Zinc OxideâZinc Phthalocyanine Interface for Hybrid Solar Cells

Article pubs.acs.org/JPCC

Zinc Oxide−Zinc Phthalocyanine Interface for Hybrid Solar Cells Giuseppe Mattioli,*,† Claudio Melis,‡,§ Giuliano Malloci,§ Francesco Filippone,† Paola Alippi,† Paolo Giannozzi,∥,⊥ Alessandro Mattoni,§ and Aldo Amore Bonapasta† †

Istituto di Struttura della Materia del CNR, v. Salaria Km 29,300 - C.P. 10 I-00015, Monterotondo Stazione (RM), Italy Department of Physics, University of Cagliari, Cittadella Universitaria, 09042 Monserrato (Ca), Italy § Istituto per l'Officina dei Materiali (CNR-IOM), UOS Cagliari SLACS, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy ∥ Department of Chemistry, Physics, and Environment, University of Udine, via delle Scienze 208, 33100 Udine, Italy ⊥ DEMOCRITOS IOM-CNR National Simulation Center, 34014 Trieste, Italy ‡

S Supporting Information *

ABSTRACT: The structural, electronic, and optical properties of a hybrid interface formed by zinc phthalocyanine (ZnPc) molecules adsorbed on the (101̅0) zinc oxide (ZnO) surface have been investigated by using ab initio and model potential theoretical methods. In particular, the attention has been focused on the effects of molecular assembling on the interface properties by considering cofacial and planar molecular aggregates on the surface. Present results show that planar aggregations provide a remarkable molecule-tosurface electronic coupling which can favor electron injection toward the substrate. Furthermore, we predict a blue shift of absorption bands in the case of cofacial aggregation and a red shift in the case of nanostructured planar J-stripes, which are in agreement with previous phenomenological models and give a firm theoretical support to observed relationships between red shift and molecular assembling. All together, present results indicate that structural and electronic properties can be achieved in ZnPc-sensitized ZnO surfaces of high potential interest for improving the efficiency of different kinds of hybrid photovoltaic cells.

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INTRODUCTION Hybrid photovoltaic (HPV) cells employing both organic and inorganic components for light harvesting and energy production have received enormous research attention. The natural abundance of the organic materials and their low cost make them, indeed, potentially highly competitive with solar cells based on thin-film technology. However, drawbacks for large-scale applications of HPV cells are still represented by their low efficiency and short durability.1−3 HPV cells are formed by an electron donor (D), often an organic molecule or a π-conjugated polymer, also acting as light absorber, or sensitizer, and an inorganic substrate acting as electron acceptor (A). The most important acceptors for HPV are metal oxides, in particular TiO2, that have been used in several HPV architectures including polymer-based hybrid solar cells and hybrid dye-sensitized solar cells (DSSC).1,2 Zinc oxide has emerged more recently in the framework of photovoltaic devices4−6 as an alternative to TiO2 because it can be synthesized with great flexibility and provides a very good electron mobility.7 For example, among polymer−oxide mixtures, blends of ZnO and P3HT are, up to date, the most efficient ones.8 Furthermore, DSSC based on ZnO have reached efficiencies comparable to titania.9 The zinc oxide nanoparticles commonly used as acceptor partners in HPV © 2012 American Chemical Society

devices are often characterized by an elongated shape along the (0001) direction and a hexagonal section.10,11 Such nanorods expose the most stable12,13 (1010̅ ) surface on their sides, with smaller contribution of another low-index nonpolar surface, namely the (112̅0) one, whose topmost atoms are arranged in Zn−O dimers, thus showing a local morphology similar to the (101̅0) surface.12 For these reasons the (101̅0) surface is often chosen, like in the present investigation, as a suitable model to represent the whole of ZnO surfaces in nanostructures.14,15 As for the organic component of HPV systems, phthalocyanine’s represent a very important class of donor molecules with good light absorption extending down to the red and near-IR regions and efficient electron transfer to the TiO2 or ZnO conduction band. Pcs (usually modified by the addition of anchoring groups) have been used indeed as efficient dyes for DSSC devices.16 Pcs molecules have been also used in polymer based hybrids to complement the optical absorption of the polymer in red region of the spectrum.17 The structure of these molecules is characterized by an aromatic macrocyclic ligand carrying clouds of π-conjugated, delocalized electrons, and a central metallic atom, in a typical 2+ oxidation state, playing the role of Received: April 19, 2012 Revised: June 20, 2012 Published: June 27, 2012 15439

dx.doi.org/10.1021/jp303781v | J. Phys. Chem. C 2012, 116, 15439−15448

The Journal of Physical Chemistry C

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Figure 1. (A) Sticks and balls representation of a zinc phthalocyanine (ZnPc) molecule and |ψ|2 plots of the π highest occupied molecular orbital (HOMO) and of the π* doubly degenerate lowest unoccupied molecular orbital (LUMO) (electronic densities sampled at 0.0005 e/au3). (B) Side and top view of the ZnO (101̅0) surface. Two reference axes of the surface are labeled X, Y. The alternation of trench grooves and rows of dimers along the X direction is shown in the lower panel. (C) Top and side views of the minimum-energy DFT configurations of an isolated ZnPc molecule adsorbed on the ZnO (101̅0) surface.

