Role of Molecular Thermodynamical Processes at Functionalized

Article pubs.acs.org/JPCC

Role of Molecular Thermodynamical Processes at Functionalized Polymer/Metaloxide Interfaces for Photovoltaics G. Malloci,† M. Binda,‡ A. Petrozza,‡ and A. Mattoni*,† †

Istituto Officina dei Materiali (CNR-IOM), Unità di Cagliari, Cittadella Universitaria, I-09042 Monserrato (Cagliari), Italy Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, I-20133 Milano, Italy

‡

S Supporting Information *

ABSTRACT: We present a combined theoretical and experimental investigation of the role of pyridine-based functionalizers (IMs) at the hybrid titania (TiO2)/poly(3-hexylthiophene) (P3HT) interface. We first used density functional theory to study the electronic structure and adsorption energy of several IMs, isolated and self-assembled, on a TiO2 anatase (101) surface. Small details in the molecular structure are found to induce strong differences in the morphology of the corresponding self-assembled monolayer. We then used model potential molecular dynamics simulations to study the translational and rotational diffusion of a P3HT oligomer on the naked and functionalized TiO2 surfaces. We correlate the photovoltaic performance measured on TiO2/IM/P3HT with the degree of order of the interface. In particular, we find that the thiol group of 4-mercaptopyridine is able to stabilize the corresponding self-assembled interlayer. This, in turn, increases the polymer mobility on the inorganic surface, yielding a larger polymer/substrate interface area.

1. INTRODUCTION The use of molecular interface modifiers (IMs) has proven to be an effective way to improve the performances of functional interfaces for photovoltaics (PV).1 In the field of hybrid PV, in particular, IMs have been reported in a large number of studies showing a critical effect on the macroscopic device performances.2 Different kinds of IMs have been proposed over the years: organic3 or inorganic4 compounds, optically active5 or optically inert,6 and physisorbed7 or covalently bonded.8 The main motivations addressed in the literature for using IMs in the development of hybrid PV devices are1,9 increasing the polymer/metaloxide interaction; enhancing charge transport; adjusting the work function of the substrate by introducing molecular dipoles; contributing to light harvesting; and reducing charge carriers recombination. The presence of IMs can directly affect the morphology of the interfaces but an accurate characterization at the nanometric scale is difficult and still remains a challenge despite the working mechanisms of excitonic solar cells are strongly dominated by the materials interplay and self-organization at the molecular level. The pyridine molecule (PYR, C5H5N, see Figure 1a) and its derivatives such as 4-tert-butylpyridine (TBP, C9H13N) have been largely employed to optimize the performances of hybrid solar cells. PYR, for example, is used in the ligand exchange treatment of colloidal TiO2 nanoparticles to replace the insulating oleic acid covering the metaloxide after synthesis.3,8,11 By enhancing charge separation and suppressing back recombination, PYR modification enabled a power conversion efficiency improvement from 0.38% to 1.14% in bulk heterojunction PV devices based on TiO2 nanorods and regioregular poly(3-hexylthiophene) (P3HT).12 The TBP derivative is well-known in the field of electrolyte based and © 2013 American Chemical Society

solid-state dye sensitized solar cells for its role as semiconductor potential determining additive and surface passivating agent.13−15 Other PYR derivatives that have resulted in interest for PV applications are 4-mercaptopyridine (4MP, C5H5NS) and 2-mercaptopyridine (2MP, C5H5NS); they consist of pyridine rings with a thiol group in the position ortho and para to the nitrogen atom, respectively (see Figure 1a). These molecules are known to form well-ordered selfassembled monolayers on silver16 and gold17 surfaces. A recent work on mesoporous films of TiO2 infiltrated by P3HT6 shows that the use of 4MP as IM, though optically transparent (see Figure 1b), is able to enhance the photocurrent of the cell, inducing a large improvement in the overall power conversion efficiency for this class of devices. At variance, 2MP is found to be detrimental for photovoltaic applications, inducing a barrier to charge injection.6 The above results show that pyridine-based IMs have a profound influence on the optoelectronic properties of the hybrid interface. However, there is no clear understanding of their role at the molecular level, which allows to disentangle electronic effects from morphological features. In particular, despite their large use in the field of hybrid PV, the correlation between IMs self-assembling and supramolecular order and the PV performance measured on actual devices has never been reported so far. In this work, we adopt a combined experimental and multiscale computational study to investigate the effect of PYR, TBP, 4MP, and 2MP as IMs of the TiO2/ P3HT interface. This solar cell system uses a conjugated Received: April 30, 2013 Revised: June 12, 2013 Published: June 13, 2013 13894

dx.doi.org/10.1021/jp4042759 | J. Phys. Chem. C 2013, 117, 13894−13901

The Journal of Physical Chemistry C

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Figure 1. (a) Sticks and balls representation of the interface modifiers considered: pyridine (PYR), 4-tert-butylpyridine (TBP), 4-mercaptopyridine (4MP), and 2-mercaptopyridine (2MP). (b) Computed density of states of each molecule; the experimental optical gap of pyridine is taken from ref 10.

