Impacts of Molecular Orientation on the Hole Injection Barrier

Article pubs.acs.org/JPCC

Impacts of Molecular Orientation on the Hole Injection Barrier Reduction: CuPc/HAT-CN/Graphene Junkyeong Jeong,† Soohyung Park,† Seong Jun Kang,‡ Hyunbok Lee,*,§ and Yeonjin Yi*,† †

Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Korea § Department of Physics, Kangwon National University, 1 Gangwondaehak-gil, Chuncheon-si, Gangwon-do 24341, Korea ‡

S Supporting Information *

ABSTRACT: The molecular orientation affected by the interaction between a substrate and deposited molecules plays an important role in device performance. It is known that the molecular orientation influences not only the charge transport property but also its electronic structure. Therefore, the combined study of morphology and electronic structure is of high importance for device application. As a transparent electrode, graphene has many promising advantages. However, graphene itself does not have an adequate work function for either an anode or a cathode, and thus the insertion of a charge injection layer is necessary for it to be used as an electrode. In this study, the hole injection barrier (HIB) reduction was investigated at the interface of copper phthalocyanine (CuPc)/graphene with the insertion of a hexaazatriphenylene hexacarbonitrile (HAT-CN) layer between them. The insertion of the HAT-CN layer roughens the originally flat graphene surface and it weakens the πinteraction between CuPc and of graphene. This induces face-on and edge-on mixed orientations of CuPc, while CuPc on bare graphene shows merely a face-on orientation. As a result, the HIB is reduced by the contribution of edge-on CuPc having lower ionization energy (0.37 eV) along with the high work function of the HAT-CN layer (0.26 eV).

1. INTRODUCTION Graphene, which consists of carbon atoms arranged in a 2D honeycomb structure inducing a linear band crossing at Dirac points, has attracted significant interest in interdisciplinary research fields as a promising material for next-generation optoelectronic devices.1,2 A transparent electrode is one of the most useful applications of graphene, which is an alternative to indium−tin oxide (ITO) due to its advantages such as very high mobility, rare-earth free fabrication, good flexibility, and transparency.3,4 However, the work function (Ψ) of graphene should be adjusted to match the charge transport level of an active material since its Ψ (∼4.5 eV) is not adequate to be used as either an anode or a cathode. Several approaches, such as chemical doping5 and O2-plasma treatment,6 have been reported to modify the Ψ of graphene. However, these methods destroy the pristine bonding of graphene, so that they undermine its unique electrical properties, even though the Ψ is controlled.7,8 On the other hand, a surface modification using an organic molecule would not deteriorate the unique electronic properties of graphene significantly due to the weak interaction of van der Waals type between the π-orbitals of an organic molecule and graphene.9 In this respect, various organic molecules have © XXXX American Chemical Society

been introduced to modify the electronic structures of graphene. For example, tetrafluorotetracyanoquinodimethane (F4-TCNQ),10 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),11 and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)12 have been used to increase the Ψ of graphene, leading to enhancement in charge injection or extraction. Recently, hexaazatriphenylene hexacarbonitrile (HAT-CN) has been recognized as an efficient surface modifier since it has a high Ψ and a very large electron affinity (EA) to locate the lowest unoccupied molecular orbital (LUMO) level close to the Fermi level of an electrode. Thus, it plays a role as an electron extraction (or equivalently hole injection) layer, which is known as a so-called charge-generation layer (CGL).13,14 Even if PEDOT:PSS is quite popular as a means to improve the power conversion efficiency of organic photovoltaics (OPVs) with a graphene anode, its hydrophobic nature would limit its versatile use as a graphene electrode.15,16 Therefore, HAT-CN would have advantages as compared to PEDOT:PSS. Received: November 25, 2015 Revised: January 4, 2016

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DOI: 10.1021/acs.jpcc.5b11535 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C The electronic properties and applications of HAT-CN are widely known.17−22 However, the morphological aspects have not been well studied. The insertion of a HAT-CN layer between graphene and an active layer would affect the morphology of active layer molecules. It is well-known that graphene works as a template which provides a flat π-surface to orient organic molecules in the face-on geometry.23,24 On the other hand, the HAT-CN interlayer would not provide a flat surface, especially as compared to graphene.17,25 This leads to changes in the direction of molecular orbitals, which affects the electronic coupling of the overlayer (active layer). Although these effects are expected to be quite different for each system, fundamental studies on them are still lacking.26 In this study, the effects of a HAT-CN layer on the morphology and electronic structures of a copper phthalocyanine (CuPc) overlayer, which is a conventional p-type organic semiconductor used in OPVs, were comprehensively investigated on the graphene substrate. CuPc has a planar molecular structure with D4h point group symmetry. It shows quite different geometry between bare graphene and a HAT-CN/ graphene surface. This structural change renders different electronic structures of CuPc at the interface region.