to further assemble into planar islands at higher concentrations. It should be also taken into account that the aggregation phenomenon has a strong impact on the optical properties of the molecules, the light absorption spectrum of aggregates being generally quite different from that of isolated molecules.19 In particular, by studying ZnPc/ZnO systems by UV−vis absorption spectroscopy,27 it was observed a continuous red shift of the optical absorption spectrum at increasing ZnPc’s coverage. These findings have been attributed to a molecular aggregation on the surface in the form of J-stripes basing on a phenomenological model (also known as the Davydov effect).19,28 Such an attribution is supported by the above MPMD study. On the other hand, a sound theoretical demonstration that ZnPc stripes induce the observed red shift is still missing (at the best of the author’s knowledge). In this scenario, in the present study, we approach the description of a ZnPc/ZnO interface by investigating the effects of molecular aggregation and intermolecular interactions on the coupling between ZnPc molecules and the (1010̅ ) ZnO surface. As major results, the present findings show that: (i) Planar aggregations of ZnPc molecules in the form of dimers and Jstripes are expected to characterize the ZnPc/ZnO interface at submonolayer and monolayer coverages. (ii) Such planar

electron donor to the ligands. A prototypical example is represented by ZnPc16 whose main electronic features are illustrated in Figure1A. In HPV cells, the electronic properties of the D−A interface play a key role by affecting both the optical absorption induced by solar light and the electron injection to the substrate. A recent, preliminary study investigating the electronic coupling between isolated ZnPc molecules on the ZnO surface18 has suggested interesting potentialities of Pcs/ZnO interfaces for photovoltaics. As a first step toward the simulation of more complex, realistic systems, the promises of such interfaces have to be checked against the characteristics of the Pcs assembling on the substrate surface as well as against its effects on the interface properties. In this regard, it has to be taken into account that Pcs tend to aggregate, as observed in gas phase and in solution,19,20 as well as on metal and semiconductor surfaces.21,22 Both ordered monolayers23,24 and island-growth features25 have been also reported in the case of Pc absorption on metal oxide surfaces. Hints of the tendencies of ZnPc molecules to form planar elongated nanoclusters on the ZnO surface (usually named and hereafter referred to as J-stripes) have been given in a recent model potential molecular dynamics (MPMD) contribution.26 These stripes are expected 15440



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atoms) rhombic surface cell has been used to simulate a closepacked ZnPc monolayer. Similar surface cells, only four atomic layers thick, have been used in the case of time-dependent density functional perturbation theory (TDDFPT) calculations (see below). Geometry optimization procedures have been performed by fully relaxing the positions of all of the atoms in a supercell, except for the atoms of the bottom layer of the semiconductor. The electronic properties of the ZnPc/ZnO system have been investigated by analyzing the electronic eigenvalues calculated at the Γ point. Particular care has been given to the simulation of dispersive interactions. In this regard, the exchangecorrelation functional has been constructed by adding an ab initio nonlocal van der Waals correlation contribution39,40 to the semilocal gradient-corrected PBE functional.41 This improved functional addresses the problem of the lack of description of long-range electronic correlation interactions in standard generalized gradient approximation (GGA) functionals. Such a drawback leads to a drastic underestimate of dispersion forces, efficiently corrected here by using the above functional.42 A further drawback of the GGA-PBE functional regards the Kohn−Sham eigenvalues related to isolated (e.g., molecules) and extended (e.g., surface slabs) systems, which can be affected at different extents by short-range electronic interaction errors leading, in particular, to underestimate the band gap or the HOMO−LUMO gap.43,44 In turn, such a drawback can affect the relative alignment of molecule and surface electronic eigenvalues which is crucial to provide a reliable estimate of the electronic and optical properties of a D−A interface, like the ZnPc/ZnO one. In this regard, we have performed further calculations in order to carefully compare GGA-PBE results with results obtained by using a GGA+U correction,45,46 as well as the screened hybrid HSE06 functional,47,48 particularly accurate to simulate the properties of hybrid organic−inorganic systems.49 We can anticipate that the picture given by PBE results closely agrees with the GGA+U and HSE ones, thus justifying the choice of a PBE approach in the case of the large molecule−surface systems investigated here. The quantitative extent of this agreement, together with the GGA+U and HSE results, is discussed in detail in the Supporting Information. Optical absorption spectra ranging from the near-IR to the near-UV regions have been calculated in the case of a gas-phase ZnPc molecule, a gas-phase ZnPc dimer, the most stable single ZnPc/ZnO interacting system, and a periodic ZnPc stripe interacting with the ZnO surface by using a recent approach to the solution of the Bethe−Salpeter equation within the framework of time-dependent density matrix perturbation theory (TDDFPT).50,51 In detail, absorption frequencies, and the related oscillator strenght, are calculated as linear response of the whole charge density of the system to the oscillating electric perturbation; A Lorentzian broadening of 0.14 eV has been applied to all the oscillators. Such an approach is expected to provide reliable results when applied to large systems, including sensitized metal oxides.18,52 A careful analysis is required for one or more ZnPc molecules interacting with the ZnO surface. More specifically, the contribution of the ZnO slab has to be separately calculated and subtracted by the ZnPc/ZnO spectra because of the generally acknowledged inaccurate contribution of periodic systems to TDDFT calculations, when performed at the adiabatic generalized gradient approximation (A-GGA) level.53,54 In the case of ZnPc/ZnO systems, the simulated spectra show therefore the

