ARTICLE pubs.acs.org/JPCC
Crystallization-Induced Charge-Transfer Change in TiOPc Thin Films Revealed by Resonant Photoemission Spectroscopy Shun Yu,*,† Sareh Ahmadi,† Chenghua Sun,†,‡ Karina Schulte,§ Annette Pietzsch,§ Franz Hennies,§ Marcelo Zuleta,^ and Mats G€othelid*,† †
Materials Physics, MAP, ICT, Royal Institute of Technology (KTH), Electrum 229, SE-16440 Kista, Sweden The University of Queensland, ARC Centre of Excellence for Functional Nanomaterials and Centre for Computational Molecular Science, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, Qld 4072, Australia § Max-lab, Lund University, Box 118, S-221 00 Lund, Sweden ^ Physical & Analytical Chemistry Department, Uppsala University, Box 259, SE-75105 Uppsala, Sweden ‡
bS Supporting Information ABSTRACT: Organic semiconductors usually demonstrate crystal structure dependent electronic properties, and through precise control of film structure, the performance of novel organic electronic devices can be greatly improved. Understanding the crystal structure dependent charge-transfer mechanism thus becomes critical. In this work, we have prepared amorphous titanyl phthalocyanine films by vacuum molecular beam evaporation and have further crystallized them through vacuum annealing. In the crystalline phase, an excited electron is rapidly transferred into neighboring molecules; while in the amorphous phase, it is mainly localized and recombines with the core hole as revealed by resonant photoemission spectroscopy (RPES). The fast electron transfer time is determined to be around 16 fs in the crystalline film, which is in good agreement with the charge-transfer hopping time estimated from the best device performance reported. The crystallized film shows more p-type characteristics than the amorphous with all the energy levels shifting toward the vacuum level. However, the greatly improved charge transfer is assigned to the molecular orbital coupling rather than this shift. From density functional theory and RPES, we specify the contribution of two differently coordinated nitrogen atoms (N2c and N3c) to the experimental results and illustrate that the N3c related orbital has experienced a dramatic change, which is keenly related to the improved charge transfer.
’ INTRODUCTION The fast development of novel electronic devices based on semiconductor organics, such as organic field-effect transistors,1 organic light-emitting diodes,2 organic photovoltaic devices,3 and nonlinear photonic devices,4 demands a better understanding of the inherent charge-transfer mechanism. The charge transfer, especially following photoexcitation, usually depends on the charge separation at the molecular or organic/inorganic interface. This process can be affected by many issues:5 energy level alignment, thermal energy and electric field, and nanomorphology. Stimulated by the significance of the applications, tremendous efforts have been made to select proper materials and to arrange them in optimized device architectures with the appropriate crystal structure in order to balance the different requirements of those factors. Thus, the elucidation of the relation between structure and electronic properties is of both scientific and technological importance. Phthalocyanines have attracted much attention in the field of organic electronics because of their good photoelectrical properties associated with their chemical and thermal stability.6 By decorating the central part of the organic macro ring with a metal, r 2011 American Chemical Society
a metal-halide, or a metal-oxo moiety, it is possible to manipulate the molecular chemical structure and to thereby modify the molecular electronic structure. Titanyl phthalocyanine (TiOPc) has been regarded as one of the best organic photoconductors.6 It has a nonplanar structure (Figure 1) converse to the structure commonly found in phthalocyanines, which results in a diversity of molecular packings. So far, four packing modes of TiOPc are observed: amorphous, phase I, phase II, and phase Y.7,8 Among them, phase II shows promising electronic properties in various devices.912 By controlling the deposition rate and substrate temperature, one can manipulate the crystal structure from amorphous to a mixture of the crystalline phase I and phase II. The latter is more favored on crystalline substrates and at higher temperature treatment.13 Photoelectron spectroscopy (PES)14 and X-ray emission spectroscopy15 have been used to characterize the intrinsic electronic properties of crystallized TiOPc, the effect of atmospheric Received: October 20, 2010 Revised: June 7, 2011 Published: June 13, 2011 14969
dx.doi.org/10.1021/jp1100363 | J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C
Figure 1. The TiOPc molecular structure.
