Enhanced Photoresponsivity of Fullerene in the Presence of

Jan 29, 2019 - Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo ... 7588, Institut des NanoSciences de Paris , F-75005 Paris , Fr...
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Enhanced Photoresponsivity of Fullerene in the Presence of Phthalocyanine: A Time-Resolved X‑ray Photoelectron Spectroscopy Study of Phthalocyanine/C60/TiO2(110) Kenichi Ozawa,*,† Susumu Yamamoto,‡ Marie D’angelo,§ Yuto Natsui,∥ Naoya Terashima,∥ Kazuhiko Mase,⊥,# and Iwao Matsuda‡ †

Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan § Sorbonne Universités, UPMC Université Paris 06, UMR 7588, Institut des NanoSciences de Paris, F-75005 Paris, France ∥ Department of Advanced Physics, Hirosaki University, Hirosaki, Aomori 036-8561, Japan ⊥ Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan # SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801, Japan

J. Phys. Chem. C Downloaded from pubs.acs.org by TULANE UNIV on 02/13/19. For personal use only.

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S Supporting Information *

ABSTRACT: Time-resolved soft X-ray photoelectron spectroscopy has been utilized to reveal a time evolution of excitation states of metal and metaloxo phthalocyanine (MPc; M = Cu and TiO) and fullerene (C60) in the ultrathin layered MPc/C60/TiO2(110) systems. C 1s core-level photoemission peaks of MPc and C60 were monitored to assess spontaneous changes induced by ultraviolet laser. The C 1s peaks of both species move toward higher binding energies, reflecting cationization of the molecules as a result of dissociation of photoexcited excitons followed by electron transfer from the molecules to TiO2. The magnitude of the C60 C 1s peak shift is as large as 13−14 meV, whereas the shift of only 7 meV is induced when phthalocyanine-free C60/TiO2(110) is excited. Photoresponsivity of C60 is enhanced when C60 coexists with the phthalocyanine molecules. An efficient energy transfer from photoexcited MPc to C60 and a resultant exciton formation in C60 are responsible for this enhancement.

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INTRODUCTION A ubiquitous light-harvesting system paves the way for a zeroemission sustainable society. An organic photovoltaic (OPV) device is a leading solution as an inexpensive renewable energy generator. Therefore, much effort has been devoted to elucidate the mechanism of the light−electricity conversion and to improve the conversion efficiency. However, the efficiency of the OPVs is still less than 15%,1 which is not enough to commercialize the OPVs to make a profit. A high conversion efficiency is directly linked to effective separation of photoexcited excitons (electron−hole pairs) as well as restricted electron−hole recombination at the heterojunction between p-type and n-type organic semiconductors. Thus, it is important to obtain the atomistic view of the photoexcited carrier behavior, especially at the p/n junction, for further improvement of the OPV performance. Since the pioneering work by Tang,2 phthalocyanine/ fullerene layered systems have been one of the most studied model OPVs.3−11 Here, UV-sensitive fullerene and visiblelight-sensitive phthalocyanine act as electron acceptor and donor, respectively, so photoexcited electrons are transferred © XXXX American Chemical Society

from phthalocyanine to fullerene once excitons are dissociated at the phthalocyanine/fullerene heterojunction. Jailaubekov et al.4 studied the carrier dynamics at the heterojunction between fullerene (C60 and C70) and copper phthalocyanine (CuPc) in femto- and picosecond time domains and found that the times required to form hot charge transfer (CT) excitons and a subsequent cooling of the CT excitons were 80 fs and 1 ps, respectively. Although a longer CT time of ∼300 fs was reported by Dutton and Robey,5 the formation of the CT excitons is completed in femtoseconds. Dissociation of the CT excitons, then, proceeds to give free carriers. The free carriers migrate to lower energy sites over the first 20 ps (in the zinc-phthalocyanine/C60 system)6 and electron− hole recombination sets in subsequently. Arion et al. examined the recombination process in the CuPc/C60 system by timeresolved X-ray photoelectron spectroscopy (TRXPS) and found that anionized C60 formed by electron donation from Received: January 7, 2019 Revised: January 28, 2019 Published: January 29, 2019 A