aggregates are characterized by a significant ZnPc−ZnO electronic coupling which induces the appearance of empty electronic levels deriving from an intimate mixing of ZnPc and ZnO electronic states and strategically located within the ZnO conduction band and below the ZnPc LUMO. These levels are, therefore, specially suitable for an efficient molecule-to-surface transfer of photoexcited electrons. (iii) In the case of planar Jstripes, intermolecular interactions induce a red shift of absorption bands which agrees with the phenomenological model mentioned above and, most important, gives a firm theoretical support to the attribution of such a red shift to the formation of J-stripes proposed in ref 27.

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THEORETICAL FRAMEWORK Previous model potential molecular dynamics MPMD investigations of ZnPc molecules adsorbed on the ZnO surface26,29 have provided equilibrium geometries for single molecules, dimers, and molecular stripes, used as a first guess for the present ab initio DFT calculations. The model potential for the ZnO wurtzite crystal was described by the sum of a Coulomb and a Buckingham-type two-body potential.30,31 The AMBER force field,32 including both bonding (stretching, bending, torsional) and nonbonding (van der Waals plus Coulomb) contributions, has been used to describe ZnPc molecules.29 Interatomic forces between atoms of the ZnPc and the ZnO substrate were calculated by including electrostatic and dispersive interactions. Coulomb terms involved interactions between atomic partial charges of the molecule and the ZnO ions. Dispersive interactions of the Lennard-Jones type were taken from the AMBER database.32 All the simulations were performed by using DL_POLY33 (version 3). The assembling of ZnPc molecules at room temperature has been studied by using the metadynamics technique, which allows to accelerate rare events and to sample the free energy as a function of suitable collective variables,34 e.g., the distance between the center of mass of two ZnPc molecules in Figure 5. Ab initio DFT calculations have been performed by using the Quantum-ESPRESSO package.35 Total energies have been calculated by using ultrasoft pseudopotentials36 for all atoms but Zn, whose electronic core was represented by a normconserving pseudopotential.37 The Zn 3d shell has been embedded into the pseudopotential. A similar approach has been used and discussed in detail in the case of the Ga atom in the wurtzite GaN crystal,38 probably the ZnO closest relative. Extensive calculations have been performed to carefully check this approach; the achieved results are presented and discussed in detail within the Supporting Information. Kohn−Sham eigenfunctions have been expanded on a plane-wave basis set; satisfactorily converged results have been achieved by using cutoffs of 35 Ry on the plane waves and of 280 Ry on the electronic density as well as by sampling the first Brillouin zone with the Γ point only. Negligible differences on the structural and electronic properties of the investigated molecule−surface systems have been obtained by using a 2 × 2 × 1 Γ-centered kpoint mesh. Several ZnO (101̅0) surface cells have been modeled by adding ≈15 Å of empty space to different crystal slabs all formed by six atomic layers of bulk ZnO parallel to the (101̅0) crystal plane. In the case of a single ZnPc molecule adsorbed on the ZnO surface, almost identical results have been achieved in the case of 4 × 6 (288 atoms), 4 × 8 (384 atoms, used also in the case of periodic ZnPc stripes), 4 × 9 (432 atoms, used also in the case of isolated ZnPc dimers), and 5 × 9 (540 atoms, 26 Å × 29 Å wide) surface cells. A 6 × 8 (288 15441



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Figure 2. Electronic eigenvalues calculated at the Γ point in the case of one or two ZnPc molecules interacting with the ZnO surface: (A) ZnPc nonbonded to the ZnO surface (i.e., kept far from the surface); (B) single ZnPc molecule bonded to the ZnO surface; (C) single ZnPc molecule bonded to the ZnO surface in a restricted open-shell electronic configuration (see text); (D) two ZnPc molecules aligned along the X direction (AA configuration, see the text); (E) two stacked ZnPc molecules (AC configuration, see the text); (F) two ZnPc molecules in a J-type aggregation pattern (see text). An example of |ψ|2 plot related to the new mixed ZnO/ZnPc states is shown in Figure 3. The electronic eigenvalues have been aligned by using the 1s level of a He atom inserted as a reference in all the supercells. CBM and VBM labels indicate the ZnO conduction band minimum and valence band maximum, respectively.

