron across the bandgap of GaAs; after a controlled time-delay, a probe photon (hν2) ejects the excited electron above the vacuum level. We show that TR2PPE provides a direct view in time and energy domains of the drift of photoexcited electrons in the GaAs conduction band by the space charge field. We determine a time scale of 0.3 ps for drift in the space charge region. We find the transfer of excited electrons from the GaAs conduction band to the unoccupied molecular orbitals of C60 on subpicosecond time scales.
2. EXPERIMENT All experiments were carried out in a vacuum system consisting of two chambers: an ultrahigh vacuum (UHV) analysis chamber (10−10 Torr) equipped with a hemispherical electron analyzer (VG100AX) for TR-2PPE and ultraviolet photoemission spectroscopy (UPS) and a high vacuum chamber (10−7 Torr) equipped with Knudsen cells and quartz crystal microbalance (QCM) for the deposition of molecular films. The samples were cut from a highly Zn-doped (∼1 × 1019 cm−3) p-type GaAs(001) wafer capped with an As film (Wafer Technology Ltd.). Each GaAs sample (∼1 × 0.5 cm) was mounted onto a direct-heating sample holder and transferred into the UHV chamber for cleaning. We removed the As capping layer in UHV by heating the sample to 550 °C to prepare a clean GaAs(001) surface,14 as confirmed by UPS. A C60 molecular film was subsequently deposited onto the clean GaAs(001) surface at a deposition rate of 0.12 nm/min, as determined by the QCM. UPS spectra were recorded at an energy resolution of about 0.14 eV at room temperature, with the He−I line (21.2 eV) from a helium discharge lamp (VG) as Received: December 12, 2013 Revised: January 20, 2014 Published: January 22, 2014 2987
dx.doi.org/10.1021/jp412180t | J. Phys. Chem. C 2014, 118, 2987−2991
The Journal of Physical Chemistry C
Article
UV light source. Photoelectrons were detected at surfacenormal geometry. A femtosecond Ti:sapphire oscillator (Coherent Mira 900, rep-rate 76 MHz) operating at a wavelength of 827 nm (1.50 eV) was used in TR-2PPE measurements. The laser output was split into two beams: one was used directly as pump (hν1 = 1.50 eV, ∼1 nJ pulse energy) and the other was frequencytripled in a nonlinear optical setup (INRAD 5-050) to generate UV probe (hν2 = 4.50 eV, ∼0.1 nJ pulse energy). The pump and probe beams were recombined and focused by an f = 50 cm lens onto the sample surface. On the basis of cross-correlation 2PPE measurement from a clean Au(111) surface, we estimated the laser pulse width of ∼150 ± 20 fs. All TR-2PPE measurements were carried out at room temperature with the p-polarized pump and probe laser. Experiments with different pump laser powers (pulse energy = 0.2−2 nJ) showed no appreciable shift in the surface Fermi level, indicating negligible photovoltage effect on the GaAs surface.
Figure 2. Energy level diagram of the C60/GaAs(100) interface and schematic illustration of the TR-2PPE process. The dashed arrows indicate electron movement, transfer, and relaxation. The energy scale (eV) is referenced to the Fermi level (EF = 0.0 eV). H, HOMO; L, LUMO; L+1, LUMO+1.
