Hybrid Zinc Phthalocyanine/Zinc Oxide System for Photovoltaic Devices: a DFT and TDDFPT Theoretical Investigation

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1 Page 10 of 18 Hybrid Zinc Phthalocyanine/Zinc Oxide System for Photovoltaic Devices: a DFT and TDDFPT Theoretical Investigation Giuseppe Mattioli, a Francesco Filippone, a Paola Alippi, a Paolo Giannozzi, b and Aldo Amore Bonapasta, a Received Xth XXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX First published on the web Xth XXXXXXXXXX 200X DOI: /b000000x By combining ab initio density functional theory (DFT) and time-dependent density functional perturbation theory (TDDFPT) methods, we investigate the structural, electronic and optical properties of a zinc phthalocyanine (ZnPc) molecule interacting with the zinc oxide (ZnO) wurtzite (10 10) surface. Our results reveal the existence of a strong molecule-surface coupling whose major effect is the appearance of a new unoccupied electronic level, deriving from an intimate mixing of ZnPc and ZnO electronic states and strategically located within the ZnO conduction band and below the ZnPc LUMO. This level induces appreciable changes in the ZnPc absorption spectrum and is expected to significantly favor a molecule to surface transfer of photo-excited electrons, a key process in the functioning of hybrid photovoltaic devices. The molecule-surface interactions are also characterized by significant van der Waals forces and by the formation of molecule-surface chemical bonds, thus resulting in appreciable molecular adhesion to the surface. 1 Introduction Organic and hybrid organic-inorganic photovoltaic devices (OPV and HPV, respectively) have received enormous research attention in the last years A potential low cost, related to the natural abundance of the organic materials and the possibility to avoid expensive fabrication procedures, makes indeed such devices potentially highly competitive with thinfilm inorganic solar cells. However, large-scale applications still suffer of a low efficiency and short durability of OPV and HPV cells. Basically, OPV and HPV devices are formed by two main elements: a light-harvesting component, or organic sensitizer, e.g., an organic molecule or a π-conjugated polymer, also acting as an electron donor (D), and a different organic species or an inorganic nanostructured substrate acting as an electron acceptor (A). 3 These components are involved in the following processes at the core of the solar cell functioning: 1 3 (i) optical absorption induced by solar light into the light harvesting D element and formation of a bound electron-hole e h + pair, i.e., a Frenkel exciton; (ii) exciton diffusion and dissociation at the D A interface, which leads to an electron transfer from the sensitizer to the inorganic substrate; (iii) charge carrier migration and collection at the electrodes, with regeneration of a Istituto di Struttura della Materia (ISM) del Consiglio Nazionale delle Ricerche, Via Salaria Km 29.5, CP 10, Monterotondo Stazione, Italy; giuseppe.mattioli@ism.cnr.it b Department of Chemistry, Physics and Environment, University of Udine, via delle Scienze 208, Udine, and DEMOCRITOS IOM-CNR National Simulation Center, Trieste, Italy a neutral D A system. Regarding HPV cells, in the case of dye-sensitized solar cells (DSSC), 2 the D A system is represented by a D organic dye chemically bonded by means of a suitable anchoring group to an A inorganic semiconducting substrate, like a TiO 2 or ZnO nanostructured crystal. Photogenerated electrons are transported into the oxide conduction band toward the anode, while a suitable electrolytic medium (e.g., the I 3 I pair as well as a p-type organic conductor) transfers the photogenerated holes to the cathode. In the case of nanostructured-heterojunction polymer-based solar cells, 1 a light-harvesting p-type conducting polymer (e.g., the poly- 3-hexyl-thiophene, P3HT) acts both as sensitizer and hole transport medium, while electrons are transferred to a suitable n-type semiconducting oxide (e.g., TiO 2 or ZnO nanostructures). In both the above architectures, a key role is played by the injection of photogenerated electrons from the D sensitizer to the A oxide, which is expected to crucially affect the efficiency of the HPVs. Within this scenario, we focus here on the interaction of a prototypical molecular sensitizer, the zinc phthalocyanine 4,5 (ZnPc), with the most stable (10 10) surface of the ZnO semiconductor (see Fig. 1). The ZnPc molecule has been chosen because phthalocyanine molecules are commonly used as dyes for photovoltaic applications due to their high molecular extinction coefficient in the visible range (higher than 10 5 dm 3 M 1 cm 1 ) combined with a long exciton diffusion length (in a 8-68 nm range for CuPc) and a high conductivity for holes ( cm 2 /(V s)) 5,6. The structure of these molecules is characterized by one or more macrocyclic lig