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annealing in O2 atmosphere),18 C60 (>99%, Kanto Chemical) was evaporated on the TiO2 surface at room temperature by the use of a Knudsen cell. The thickness of the C60 layer was adjusted to a monolayer (ML)-equivalent thickness (0.8 nm19) by annealing the C60-deposited sample. CuPc and TiOPc (>99%, Sigma-Aldrich) with a monolayer-equivalent thickness (0.34 nm20) were then evaporated on the C60-covered TiO2 surface using another Knudsen cell. The film thicknesses were deduced from the intensity decrease of the Ti 2p core-level peak (for the experiments at BL07LSU and BL-13B; an example is shown in Figure 1a) or the Ti LMM Auger peak

photoexcited CuPc went back to the neutral one by 328 ns after laser irradiation.7 Namely, electron−hole recombination should proceed in the nanosecond time domain. However, this view contradicts a possible formation of a metastable fullereneanion/phthalocyanine-cation exciplex, whose lifetime extends to the microsecond range.6 Thus, a more detailed picture of the carrier behavior in the time scale between nanoseconds and microseconds is required. In the present study, layered systems fabricated by metal and metaloxo phthalocyanine (MPc; M = Cu and TiO) and C60 on a rutile TiO2(110) surface were investigated by TRXPS utilizing UV-laser-pump/synchrotron-radiation-probe technique to clarify the photoexcited carrier dynamics between 0.1 ns to 4 μs. TiO2 is often used in dye-sensitized solar cells12 and perovskite solar cells13 as an electron-transport electrode. Transient photoexcited states of MPc and C60, generated by the UV laser (402 nm), were examined through the shift of the C 1s spectrum. Both C60 and MPc turn into cationic states, and these states are preserved even after 4 μs of UV irradiation. An interesting finding is that, although C60 in the C60/TiO2(110) system also responds to the UV light, photoresponsivity is enhanced when C60 coexists with the phthalocyanine molecules in the CuPc/C60/TiO2(110) and TiOPc/C60/ TiO2(110) systems. Phthalocyanine-assisted excitation of C60 is responsible for this enhancement.

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EXPERIMENTAL PROCEDURE The TRXPS measurements were performed at beamline BL07LSU of SPring-8.14 The pump laser with a wavelength of 402 nm, a pulse duration of 60 fs, and a repetition rate of 208 kHz was generated from a Ti/sapphire laser system. The laser with a power of 670 μJ/cm2/pulse was focused on the sample surface to the spot size of 250 × 210 μm2 at a full width at half-maximum. The probe synchrotron radiation was provided with a pulse duration of 50 ps and a repetition rate of 208 kHz (an H-mode operation). The spot size of the synchrotron radiation was 55 × 7 μm2 at the sample position. The pulse interval of both the laser and the synchrotron radiation was 4.8 μs. A time-of-flight electron energy analyzer (VG Scienta ARTOF 10k) was used to acquire the TRXPS spectra. An overall energy resolution, assessed from the Au 4f7/2 peak width of a gold foil, was determined to be 0.70 eV at the photon energy (hν) of 385 eV at room temperature. Details of the measurement system are given elsewhere.15 We also carried out conventional photoelectron spectroscopy measurements at beamlines 3B16 and 13B17 of the Photon Factory to characterize the electronic structures of the adsorption systems. Hemispherical electron energy analyzers were used to measure the valence-band spectra (VSW HA54 at BL-3B) and the core-level spectra (Gamma Data/Scienta SES200 at BL-13B). Typical energy resolutions were 0.30 eV at hν = 70 eV (BL-3B) and 0.27 eV at hν = 600 eV (BL-13B). All measurements at BL07LSU, BL-3B, and BL-13B were done at room temperature. Electron-binding energies are referenced to the Au 4f7/2 peak position (a binding energy of 84.0 eV) for the TRXPS spectra and the Fermi cutoff position of the Ta foil for the spectra measured at BL-3B and BL-13B. Commercially available rutile TiO2 single crystals with (110) orientation (Crystal Base and MTI) were used. To suppress the possible charging of TiO2 by photoemission, 0.05 wt % Nb-doped crystals were used throughout the experiment. After cleaning of the TiO2 surface in an ultrahigh vacuum condition by an established method (cycles of Ar+ sputtering and