doping,16 TiOPc/C60 donoracceptor dimers,12,17 and so forth. Discussions on intramolecular and intermolecular charge transfer, such as the TiN and TiO charge transfer, can also be found in such investigations based on resonant inelastic X-ray scattering (RIXS) at the Ti L-edge.15,18 In this work, we combine PES and density functional theory (DFT) to investigate the molecular crystal structure dependent electronic properties of thin TiOPc films on rutile TiO2(110). Deposition at room temperature resulted in an essentially amorphous film. Subsequently, the crystallized film was achieved by annealing at 250 C. We have monitored molecular orbital changes during this phase transition. Special attention has been paid to compare and analyze the charge transfer of photoexcited electrons in different phases by the implementation of resonant photoemission spectroscopy (RPES).19 This technique has been successfully used to determine the charge-transfer time and the electronic structures at heterojunctions, for instance, ruthenium complex dyes20,21 and their components22 on TiO2, organics on metal surfaces,23 and polymers system.24 For other phthalocyanine systems, RPES revealed an ultrafast charge transfer for CuPc monolayer on Au, whereas a much slower process was observed for thick CuPc films.25 In a spin-coated metal-free phthalocyanine (H2Pc) derivative liquid crystal film, a faster delocalization is observed in ordered regions than in amorphous regions.26 By combining the element sensitivity of RPES and DFT molecular orbital calculations, we can distinguish the contribution to the molecular frontier orbitals from the two differently coordinated nitrogen atoms and can reveal the influence of phase transformation on those orbitals and charge transfer in the excited state. The chargetransfer time in the crystallized film is determined to be around 16 fs, which is in good agreement with the time scale estimated from the best TiOPc organic field-effect transistor (OFET) device measurements.10
’ EXPERIMENTAL METHODS The TiO2 substrate was purchased from Surface Preparation Laboratory, The Netherlands, and was pretreated by heating to 1000 K in ultrahigh vacuum (UHV) for 2 h in order to increase the conductivity resulting in a change of sample color from transparent to light blue. Before molecular deposition, the substrate surface was cleaned by cycles of Ar+ sputtering and ultrahigh vacuum annealing until the surface displayed a clear 1 1 low-energy electron diffraction (LEED) pattern and it was free from contamination as judged by PES. TiOPc was purchased from Sigma-Aldrich (95% purity). Before deposition, the molecules were thoroughly outgassed until no impurities (mainly water) were detected. The thickness of the molecular layer was estimated to about 2.5 nm on the basis of the intensity attenuation of the substrate Ti2p3/2 using a calculated inelastic mean free path within the organic layer around ∼0.8 nm according to Seah and Dench.27 Annealing was realized by resistively heating a 99.9% pure tantalum ribbon (0.03 mm 0.75 mm 10 mm) at
ARTICLE
2 A for 10 min, which was clipped at the back side of the TiO2 substrate. The heating temperature was measured by a pyrometer and was estimated to be ∼250 C on the TiOPc film. According to Coppede et al.,13,28 the film can transform from amorphous to mainly crystalline phase II at this temperature on a crystalline substrate (e.g., sapphire). A strong desorption of the molecule from the surface can be excluded on the basis of the small changes of the N1s and C1s integrated intensities and the small intensity increase of the substrate Ti2p3/2 signal (the estimated thickness after annealing is ∼2.3 nm27). The experiments were carried out at beamline I511, MAX-lab synchrotron light source in Lund, Sweden. PES spectra were recorded by a Scienta R4000 analyzer at normal emission and a p-polarized beam at a grazing incidence angle of 7. The total experimental resolutions were as follows: 154 meV (hν = 514 eV, Ti2p), 133 meV (hν = 455 eV, N1s), 106 meV (hν = 347 eV, C1s), and 18 meV for valence band (VB, hν = 110 eV) spectra. All core level spectra were normalized to the background at the lower binding energy side, and VB spectra were normalized to the intensity at 25 eV, where neither substrate nor molecular orbitals contribute. The binding energy scale was calibrated with respect to the Fermi level measured from a tantalum sample holder in electrical contact with the sample. Numerical curve fitting was performed with the XPSpeak41 software.29 RPES were recorded at the same geometry as PES by the Scienta R4000 with 150 meV electron energy resolution and 95 meV photon resolution, respectively. The binding energy of RPES is aligned with their counterparts measured at 110 eV photon energy. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) was also collected under p polarization by the Scienta spectrometer in the Auger yield mode (the kinetic energy window 369.0 ( 10.0 eV) with the emission angle at 45 to the photon polarization-vector. The photon energy was calibrated by measuring the kinetic energy difference of Ti2p3/2 from first- and second-order light. To avoid beam damage to the molecular film during measurement, the sample was scanned with a speed of 1.7 μm/s.
’ COMPUTATIONAL PARAMETERS Spin-polarized density functional theory (DFT) calculations have been carried out using the DMol3 code.30,31 We used the generalized gradient approximation (GGA) with the Perdew BurkeErnzerhof functional (PBE)32 for the geometry optimization and the electronic structure calculations. Effective core potentials with double-numeric quality basis and all electron potentials with double-numeric polarized basis have been employed for the description of core electrons during the optimization and the energy calculation, respectively. During our calculations, the convergence criteria for structure optimizations were set to (a) energy tolerance of 1.0 106 Ha per atom, (b) maximum force tolerance of 1.0 104 Ha/Å, and (c) maximum displacement tolerance of 1.0 103 Å. ’ RESULTS AND DISCUSSION C1s and N1s photoelectron spectra recorded from as-deposited (amorphous) and annealed (crystalline) TiOPc thin films are shown in Figure 2 with their numerical fits. The typical C1s spectrum (Figure 2a) of a transition-metal Pc molecule is a threepeak structure which usually consists of four components: benzene carbon (CB), pyrrole carbon (CP), and their corresponding shakeup components.33 We have fitted the spectra using these components, but their position, intensity, and width 14970
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C
ARTICLE
Figure 2. C1s (a) and N1s (b) core level spectra of TiOPc before (black) and after (red) annealing. Each spectrum has been numerically fitted. For C1s (a), CP and CB with their corresponding shakeup structures are shadowed in blue and green. The CP/CB intensity ratio reduces from 0.28 to 0.23 after heat treatment. For N1s (b), the shakeup structure is shadowed in dark gray.
