Semiconductor Interfaces

Article pubs.acs.org/JPCC

Tuning Energy Level Alignment At Organic/Semiconductor Interfaces Using a Built-In Dipole in Chromophore−Bridge−Anchor Compounds Sylvie Rangan,*,† Alberto Batarseh,‡ Keyur P. Chitre,‡ Andrew Kopecky,‡ Elena Galoppini,‡ and Robert Allen Bartynski† †

Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers University, 136 Frelinghuysen Road, Piscataway, New Jersey 08854, United States ‡ Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States S Supporting Information *

ABSTRACT: A chromophore−bridge−anchor molecular architecture is used to manipulate the molecular level energy position, with respect to the band edges of the substrate, of a chromophore bound to a surface via an anchor group. An energy shift of the chromophore’s frontier orbitals is induced by the addition of an oriented molecular dipole into the bridge part of the compound. This principle has been tested using three Zinc Tetraphenylporphyrin derivatives of comparable structure: two of which possess a dipole, but pointing in opposite directions and, for comparison, a compound without a dipole. UV− vis absorption and emission spectroscopies have been used to probe the electronic structure of the compounds in solution, while UV photoemission spectroscopy has been used to measure the relative position of the molecular levels of the chromophore with respect to the band edges of a ZnO(11−20) single crystal substrate. It is shown that the introduction of a molecular dipole does not alter the chromophore’s HOMO−LUMO gap, and that the molecular level alignment of the compounds bound to the ZnO surface follows the behavior predicted by a simple parallel-plate capacitor model.

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INTRODUCTION Energy level alignment is central to the electronic behavior of organic/inorganic interfaces, affecting properties such as charge separation and charge transport. For example, in photovoltaic applications, energy level alignment plays a critical role in establishing the open circuit voltage and (somewhat less directly) the short circuit current of a device by determining barriers to charge transport.1,2 For an interface at equilibrium, the energy alignment of electronic levels is given by their relative orbital energies with respect to a common vacuum level, shifted by a potential difference resulting from charge redistribution at the interface. In this work, this shift is referred to as the intrinsic interface dipole. A large body of literature is devoted to understanding the key physical phenomena that determine the intrinsic dipoles at interfaces involving organic molecules.3−9 For instance, at metal/organic interfaces, intrinsic dipole formation is often understood in terms of the “pillow” effect, induced gap states, and alignment between the Fermi level of the metal and the charge neutrality level (CNL) of the organic molecule.9,10 Many of these concepts have been extended to address interfaces of organic molecules with nonmetal systems.11 At semiconductor/organic or organic/ organic interfaces, interfacial charge transfer (e.g., fractional thermal population of the LUMO from the substrate valence electrons) can play a leading role in driving level alignment.12−17 In all cases, the nature of bonding to the substrate © 2014 American Chemical Society

(i.e., chemisorption or physisorption) is an important consideration.7 A number of different approaches have been pursued to tune the energy level alignment at the organic molecule/substrate interface. These include synthetic modification of the chromophoric molecule (e.g., perfluorination or metallic-ion substitution),4,7,15,18−20 manipulating molecular orientation,4,21 codepositing two organic species,22 or introducing an interfacial buffer layer.23−25 What is clear from the literature is that the intrinsic interface dipole, as well as approaches designed to modify it, are highly dependent upon the specific species present at the interface, and the effects are not readily predicted or controlled. For example, the buffer layer approach comes at the expense of creating two intrinsic interfacial dipoles.23−25 In this work, we demonstrate a new paradigm for controlling the energy level alignment of a chromophore, while leaving its absorption properties unaffected: using a single layer of chromophore-containing molecules with an internal electric dipole moment. In the work described here, this goal is accomplished using a chromophore−bridge−anchor molecular architecture where the chromophoric unit is electronically decoupled from the bridge. By adding to the bridge electron Received: March 24, 2014 Revised: May 20, 2014 Published: May 21, 2014 12923