3. RESULTS AND DISCUSSION Figure 1 shows UPS spectra obtained for the As-capped, clean, and C60 thin film covered GaAs(001) surfaces. The binding
C60 is 2.3 eV, obtained from the threshold energies determined in UPS and IPES, while the peak-to-peak gap from the same measurements is 3.5 eV.17,20−22 Following common practices, we use the threshold gaps for easy description of charge transfer.23 Also shown in Figure 2 is the position of the LUMO +1 level, which lies at ∼1 eV above LUMO.21 In the first step of a TR-2PPE measurement, the pump photon energy of hν1 = 1.50 eV (red arrow in Figure 2) is within the optical gap of C60 (∼1.7 eV) but slightly above the band gap of GaAs (Eg = 1.42 eV at room temperature). Thus, the absorption of hν1 creates an electron in the conduction band (and a hole in the valence band) of GaAs with little excess energy. Because the effective electron mass is 1/7 of that of the hole in GaAs,24 the excess electron energy in the conduction band is ΔEe = 6/7·(1.50 − 1.42) = 0.069 eV, which is almost 1 order of magnitude smaller than the amount of band bending in the near surface region. On the basis of the band bending (∼0.5 eV) and doping density (∼1019 cm−3), we estimate the thickness of the space charge field to be of the order of 10 nm.25 Given the extinction coefficient of GaAs at 1.5 eV,26 we calculate a light absorption depth of ∼1 μm, which is larger than the thickness of the space charge region. The electron escape depth at low kinetic energies (≤1 eV) is ≥10 nm from the universal curve.27 In the second step of a TR-2PPE experiment, the probe pulse (hν2 = 4.50 eV, blue arrows in Figure 2) ionizes the excited electron for detection, and the time-evolution of photoelectron energy is expected to mainly track the movement of the excited electron at/near the CBM edge as governed by the space charge field. Figure 3 shows TR-2PPE spectra obtained from the Asterminated GaAs(001) surface. The spectra (with time intervals of 80 fs) are offset for clarity in panel A and as a pseudocolor plot in panel B (electron energy vs pump−probe delay, with photoelectron intensity on a pseudocolor scale). Around zero pump−probe delay (Δt), the photoelectron distribution is broad (0.3−1.4 eV), with a peak ∼1.1 eV. As Δt increases, we see a subpicosecond energy relaxation, resulting in a transfer of intensity around 1.1 eV at Δt ≈ 0 ps to that at ∼0.55 eV for Δt ≥ 1 ps. The amount of energy relaxation is much larger than the initial excess electron energy of 0.069 eV above the CBM, but close to the total band bending on this highly doped
Figure 1. Ultraviolet photoemission spectra of clean GaAs(100) (black), As-capped GaAs(100) (red), 1 nm (blue), and 5 nm (green) C60 thin film deposited on clean GaAs(100). The arrows indicate the onset of GaAs valence band (VBM) and C60 HOMO.
energies are referenced to the Fermi level (EF). The band structure of the As-capped GaAs(001) surface shows a valence band maximum (VBM) cutoff at ∼0.5 eV. With the desorption of the As overlayer, the valence band structure becomes clearer, with a VBM cutoff at 0.55 eV below EF, in agreement with previous reports.15 It is known that desorption of the As cap from the GaAs(001) surface leads to the formation of a c(2 × 8)/(2 × 4) reconstruction with surface As dimers.14,16 Upon adsorption of 1 or 5 nm C60, the band structure of GaAs(001) is replaced by features of C60 molecular orbitals (MOs).9,17 The onset of the highest occupied molecular orbital (HOMO) is at 1.90 ± 0.05 eV below EF, nearly independent of C60 coverage. On the basis of values obtained from UPS spectra, we construct an energy-level alignment diagram in Figure 2. The pGaAs(001) face has a strong downward band bending of about VB = 0.55 eV toward the surface due to surface states.15,18 The MOs of C60 molecules are aligned to have a type II semiconductor junction, in excellent agreement with previous suggestions.9,19 Here, we position the lowest unoccupied molecular orbital (LUMO) of C60 based on the threshold transport level as determined from inverse photoemission spectroscopy (IPES) measurements. The transport gap in solid 2988
dx.doi.org/10.1021/jp412180t | J. Phys. Chem. C 2014, 118, 2987−2991
The Journal of Physical Chemistry C
Article
Figure 3. (A) TR-2PPE spectra obtained at different pump−probe delays (Δt = −0.04 → 3.64 ps, with 0.08 ps steps) from As-terminated GaAs(001) surface. The spectra are offset vertically for clarify. (B) Two-dimensional pseudocolor representation of the same spectra in (A). The white curve is an exponential fit to the peak positions. The pump and probe photon energies are hν1 = 1.50 eV and hν2 = 4.50 eV, respectively, and the sample temperature is 298 K.