Löwdin charges reported in Table 1. Two new electronic features accompany such polarization effects and characterize the interacting molecule−surface system with respect to a noninteracting one (compare parts A and B of Figure 2, respectively): (i) an appreciable lowering of both the HOMO and LUMO,55 which agrees with the above charge density displacements;56 (ii) the occurrence of a novel, unoccupied electronic level in the ZnO CB. This level presents two very interesting features: it originates from a strong mixing of the ZnPc LUMO and the ZnO CB levels, as clearly shown by the charge density plot in Figure 3, and it is located about 0.2 eV below the native ZnPc LUMO. The electronic features of this ZnPc−ZnO configuration have been further investigated here by performing a restricted open-shell calculation starting from the same molecule−surface configuration. In this calculation, made to get an approximate description of the lowest excited state of the system, the occupation of the Kohn−Sham electronic levels is kept fixed in order to force, in an unbiased way, a hole in the highest occupied electronic level and an electron in the lowest unoccupied electronic level of the whole system (circled + and − in Figure 2C, respectively). The achieved results confirm the D−A character of the ZnPc−ZnO interaction. The resulting electronic structure is characterized indeed by the spontaneous localization of the hole and the electron in the ZnPc HOMO and the ZnO CB minimum, respectively. This is accompanied by a further lowering of both the HOMO and LUMO levels, as expected in the case of a molecule−surface charge-transfer process.56 Finally, even these results confirm the occurrence of mixed ZnPc−ZnO levels. ZnPc Dimers on the ZnO Surface. The assembling of Pc molecules on metal oxides can be put in close relationship with the properties of molecular dimers interacting with the oxide

changes occurring in the ZnPc spectrum due to the adsorption of one or more molecules on the ZnO surface. A reduced 192 atoms four-layer 4 × 6 (3 × 8) ZnO slab has been used in the case of TDDFPT calculations to simulate the molecule/surface (periodic stripe/surface) system. The DFT results indicate that the structural and electronic properties of the ZnPc/ZnO system calculated with the reduced slab are in a good agreement with the ones obtained by using the above sixlayer slabs.

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RESULTS AND DISCUSSION Isolated ZnPc on the ZnO Surface. The most stable nonpolar (101̅0) ZnO surface exhibits trench grooves alternated with rows of dimers both oriented along the direction labeled X in Figure 1B. The minimum-energy configuration of an adsorbed molecule, hereafter referred to as A configuration,18,29 is shown in Figure 1C. The molecule binds to the surface by a combination of two joined interactions: the formation of a chemical bond involving the molecular Zn and one of the 3-fold coordinated O atoms belonging to the surface dimers rows and a “face-to-face” interaction involving the broad molecular macrocyclic system and the rather flat ZnO surface primarily due to strong van der Waals forces. These two bonding contributions result in a ZnPc adsorption energy, Eads, equal to 3.8 eV. On the side of the electronic properties of the ZnPc/ZnO system in the A configuration, the relevant “face-to-face” interaction induces a high polarization of electronic charge at the molecule−surface interface. More specifically, a fraction of the ZnPc charge density is shared with the ZnO surface in agreement with the donor tendency of the molecule and the acceptor tendency of the surface;18 see, e.g., the values of the 15442



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energy, the molecules are partially (AC′ in Figure 4B) or totally (AC in Figure 4C) overlapped. Table 1. Löwdin Charges Calculated in the Case of the Bonded ZnPc−ZnO Configuration and Corresponding Shifts with Respect to the Nonbonded Configuration (See Text)a Bonded ZnPc atom Zn N C H total

total charge

s charge

p charge

10.42 0.46 0.00 5.27 1.32 3.95 3.95 0.92 3.02 0.78 0.78 0.00 191.47 53.10 128.39 Bonded ZnPc−Nonbonded ZnPc

d charge 9.96 0.00 0.00 0.00 9.96

atom

total shift

s shift

p shift

d shift

Zn N C H total

−0.15 −0.10 −0.03 0.00 −1.85

−0.16 −0.03 −0.01 0.00 −0.72

0.00 −0.07 −0.02 0.00 −1.22

+0.01 0.00 0.00 0.00 +0.01

All values are averaged on atomic species and summed in the “total” rows. Total and projected (on s, p, and d angular momentum channels) charges related to the bonded ZnPc−ZnO configuration are reported in columns “Bonded ZnPc”. Total and projected charge shifts are reported in columns “Bonded ZnPc−Nonbonded ZnPc”.

a

Figure 3. |ψ|2 electron density plots of the unoccupied, delocalized ZnO/ZnPc electronic level located below the LUMO level in Figure 2 (electronic densities sampled at 0.0005 e/au3).