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The remainder of this work is organized as follows. First we present the motivations behind the molecular design of the chromophoric compound, a ZnTPP derivative, as well as a simple model based on DFT calculations attempting to predict the magnitude of the electronic level shifts. Next we present results from UV−vis absorption spectroscopy and emission spectroscopy, used to probe the electronic structure of the three isolated compounds in solution. Finally, UV photoemission spectroscopy was used to determine the alignment of the chemisorbed compounds molecular levels, with respect to the band edges of a ZnO(11−20) single crystal surface. It will be shown that the introduction of a molecular dipole does not alter the chromophore’s HOMO−LUMO gap, and that the molecular level alignment of the compounds bound to the ZnO surface follows the behavior predicted by our simple model.

donor (D, NMe2) and acceptor (A, NO2) groups, an intramolecular dipole is introduced between the chromophore and the anchor, as illustrated in Figure 1. When a monolayer of

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EXPERIMENTAL SECTION Sample Preparation. The ZnO sample was a commercially produced single-crystal from MTI Corporation, cut to within 0.5° of the (11−20) plane. The sample was degassed and prepared in ultrahigh vacuum (UHV) using several cycles of 1 keV Ar+ ion sputtering (while maintaining a maximum sample current of 2 μA) and annealing in UHV at 800 K. The cleanliness of the surface was checked using XPS, and the surface termination was assessed by low energy electron diffraction. Following an ex-situ 60 min sensitization in a solution of dye in anhydrous THF, and rinsing with anhydrous THF to prevent dye accumulation, the sample was reintroduced into the UHV analysis chamber. Electronic Structure Determination. The valence band electronic states were examined using He II (40.8 eV) excited ultraviolet photoemission spectroscopy (UPS). The experimental broadening in UPS is extracted from the width of the Fermi edge of a Au sample and has a fwhm smaller than 0.3 eV. The energy scales of the spectra were calibrated using the measured position of the Fermi level of a gold sample in contact with the oxides sample. For both the clean and

Figure 1. A molecular dipole in the bridge (B) shifts the energy levels of a chromophore (C) with respect to those of the surface of the semiconductor, to which the complex is bound through an anchor (A). Arrows indicate the direction of electron transfer in the bridge unit, leading to a molecular dipole.

such molecules is bound to a metal oxide surface, the resulting dipole layer establishes a potential difference that shifts the chromophore HOMO and LUMO levels with respect to those of the substrate without altering the intrinsic interfacial dipole. Furthermore, as suggested by Figure 1, the relative positions of the D and A groups can be interchanged, to confirm that reversing the intramolecular dipole direction leads to level shifts of the same magnitude but in the opposite direction.

Figure 2. Molecular structure of the chromophore−bridge−anchor architecture studied in this work. Compounds 1 and 3 have an electron donor (NMe2) and acceptor (NO2) group in the bridging unit to form an intramolecular dipole, but with opposite orientation. Arrows indicate the direction of electron donation. Compound 2 was used for comparison. 12924

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sensitized surfaces, secondary electron cutoff have been measured on biased samples using He I (21.1 eV). For optical properties measurements, the compounds were dissolved in acetonitrile (HPLC grade) and sonicated for 10 min. The solutions were filtered (0.2 μm) and the filtrates were used to determine the UV−vis absorption and steady-state fluorescence emission spectra.