Figure 1. (a−c) Photoelectron spectra of clean TiO2(110), a monolayer (ML) thick C60-covered TiO2(110), and CuPc (1 ML)/ C60 (1 ML)/TiO2(110) in (a) Ti 2p, (b) C 1s, and (c) N 1s corelevel regions. The photon energy used was 600 eV. (d) Results of peak fitting of the C 1s spectra for C60/TiO2 and CuPc/C60/TiO2 systems. Lines formed by dots are the experimental spectra whose backgrounds were subtracted by Shirley-type curves, and lines through the dots are the best-fitted results. The C 1s spectrum for C60/TiO2 (bottom) is reproduced by a single Voigt function for the main peak and two Gaussian functions for the satellite peaks. The spectrum for CuPc/C60/TiO2 is reproduced by a C60 component (a dashed line), whose line shape is the same as that of the C 1s spectrum for C60/TiO2 except for the spectral width, and a CuPc component (a solid line), which is reproduced by two Voigt functions for the components of benzene C and pyrrol C and two Gaussian functions for the satellite peaks. The satellite peaks whose intensities are enlarged by 5 times are also shown.

(measured using a cylindrical mirror analyzer installed in the measurement system at BL-3B) by deposition of C60 and the phthalocyanine molecules. Note that, for the TiOPc/C60/ TiO2(110) system, the addition of Ti by TiOPc was taken into account by assuming a TiOPc density in a monolayer film (5.1 × 1013 cm−2).20

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RESULTS Figure 1a−c shows, respectively, Ti 2p, C 1s, and N 1s spectra acquired from clean TiO2(110), C60/TiO2(110), and CuPc/ C60/TiO2(110) systems. The C 1s spectrum of C60/TiO2(110) (Figure 1d, lower) bears a peak at 284.6 eV with small B

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The Journal of Physical Chemistry C contributions at 286.5 and 288.5 eV, which are attributed to the first two intense shake-up satellite peaks.21 As CuPc covers the C60 layer, the main C 1s peak is suppressed, whereas the tails at both sides of the main peak become somewhat intense (Figure 1b). To extract the contribution of CuPc, the C 1s spectrum is deconvoluted into C60 and CuPc components (see the Supporting Information for the deconvolution procedure). A peak of benzene carbon and that of pyrrol carbon of CuPc are observed at 284.3 and 285.3 eV, respectively (Figure 1d, upper). Furthermore, additional two small contributions emerge at 286.1 and 287.2 eV, both of which are, again, attributed to the shake-up satellites.10,11,22 The shake-up satellite structure is observed to be more intense in the N 1s spectrum (Figure 1c), in which a shoulder at ∼400 eV is attributed to the shake-up structure.22 As shown later in Figure 4b, the C 1s spectrum for the TiOPc/C60/TiO2 system is also deconvoluted into the C60 and phthalocyanine components. Figure 2 shows the absorption spectra of C60, CuPc, and TiOPc films in the UV−visible light region. An intense