The above MPMD dimer configurations have been regarded as starting points for further geometry optimizations carried out here by ab initio DFT methods. A quite good agreement has been found between MPMD and DFT results; e.g., both techniques indicate that the aligned, partially stacked and stacked configurations shown in Figure 4 correspond to stable adsorption sites for a ZnPc dimer on the ZnO surface. Adsorption energy values of 7.5, 6.1, and 5.9 eV have been calculated for the AA aligned configuration (i.e., the energy value corresponds to twice the isolated molecule value) and the AC′ and AC configurations, respectively; that is, the partially stacked AC′ configuration is found to be slightly lower in energy than the stacked AC one. This is at variance with MPMD results indicating that the AC and AC′ configurations are 0.3 and 0.8 eV higher in energy than the aligned AA

surfaces. Pc dimers can be easily formed during synthesis from gas phase20 or from solution27 as well as due to thermally activated molecule diffusion on the surface. They can also represent centers of aggregation for the formation of larger clusters and be characterized by different electronic properties with respect to the case of isolated molecules. Two different kinds of dimer configuration have been identified in a previous MPMD study:26 in the most stable one, the two molecules lay aligned on the surface along one of the topmost O rows (see Figure 4A), basically corresponding to a couple of AA molecules. In stacked configurations, which are higher in

Figure 4. Sticks and balls representation of the three minimum-energy configurations of a ZnPc dimer adsorbed on a ZnO (101̅0) surface, as given by DFT calculations. AA: molecules aligned along the X direction (see Figure 1B); AC: stacked molecules; AC′: partially stacked molecules. 15443



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top molecule of the AC dimer slips down to the AA configuration and performed a new MPMD-based metadynamics calculation choosing as a collective variable the distance between the centers of two ZnPc molecules adsorbed on the ZnO surface (details on this kind of simulations are given elsewhere26). The corresponding one-dimensional free energy profile (see Figure 5) is characterized by three minima

configuration, respectively. Such a difference between MPMD and DFT adsorption energies can be ascribed to the delicate interplay between covalent bonding and dispersion forces in the molecule−surface interaction. On the other hand, it should be noted that the two techniques find an identical value in the case of a gas-phase ZnPc dimer, 1.6 eV, in line also with previous results.20 Finally, in the case of the topmost molecule, DFT calculations give adsorption energy values of 2.1 and 2.3 eV for the stacked and partially stacked configurations, respectively. A comparison of these values with that estimated for the gasphase dimers indicates that in both configurations the topmost ZnPc molecules are still weakly interacting with the underlying ZnO surface. On the side of the electronic properties, the stable AA configuration is characterized by charge density distributions and electronic levels hardly distinguishable from those related to the A configuration of a single molecule. Both molecules undergo indeed a strong coupling with the ZnO surface which leads to similar changes of the electronic structure, that is, a lowering of the HOMO and LUMO levels (see Figure 2D), and the occurrence of mixed ZnPc/ZnO levels like those shown in Figure 3. It is worth noticing that, in this dimer configuration, the closeness of the two molecules does not induce any mixing between their electronic states; that is, there is no evidence of any intermolecular electronic interaction. In the case of the stacked AC configuration, the upper molecule is only weakly coupled with the ZnO surface; its electronic levels (see Figure 2E) are mainly aligned with the ones corresponding to a ZnPc molecule kept far from the surface and noninteracting with the electronic states of the ZnO substrate. The lower molecule shows instead an evolution of the electronic states consistent with a strong coupling with the surface. This dimer configuration is also characterized by some mixing between the electronic states of the two molecules which, however, is significantly smaller than that occurring in gas-phase dimers with the same “face-to-face” arrangement. In the AC′ case, an intermediate behavior between the AA and AC configurations has been found (not reported in Figure 2). The above results show the existence of marked differences between the electronic properties of AC and AA dimers. Accordingly, the electronic and optical properties of a ZnPc/ ZnO interface may depend on their relative abundance. Aligned AA dimers are energetically more stable and are expected to be more abundant at equilibrium. However, stacked dimers can easily form in solution or in gas phase19,20,27 and can be eventually adsorbed on the ZnO surface as AC dimers. At finite temperature this AC configuration can also evolve into the lower energy AA configuration by overcoming a free energy barrier. Thus, the height of such barrier, which controls the AC lifetime and relative population, becomes a delicate parameter affecting the molecular assembling on the ZnO surface. In order to take into account possible effects of dimer formation on the properties of the ZnPc/ZnO system, we have investigated some features of the dimer dynamics in a combined MPMD-DFT approach. In this regard, as detailed in the Supporting Information, we have preliminarily checked the accuracy of the MPMD approach by comparing the energy barriers estimated along a same path connecting AC and AA configurations by MPMD and by DFT-based nudged elastic band (NEB) simulations. The achieved results show a good agreement between the two methods, thus strengthening the description of the dynamical behavior of ZnPc dimers given by metadynamics. Then, we have considered a process where the

Figure 5. Curve in the figure indicates the free energy sampled by a MPMD-based metadynamics calculation along the minimum free energy path related to the Zn−Zn intermolecular distance. The AA, AC, and AC′ sketched configurations are shown in Figure 4 and described in the text.