2 Page 11 of 18 Journal of Materials Chemistry ands carrying clouds of π-conjugated, delocalised electrons see, e.g., the and LUMO electron densities in Fig. 2, and by a central metal or group, in a typical (2+) oxidation state, playing the role of electron donor to the ligands. As for the acceptor inorganic substrate, many recent studies have investigated the zinc oxide as a component in HPV cells, like DSSC. 2,7 10 ZnO presents indeed several attractive properties: it has a wide direct band gap (3.4 ev) similar to that of TiO 2 (generally used in HPV cells), it is highly transparent and stable against degradation, 11 providing also a very good electron mobility. 12 It presents a low rate of direct electron-hole recombination processes 13 and can be synthesized with great flexibility in different nanostructures, even in 1-D single crystal structures, thus enabling a higher surface-to-volume ratio for greater loading of organic dyes. 7 Here, we have considered the most stable (10 10) wurtzite ZnO surface, which should be abundant in ZnO nanocrystals. Such a non-polar surface exhibits trench groves alternated with rows of Zn-O dimers both oriented along the [1 210] direction. The Zn-O topmost dimers present occupied and unoccupied dangling bonds on the O and Zn topmost sites, respectively. This results in regions of locally different electron density, clearly shown by the calculated charge density plots in Fig. 1, which are also consistent with the almost flat sp 2 -like configuration of the Zn sites and the sp3-like configuration of the O sites. The above mentioned dye-to-substrate electron transfer, a key step in the HPV functioning, is crucially affected by the molecule-surface chemical bonding, by the alignment of the molecule and substrate electronic levels, as well as by their spatial localization and degree of mixing. These features, in turn, are closely related to the atomic scale details of the molecule-substrate morphology such as the molecule adsorption sites on the surface and the molecule orientation and distance with respect to the substrate. In this concern, we have investigated the structural, electronic and optical properties of the ZnPc/ZnO system by using ab initio density functional theory (DFT), and time-dependent density functional perturbation theory (TDDFPT) methods. 14 Our results show that: (i) the molecule-surface interaction is characterized by large van der Waals (VDW) forces and, in the most stable adsorption sites on the ZnO surface, by the formation of an appreciable chemical bond between the Zn of ZnPc and an O atom of the ZnO surface. Interestingly, such twofold contribution results in a quite large molecule-to-surface adsorption energy, thus indicating that an appreciable molecular adhesion to the surface can be achieved without modifying the molecular architecture with suitable, anchoring functional groups. (ii) the adsorbed molecule polarizes its electronic clouds towards the surface atoms. At the same time, it displaces some electronic charge from molecular regions facing surface O rows to regions facing surface Zn atoms, acting therefore as a sort of bridge-connector which tends to balance O Dimers Row Zn Trench Grove [0001] [0001] [1010] [1210] Fig. 1 Side and top view of the ZnO (10 10) surface showing the alternation of trench groves and rows of dimers along the [1 210] direction. Charge density plots corresponding to unoccupied dangling bonds on the topmost Zn row and occupied dangling bonds on the topmost O rows are shown in the left and right part of the upper panel inset, respectively