Figure 3. (a) C 1s spectra of C60/TiO2(110) acquired without UV laser irradiation (denoted as “Laser OFF”) and at 0.1 ns after irradiation (“Laser ON”). The power density of the pump laser (402 nm) was 670 μJ/cm2, and the photon energy of the probe light was 385 eV. Curves formed by dots are experimental data and solid lines are the best-fitted results using a Shirley background curve and Voigt functions. Enlarged spectra around the peak maximum are displayed in the inset. These spectra were measured consecutively with and without laser irradiation. The peak maximum is indicated by a triangle. (b) Dependence of the laser-induced C 1s peak shifts on the delay time t. Figure 2. UV−visible light absorption spectra of C60, CuPc, and TiOPc deposited on glass plates in ultrahigh vacuum (an inset picture). A double-beam spectrometer (Jasco V-630) was used to acquire the spectra, in which the contribution of the glass plate was subtracted. Thicknesses of the organic films, estimated by a quartz thickness monitor, were 31, 16, and 30 nm for C60, CuPc, and TiOPc, respectively. Absorbance of the spectra are normalized to monolayerequivalent absorbance; namely, the measured values of absorbance are multiplied by 0.026 (=0.8/31), 0.021 (=0.34/16), and 0.011 (=0.34/ 30) for C60, CuPc, and TiOPc, respectively.

error but reflects some physical phenomenon. Although the magnitude of the shift is different, the laser-induced C 1s peak shift of C60 was also reported in our preceding study.26 As shown in Figure 3b, the amount of the peak shift is insensitive to the change of the delay time t (a time difference between the pump pulse and the probe pulse), indicating that a laser-induced state of C60 has a long lifetime in the C60/TiO2 system. It should be noted that, although the laser energy (402 nm = 3.1 eV) exceeded the band gap energy of rutile TiO2 (3.0 eV27), a surface photovoltage was not induced on the TiO2 surface, as confirmed by the insensitivity of the Ti 2p peak position (not shown). This is reasonable because the laser power density used in the present study (670 μJ/cm2) was not enough to induce an appreciably large photovoltage.18 Figure 4a,b shows the C 1s spectra of the CuPc/C60/ TiO2(110) and TiOPc/C60/TiO2(110) systems, respectively. Solid lines through the dots (the experimental data) are bestfitted results of the spectral line shape, which is decomposed into C60 and MPc components (drawn by thick solid lines). It is noted that the C60 C 1s peaks appear at 284.6 eV in the spectra of the CuPc/C60/TiO2 system shown in Figures 1d and 4a, whereas the energy position of the CuPc C 1s component is rather different (284.3 and 284.0 eV for the benzene C peaks in Figures 1d and 4a, respectively). A reported energy difference between the C 1s peaks of C60 and benzene C of CuPc ranges from 0.211 to 0.9 eV,7 and the values found in the present study fall in this energy range. Although the exact origin of the discrepancy is not understood yet, experimental results from the TRXPS measurements given below are not

absorption peak at 345 nm and a broad shoulder at 450 nm in the C60 spectrum are associated, respectively, with the HOMO − 1 → LUMO + 1 transition and the HOMO − 1 → LUMO (and/or HOMO → LUMO + 1) transition23 (highest occupied molecular orbital, HOMO; lowest unoccupied molecular orbital, LUMO). The CuPc and TiOPc spectra bear adsorption bands centered at 300−400 nm (the B band) and between 500 and 900 nm (the Q band).24,25 At the monolayer-equivalent thickness, absorbance of C60 is larger than that of CuPc and TiOPc at 402 nm. Thus, the pump laser should mainly excite C60. To examine the photoresponsivity of C60, we first studied the C60/TiO2(110) system. Figure 3a shows the C 1s spectra acquired with and without pump laser irradiation (the spectrum “with” the laser was obtained at 0.1 ns after laser pulse irradiation). Two spectra are almost identical, but a slight shift of the peak maximum towards higher binding energies is recognized. The magnitude of the shift is estimated to be 7 ± 1 meV. Although the peak shift is very small, the peak position moves back and forth in response to laser irradiation (the inset of Figure 3a). Thus, the shift is not caused by a measurement C

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should reflect a physical phenomenon induced in the phthalocyanine molecules by laser irradiation. It is worth noting that the magnitude of the laser-induced C60 C 1s peak shifts that we are focusing on is much smaller than the energy resolution of the system (0.70 eV) and is even smaller than the thermal broadening at room temperature (25 meV). However, it is not surprising to detect such a small shift because the shift reflects the move (in the energy scale) of the center of the gravity of the observed peak.