are different in the two preparations. On the basis of the molecular stoichiometry, the relative intensity between the carbon peaks (CP and CB) should be CP:CB = 1:3. In the present case, the ratios are different before and after annealing as shown by the curve fitting. The molecular orientation may affect the PES intensity. For a randomly oriented molecular film, the CP:CB value should be close to 1:3, while for a more ordered film, a deviation from this value can be generated because of the surface sensitivity of PES34 unless the molecules are lying flatly, which is not supported by our NEXAFS observation (see below). In our measurement, we used the same photon energy and p polarization to keep a constant surface sensitivity. The N1s spectrum (Figure 2b) usually shows as a single peak with a small shakeup structure behind. A 0.4 eV chemical shift between N2c and N3c (see Figure 1) has been claimed by theoretical calculations,35 but other independent experiments14,15 also point to an identical line profile as we present here. In Figure 2, both C1s and N1s spectra narrow by 0.2 eV following annealing. In general, an amorphous structure renders a broad peak, while crystalline order gives sharp features. In the disordered film, molecules adopt a variety of different configurations with respect to their nearest neighbors and experience different local electronic environments resulting in a broadened peak. Our observation points to a transformation from amorphous to crystalline. The C1s spectrum entirely shifts by 0.25 eV to lower binding energy after annealing and N1s shifts by 0.20 eV in the same direction. Conventionally, a shift to the lower binding energy indicates a higher electron density around the emitter. An increased conductivity within the film would lead to a more effective core hole screening, which also will shift a PES peak
to lower binding energy. Harada et al. have reported that the apical oxygen of TiOPc in phase II (R-phase) induces additional intermolecular orbital hybridizations,18 which may contribute to an increased electron density and mobility compared to the amorphous film. The position of the band edges and core levels with respect to the Fermi level can thus change depending on crystal order and defects just like in an inorganic semiconductor. To get a good fit, CP and CB are shifted different amounts between the two spectra. Although we should avoid overinterpreting the results from curve fitting, this observation reflects variations in the electronic surrounding leading to differences in chemical shifts and local charge transfer screening process on different parts of the molecule.36 Previous studies of monolayers of TiOPc,37 H2Pc,38 and FePc39,40 on TiO2 have shown that a single-layer Pc experiences a charge transfer from molecule to substrate. Consequently, the C1s and N1s from interface molecules appear at higher binding energy typically shifted by more than 1.0 eV from the thick film peak. Thus, the shift to lower binding energy in the present case is not a shifted interface peak. In addition, although changes in line profile and peak position are observed, a quantitative analysis reveals fairly constant integrated peak intensity before and after the heat treatment, which contradicts severe desorption. The shakeup satellite peak stems from photoelectrons having lost kinetic energy by exciting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) on its way out. This shakeup appears on the higher binding energy side of the main photoemission peak shifted by approximately the HOMO/LUMO gap (around 1.7 eV for TiOPc41). Structural changes of TiOPc might actually 14971
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C
ARTICLE
Figure 5. N1sfπ* transition region of N K-edge NEXAFS before (black) and after (red) annealing recorded with p-polarized light. N1sfLUMO transition is marked by A. Transitions B and C represent the transitions to higher empty π* orbitals. After the heat treatment, transition B splits into two components B0 and B00 . Figure 3. Top panel: Ti2p3/2 core level of clean TiO2; bottom panel: Ti2p core level spectra of TiOPc before (black) and after (red) annealing. Curve fitting: black lines are TiO2 components with 1:9 Lorentzian to Gaussian ratio; blue line is TiOPc component with 3:7 Lorentzian to Gaussian ratio, which gives the best fit.
Figure 4. Valence band spectra. Top panel: clean TiO2; bottom panel: TiOPc on TiO2 before (black) and after (red) annealing. TiO2 related components are referred to the clean TiO2 by the vertical line.
decrease the Q band absorption energy by 0.2 eV from 722 nm in the amorphous phase to 835 nm in R phase (phase II).42 We do not observe any clear changes in the shakeup in our spectra, but the relative intensity is weak and the presence of the core hole during the excitation may modify the apparent energy. Thus, we assign the as-deposited film to an amorphous phase while the annealed film holds crystalline order. The two phases have different electron densities, conductivities, and energy-level
alignment with respect to the Fermi level. Annealing does not change the surface coverage significantly. Ti2p3/2 spectra are shown in Figure 3. As a reference, the Ti2p3/2 spectrum from the clean TiO2 surface is shown in the top panel. The dominant peak at 459.1 eV is assigned to Ti4+ ions in the bulk and at the surface. The smaller hump at lower binding energy stems from Ti3+ ions at defects in the substrate, such as oxygen vacancies43 and Ti interstitials.44 Deposition of TiOPc is displayed in the two lower spectra. The spectra have been fitted with three components: substrate Ti4+ and Ti3+ (black) and TiOPc (blue). Annealing causes a narrowing and a 0.35 eV shift of the TiOPc component (blue) toward lower binding energy. This is the same direction but is slightly larger than C1s and N1s. Additionally, the substrate component becomes slightly stronger after annealing. This observation is related to the fact that more of the substrate may be exposed after the crystallization and island formation of the overlayer. A similar crystallization process of TiOPc has been observed on sapphire and mica.28 Valence band spectra from the clean surface, the as-deposited TiOPc film, and the annealed TiOPc film are shown in Figure 4. The line profile development of the valence band spectra underlines formation of crystalline islands since the contribution from the substrate becomes stronger after annealing. In the regions away from the strong substrate VB contribution (4.010.0 eV), the molecular spectra are rather similar, but there is a general shift to lower binding energy of all peaks after annealing. For instance, HOMO shifts by 0.2 eV. In addition, there is a split and a new peak appears closer to the Fermi level. On one hand, such splits have been observed previously and were explained by the interaction between the oxygen moiety and the adjacent organic ring.45 Thus, one would expect that amorphous or disordered bilayers would generate such HOMO splitting. On the other hand, this split can also be explained by molecules having their intrinsic dipoles pointing in opposite directions for the TiOPc bilayer structure, where the molecules typically pair up with the oxygen pointing inward.46,47 Hence, more theoretical work is needed in order to safely exclude a split HOMO in the crystalline 14972
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C
Figure 6. The total density of states (black solid line) of TiOPc and the partial density of states from N2c (red solid line) and N3c (blue solid line) are plotted in the bottom. The energy scale is expanded by 30% (middle) in order to match with the experimental valence band data (black circle, top) and NEXAFS (void circle, top) before annealing. The smearing width of the calculation result is 0.05 eV, and the zero energy for DOS and PES is chosen at HOMO. On the basis of the appearance of VB, three molecular orbitals are schematically divided as orbitals I, II, and III, which are aligned with the counterparts of the stretched DOS. The nitrogen PDOS is multiplied by 10 to be conveniently compared with experimental data. NEXAFS is shifted to align transition A (N1sfLUMO) with the simulated LUMO. Transitions B and C also match with higher empty orbitals of DFT simulations.