Å thickness was used as an insertion layer after confirming the saturation of HAT-CN Ψ on graphene (i.e., the pristine bulk HAT-CN layer; see Figure S1 in Supporting Information). The base pressures of the analysis and the preparation chambers were maintained at ultrahigh vacuum below ∼10−10 and ∼10−8 Torr, respectively. All samples were kept at room temperature during the deposition and analysis process.

2. EXPERIMENTAL DETAILS Monolayer graphene was grown on Cu foil with the thermal chemical vapor deposition method. Then it was transferred to the SiO2/Si substrate. For the transfer, poly(methyl methacrylate) (PMMA, 950 K PMMA C4) was coated on the graphene/Cu surface as a supporting layer. Etching the Cu, PMMA/graphene film was place on the SiO2/Si substrate and the PMMA film was removed with acetone. These sequential processes were repeated three times to make the three-layer graphene substrate. Detailed processes are described elsewhere.12 To investigate the orientation of CuPc, Raman spectra were measured in backscattering geometry. A linearly polarized Nd:YAG laser (532 nm) was used as a excitation source. The scattered light was detected in two different modes: polarized in parallel and perpendicular to the incident light. By comparing these two spectra, molecular orientations with respect to the substrate were analyzed.27−29 Grazing incidence wide-angle Xray scattering (GIWAXS) measurements were also conducted to figure out and cross-check the molecular orientations of CuPc. The measurements were conducted at the 3C beamline of the Pohang Accelerator Laboratory. The sample-to-detector distance was 0.2 m, and a MAR165 2D CCD detector was used. In situ ultraviolet photoelectron spectroscopy (UPS) and Xray photoelectron spectroscopy (XPS) measurements were performed in an analysis chamber composed of a PHI 5700 spectrometer, ultraviolet light source (He I, 21.22 eV), and monochromatic X-ray source (Al Kα). During UPS measurements, a sample bias of −10 V was applied to obtain the secondary electron cutoff (SEC). The total broadening of UPS was 90 meV, which comes from the broadening of the Fermi edge measured from a clean Au substrate. Samples were transferred between a preparation and an analysis chamber without breaking vacuum. In the preparation chamber, HATCN and CuPc were thermally evaporated. Both evaporation rates were kept at 0.1 Å/s, and their evaporation temperatures were 244 °C for HAT-CN and 296 °C for CuPc. The deposition rate and total effective thickness of HAT-CN and CuPc layers were deduced from an attenuation of XPS core levels and quartz crystal microbalance (QCM). HAT-CN of 68

To analyze the active Raman tensor from the measured spectra, the scattered light was detected in two different polarization modes. One was the scattered light polarized parallel to the incident light (HH), and the other was polarized perpendicular to the incident light (HV). Thus, the HH spectrum gives information about diagonal elements of the Raman tensors, while the HV spectrum contains off-diagonal elements of the Raman tensors. If the sample is a single crystal and its molecular axis is the same to the substrate axis, then A1g and B1g are seen in the HH spectra, while B2g, E1g, and E2g are not. However, since the axes of molecules and the substrate do not coincide, Raman tensors are transformed along the rotations of molecule. If there are preferred molecular orientations, they can be deduced from which tensor is activated in the Raman spectra.28,29,31 Figure 1 shows the Raman spectra of (a) CuPc (30 Å)/ graphene and (b) CuPc (50 Å)/HAT-CN (68 Å)/graphene. The same intensity of graphene G-peak (1574 cm−1) is observed at both polarizations in each panel. The sharp and intense peak at 509 cm−1 and a hill in the red line around 950 cm−1 are from the silicon substrate (Figure S2). The former corresponds to T2g and the latter a two-phonon Raman peak dominated by A1g symmetry from the space group of Fd3̅m.32,33 The A1g Raman peaks of CuPc thin films are labeled from literature.34−36 In Figure 1a, A1g peaks (580, 670, 825, and 1419 cm−1) are seen in the HH spectrum but are not in the HV spectrum. (The small HV peak near 670 eV is not from A1g but from B2g that is seen as a right shoulder in the HH spectrum.) This is possible only when the A1g does not have off-diagonal elements as described above. The other Raman tensors (other peaks) are activated in both HH and HV modes since they have off-diagonal elements corresponding to the azimuthal rotation. This means that CuPc molecules are in face-on geometry with random azimuthal rotation with respect to the substrate z-axis because the Raman spectroscopy measurements were conducted in backscattering geometry. To summarize, CuPc molecules are faced on bare graphene, which is in good agreement with the GIWAXS results shown later. In Figure 1b, however, A1g peak intensities in the HV spectrum are nonzero, indicating off-diagonal elements in A1g. This implies that CuPc