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DISCUSSION Behavior of Photoexcited Carriers. The laser-induced changes in the MPc/C60 systems (M = Cu and TiO) observed in the present study are different from those reported for the CuPc/C60 system in the preceding study by Arion et al.7 They found that, though the CuPc C 1s peak was insensitive, the C60 C 1s peak moved toward the lower binding energy side by a visible-light laser (532 nm). The shift is understood by the formation of anionic C60 (Cδ− 60 ) as a result of an electron transfer from excited CuPc that absorbs the visible light. The phthalocyanine → C 60 electron transfer is a natural consequence because phthalocyanine and C60 are, respectively, p-type and n-type organic semiconductors and act as hole and electron transport layers in OPVs.28 Thus, many phenomena induced in the phthalocyanine/C60 systems by visible-light irradiation have been interpreted on the basis of the phthalocyanine → C60 electron transfer.4−9 To understand the laser-induced changes observed in the present study, i.e., the shift of the C 1s peaks of both C60 and MPc towards the higher binding energies, knowing an energy level diagram of the layered system is of great help. Figure 5a shows valence band spectra of the clean, C60-covered, CuPc/ C60-layered, and TiOPc/C60-layered systems on TiO2(110). The valence band maximum (VBM) of the clean TiO2(110) surface lies at 2.9 eV below the Fermi level. As the TiO2 surface is covered with a monolayer-thick C60 layer, the emission from TiO2 is largely suppressed and the spectrum is dominated by the C60 states. HOMO and HOMO − 1 peaks of C60 are observed at 1.8 and 3.2 eV, respectively, with an upper edge of the HOMO state at 1.2 eV. Upon deposition of CuPc, a HOMO state of CuPc is visible at 0.7 eV at 0.5 ML and is shifted to 0.9 eV at 1 ML. A leading edge of CuPc HOMO is 0.3 eV at 1 ML. For the TiOPc/C60 system, the HOMO of TiOPc is much deeper with 1.5 eV at 1.1 ML, whereas the leading edge is slightly shallower at 0.2 eV. For both systems, the C60 valence states shift toward higher binding energies by phthalocyanine deposition. As determined from the C60 HOMO position, falls in an energy region between the HOMO and HOMO − 1 levels of MPc (Figure S1 in the Supporting Information), the energy shift amounts to ∼0.3 eV up to 1 ML of CuPc deposition and to ∼0.35 eV up to 0.5 ML of TiOPc (a shift is uncertain at 1.1 ML because the HOMO of C60 does not form a clear peak). The cause of the shift is an interface dipole formed at the MPc/C60 interface,11 as is evident from the shift of the secondary electron cutoff position by MPc deposition on top of the C60 layer (Figure S2 in the Supporting Information). Based on the valence band observations, energy-level diagrams of the MPc/C60/TiO2(110) systems are reconstructed as shown in Figure 5b. Here, positions of the LUMO levels are estimated by using the band gap of rutile TiO2 (3.0 eV) and HOMO−LUMO gap energies of C60 (1.9 eV) and CuPc (1.8 eV), both of which correspond to the shake-up

Figure 4. (a, b) C 1s spectra of CuPc/C60/TiO2(110) and TiOPc/ C60/TiO2(110) measured with and without UV laser irradiation. Enlarged spectra around the peak maximum are shown in the inset. Parameters for the pump laser and the probe light are the same as those described in Figure 3. Each C 1s spectrum is decomposed into the C60 and phthalocyanine components (solid lines). (c, d) Plots of the C60 C 1s peak shift (upper panels) and those of the phthalocyanine C 1s peak shift (lower panels) as a function of the delay time t. Error bars cover scatter of the data points of multiple measurements. Solid lines in the upper panels is the best-fitted result of the experimental data to a biexponential function, Δ1 exp(−t/τ1) + Δ2 exp(−t/τ2), where Δ and τ are the amount of shift and the decay time constant, respectively.