TiOPc bulk. The systematic peak shift including both core level and VB of TiOPc indicates that the energy alignment is strongly dependent on the packing mode of the organic film. Phase II is reported being a p-type semiconductor with better electronic performances than the other phases.48 Our observed energy shift toward the vacuum level advocates for the formation of a ptype film. Figure 5 shows N K-edge NEXAFS by which we investigate the empty orbitals in the presence of a N1s core hole. Transition A, ascribed to an excitation from N1s into LUMO, is observed at 398.6 eV photon energy in the amorphous thin layer, whereas after annealing, this transition occurs at 398.5 eV. The transitions B and C are due to excitations from N1s into higher empty π* orbitals. TiOPc possesses a degenerate LUMO because of the C4v symmetry.48,49 The molecular distortion (symmetry reduces from C4v to C1 when crystallization to phase II takes place) lifts the degeneracy of LUMO giving rise to doublet absorption structure of phase II in Q bands (660 and 840 nm, ∼0.4 eV separation) which have orthogonal transition momentum.42,49 However, the spectra do not show any splitting of LUMO either before or after annealing within the experimental resolution in that NEXAFS does not necessarily represent the ground-state density of unoccupied states because of the Coulomb electron core hole interaction.50 Transition B is shown as an asymmetric peak at 399.9 eV (B0 ) before annealing. After annealing, this peak splits into two components B0 and B00 accompanied with a clear intensity reduction of the transition B0 and a broadened peak width. The intensity of NEXAFS resonant transition is related to the total number of electron decays (normal Auger + autoionization, see Supporting Information), which are proportional to the absorption cross section. Clearly, this reduced intensity at
ARTICLE
transition B points to the fact that crystallization modifies the electronic structure of unoccupied orbitals through changing the molecular symmetry or molecular coupling and further reducing the absorption cross section. Consequently, fewer transitions from N1s to this empty state were allowed after annealing. The angle-dependent NEXAFS did not show any large variations because of random orientation of the as-deposited molecules and also of the polycrystalline TiOPc islands resulting in the equal intensity of N1s to π* transition at different light polarization (not shown here). A DFT calculation has been done on an isolated TiOPc molecule. Figure 6 presents the total density of states (DOS) of the molecular orbitals (MO) and the partial density of state (PDOS) of two-coordinated nitrogen (N2c) and three-coordinated nitrogen (N3c) at the bottom with the zero energy chosen at the HOMO peak. The overall features of DOS resemble the measured PES and NEXAFS spectrum reasonably (Figure 6 top) before annealing but with an evident energy scale contraction, which has been reported for several phthalocyanines calculated using the GGA-PEB method5155 and for other organic molecules.56 This compression of DOS stems from the comparison of KohnSham eigenvalues with quasiparticle excitation energies.57,58 Generally, a stretch factor is required to fit the simulated DOS with the experimental data.5254,56,59 In this case, we have to expand the energy scale of the simulated DOS by 30% to match the measured orbitals IIII, which is close to the earlier reports for CoPc and NiPc.53 The discrepancy of the relative intensity between the simulated results and the experimental data can be due to the lack of correction for the photoionization cross section in the simulation and the overlap of the molecular orbital and TiO2 VB in the experiments. Nevertheless, given all the issues above and that we did the calculation on a single molecule, the simulation fits reasonably well with the experiments. According to these results, we conclude that both N2c (red) and N3c (blue) do not contribute much to the HOMO orbital, which is mainly located on the pyrrole and carbons benzene rings. It is also supported by the result from other phthalocyanines.60 N2c has a larger contribution to orbital I while N3c dominates in orbital II similar to the case of copper hexadecafluorophthalocyanine.61 For the empty orbital, N2c contributes more on transition A and B than transition C, while N3c has about equal contribution to transitions AC. The electronic structure is evidently modified as a consequence of the phase transformation from amorphous to crystalline. How does this influence the lifetime of excited states and charge-transfer times within the molecular structure? The charge-transfer process was investigated by resonant photoelectron spectroscopy as shown in Figure 7. Spectra were collected before (b) and after (c) annealing and were aligned with the VB spectra. The RPES images are essentially a collection of valence band spectra recorded at different photon energies across the N K edge threshold in the colorful intensity scale. There is also a contribution from N1s photoelectrons generated by secondorder light from the monochromator, which superimposes on the RPES as a linear trace developing from higher to lower binding energy with increasing photon energy (in the left bottom corner). The HOMO should display at a binding energy around 2.0 eV, which is not obvious in this case because of the low cross section at this photon energy. In addition, no resonant enhancement has been observed along the HOMO distribution which is in agreement with the theoretical calculation: N atoms do not give any major contribution to the HOMO. Spectra are divided 14973
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C
ARTICLE
Figure 7. Resonant photoelectron spectra at N K-edge of TiOPc on TiO2 aligned with VB spectra measured at 110 eV before (a, b) and after (c, d) annealing. Resonant transitions have been marked by vertical lines at the corresponding photon energy measured by NEXAFS. Along the binding energy scale, the VB has been divided in three regions R1 (orbital I), R2 (orbital II), and R3 (deeper orbitals, III). The horizontal dotteddashed lines through c and d indicate the substrate contribution after annealing as shown in Figure 4.