3. RESULTS AND DISCUSSION In order to figure out the orientation of the CuPc molecule, Raman spectra were measured. Since the CuPc molecule has D4h point group symmetry, the corresponding Raman tensors are30 ⎛a 0 0⎞ ⎜ ⎟ A1g = ⎜ 0 a 0 ⎟ ⎜ ⎟ ⎝0 0 b ⎠

⎛0 0 e ⎞ ⎜ ⎟ E1g = ⎜ 0 0 0 ⎟ ⎝e 0 0⎠

B

⎛ c 0 0⎞ ⎜ ⎟ B1g = ⎜ 0 −c 0 ⎟ ⎝0 0 0⎠

E 2g

B2g

⎛0 d 0⎞ ⎟ ⎜ = ⎜d 0 0⎟ ⎟ ⎜ ⎝0 0 0⎠

⎛0 0 0⎞ ⎜ ⎟ = ⎜0 0 e ⎟ ⎝0 e 0⎠


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The Journal of Physical Chemistry C

pattern around (Qxy = 0 Å−1, Qz = 0.5 Å−1) in Figure 2d, which is not seen in Figure 2c. This pattern indicates the edge-on orientation of CuPc as reported.24,37 In addition, the face-on patterns are also seen in Figure 2d, even if they are blurred compared to the face-on patterns in Figure 2c. These particular patterns in Figures 2c and 2d indicate that CuPc has both edgeon and face-on orientations on HAT-CN, while the face-on orientation is dominant on bare graphene. This is consistent with Raman results showing the existence of edge-on CuPc with HAT-CN. This mixed orientation could be attributed to the random orientation of HAT-CN (Figure 2b) and its surface roughening on an originally flat graphene surface. These different surface conditions were also observed with scanning electron microscope (SEM) and atomic force microscope (AFM) measurements (Figures S3 and S4). These images directly show the roughened surface which weakens the πinteraction between graphene and CuPc. To see the interfacial electronic structures and detailed molecular orientation inside the CuPc film, UPS measurements were conducted. The Raman and GIWAXS results contain full information about surface/interface and bulk due to their deep probing depth while UPS data contain only surface information. Such different probing depths were used for detailed analysis: The existence of two orientations was confirmed with Raman and GIWAXS as already described and detailed orientation inside the film can be analyzed with UPS. UPS experiments were conducted at the interfaces of CuPc/graphene and CuPc/HAT-CN. The SEC (a) and HOMO region (b) of UPS spectra during the CuPc deposition on bare graphene are shown in Figure 3. The SEC region was normalized, and the Shirley-type background was removed from the HOMO region. The SEC spectra (Figure 3a) are shown on kinetic energy abscissa so that the SEC indicates the Ψ directly. The Ψ of bare graphene is 4.40 eV. As CuPc molecules are deposited, the SEC position gradually shifts to lower kinetic energies indicating Ψ decrease. The Ψ change is saturated at the 30 Å step with the value of 3.95 eV. In the HOMO region (Figure 3b), the HOMO onset of CuPc was observed at 0.98 eV below the Fermi level at the 1 Å step, and it slightly shifts to higher binding energies by 0.13 eV. As compared to the HOMO level shift, a relatively large Ψ shift is observed which originated from the induced interface dipole (eD) due to π−π interaction between CuPc and graphene.39 Figure 4 shows the UPS spectra of CuPc/HAT-CN/ graphene. When HAT-CN (68 Å) is deposited on graphene, the Ψ increases significantly from 4.40 to 5.50 eV (Figure 4a). Following that CuPc is deposited step by step up to 50 Å thickness until the ionization energy (IE) is saturated. The Ψ is gradually decreased by 1.34 eV during the CuPc deposition. In Figure 4b, the HOMO feature of CuPc starts to appear around 1 eV and shifts with further deposition. In addition, the shape of the HOMO feature cannot be fitted with a single Gaussian curve at low coverage. Also, the HOMO feature slightly shifts to lower binding energies up to the 6 Å step and then moves back to higher binding energies while the SEC shifts consistently to lower kinetic (higher binding) energies. These are quite different from the HOMO shifts on bare graphene (Figure 3b). In the CuPc/graphene interface, the CuPc HOMO feature is well fitted with a single Gaussian curve with a constant Gaussian width of ca. 0.5 eV and shows only monotonic shift without any shape changes. The nonmonotonic shift and the asymmetrical shape of the HOMO feature at low CuPc coverage are attributed to the different