affected, since TRXPS is a site-specific technique, which is ideal for studies of complex systems. Insets in Figure 4a,b show magnified views around the peak maximum, where the C60 contribution is dominant. Clearly indicated is a laser-induced peak shift toward higher binding energies. The magnitude of the C60 C 1s peak shift is 13−14 meV at the delay time of 0.1 ns for both systems. This value is diminished to less than 10 meV as the delay time is prolonged to more than 100 ns (upper panels of Figure 4c,d). The delay time dependence of the C60 C 1s peak shifts for both CuPc/C60/TiO2(110) and TiOPc/C60/TiO2(110) systems are reproduced well by a biexponential function, and the best-fitted results (solid lines) are obtained with decay time constants of 95 ns and >100 μs for CuPc/C60/TiO2 and 20 ns and >100 μs for TiOPc/C60/TiO2. This means that laser irradiation gives rise to at least two different transient excited states of C60; one has a relatively short lifetime (less than 100 ns) and the other is a very stable state with a μs-order lifetime. Regarding the CuPc and TiOPc C 1s peaks, it is more difficult to accurately determine the positions because the peak maximum is determinable only after deconvolution. Plotted points in the bottom panels of Figure 4c,d are the determined C 1s peak shifts of CuPc and TiOPc, respectively. Since each point has a relatively large uncertainty (as large as ±20 meV) and the points seem to be distributed randomly, it is difficult to grasp a clear trend of the shift as a function of the delay time. Nevertheless, the points are distributed on the “plus” side, indicating the shift towards higher binding energies. The shift to the same direction is also observed in the N 1s peak at t = 0.1 ns for both CuPc and TiOPc (not shown). Thus, the shift D

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LUMO is not recognized in the valence spectra in Figure 5a. Thus, the charged state of C60 should be more carefully examined by an elaborate study. However, if C60 is anionized, absence of the C60 C 1s peak shift (Figure 1d) may be explained by cancellation between the shift by the interface dipole shift and that by additional charge on C60. C60 and MPc are typical n-type and p-type semiconductors, respectively, and our valence-band measurements confirm the formation of the p/n heterojunction at both CuPc/C60 and TiOPc/C60 interfaces (Figure 5b). Thus, as Arion et al. have proposed in their study of the CuPc/C60 junction,7 the formation of Cδ− 60 upon phthalocyanine excitation by the visible-light laser is naturally understood on the basis of the energy-level alignment. In the present study, however, the C60 C 1s peak moves toward the higher binding energy side, which is contrary to that observed by Arion et al.7 Moreover, the CuPc and TiOPc C 1s peaks also shift in the same direction. These observations are explainable if both C60 and MPc are cationized. Since the energy of the pump laser was 3.1 eV (402 nm), an electron promotion should be between the HOMO (or HOMO − 1) and LUMO + 1 (or LUMO) levels in both C60 and MPc because the HOMO−LUMO gaps of these molecules (1.7−1.9 eV) are much smaller than the photon energy. However, an involvement of the LUMO state is less probable in the photoexcitation process of C60 because the LUMO may be partially occupied in the MPc/C60/TiO2 systems (Figure 5b). According to the study by Enkvist et al.,21 the C60 HOMO → LUMO + 1 transition energy is determined to be ∼3.0 eV, which matches the laser energy used in our study (3.1 eV). Thus, the HOMO → LUMO + 1 transition is probable when C60 is excited by the 3.1 eV laser. Since the LUMO + 1 state of C60 should be located well above the conduction band minimum (CBM) of TiO2(110) even in the MPc/C60/TiO2 systems as judged from the C60 HOMO positions (Figure 5b), an electron transfer from C60 to TiO2 is energetically favored. Regarding MPc, on the other hand, not only electrons excited in the LUMO + 1 level but also those in the LUMO level can be transferred to TiO2 because both levels lay above the CBM of TiO2. Since the C60 layer (0.8 nm thickness) is inserted between MPc and TiO2, a C60-mediated transfer rather than a direct transfer may proceed. Photoexcitation and the subsequent electron transfer are depicted in the left part of Figure 6a. A reason why spontaneously formed Cδ− 60 is not observed in the present study is expected to be a fast C60 → TiO2 electron transfer. However, if the C60 layer is much thicker as fabricated