into three regions: R1 (3.05.6 eV), R2 (5.68.6 eV), and R3 (8.613 eV) with the corresponding molecular orbitals: I in R1, II in R2, and deeper orbitals (III) in R3. On the basis of NEXAFS, the photon energies required for the resonant transitions are marked as A, B0 , and B00 in dashed line. To interpret the RPES spectra, it is important to specify the electron decay channels at the resonant excitation, which are normal Auger decay, spectator decay, and participator decay. The last two decays together are also known as autoionization. Basic characteristics of these decay channels are given in the Supporting Information. A thorough explanation can be found in the review by Br€uhwiler et al.19 The typical participator decay shows an Auger Raman characteristic: the kinetic energy of the Auger electron depends on the photon energy such that it appears at constant binding energy similar to photoemission. Before annealing, one observes two strong resonant enhancements in the N1sfLUMO transition (dashed line A) at 4.5 eV and between 6.5 and 13.0 eV binding energy. It is reasonable to assign the first to a participator decay which evidently superimposes with orbital I; the second, a broad feature, should be related to a spectator decay; a similar assignment has been reported for CuPc.62 At transition B0 , only three enhanced features can be resolved along the binding energy scale: a small one in R1, a stronger one in R2, and a broad feature in R3. The first two are participator decays because of their Auger Raman characteristic that they coincide with orbitals I and II, respectively. Finally, the third is attributed to a spectator decay. At even higher photon energy, no clear enhancement can be resolved in R1 and R2, and only some weak features can be resolved at high binding energy in R3 (Figure 8, panel R3). After annealing, the background of RPES has changed because of some contribution from the exposed TiO2 substrate (Figure 4, bottom). It will, however, not disturb the resonant enhancement from the molecular crystal. One can observe similar enhancements at the N1sfLUMO transition as before annealing, where the participator decay superimposes with orbital I and where the spectator decay is observed at higher binding energy. Large changes are observed at transition B (Figure 8, panels R1, R2, and
R3). In the R1 region, the apparent resonant enhancements related to transition B0 are quenched to the noise level. A similar intensity reduction along B0 is also shown in the R2 and R3 regions. Providing that the change of absorption cross section is taken into account, a decreased intensity of a participator decay usually points to a rapid electron transfer away from the excited state. In the R3 region, instead of the asymmetric peak before annealing, a more resolved doublet structure corresponding to transitions B0 and B00 can be observed following this heat treatment (Figure 8, panel R3). This result is in good agreement with the NEXAFS results. Additionally, we find no evident resonant enhancements in R1 or R2 when transition B00 is realized either before or after annealing leading to a rational assumption that excited electrons are more delocalized. As pointed out by the DFT calculations in Figure 6, N3c has a dominant contribution to orbital II. Thus, it is reasonable to assume that the strong participator enhancement at (R2, B0 ) before annealing (Figure 7a) is mainly from the N3c related orbital. After annealing, the crystallization process renders a closer packed molecular structure, and the concomitant shortrange intermolecular ππ interactions can yield better electronic coupling and hole transport bandwidth,48 which is illustrated by the peculiarly high performance of TiOPc devices.10 The DFT and experimental data point out that the N3c (inner nitrogen) has experienced a dramatic change in the chemical/electronic environment, which can be related to the molecular symmetry change from C4v to C1,49 coupling of molecular orbitals,48 and better energy alignment from the ordering process. This delocalization of N3c orbitals opens a channel for fast removal of excited electrons rather than taking part in the autoionization process. From the energy level point of view, the alignment of the NEXAFS to the Fermi level of TiOPc/TiO2 by subtracting the binding energy of the PES peak from the photon energy shows that the transition from N1s to LUMO ends up with an intermediate state below the Fermi level (seen in the upper panel of Figure 8). In this case, the excited electron is trapped locally within the HOMO/LUMO gap which is 1.7 eV.49 Because of the 14974
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C
ARTICLE
Figure 9. Comparison of the normalized intensity of integrated RPES R1 and NEXAFS: before annealing (top) and after annealing (bottom). The dashed lines mark the background.