Figure 1. Raman spectra of (a) CuPc/graphene and (b) CuPc/HATCN/graphene. Black squares, white triangles, and the symbol G represent vibrational modes of A1g symmetry, B1g symmetry, and G peak of graphene, respectively.

molecules are inclined; that is, there exists edge-on CuPc. This is a simple and fast procedure to figure out the orientation of a symmetric molecule like CuPc. The versatility of this method can also be seen from the observed Si peaks as the A1gdominated two-phonon peak (around 950 cm−1 ) has disappeared in HV. To cross-check the Raman results, we carried out GIWAXS measurements on each sample. Figure 2 shows GIWAXS patterns of (a) graphene, (b) HAT-CN/graphene, (c) CuPc/ graphene, and (d) CuPc/HAT-CN/graphene. First, graphene does not have particular patterns in a GIWAXS image as shown in Figure 2a. However, in the case of a HAT-CN layer on graphene (Figure 2b), some concentric semicircles are seen, meaning HAT-CN molecules form small domains with random orientation like crystal powder. An overall pattern from CuPc/ graphene (Figure 2c) is not dispersed much while that of a CuPc/HAT-CN/graphene (Figure 2d) shows a widespread from side to side. This indicates that CuPc molecules are welloriented on bare graphene but not on the HAT-CN layer. The scattered patterns shown in Figure 2c are known as the face-on orientation of CuPc.24,37 The strong semicircular patterns from HAT-CN between about Qz = 1.0 Å−1 and Qz = 2.0 Å−1 are seen in both Figures 2b and 2d. This indicates random orientation of the small crystalline domain. Although HAT-CN also has a planar structure like CuPc, it is not oriented in face-on geometry (having instead a random orientation) to minimize surface energy.17,38 This would affect the orientation of the CuPc overlayer in contrast to the flat graphene surface. The remarkable difference between Figures 2c and 2d is the point C


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Figure 2. GIWAXS images of (a) CuPc/graphene, (b) CuPc/HAT-CN/graphene, (c) graphene, and (d) HAT-CN/graphene.

Figure 3. UPS spectra of CuPc (1, 2, 4, 6, 10, 15, 30 Å)/graphene. (a) Normalized secondary cutoff and (b) HOMO region are presented.

Figure 4. UPS spectra of CuPc (1, 2, 4, 6, 10, 15, 30, 50 Å)/HAT-CN (68 Å)/graphene. (a) Normalized secondary cutoff, (b) HOMO region, and (c) deconvolution of the HOMO peak with two different molecular orientations of edge-on (blue line) and face-on (green line). Measured spectra are shown with red circles, and the sum of fitting curves is displayed with a black line.

orientation of the CuPc molecules. This idea is supported by the fact that the IE changes with the molecular orientation of CuPc,40 and thus two distinct HOMO levels (asymmetric shape) can be detected at the same time. Accordingly, taking into account the Raman and GIWAXS results, the HOMO feature can be deconvoluted with two Gaussian curves corresponding to face- and edge-on orientations as shown in Figure 4c. The blue fitted line is the edge-on orientation (lower

IE), and the green one is the face-on (higher IE) orientation. Their sum is the black line. Since the IE of edge-on CuPc is smaller than that of face-on by 0.4 eV,40 the blue and green D


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CuPc HOMO to the HAT-CN LUMO (or equivalently hole injection into the CuPc HOMO) is expected.