3 Page 12 of 18 E (ev); He (1s) = 0.0 Non bonded ZnPc Bonded ZnPc LUMO (π ) 13.0 LUMO+1 Mixed LUMO 12.0 "CBM" "VBM" 10.0 LUMO+1 Mixed LUMO "CBM" 1.6 "VBM" LUMO 1.4 CBM VBM LUMO+1 Mixed LUMO CBM 1.2 VBM LUMO LUMO CBM CBM VBM (π) 9.0 (A) PBE ZnO cluster/znpc (B) HSE (C) PBE (D) PBE ZnO surface/znpc VBM (E) HSE (F) PBE Separated ZnO surface/znpc Fig. 2 Left: ψ 2 plots of the π highest occupied molecular orbital () and of the π doubly degenerate lowest unoccupied molecular orbital (LUMO) of an isolated ZnPc molecule. Right: Electronic eigenvalues calculated at the Γ point in the case of: (A-B) a ZnPc molecule interacting with a 44-atom ZnO cluster; (A): GGA-PBE calculation; (B): Hybrid-HSE calculation. (C-D) GGA-PBE calculations of a ZnPc molecule interacting with a 432-atom ZnO surface; (C) ZnPc non bonded to the ZnO surface (i.e., kept far from the surface); (D) ZnPc molecule bonded to the ZnO surface. (E-F) a ZnPc molecule and a 24-atom ZnO surface accommodated in separated supercells; (E): GGA-PBE calculations; (F): Hybrid-HSE calculations. ψ 2 plots of the split LUMO orbitals (mixed ZnO/ZnPc and LUMO+1 (π levels) are shown in Fig. 5. 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. CBM and VBM labels have been used in the case of ZnO clusters for the sake of clarity

4 Page 13 of 18 Journal of Materials Chemistry the charge distribution on the ZnO surface. (iii) as a major result, the electronic structure of the ZnPc/ZnO system reveals the existence of an unoccupied level deriving from an intimate mixing of ZnPc and ZnO electronic states and strategically located within the ZnO conduction band and below the ZnPc LUMO, which could significantly favor a molecule to surface transfer of photo-excited electrons. Accordingly, absorption spectra calculated for the isolated ZnPc and the molecule adsorbed on the ZnO surface indicate that this unoccupied level is involved in an electronic excitation from the ZnPc, which corresponds to a new line red-shifted with respect to the original ZnPc lines. 2 Theoretical Framework Ab initio calculations have been performed in the framework of Density Functional Theory methods by using the Quantum- ESPRESSO package 14. Total energies have been calculated by using ultrasoft pseudopotentials 15 for all atoms but Zn, whose electronic core was represented by a norm-conserving pseudopotential 16. 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. Particular care has been given to the effects of dispersive interactions. In this regard, the exchange-correlation functional has been constructed by adding an ab initio non-local van der Waals correlation functional (VDW-DF 17,18 ) to the semilocal gradientcorrected GGA-PBE functional. 19 Careful checks have been performed in the case of such a rather new VDW-DF implementation. Regarding ZnPc gas-phase dimers, results given by the present VDW-DF approach have been compared with empirical model-potential molecular dynamics (MPMD) results 20 and with results obtained by using a semiempirical dispersion correction (DFT-D2 21,22 ) applied to the PBE calculation. All the techniques find an almost identical value in the case of the interaction energy of a ZnPc dimer (1.6 ev), in line also with previous results achieved with a different ab initio approach. 23 Moreover, regarding molecule-surface interactions, first, the present VDW-DF approach has been already successfully checked against several molecule-surface systems. 24 Second, we have performed DFT-D2 calculations of the ZnO/ZnPc system, finding a quite high absorption energy value of 5.7 ev, to be compared with the corresponding VDW-DF estimate of 3.8 ev. As the semiempirical DFT-D2 is known to overestimate the molecule surface binding energy, 22 we can regard the present results as quite reliable for providing an accurate estimate of the structural properties of the ZnO/ZnPc system. The ZnO (10 10) surface cell has been modelled by adding 15 Å of empty space to a 4 9 crystal slab, formed by 6 atomic layers of bulk ZnO (432 atoms) cut along the [10 10] crystal axis. Different starting molecule-surface configurations have been considered, on the ground of previous MPMD calculations. 