corresponding to the previously discussed AC, AC′, and AA dimers. The AC configuration is separated from the AA configuration by a free energy barrier of ∼0.4 eV, which is the sum of two contributions: a free energy barrier of 0.2 eV, which brings the AC dimer to the AC′ configuration; a further free energy barrier of 0.2 eV which allows the top molecule to further move, reaching the AA configuration. It is important to note that the reverse reaction from AA to AC is quite unlikely to occur because of the large free energy barrier (1.0 eV). All together, the above results indicate that adhesive molecule− surface interactions dominate over cohesive molecule− molecule ones and suggest a dissolution of ZnPc dimers, thus favoring the formation of a two-dimensional ZnPc monolayer instead of three-dimensional structures. J-Type Aggregates on the Surface. According to the mentioned MPMD-based analysis,26 the energetically favored aggregate of a large number of ZnPc molecules interacting with the ZnO surface is represented by close-packed linear stripes of ZnPc along the X direction (see Figure 6). Such a J-stripe configuration, also observed in the case of a similar CuPc/TiO2 interface,24 has been proposed as responsible of red-shifted absorption features observed in the ZnO/ZnPc system.27 Regarding DFT calculations, we simulate the structural and electronic properties of J-stripes by enforcing periodic boundary conditions around an AA dimer along the X-axis; this corresponds to a supercell (enclosed into the red rectangle in Figure 6A) containing two molecules. A Zn−Zn distance of 13 Å along the X-axis, corresponding to a ×4 periodicity along the surface Zn−O dimers row, indicates that nearest-neighbors molecules are close packed at the minimum distance allowed by the intrinsic dimensions of the system. At the same time, 21 Å separate instead a molecule from its periodically repeated image along the Y-axis, ensuring negligible interaction effects. It is worth noticing that, due to the use of periodic boundary conditions, in the following discussion of the J-stripes 15444



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Figure 6. (A) Sticks and balls representation of a J-type aggregate of ZnPc molecules along the X direction (see Figure 1B) of the ZnO surface. The simulated periodic supercell, containing two ZnPc molecules, has been enclosed into a red rectangle. (B) Electron density plots of the mixed HOMO orbitals of two ZnPc molecules arranged in a J-type aggregate. (C) Sticks and balls representation of a close-packed ZnPc monolayer adsorbed on the ZnO surface. The simulated periodic supercell, containing two ZnPc molecules, has been enclosed into a red rhombus. (D) Electron density plots of the mixed HOMO orbitals of ZnPc molecules in a close-packed monolayer. The distances (in Å) between nearest-neighbors molecules along the X and Y directions have been highlighted.

properties it will be assumed that present results can be applied to stripes long enough to minimize boundary effects. On the side of the structural properties of this J-type system, both the Zn−O molecule-surface bond and ZnPc orientation result to be quite similar to those found for the AA dimer (compare Figures 6 and 4). On the side of the electronic properties, the position of electronic eigenvalues with respect to the ZnO bands, sketched in Figure 2F, is also almost identical to the AA dimer one in Figure 2D. Thus, this J-type assembling does not affect the main features of the molecule−surface electronic coupling which characterize isolated molecules and AA dimers. Relevant differences have been found instead concerning the nature and spatial distribution of the J-stripe electronic states. In addition to the above mixing of molecule and surface electronic states occurring in the case of the nonperiodic interacting systems (i.e., the AA dimer configuration), a periodically repeated molecular structure induces indeed a strong mixing of molecular orbitals between the two ZnPc molecules forming the recurring unit in the stripe. As an

example, electron density plots of HOMO orbitals, corresponding to the eigenvalues represented by blue and light blue continuous lines in Figure 2F, clearly show an electronic charge distribution on both molecules (see Figure 6B). Similar features characterize LUMO orbitals in addition to the above-discussed mixing with the ZnO conduction band. This mixing of molecular electronic states affect the optical properties of Jtype stripes, which result to be different from those of isolated molecules and dimers interacting with the ZnO surface, as it will described in the next section. The orbital mixing between molecules arranged in the J-type configuration indicates the occurrence of intermolecular interactions. Moreover, such interactions represent an intrinsic property of such a molecular arrangement. The same orbital mixing has been found indeed for ZnPc molecules arranged in the same J-type geometry and not interacting with the ZnO surface. In order to gain further insight on the effects of intermolecule interactions, a closer packing of ZnPc molecules has been also considered (see Figure 6C). In such a molecular 15445