components (0.4 eV apart each other) could be assigned as edge-on and face-on, respectively. The fitted line shows good agreement with the measured spectrum. During the successive depositions of CuPc on HAT-CN, the region of mixed CuPc orientations is highly confined in the interface. Over the thickness of 30 Å, the HOMO feature is not fitted with two Gaussian curves anymore, meaning edge-on dominant orientation. These indicate that CuPc molecules are oriented in both face-on and edge-on near the HAT-CN interface, while the face-on orientation disappears in the region far from HATCN. This is attributed to the reduced interaction between HAT-CN and CuPc as the CuPc coverage increases. The intermolecular interaction between CuPc molecules increases more and more and induces edge-on ordering.41,42 Combining all the information measured above, the energy level alignments at (a) CuPc/graphene and (b) CuPc/HATCN/graphene were evaluated (Figure 5). At a CuPc/graphene

4. CONCLUSION We investigated the molecular orientations and their influence on electronic structures of CuPc/graphene and CuPc/HATCN/graphene. HAT-CN is an efficient interlayer to increase the Ψ of graphene, but the HIB reduction via HAT-CN deposition cannot be simply explained with the Ψ increase. The changes on molecular orientation during the successive deposition of HAT-CN and CuPc were considered. Raman and GIWAXS spectra show that CuPc has a face-on orientation on graphene, whereas it has face-on/edge-on mixed orientation on HAT-CN/graphene. The former is attributed to the interaction between a graphene substrate and CuPc molecules, while the latter is attributed to the intermolecular interaction between CuPc molecules. These two different orientations show two different HOMO levels in UPS measurements due to their different IE. The edge-on CuPc has a lower IE, and its HOMO is closer to the Fermi level by 0.37 eV than the face-on CuPc. HAT-CN reduces the HIB not only by the Ψ increase of graphene but also by the changes in CuPc orientation. Therefore, both electronic and morphological effects should be considered to evaluate the energy level alignment of organic molecules accurately. Such precise analysis would give an opportunity to find relevant materials and interfaces for high performance devices.

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Figure 5. Energy level diagrams of (a) CuPc/graphene and (b) CuPc/ HAT-CN/graphene. ΨGr is the work function of graphene, eD the interface dipole, Vb the HOMO bending, and IE the ionization energy, Evac the vacuum level, and EF the Fermi level. The LUMO level of HAT-CN is estimated from the previous report of the transport gap.13

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11535. Thickness-dependent work function of a HAT-CN layer on graphene, the Raman spectra, and the SEM and AFM images of HAT-CN/graphene and graphene substrates (PDF)

interface (a), there are symmetric HOMO peak in every step (Figure 3b). That is induced by the consistent face-on orientation as discussed. Thus, the IE of CuPc on graphene is not changed from the interface to the bulk region. The hole injection barrier (HIB) from graphene to CuPc was evaluated to be 0.98 eV. The eD of 0.30 eV was calculated by subtracting the IE of CuPc (5.06 eV) from the sum of the HOMO onset (0.98 eV) and Ψ (4.38 eV) at the first deposition step (1 Å). It is quite similar to the previous report.6 On the other hand, at the CuPc/HAT-CN/graphene interface (b), two HOMO levels are assigned as the face-on and edge-on CuPc due to the mixed orientation. The IE of the face-on CuPc is 5.15 eV while that of the edge-on is 4.78 eV. Since the edge-on CuPc has a smaller IE, the edge-on CuPc HOMO is closer to the Fermi level than the face-on CuPc. Therefore, the HIB from graphene to (edgeon) CuPc is significantly reduced to be 0.35 eV by the HATCN layer as compared to the CuPc/graphene interface. The HOMO bending and eD were calculated to be 0.27 and 0.95 eV in the same manner. They are quite large compared to those of the CuPc/graphene interface (0.13 and 0.30 eV, respectively). This originates from the charge transfer from CuPc to HAT-CN since HAT-CN has a higher EA than the IE of CuPc. It induces a large amount of HOMO bending and eD for thermal equilibrium. Collectively, the energetic difference between the LUMO of HAT-CN (deduced from reported transport gap13) and the HOMO of edge-on CuPc becomes only 0.56 eV. Therefore, a direct electron extraction from the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.L.). *E-mail: [email protected] (Y.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a research project of the National Research Foundation of Korea (2012M3A7B4049801, 2013R1A1A1004778, and 2015R1C1A1A01055026), Samsung Display and Defense Acquisition Program Administration (DAPA), and the Agency for Defense Development (ADD).

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Impacts of Molecular Orientation on the Hole Injection Barrier

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