V = Dproj/(εA) where ε is the permittivity across the capacitor plates, and A is the molecular footprint of the molecule. The molecular footprint can be approximated by considering the square of largest dimension of the ZnTPP molecule (about 17.8 Å) to which can be added an intermolecular distance of 4 Å. A is then equal to 475 Å2. In this model, ε is chosen as the vacuum permittivity ε0 = 8.85 × 10−12 C·V·−1 m−1. The expected potential difference between the capacitor plates is thus 300 meV. Thus, a significant shift of the molecular levels is expected when the A and D groups are present, and upon reversing the orientation of the dipole. Electronic Structure. The first step in the analysis of the chromophore compounds consists in ensuring that the HOMO−LUMO gap is not altered by the addition of the dipole unit. A comparison of the UV−vis absorption and emission spectra of acetonitrile solutions of 1, 2, and 3 is shown in Figure 4. The UV−vis spectra for all three compounds show the absorptions characteristic of ZnTPP with bands at 422 nm (Soret), and 556 and 597 nm (Q bands). The steady-state fluorescence emission spectra exhibited the same shape for all three samples. Two bands at 603 and 656 nm were observed, which are consistent with emission spectra observed for other ZnTPP.26,32 No spectral shifts were observed in the absorption and emission for compounds 1, 2, and 3, indicating that the energy gap between excited states and ground states are the same for all three compounds. Therefore, the absorption properties of the chromophore are not affected by the presence, or functionalization, of the bridge group. As a second step, UV photoemission spectroscopy (UPS) was used to determine the energy alignment of the electronic states of the molecules with respect to the oxide semiconductor substrate.30,31 UPS spectra were obtained from a ZnO(11−20) single crystal surface, chosen as the wide band gap oxide, before and after coating the surface with 1, 2, or 3. The secondary electron cutoff of each sensitized surface is reported in Figure 5A, while the valence band region is shown in Figure 5B. The zero of energy refers to the Fermi level of the system and is the common point of reference for all samples. The valence band spectrum of the clean ZnO(11−20) surface (shown in Figure 5) is characterized by a broad feature ranging from binding energies of −3.6 eV (the valence band maximum) to −9.2 eV, associated with the O 2p levels. The strong peak centered at −11 eV that is associated with the shallow Zn 3d core level of ZnO. After the ZnO surface was functionalized with 1, 2, or 3, the spectra were dramatically different, showing strong new features that are clear signatures of the adsorbed chromophorecontaining compounds. The new spectral features can be readily interpreted in the light of earlier work that has established the electronic structure of such adsorbates.30,31 In particular, the feature at a binding energy near −2.3 eV is associated with the HOMO which resides on the ZnP part of each molecule. The inset of Figure 5B is the HOMO region of the spectrum on an expanded scale. It is clear from the inset that there is a systematic shift of the leading edge of the HOMO feature: the HOMO of 3 is more negative than that of 2 by ∼100 meV, while that of 1 is more positive than 2 by the same amount. The direction of the observed shifts is in accordance with the predictions of the parallel-plate capacitor model, as illustrated Figure 3.

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RESULTS AND DISCUSSION Molecular Strategy. The compounds we synthesized to implement this strategy are shown in Figure 2. The synthesis of compounds 1 and 3 will be described elsewhere, while the synthesis of 2 has already been reported.26 The chromophoric unit, a zinc tetraphenylporphyrin (ZnTPP), was chosen in part owing to the growing importance of porphyrin-based systems in photovoltaic applications,27,28 but primarily because the photophysical properties of a ZnTPP ring are largely unaffected by the functionalization of the meso-phenyl groups.26,29−32 Furthermore, compounds 1−3 allow a direct comparison with earlier studies of ZnTPP derivatives bound to TiO2 and ZnO.26,30−32 The isophthalic acid (Ipa) anchor group was chosen as it binds efficiently to metal oxides through the carboxylate groups and was used in previous studies.29,33,34 A key aspect of this molecular design was to introduce an intramolecular electric dipole moment that could be reversed without influencing the photophysical properties of the chromophore. This was achieved by adding to the central pphenylene moiety of the linker an electron donor group (D, NMe2) in the para position and an electron acceptor group (A, NO2). Depending on the relative position of the NO2 and the NMe2 groups in 1 and 3, the dipole moment has a component that is oriented toward or away from the anchoring group (Figure 2). Anticipated Molecular Dipole. For monolayer coverage of the surface by the molecules, the molecular arrangement at the surface can be represented as shown in Figure 3: each

Figure 3. A simple parallel plate capacitor model can help anticipate the potential offset between the chromophore (green circles) and the ZnO substrate.