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arrangement, basically, the J-type stripes are arranged in a closepacked configuration both along the X and Y axes in order to sample intermolecular interactions in both directions. This has been achieved by setting up a rhombic surface periodic unit containing two molecules which are nearest neighbors both along the X axis and Y axis, as shown in Figure 6C. The above arrangement can be also considered as a model of an ordered close-packed ZnPc monolayer adsorbed on the ZnO surface, with intramolecular distances of 13.0 and 15.5 Å along the X and Y axes, respectively. Present DFT results on that arrangement clearly show the occurrence, in addition to the expected mixing of molecule and surface electronic states, of a strong mixing of molecular orbitals between the two ZnPc molecules forming the recurring unit; see, e.g., the electronic charge distributions in Figure 6D. Thus, such an orbital mixing, whose extent is larger than that of the isolated J-stripe, depends only on the configuration of the neighboring molecules on the ZnO surface. Optical Properties of ZnPc Molecules and ZnPc/ZnO Systems. In the case of gas-phase, isolated ZnPc molecules, the absorption spectrum is characterized by two strong features known as Q-band and Soret band, common to almost all phthalocyanine and porphyrin molecules, which fall in the red part of the visible region (1.88 eV) and in the near-UV region (3.58 eV), respectively.57 Such absorption peaks are very well reproduced by our TDDFPT calculations, as shown in Figure 7B. When a single isolated ZnPc molecule is chemisorbed on the surface, both the spectral features are red-shifted with respect to the gas-phase spectrum (compare Figure 7B and Figure 7C). The Q-band, in particular, undergoes a 0.10 eV (37 nm) red shift, in a full agreement with the above DFT indications of a smaller HOMO−LUMO energy difference arising from the ZnPc/ZnO interaction. In the cases of ZnPc dimers and stripes, the close proximity of two or more macrocyclic rings can lead to a strong coupling of their electronic states (the Davydov effect mentioned above), which induces a splitting of the related excited states.19,28 This, in turn, is responsible for relevant shifts of the absorption bands. In particular, a blue shift of the Q-band is predicted by the Davydov model in the case of a cofacial alignment of two or more molecules; a red shift of the Q-band, on the contrary, is expected in the case of coplanar dimers or stripes, as long as a strong coupling of electronic states still occurs. The results achieved here for ZnPc molecules, both in gas phase and when interacting with the ZnO surface, agree with the above Davydov model. In the former case, a gas phase ZnPc dimer (cofacial alignment) undergoes indeed a 0.06 eV (21 nm) blue shift of the Q-band, as shown in Figure 7A. In the case of ZnPc molecules interacting with the ZnO surface, besides the isolated ZnPc molecule, we have considered a J-type aggregation where a coplanar alignment is accompanied by a strong mixing between ZnPc molecular orbitals (shown in Figure 6) which adds to the mixing between molecular and surface states. The corresponding TDDFPT absorption spectra, shown in Figure 7C,D, confirm the existence of significant relationships between aggregation patterns, the electronic properties of molecular aggregates on the surface, as described by “single particle” DFT calculations, and their optical properties, as estimated by a “many body” TDDFPT approach. In the case of the J-type configuration, a 0.18 eV (71 nm) red shift of the Q-band is indeed calculated, larger than that found for an isolated molecule adsorbed on the ZnO surface, thus indicating that the intermolecular mixing characterizing the J-stripes enhances the

Figure 7. TDDFPT absorption spectra of (A) a gas-phase ZnPc dimer, (B) an isolated gas-phase ZnPc molecule, (C) a ZnPc molecule adsorbed on the ZnO surface, and (D) two ZnPc molecules arranged in a J-type aggregate on the ZnO surface. The absorption energy value of the typical phthalocyanine Q-band is reported in both eV and nm (the latter one in parentheses) to ensure the best comparison with experimental measurements. The vertical dotted line is a guide to the eye which indicates the extent of red and blue shifts of the Q-band. (C) and (D) spectra involve the contribution of ZnO surface slabs underlying the ZnPc molecules. As detailed in the Theoretical Framework section, such a contribution has been subtracted out from the spectra and the resulting thin black lines have been smoothed by using spline functions.58

red-shifting induced by the molecule−surface coupling. This result is fully consistent with the red shift predicted by the Davydov model. Most important, it gives a firm theoretical support to the results of the mentioned experimental study investigating the properties of ZnPc-impregnated ZnO nanocrystals.27 This study reports indeed a pronounced red shift of the Q-band in the case of heavily loaded ZnO nanoparticles, by suggesting also a key role of J-type aggregates in the optical properties of the hybrid interface. It is worth noticing that, in the case of the monolayer coverage formed by dense ZnPc stripes, the above DFT results suggest an absorption spectrum for such an arrangement quite similar to that calculated for the J-type aggregation, possibly characterized by an even more pronounced red shift of the Q-band. The role of the molecular aggregation in the optical properties can be further acknowledged by the fact that AA dimers, although arranged in a coplanar fashion, do not show any appreciable intermolecular mixing of their degenerate HOMO orbitals.59 All together, the above results indicate that a direct link can be established between the structure of molecular aggregates and the 15446