Figure 8. Upper panel: NEXAFS presented on a binding energy scale after subtraction (Ehν EB(N1s)) with the zero energy at Fermi level. Lower panel: Integrated RPES in different regions in Figure 7 and normalized to the peak height of the N1sfLUMO transition. The dashed lines mark transitions B0 and B00 . Spectra collected before and after annealing are marked in black and red.
experimental setup, only core holes annihilated through the autoionization (participator or spectator decay) can be observed regardless of the order in the film. Adjacent molecules to a photoexcited one are disturbed by the core hole slightly pulling their LUMO down (schematic in the Supporting Information) as reported for solid C60.19 Thus, the higher empty π* orbitals (B and C) in the photoexcited molecule are well above the LUMO of the surrounding molecules, but no charge transfer is deduced from the RPES spectra. The alignment shows that the excited state of the molecule shifts closer to the vacuum level after annealing by 0.1 eV, which is consistent with the shift in PES. However, this shift cannot explain the ultrafast charge transfer completely because the systematic energy shift induced by annealing does not particularly influence the relative position of the different orbitals. Thus, the change of local energy level alignment has a limited impact on the charge transfer. Conversely, the molecular coupling as suggested by the theoretical study plays a dominant role. However, when considering a more general system at a larger scale, a change in the energy-level alignment might be critical for the performance of a device.26
To determine the charge-transfer time, one first has to calibrate the absorption matrix change by normalizing to the peak height of the N1sfLUMO transition for both NEXAFS and the integrated RPES in the corresponding region19,63 (Figure 9). Assuming that each molecule in the amorphous thin film behaves like the one in the isolated state with an exponential core hole decay, a charge-transfer time of the crystallized film can be estimated from the relation below (eq 1)19,21,63 τCT ¼ τCH 3
Icoup, RPES =Icoup, XAS Iiso, RPES =Iiso, XAS Icoup, RPES =Icoup, XAS
ð1Þ
where τCH(CT) stands for core hole decay time (charge-transfer time) with τCH about 6 fs for N1s of Pc.25 IRPES is the intensity of the integrated RPES in R1 at transition B which only contains the information from the resonant photoemission signal; IXAS is the NEXAFS intensity at the same transition. The abbreviations of coup and iso stand for coupled state and isolated state. In this case, we use the intensity of the peak before and after annealing to replace Iiso and Icoup, respectively. As a result, Iiso,RPES/Iiso,XAS is 0.55. For Icoup,RPES/Icoup,XAS, the maximum value is then roughly determined as 0.42 since the RPES signal is quenched to a noisy background level. Thus, the charge-transfer time can be calculated with an upper limit of 16 fs in the crystallized film. Previously, de Jong et al.26 compared the RPES estimation with the charge-carrier hopping rate determined by the relation between zero field mobility and attempt frequency for hopping64 (eq 2) from measurements on amorphous and crystalline parts in a H2Pc liquid crystal thin film. They found a much faster charge transfer, more than 3 orders of magnitude difference for the charge-transfer bandwidth, compared to the fastest H2Pc devices, where the long-range effect of the device scale was suggested to 14975
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C be considered. Here, we will process a similar comparison: ea2 νh 2a exp μ0 ¼ ð2Þ L σ where a is the charge-carrier displacement per jump for the nearest-neighbor hopping. In this case, this distance is comparable with the intermolecular distance. For TiOPc, the convex and concave dimers hold the distance about 3.1 and 3.3 Å,48 respectively. Thus, we use the average value 3.2 Å for a. L is the localization length, which we assume is comparable to the single molecular size (12 Å) for electrons in the excited state. σ is the root-mean-square width of the density of states available for hopping approximated to kBT at room temperature (∼26 meV).26 The highest hole mobility (μ0) was reported between 1 and 10 cm2 V1 s1 for phase II dominant TiOPc thin film OFET.10 With all the parameters above, we obtain a time for hopping τh (τh = 1/υh) in the region between 2.3 and 23 fs. Our experimental estimation is clearly located within this time region.
’ CONCLUSION We have prepared TiOPc thin films on TiO2 single crystal in vacuum. The thin film is predominantly amorphous as deposited and subsequently crystallizes by thermal treatment. A systematic shift of core level and valence band levels toward lower binding energies indicates that the film becomes more p-type. RPES shows that the autoionization of electrons excited above LUMO is severely quenched following the heat treatment indicating that an ultrafast charge transfer takes place accompanying the structural change. The decay process is explained by DOS of the molecular orbitals on the basis of DFT calculations. Energy-level alignment is estimated as a minor driving force for the charge transfer compared with the molecular coupling effect. Further chargetransfer analysis points out that the charge-transfer time in crystallized film is less than 16 fs, which matches with the reported record of the fastest transistor device using TiOPc. ’ ASSOCIATED CONTENT
bS
Supporting Information. Schematic of X-ray photoemission, X-ray absorption, and the following electron decay process; schematic of energy-level alignment of TiOPc at excitation states. This material is available free of charge via the Internet at http:// pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (S.Y.);
[email protected] (M.G.).
’ ACKNOWLEDGMENT Great thanks go to the kind staff at Max-lab. The Swedish Energy Agency (STEM), the Swedish Research Council (VR), the G€oran Gustafsson Foundation, the Carl Trygger Foundation, and the Queensland government are kindly acknowledged for financial support. Computational resources used in this work were provided by the University of Queensland (Centre for Computational Molecular Science) and the Australian Research Council (LIEF grant LE0882357: A Computational Facility for Multiscale Modeling in Computational Bio and Nanotechnology).