GaAs(001) surface. We conclude that the time-evolution of electron energy observed in TR-2PPE in Figure 2 indeed maps out the drift dynamics of the photoexcited electron in the space charge field, as suggested before.28 In this process, photoexcitation by the pump laser pulse creates conduction band electrons near the CBM within the light absorption depth, which is much larger than the width of the space charge field. Drift of these photoelectrons in the space charge field concentrates them to the surface of the semiconductor. This drift process is most obvious in the two-dimensional pseudocolor plot, panel B. A single exponential fit (white curve) to the peak positions (red region) of the TR-2PPE spectra as a function of pump−probe delay time gives a time constant of τd = 0.30 ± 0.05 ps. Given a saturation electron drift velocity (at high field) of νd ≈ 0.07 nm/fs,29 this time constant corresponds to a width of the space charge region of ∼20 nm. Note that the above discussion focuses on a semiquantitative analysis of the time-dependent energetic position of the photoelectron, not its intensity. Analyzing photoemission intensity is more difficult for a number of reasons. In the measurement, photoelectrons are detected at surface normal direction, without angular resolution. Complication can come from the disordered As-cap, which may scatter the outgoing photoelectrons to modify the initial angular distribution of photoelectrons. TR-2PPE reveals similar drift-dominated electron dynamics on the clean GaAs(001) surface, as shown in Figures 4A and B for pesudo-color plot and offset spectra, respectively. The initial broad photoelectron distribution peaking at ∼1.1 eV is tranformed to a distribution peaked at ∼0.5 eV on a subpicosecond time scale. This again corresponds to the drift dynamics within the space charge region. A comparison of TR-2PPE spectra from clean GaAs(001) (Figure 4A and B) with those on 1 nm C60/GaAs(001) (Figure 4C and D) reveals two major differences. As compared to spectra from clean GaAs(001), the high-energy peak (∼1.1 eV) near Δt = 0 ps from C60/GaAs(001) is much weaker, while the lower energy peak at Δt > 1 ps is broader and extends to a lower energy by ∼0.1 eV. The faster decay of the higher energy peak (∼1.1 eV) initially can be attributed to injection of nascent photoexcited electrons from GaAs to C60, while the broader peak at lower energy (and longer time delays) results
Figure 4. (A,B) TR-2PPE spectra from clean GaAs(001) surface in 2D pseudocolor and offset representations, respectively; (C,D) TR-2PPE spectra from 1 nm C60/GaAs(001) in 2D pseudocolor and offset representations, respectively. In panels B and D, the spectra are shown in 0.1 ps steps. The pump and probe photon energies are hν1 = 1.50 eV and hν2 = 4.50 eV, respectively, and the sample temperature is 298 K.
from a mixture of photoemission signal from the C60 LUMO (due to electron transfer from GaAs) and the GaAs conduction band, as we detail below. The photoemission spectrum in each case originates from contributions of electrons with varying energy in the space charge field and, in the case of C60/GaAs, also from transiently populated molecular orbitals. It is difficult to quantitatively delineate these contributions in the absence of knowledge on the energy and angle-dependent photoionization cross sections. In the following, we carry out a semiquantitative or qualitative analysis. We emphasize the relative changes in photoemission intensity as a function of pump−probe delay time, not the absolute intensities. We find that each TR-2PPE spectrum can be well described by the sum of three Gaussian peaks with fixed width and positions, independent of pump−probe delay time, as shown in Figure 5A and B for both clean and C60 monolayer covered GaAs(001) surfaces. On clean GaAs(001), the three Gaussians are at 1.1, 0.75, and 0.4 eV, respectively, with a common width (fwhm) of 0.36 eV. Here, the high-energy peak (1.1 eV) can be attributed mostly to photoemission from the top of the space charge field in GaAs (≥10 nm beyond the surface) and the lowenergy peak (∼0.4 eV) mainly from the bottom of the space charge field (at or near the surface). The 0.75 eV peak accounts for the middle of the space charge field. The time evolutions of the intensities of these three peaks support the approximate assignments, as shown by the blue data points in Figure 5C−E. The intensity of the 1.1 eV peak decays monotonically with time, as expected from drift in the space charge field. This decay can be described by a biexponential (blue curve) with time constants of 0.3 and 2 ps, respectively; the fast channel can be attributed to the drift toward the surface, while the slow channel likely has contribution from diffusion of excited electrons from deeper in the bulk to the space charge region. 2989
dx.doi.org/10.1021/jp412180t | J. Phys. Chem. C 2014, 118, 2987−2991
The Journal of Physical Chemistry C
Article
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