20 These starting configurations have been given as input to geometry optimization procedures which 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 slab. The electronic properties of the ZnPc/ZnO system have been investigated by analyzing the electronic eigenvalues calculated at the Γ point. Negligible differences have been obtained by using a Γ-centered k-point mesh. In principle, Kohn-Sham eigenvalues related to isolated (e.g., molecules) and extended (e.g., surface slabs) systems as given by the GGA-PBE functional can be affected at different extents by delocalisation errors which, in particular, lead to underestimate the band gap or the -LUMO gap. 25,26 On the other hand, a correct alignment of electronic eigenvalues is crucial to provide a reliable estimate of the electronic and optical properties of a D-A interface. Thus, we have performed a series of calculations which carefully compare results achieved by the above PBE approach with results obtained by using an accurate hybrid HSE approach, 27,28 which have been summarized in Fig. 2. HSE calculations have been performed by using norm-conserving pseudopotentials 16 and cutoffs of 80 Ry on the plane waves and of 320 Ry on the electronic density. In detail, we have performed two sets of calculations. First, we have investigated both at the PBE and HSE levels the properties of the ZnO surface and of the ZnPc molecule as separated systems. The ZnO surface has been modelled by a small 1 2 crystal slab, formed by 6 atomic layers of bulk ZnO (24 atoms) cut along the [10 10] crystal axis. Satisfactorily converged results have been achieved by using a Γ-centered k-point mesh. A similar setup has been used to calculate the electronic properties of an isolated ZnPc molecule, but for the sampling of the Brillouin zone, restricted to the Γ point. The calculated PBE and HSE band edges have been aligned with the calculated PBE and HSE ZnPc frontier orbitals by using the 1s level of a He atom inserted as a reference in all the supercells. Then, we have also modelled a stoichiometric 44-atom ZnO cluster cut out from the (10 10) surface slab and investigated its interaction with a ZnPc molecule by performing both PBE and HSE calculations. We can anticipate that PBE results closely agree with the HSE ones, thus justifying the choice of a PBE approach in the case of the large investigated molecule-surface systems investigated in the present study. Moreover, a recent experimental investigation reported in the case of a similar CuPc/ZnO interface 29 indicates that the measured alignment between Pc molecular orbitals and ZnO band edges is in a quite good agreement with the present results. Optical absorption spectra ranging from the near-ir to the near-uv regions have been calculated in the case of an iso

5 Page 14 of 18 O Zn C N H [0001] [0001] [1010] [1210] Fig. 3 Top and side views of the minimum energy configuration of an isolated ZnPc molecule adsorbed on the ZnO (10 10) surface. lated ZnPc molecule and of the most stable ZnPc/ZnO interacting system by using a recent approach to the solution of the Bethe-Salpeter equation within the framework of density matrix perturbation theory. 14,30 Such an approach is expected to provide reliable results when applied to large systems, including sensitized metal oxides. 31 A reduced 6-layer 3 5 ZnO slab (180 atoms) has been used in the case of TDDFPT calculations to simulate the molecule-surface system. A careful comparison between DFT results indicates that the structural and electronic properties of the reduced ZnO/ZnPc system are in good agreement with the ones obtained in the case of 4 9 slab. 3 Results and Discussion Two almost degenerate configurations of the adsorbed molecule have been found by DFT calculations in agreement with the above mentioned MPMD results 20 : the minimum energy configuration, shown in Fig. 3, and a second configuration where the molecule is slightly rotated and shifted by 0.8 Å (not reported here). The two configurations are characterized by very similar electronic properties; we discuss henceforth the results achieved for the former one. The molecule binds to the surface by a combination of two joined interactions: the formation of a chemical link involving the molecular Zn and one of the threefold coordinated O atoms belonging to the surface dimers rows (see Fig. 1), and a face-to-face interaction primarily due to strong VDW forces arising between the quite delocalised electron states which characterize both the molecule and the surface. A first evidence of the formation of a chemical bond between the molecular Zn and the surface O atoms is given by the difference density plot (ρ diff ) shown in the upper panel of Fig. 4. The electronic density of the ZnPc only and the analogue for the ZnO surface, both blocked in the geometry of the interacting configuration, have been subtracted from the electronic density of the ZnO/ZnPc system. Thus, a plotted ρ diff in Fig. 4 indicates the charge displacements induced by the interaction between the molecule and the surface. In the present