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calculations, and confirmed by the present results, ZnPc molecules show a general tendency to form planar aggregates on the surface rather than multilayered islands. Previous MPMD results also indicated that isolated molecules, aligned dimers, and J-stripes should characterize the ZnPc/ZnO interface at submonolayer coverages. These indications have been taken into account here by considering also a full monolayer, formed by packed J-stripes, as a possible model of the ZnPc/ZnO interface. As major results, we have found that: (i) The above different kinds of molecular arrangement present similar electronic properties deriving by a same kind of strong molecule−surface coupling. The main features of such a coupling are represented by the existence of unoccupied electronic levels which, due to their nature and location, can play a key role in the ignition of photogenerated electrons from the molecule to the surface, that is, in processes at the core of the hybrid photovoltaic cell operation. (ii) Intermolecular interactions induce a blue shift of absorption bands in the case of cofacial alignment of two molecules (stacked dimer) and a red shift in the case of planar J-stripes. This agrees with the Davydov phenomenological model and, most important, gives a firm theoretical support to the relationships between the red shift of absorption lines and the formation of J-type aggregates proposed in ref 27. All together, the present results reveal structural and electronic properties of the ZnPc/ZnO interface which make ZnPc-sensitized ZnO surfaces of high potential interest for improving the efficiency of different kinds of hybrid photovoltaic cells.

simulated (and measured) absorption spectra, thus permitting to achieve a sound, theoretical picture of the properties of a ZnPc/ZnO interface. Sensitization of the ZnO Surface. The dynamical behavior of ZnPc molecules on the ZnO surface indicates a general tendency of ZnPc molecules to form planar aggregates on the surface, up to a monolayer coverage, rather than multilayered islands. Moreover, isolated molecules, aligned dimers, and J-stripes are expected to characterize submonolayer coverages of the ZnO surface. In the case of isolated molecules, a ZnPc molecule exhibits a strong chemical and electronic coupling with the zinc oxide, characterized by: (i) The occurrence of a strong molecule−surface interaction, due to the combined actions of chemical bonding and dispersion forces; this condition is noticeably realized without requiring a molecular functionalization for anchoring the molecule to the surface. (ii) The appearance of a new unoccupied and mixed ZnPc/ZnO orbital close to the ZnO conduction band minimum and slightly below the ZnPc LUMO, as shown in Figure 2. The same kind of structural and electronic coupling characterizes ZnPc aligned dimers and J-stripes. In addition, intermolecular interactions occurring between ZnPc molecules arranged in J-stripes enhance the red shift of the Q-band. All together, these results permit a deeper understanding of the properties of the ZnPc/ZnO interface and, at the same time, shed a few beams of light on the complex mechanisms of generation, diffusion, and splitting of excited charge carriers underlying the sensitization of the ZnO surface by means of the adsorption of ZnPc molecules. These molecules could act indeed as dyes for harvesting solar light up to the near-infrared region in novel, solid-state architectures of dye sensitized solar cells. In this regard, a key role can be played by the mixed ZnPc−ZnO orbitals, sketched in Figure 2 and shown in Figure 3, laying just below the “native” LUMO. In fact, in the case of e−−h+ pairs photogenerated inside the ZnPc molecules by π → π* HOMO−LUMO excitation processes (the so-called Qbands, dominating the visible absorption spectra of phthalocyanine molecules57), such mixed orbitals may provide an easy route to transfer the excited electrons to the ZnO conduction band. The same orbitals could also be involved in a direct injection from the ZnPc HOMO to the ZnO. The red shift effects related to the formation of J-stripes could further improve the light harvesting of such sensitized surfaces, which is extended to the near-IR spectral region. It can be observed that even the electronic properties of a possible second molecular layer can favor molecule to surface electron-transfer processes: e−h pairs photogenerated into this layer could be indeed efficiently split, with electrons raised to the LUMO falling downward into the ZnO conduction band and holes raising upward and transferred to a suitable transport medium, as required in DSSCs.

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ASSOCIATED CONTENT

S Supporting Information *

Assessment of theoretical methods (ZnO and ZnO/ZnPc systems); details on the ZnO/ZnPc interaction (isolated molecule and ZnPc dimers). 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge computational support by CYBERSAR (Cagliari, Italy), CINECA (Casalecchio di Reno, Italy; grant IscrA_SIMuLATe), and CASPUR (Rome, Italy; grants std11478 and std11-666). We acknowledge the platform computation of the Italian Institute of Technology (IIT) and financial support under Project IIT SEED “POLYPHEMO” and Project CNR “EFOR”. A.M. acknowledges Regione Autonoma della Sardegna for funding under L.R. 2007. G.M. is glad to thank A. Paoletti for the useful discussions on phthalocyanine UV−vis spectra.

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CONCLUSIONS In the present study, we have performed an ab initio theoretical investigation of the electronic and optical properties of the hybrid interface formed by ZnPc molecules adsorbed on the ZnO surface. Such interface properties have been investigated by considering the evolution of the properties of single molecules induced by the molecular assembling on the ZnO surface. Dimers and stripes have been considered as basic units for molecular assembling, while the dynamical behavior of dimers has been investigated in detail in a combined DFT and MPMD approach. As already suggested by previous MPMD

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