chromophore (green circles) is separated from the ZnO surface by a dipole contained in the bridge group. Assuming for simplicity that the molecular axis is normal to the surface of the sample, the total molecular dipole D induced by the bridge makes an angle of 60° with respect to the surface normal. A value of D = 9.025 D is obtained from ab initio calculations (DFT-B3LYP, 6-31G basis set35−38) of the bridge unit of Figure 2. At full coverage, the system can be compared to a parallel-plate capacitor mode as illustrated Figure 3. The potential difference between the two plates of the capacitor can be expressed as a function of the dipole projected on the normal to the surface (Dproj): 12925

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Figure 4. Left: Normalized UV−vis absorption spectra measured for 1, 2, and 3 in acetonitrile. Inset: Expanded Q-band region. Right: Fluorescence emission spectra normalized at 603.5 nm for 1, 2 and 3, in acetonitrile (λexc = 556 nm).

Figure 5. Measured shifts of characteristic molecular levels (HOMO, Secondary electron cutoff), as a function of the presence and of the orientation of the dipole group. UPS spectra of the pristine ZnO(11−20) surface (gray curve), and of the same surface functionalized with the molecules 1 (red), 2 (black) or 3 (blue). (A) Secondary electron cutoff; (B) Valence band region; inset: zoomed-in plot of the position of the molecular HOMO.

surface.30,39 We anticipate that larger energy shifts could be obtained with the same chromophore, bridge, and A, D groups by using, for example, tripodal anchor groups to ensure that the molecular axis is normal to the surface.40,41

As the presence of a dipole in the bridge group changes the electrostatic potential at the chromophore, the work function of the system should shift as well. The accompanying change in work function can be obtained from the data displayed Figure 5A. Once again, there is a systematic shift of the secondary electron cutoff, in the same direction and of the same magnitude as that of the HOMOs. In fact, essentially all of the spectral features exhibit a systematic energy shift similar to the HOMOs. DFT calculations (see SI) show that the electronic states from the linker portion of the molecule have a relatively small contribution to the total density of states (DOS). Therefore, features in the UPS spectra are dominated by the ZnTPP states which, in the presence of the bridge dipole, are shifted along with the HOMOs. Validity of the Model. While clearly an approximation, the capacitor model presented above retains the essential physics of the effect and allows a simple evaluation of the internal dipole. Quantitatively, the experimentally observed 100 meV shift is smaller than the calculated value of 300 meV. One likely reason is that the ideal binding geometry (i.e., molecular axis perpendicular to the surface) and packing density (i.e., full coverage according to molecular footprint) assumed in the model are different from the one observed experimentally. Indeed, theoretical calculations of ZnTPP derivatives bound to a TiO2 surface, as well as near edge adsorption fine structure (NEXAFS) and UV−vis spectroscopy studies of ZnTPP bound TiO2(110) and ZnO(11−20) with Ipa anchor groups, indicate that the axis of the molecule is tilted ∼45° with respect to the

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CONCLUSIONS Using a ZnTPP-based chromophore−bridge−anchor architecture, we have shown that chromophore’s molecular level alignment can be manipulated, i.e., shifted upward or downward in energy with respect to the band edges of a substrate. The ability to use molecular design to shift a bound chromophore’s orbital energies with respect to those of the substrate, without altering the absorption properties of the chromophore, can have important implications in a number of areas. With a (±) ∼100 meV level shift, photoexcited electrons would sample a different unoccupied density of states in the substrate, altering electron injection properties at the interface. Moreover, this change occurs without altering the chromophore−substrate separation. Although the energy shift is small in this test-case, this concept provides a general molecular design applicable to a large number of anchoring functional groups, built-in dipole bridges, and redox-active centers. The approach used here could be expanded to address energy level alignment at other relevant organic/organic or organic/ inorganic interfaces influencing, for example, the barrier height for charge carrier transport between organic semiconductor layers or from an organic layer to an electrical contact. 12926

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ASSOCIATED CONTENT

S Supporting Information *

Calculated DOS for the dipole unit and a Zinc Tetraphenylporphyrin. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Jonathan Rochford for the synthesis of compounds 2. Funding by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-01ER15256, is gratefully acknowledged.

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