case, the upper panel of Fig. 4 indicates that an amount of charge density is displaced from molecular Zn-N orbitals and from the O antibonding region (blue isosurfaces) to the Zn-O interatomic region (red isosurfaces). Even the geometry of the ZnPc/ZnO system supports the formation of an appreciable Zn-O chemical bond: a Zn-O distance of 2.07 Å estimated in the case of the most stable configuration should be compared to the 1.97/1.99 Å Zn-O distances calculated in wurtzite bulk ZnO crystal. Finally, we have estimated an adsorption energy (E ads ) value of 3.8 ev for the ZnPc molecule on the ZnO surface. This value includes the contribution of both the chemical bond and the VDW forces. However, regarding the former contribution, by switching off the long range VDW correlation contribution (i.e., by using a non-corrected PBE functional) we estimate an E ads of 0.9 ev which can be considered as a lower limit of the Zn-O bond strength due to the concomitant occurrence of a molecule-surface repulsion (our results show that, in absence of the VDW contribution, the molecule bends upward, almost detaching from the surface but without breaking of the Zn-O bond). On the side of the electronic properties, first, let us focus again on the ρ diff plots displayed in Fig. 4. The left and right lower panels show side and top views of charge accumulation (red isosurfaces) and charge depletion (blue isosurfaces) regions induced by the ZnPc-ZnO interaction, respectively. By neglecting the contributes assigned to the formation of a Zn-O bond, the red and blue arrows and rectangles indicate that an amount of electronic charge is subtracted from the molecular π cloud in the zones where it faces surface O atoms (corresponding to the stripes of occupied dangling bonds in Fig. 1), and added to π cloud zones facing surface Zn atoms (corresponding to stripes of empty dangling bonds in Fig. 1). Then, the ZnPc seems to be highly polarised when chemisorbed on the ZnO surface and to act as a sort of molecular connec

6 Page 15 of 18 Journal of Materials Chemistry N C H O Zn Zn O N C H Fig. 5 ψ 2 plots of the split LUMO levels, the mixed ZnO/ZnPc level (left panels) and of the LUMO+1 level (right panels), induced by the ZnPc-ZnO strong interaction. The positions of these levels with respect to the ZnO bands as well as to the original -LUMO levels of an isolated ZnPc molecule are shown in Fig. 2. The electronic densities have been sampled at e/a.u. 3. Fig. 4 Difference density plots showing the displacements of electronic charge induced by the ZnO-ZnPc interaction. Blue isosurfaces cover regions where the difference is negative, red isosurfaces where it is positive, that is, charge density flows from blue regions to red regions when the molecule-surface interaction is turned on. Details of the electronic charge displacement towards the interatomic Zn-O region due to the formation of a molecule-surface bond are shown in the upper panel (electronic densities sampled at e/a.u. 3 ). An overall picture of the molecule-surface charge displacement (with red zones and blue zones separately drawn, for the sake of clarity) is displayed in the lower panels (electronic densities sampled at e/a.u. 3 ). Rectangles and arrows indicate that the electronic charge is displaced from molecular regions facing topmost O rows (blue) to molecular regions facing topmost Zn rows (red). tor between locally negative and positive surface sites. The same plots indicate that a fraction of the ZnPc charge density is shared with the ZnO surface, in agreement with the donor tendency of ZnPc and the acceptor tendency of ZnO. Such a resulting strong molecule-surface coupling has important effects on the electronic structure of the whole system, as shown by a comparison between the electronic eigenvalues calculated for the non-interacting and interacting ZnPc-ZnO systems, see the Non-Bonded ZnPc and Bonded ZnPc sketches in Fig. 2 (C) and (D), respectively. In the former case, Fig. 2 (C), the electronic structure of the molecule-surface system is very close to that corresponding to the sum of the electronic levels of the isolated components (Fig. 2 F). On the other hand, new electronic features characterize the interacting system, see Fig. 2 (D): (i) an appreciable lowering of both the and LUMO, which agrees with the above charge density displacements 32 ; (ii) a splitting of the doubly degenerate molecular LUMO in two contributions, the former substantially retaining its molecular character, see the LUMO+1 plots in the right panels of Fig. 5, the latter presenting two very interesting features: it originates from a strong mixing of the ZnPc LUMO and the ZnO levels, as clearly shown by the ψ 2 plot in the left panels of Fig. 5, and it is located about 0.2 ev below the native ZnPc LUMO but still above the ZnO conduction band minimum (CBM). In order to further support the above theoretical picture, we have performed other calculations by comparing the GGA