ARTICLE
’ REFERENCES (1) Yamashita, Y. Sci. Technol. Adv. Mater. 2009, 10, 024313. (2) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (3) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Chem. Rev. 2010, 110, 6689–735. (4) Nalwa, H. S. Adv. Mater. 1993, 5, 341–358. (5) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736–6767. (6) Law, K. Y. Chem. Rev. 1993, 93, 449–486. (7) Hiller, W.; Str€ahle, J.; Kobel, W.; Hanack, M. Z. Kristallogr. 1982, 159, 173. (8) Oka, K.; Okada, O.; Nukada, K. Jpn. J. Appl. Phys. 1992, 31, 2181–2184. (9) Saito, T.; Sisk, W.; Kobayashi, T.; Suzuki, S.; Iwayanagi, T. J. Phys. Chem. 1993, 97, 8026–8031. (10) Li, L.; Tang, Q.; Li, H.; Yang, X.; Hu, W.; Song, Y.; Shuai, Z.; Xu, W.; Liu, Y.; Zhu, D. Adv. Mater. 2007, 19, 2613–2617. (11) Yamaguchi, S.; Sasaki, Y. Chem. Phys. Lett. 2000, 323, 35–42. (12) Placencia, D.; Wang, W.; Shallcross, R. C.; Nebesny, K. W.; Brumbach, M.; Armstrong, N. R. Adv. Funct. Mater. 2009, 19, 1913–1921. (13) Coppede, N.; Toccoli, T.; Pallaoro, A.; Siviero, F.; Walzer, K.; Castriota, M.; Cazzanelli, E.; Iannotta, S. J. Phys. Chem. A 2007, 111, 12550–12558. (14) Alfredsson, Y.; Rensmo, H.; Sandell, A.; Siegbahn, H. J. Electron Spectrosc. Relat. Phenom. 2009, 174, 50–54. (15) Zhang, Y.; Wang, S.; Demasi, A.; Reid, I.; Piper, L. F. J.; Matsuura, A. Y.; Downes, J. E.; Smith, K. E. J. Mater. Chem. 2008, 18, 1792. (16) Nishi, T.; Kanai, K.; Ouchi, Y.; Willis, M. R.; Seki, K. Proc. Int. Symp. Super-Functionality Organic Devices IPAP Conf. Series 6, 2005, 3537. (17) Brumbach, M.; Placencia, D.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 3142–3151. (18) Harada, Y.; Tokushima, T.; Takata, Y.; Kamakura, N.; Takeuchi, T.; Kobayashi, K.; Shirota, Y.; Shin, S. Jpn. J. Appl. Phys. 2005, 44, L1147–L1149. (19) Br€uhwiler, P. A.; Karis, O.; Martensson, N. Rev. Mod. Phys. 2002, 74, 703–740. (20) Johansson, E. M. J.; Hedlund, M.; Odelius, M.; Siegbahn, H.; Rensmo, H. J. Chem. Phys. 2007, 126, 244303. (21) Mayor, L. C.; Taylor, J. B.; Magnano, G.; Rienzo, A.; Satterley, C. J.; O'Shea, J. N.; Schnadt, J. J. Chem. Phys. 2008, 129, 114701. (22) Schnadt, J.; Br€uhwiler, P. A.; Patthey, L.; O'Shea, J. N.; S€odergren, S.; Odelius, M.; Ahuja, R.; Karis, O.; B€assler, M.; Persson, P.; Siegbahn, H.; Lunell, S.; Martensson, N. Nature 2002, 418, 620–623. (23) Wang, L.; Chen, W.; Wee, A. T. S. Surf. Sci. Rep. 2008, 63, 465–486. (24) Hintz, H.; Peisert, H.; Ayg€ul, U.; Latteyer, F.; Biswas, I.; Nagel, P.; Merz, M.; Schuppler, S.; Breusov, D.; Allard, S.; Scherf, U.; Chasse, T. Chemphyschem 2010, 11, 269–275. (25) Chen, W.; Wang, L.; Qi, D. C.; Chen, S.; Gao, X. Y.; Wee, A. T. S. Appl. Phys. Lett. 2006, 88, 184102. (26) de Jong, M. P.; Osikowicz, W.; Sorensen, S. L.; Sergeyev, S.; Geerts, Y. H.; Salaneck, W. R. J. Phys. Chem. C 2008, 112, 15784–15790. (27) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2–11. (28) Walzer, K.; Toccoli, T.; Pallaoro, A.; Verucchi, R.; Fritz, T.; Leo, K.; Boschetti, A.; Iannotta, S. Surf. Sci. 2004, 573, 346–358. (29) XPSpeak 41 is a free software available from rmkwok@cunk. edu.hk. Accessed 1999. (30) Delley, B. J. Chem. Phys. 1990, 92, 508. (31) Delley, B. J. Chem. Phys. 2000, 113, 7756. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868. (33) Brena, B.; Luo, Y.; Nyberg, M.; Carniato, S.; Nilson, K.; Alfredsson, Y.; Åhlund, J.; Martensson, N.; Siegbahn, H.; Puglia, C. Phys. Rev. B 2004, 70, 195214. (34) Zhang, L.; Peisert, H.; Biswas, I.; Knupfer, M.; Batchelor, D.; Chasse, T. Surf. Sci. 2005, 596, 98–107. 14976
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