Figure 5. (a) Normal emission spectra in the valence band region of TiO2, C60/TiO2(110), CuPc/C60/TiO2(110), and TiOPc/C60/ TiO2(110). The photon energy of 70 eV was used. Spectra around the HOMO states of CuPc (1 ML) and TiOPc (1.1 ML) are also shown with a 5-fold enlarged intensity scale. Arrows indicate the energy positions of the upper edges of the HOMO states of C60, CuPc, and TiOPc determined by extrapolating the leading edge of the structures. (b) Energy-level diagrams of C60/TiO2(110), CuPc/C60/ TiO2(110), and TiOPc/C60/TiO2(110). Energy positions of the VBM, the HOMO levels, and the upper edges of the HOMO levels are experimentally determined, whereas the conduction band minimum (CBM) and LUMO positions are deduced from the VBM and HOMO positions and the band gap of TiO2 (3.0 eV) and the HOMO−LUMO gaps of the molecules (1.9 eV for C60, 1.8 eV for CuPc, and 1.7 eV for TiOPc).

energies determinable from the data in Figure 1d, and the HOMO−LUMO gap energy of TiOPc (1.7 eV), which is a literature value.29 Note that the center of the LUMO level of C60 is below the Fermi level in both CuPc/C60/TiO2 and TiOPc/C60/TiO2 systems. This implies anionization of C60 upon MPc adsorption. However, a trace of the occupied

Figure 6. (a) Schematic illustrations showing the photoexcitation process followed by an electron transfer (left) and the electron−hole recombination process (right). (b) Possible MPc → C60 energy transfer processes by the Förster mechanism (dipole−dipole coupling) and the Dexter mechanism (an electron exchange). E

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The Journal of Physical Chemistry C in the preceding studies3−7,9−11 or the stacking order of MPc and C60 is reversed as the study by Arion et al.,7 a retention time of the transferred electron from MPc should be longer so that there would be an increasing chance to detect Cδ− 60 . The laser-induced spectral changes observed in the present study are not limited to the peak shift. Although very small, the width of the C60 C 1s peak increases by 0.5−1% (Figure S3 in the Supporting Information). Peak broadening implies that C60 is not homogeneously charged upon laser irradiation, i.e., there should exist several charged states between neutral C60 and δ+ cationic C60 . Furthermore, neutral C60 feels a different potential depending on the distance from charged C60 in the same layer as well as charged MPc in the above layer so that the C 1s core levels in these neutral C60 species should be slightly different. Although these multiple states of C60 are not resolved in the present measurement condition, the laserinduced peak shift is a good measure for evaluating the photoresponsivity of the layered system; namely, the larger the peak shift is, the higher the C60 photoresponsivity is. It is important to exclude other phenomena that may induce the C 1s peak shift. There are two possible explanations: one is sample charging and the other is laser heating. The former possibility can be easily denied, since the laser energy (3.1 eV) is not enough to yield emitted photoelectrons. Laser heating is more difficult to negate, but because an 800 nm pump laser also causes the C 1s peak shift, this possibility might be excluded. In Figure S4 in the Supporting Information, the C60 C 1s peak shifts for CuPc/C60/TiO2(110) induced by 402 and 800 nm pump lasers are compared. It is obvious that the 800 nm laser is more efficient than the 402 nm laser. On the other hand, as shown in Figure 2, light absorbance of C60 is much lower at 800 nm than at 402 nm. Thus, though not decisive, these observations imply that laser-induced sample heating may not be the cause of the C 1s peak shift. Enhancement of Photoresponsivity of C60. One of most interesting findings in the present study is that the UVinduced shift of the C60 C 1s peak is larger for the MPc/C60/ TiO2 systems than for the C60/TiO2 system. Figure 7 summarizes the delay-time dependence of the C60 C 1s peak shifts for the three systems. The magnitude of the shift is nearly twice as large for the MPc/C60/TiO2 systems than the C60/TiO2 system at short delay times (