7 Page 16 of 18 PBE results with accurate hybrid-hse simulations. First, we have investigated the alignment of electronic eigenvalues corresponding to a ZnPc molecule separated from a ZnO surface slab, as given by HSE and PBE calculations, see Figs. 2 (E) and 2 (F), respectively. HSE calculations are able to recover an almost perfect agreement with the experimental ZnO band gap and ZnPc optical absorption edges. They provide also results in agreement with the above cited experimental measurements related to a similar CuPc/ZnO system. 29 Even the PBE results provide a correct, although less accurate, description of the eigenvalues line-up. Then, the change of the ZnPc electronic properties occurring when the molecule interacts with the ZnO surface has been checked by performing a further set of HSE and PBE calculations where the ZnPc molecule is bonded to a small ZnO cluster cut out from a larger ZnO (10 10) surface slab, see Figs. 2 (A) and 2 (B), respectively. Even in this case, the ZnPc molecule forms a strong Zn O chemical bond. Moreover, both PBE and HSE results describe in the same way the evolution of the electronic structure of the isolated ZnPc induced by its interaction with the ZnO cluster (compare Figs. 2 (A) and 2 (B) with Figs. 2 (E) and 2 (F), respectively) and closely agree with the picture given by the PBE results in the case of a ZnPc interacting with the ZnO surface, see Figs. 2 (C) and 2 (D). The ZnO(cluster)/ZnPc results reproduce indeed the -LUMO lowering, the splitting of the LUMO orbitals, and the mixing between ZnPc LUMO and the ZnO conduction band. The above strong molecule-surface coupling affects even the optical properties of ZnPc molecules in contact with the ZnO surface. In the case of an isolated ZnPc molecule, a calculated TDDFPT absorption spectrum is in a very good agreement with previous UV-VIS spectra measured for gas-phase ZnPc molecules 33 (see the blue curve in Fig. 6). In detail, two main strong absorption peaks have been calculated, the so-called Q band and Soret band, which fall in the red part of the visible region (1.9 ev) and in the near UV region (3.4 ev), respectively. When the ZnPc molecule is chemisorbed on the ZnO surface, both the spectral features are broadened and red-shifted with respect to the gas-phase spectrum (see the black curve in Fig. 6). The contribution of the ZnO slab has been separately calculated and subtracted by the ZnPc/ZnO spectrum, because of the generally acknowledged inaccurate contribution of periodic systems to TDDFT calculations, when performed at the adiabatic generalized gradient approximation (A-GGA) level. 34,35 Slight shifts between such ZnO-related contributions produce wide oscillation in the ZnPc-related features; we have therefore smoothed the black curve by using the stable Casteljau s algorithm: this procedure leads to draw the red Bézier curve shown in Fig In the case of the Q band, the calculated red shift (from 1.9 ev to 1.7 ev) is in a good agreement both with the above discussed DFT indications of a red-shifted LUMO arising from the ZnPc/ZnO interaction I( ω ) (arb. un.) Gas Phase ZnPc ZnPc/ZnO ZnO Bezier Curve Q ω (ev) Fig. 6 Blue curve: TDDFPT absorption spectrum of a gas-phase ZnPc molecule. The typical strong Q and Soret phthalocyanine bands are labelled in the figure. Black curve: TDDFPT absorption spectrum of the ZnPc molecule strongly chemisorbed on the ZnO surface. The plot has been obtained by subtracting out the ZnO slab contribution from the ZnPc/ZnO spectrum, in order to neglect the ill-defined contribution to the spectrum coming from the surface slab. Red curve: A Bézier curve has been obtained from the black curve by applying the Casteljau s algorithm to gain cleaner indications of spectral changes induced by the molecule-surface interaction. The red shifts of both the Q and Soret bands are indicated by red arrows. S 1 9 7