The Journal of Physical Chemistry C
ARTICLE
(35) Park, K.; Miller, A.; Klier, K.; Opila, R. L.; Rowe, J. E. Surf. Sci. 2003, 529, L285–L292. (36) Peisert, H.; Chasse, T.; Petershans, A. J. Phys. Chem. C 2008, 112, 5703–5706. (37) Yu, S.; Ahmadi, S.; Palmgren, P.; Hennies, F.; Zuleta, M.; G€othelid, M. J. Phys. Chem. C 2009, 113, 13765–13771. (38) Palmgren, P.; Priya, B.; Niraj, N.; G€othelid, M. Sol. Energy Mater. Sol. Cells 2006, 90, 3602–3613. (39) Palmgren, P.; Yu, S.; Hennies, F.; Nilson, K.; Åkermark, B.; G€othelid, M. J. Chem. Phys. 2008, 129, 074707. (40) Palmgren, P.; Nilson, K.; Yu, S.; Hennies, F.; Angot, T.; Nlebedim, C. I.; Layet, J.-M.; Le Lay, G.; G€othelid, M. J. Phys. Chem. C 2008, 112, 5972–5977. (41) Yanagi, H.; Chen, S.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R.; Fujishima, A. J. Phys. Chem. 1996, 100, 5447–5451. (42) Nalwa, H. S.; Saito, T.; Kakuta, A.; Iwayanagi, T. J. Phys. Chem. 1993, 97, 10515–10517. (43) Diebold, U. Surf. Sci. Rep. 2003, 48, 53–229. (44) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.; Hansen, J. Ø.; Matthiesen, J.; Blekinge-Rasmussen, A.; Lægsgaard, E.; Hammer, B.; Besenbacher, F. Sciences (N Y) 2009, 320, 1755–1759. (45) Brena, B.; Palmgren, P.; Nilson, K.; Yu, S.; Hennies, F.; € Agnarsson, B.; Onsten, A.; Mansson, M.; G€othelid, M. Surf. Sci. 2009, 603, 3160–3169. (46) Yamane, H.; Honda, H.; Fukagawa, H.; Ohyama, M.; Hinuma, Y.; Kera, S.; Okudaira, K. K.; Ueno, N. J. Electron Spectrosc. Relat. Phenom. 2004, 137140, 223–227. (47) Fukagawa, H.; Yamane, H.; Kera, S.; Okudaira, K. K.; Ueno, N. Phys. Rev. B 2006, 73, 2–5. (48) Norton, J. E.; Bredas, J.-L. J. Chem. Phys. 2008, 128, 034701. (49) Mizuguchi, J.; Rihs, G.; Karfunkel, H. R. J. Phys. Chem. 1995, 99, 16217–16227. (50) Salomon, E.; Papageorgiou, N.; Angot, T.; Verdini, A.; Cossaro, A.; Floreano, L.; Morgante, A.; Giovanelli, L.; Le Lay, G. J. Phys. Chem. C 2007, 111, 12467–12471. (51) Brena, B.; Puglia, C.; de Simone, M.; Coreno, M.; Tarafder, K.; Feyer, V.; Banerjee, R.; G€othelid, E.; Sanyal, B.; Oppeneer, P. M.; Eriksson, O. J. Chem. Phys. 2011, 134, 074312. (52) Marom, N.; Kronik, L. Appl. Phys. A: Mater. Sci. Process. 2008, 95, 165–172. (53) Marom, N.; Kronik, L. Appl. Phys. A: Mater. Sci. Process. 2008, 95, 159–163. (54) Marom, N.; Hod, O.; Scuseria, G. E.; Kronik, L. J. Chem. Phys. 2008, 128, 164107. (55) Bhattacharjee, S.; Brena, B.; Banerjee, R.; Wende, H.; Eriksson, O.; Sanyal, B. Chem. Phys. 2010, 377, 96–99. (56) Segev, L.; Salomon, A.; Natan, A.; Cahen, D.; Kronik, L.; Amy, F.; Chan, C.; Kahn, A. Phys. Rev. B 2006, 74, 1–6. (57) Hybertsen, M.; Louie, S. Phys. Rev. B 1986, 34, 5390–5413. (58) Jones, R. O.; Gunnarsson, O. Rev. Mod. Phys. 1989, 61, 689–746. (59) Hwang, J.; Kim, E.-G.; Liu, J.; Bredas, J.-L.; Duggal, A.; Kahn, A. J. Phys. Chem. C 2007, 111, 1378–1384. (60) Liao, M.-S.; Scheiner, S. J. Chem. Phys. 2001, 114, 9780. (61) Piper, L. F. J.; Cho, S. W.; Zhang, Y.; DeMasi, A.; Smith, K. E.; Matsuura, A. Y.; McGuinness, C. Phys. Rev. B 2010, 81, 1–12. (62) Aristov, V. Y.; Molodtsova, O. V.; Maslyuk, V. V.; Vyalikh, D. V.; Zhilin, V. M.; Ossipyan, Y. A.; Bredow, T.; Mertig, I.; Knupfer, M. J. Chem. Phys. 2008, 128, 034703. (63) Schnadt, J.; O’Shea, J. N.; Patthey, L.; Kjeldgaard, L.; Åhlund, J.; Nilson, K.; Schiessling, J.; Krempasky , J.; Shi, M.; Karis, O.; Glover, C.; Siegbahn, H.; Martensson, N.; Br€uhwiler, P. A. J. Chem. Phys. 2003, 119, 12462. (64) Martens, H. C. F.; Blom, P. W. M.; Schoo, H. F. M. Phys. Rev. B 2000, 61, 7489–7493.
14977
dx.doi.org/10.1021/jp1100363 |J. Phys. Chem. C 2011, 115, 14969–14977