8 Page 17 of 18 Journal of Materials Chemistry (i.e., the mixed ZnPc/ZnO level), and with experimental data obtained in the case of ZnPc-impregnated ZnO nanocrystals. 4 The above results suggest that the ZnO surface can be successfully sensitised by means of the adsorption of ZnPc molecules in view of an improvement of different kinds of HPV device. As an example, ZnPc molecules (or similar analogues) could act themselves as dyes for light harvesting in novel, solid-state architectures of DSSCs. In this regard, a key role can be played by the mixed ZnPc/ZnO orbital sketched in Fig. 2 and shown in Fig. 5. In fact, in the case of e h + pairs photogenerated inside the ZnPc molecules by π π - LUMO excitation processes (the above mentioned Q-bands, dominating the visible absorption spectra of phthalocyanine molecules), such an orbital can provide an easy route to transfer the electrons to the ZnO conduction band. Moreover, the simulated optical spectra in Fig. 6, show that the above mixed level can be involved in direct electronic excitation from the ZnPc, corresponding to the line red-shifted with respect to the Q band of the isolated molecule, further favoring the transfer of a photoexcited electron. It should be also considered that, as mentioned above on the ground of calculated E ads values, a stable adhesion of the ZnPc to the ZnO surface should be achieved without requiring molecular anchoring groups. As a second example for a different application, a ZnO- ZnPc sensitised photoanode could be coupled to a π- conjugated conducting polymer in a three-component photoactive junction of interest for nanostructured-heterojunction polymer-based solar cells. ZnPc molecules and similar related compounds provide indeed a functionalisation layer which may improve the efficiency of such kind of cells by (i) extending towards the red or near-ir the absorption window 5 of photosensitive polymers; (ii) providing an active electronic spacer which separates electrons running through the ZnO CB and holes running along the polymer orbital 1 ; (iii) acting as a bridge-connector between negative and positive regions of the ZnO surface in order to build a non-polar interface between a sensitised ZnPc/ZnO A system and a D polymer which may improve the chemical affinity between these two components. 4 Conclusions In conclusion, the hybrid ZnPc/ZnO system has been investigated in a combined DFT and TDDFPT approach. The achieved results provide a detailed description of the structural and electronic molecule-surface coupling and reveal the existence of unoccupied electronic levels which, due to their nature and location, can play a key role in the optical absorption and molecule-to-surface electron transfer, thus significantly affecting processes at the core of the HPV-devices operation. The achieved results also indicate that an appreciable molecular adhesion should be reached without requiring the insertion of anchoring groups in the molecular structure. All together present results indicate that ZnPc-sensitized ZnO surfaces show structural and electronic properties of high potential interest for improving the efficiency of different kinds of organic-inorganic hybrid photovoltaic cells, like DSSC or nanostructured hetero-junction photovoltaic devices. Acknowledgement We are glad to thank D. Rocca for the excellent technical support and useful discussions on TDDFPT calculations, and C. Melis, L. Colombo and A. Mattoni for early sharing of MPMD results on the ZnPc/ZnO interaction. We also thank A. Paoletti, G. Pennesi, G. Rossi and G. Zanotti for useful discussions on phthalocyanine optical absorption spectra. We acknowledge computational support by the CINECA consortium (grant IscrA SIMuLATe), by the CASPUR consortium (grant std11-478), and financial support by the Italian Institute of Technology (IIT) under Project SEED POLYPHEMO. Notes and references 1 A. C. Mayer, S. R. Scully, B. E. Hardin, M. W. Rowell and M. D. McGehee, Mater. Today, 2007, 10, M. Grätzel, Acc. Chem. Res., 2009, 42, J.-L. Bredas, J. E. Norton, J. Cornil and V. Coropceanu, Acc. Chem. Res., 2009, 42, C. Ingrosso, A. Petrella, P. Cosma, M. L. Curri, M. Striccoli and A. Agostiano, J. Phys. Chem. B, 2006, 110, M. Garcìa-Iglesias, J.-J. Cid, J.-H. Yum, A. Forneli, P. Vàzquez, M. K. Nazeeruddin, E. Palomares, M. Grätzel and T. Torres, Energy Environ. Sci., 2011, 4, A. W. Hains, Z. Liang, M. A. Woodhouse and B. 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