adhesion on nanostructured TiO2.22 To describe PYR, 4MP, 2MP, TBP, and P3HT, we used the AMBER force field23 that includes either bonding (stretching, bending, and torsional) and nonbonding (van der Waals plus Coulomb) contributions. The interatomic potential for the TiO2/P3HT, TiO2/IMs, and IMs/P3HT interactions is a sum of Coulomb and LennardJones contributions; the Lennard-Jones parameters were taken from the AMBER database.23 The atomic partial charges for PYR, TBP, 4MP, 2MP, and P3HT were estimated by DFT calculations with the ESP Kollmann procedure using the TURBOMOLE code.24 The same code has been used to compute the dipole moment of each molecule. The velocity Verlet algorithm with a time step of 1 fs was used to solve the equations of motion. Long-range electrostatic forces were evaluated using a 3-D particle mesh Ewald algorithm.25 A 9.5 Å cutoff was used to accurately calculate the van der Waals interactions. The simulation protocol used for geometry optimization consisted of a low-temperature annealing as long as 0.1 ns followed by a conjugate gradients relaxation. Finite temperature calculations consisted of 2 ns long annealing at room temperature within the NVT ensemble (Berendsen thermostat). The above computational scheme has been previously validated against available experimental and ab initio data.22,26 DFT calculations have been performed using a planewave pseudopotential approach as implemented in the QUANTUM-ESPRESSO package. We used the Perdew− Burke−Ernzerhof (PBE) exchange-correlation functional27 and ultrasoft pseudopotentials.28 Dispersion corrections have been included according to Grimme’s parametrization.29,30 Satisfactorily converged results have been obtained by using kinetic energy cutoffs of 25 Ry for wave functions and of 200 Ry for charge density. The k-point sampling of the Brillouin zone was limited to the Γ point. For geometry optimizations, we used the Broyden−Fletcher−Goldfarb−Shanno algorithm, with threshold values of 0.0026 eV/Å and 1.4 × 10−4 eV for residual forces and energy variation, respectively. The anatase (101) surface was modeled with a periodically repeated slab cut from the bulk. The optimized bulk lattice parameters (a,b,d = 3.79, 9.74, 2.01 Å) are consistent with previous calculations (a,b,d = 3.786, 9.737, 2.002 Å31,32) and agree with experimental data (a,b,d = 3.782, 9.502, 1.979 Å33). To simulate IMs interacting with the anatase (101) surface, we used supercells containing four atomic layers (72 atoms, 5.9 Å thick) separated by a vacuum of ∼15 Å. The surface has dimensions 2 × 3 in the

polymer behaving both as light antenna system and hole transporting layer, while electrons are injected in the high dielectric, high mobility metal oxide, thus reducing the complexity and related intrinsic losses (e.g., in photovoltage) of dye sensitized solar cells in a two-component solid-state device. In spite of this, however, hybrid solar cells of this kind have not expressed their full potential yet, and even more striking, the reason why they are so inefficient is still not clearly understood.18 We first use density functional theory to study the electronic structure and adsorption energy of the IMs, both isolated and self-assembled, on a TiO2 anatase (101) surface. We show that small details in the molecular structure of the IM imply different adhesion configurations on the metaloxide surface. These configurations, in turn, induce strong differences in the morphology of the corresponding self-assembled monolayer. We then use model potential molecular dynamics to study the thermodynamics of the systems at finite temperature. In particular, we study the translational and rotational diffusion of a P3HT oligomer on the naked and functionalized TiO2 surfaces. Interestingly, we found a tight correlation between the PV performances measured on P3HT/ IM/TiO2 based solar cells and the degree of order of the functionalized interface. Overall, our combined theoretical− experimental work demonstrates that, besides the electronic properties of photovoltaic interfaces, which have often been taken into account, thermodynamical effects such as selfassembling and supramolecular order are critical to predict the properties of these interfaces and to infer new routes for improving hybrid solar cells. Along these lines we indentify specific molecular groups, which are able to stabilize homogeneous and ordered interlayers that increase the polymer mobility on the inorganic surface, thus yielding a larger and optimized polymer/substrate interface area.

2. METHODS 2.1. Computational Methods. We used a combination of model potential molecular dynamics (MPMD) and density functional theory (DFT) calculations.19 MPMD calculations have been performed using the DLPOLY code20 by combining existing force fields and by adding long-range Coulomb and dispersive contributions to model interactions across the hybrid interface. The interatomic potential for TiO2 is described by the sum of Coulomb and Buckingham-type two-body contributions,21 which have been successfully applied to study P3HT 13895



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Figure 2. (a) Minimum-energy configurations for the pyridine-based IMs considered, as computed by DFT; the 5-fold coordinated Ti atoms of the substrate are marked in violet. (b) Charge redistribution ρdiff obtained as ρTOT − (ρSUB + ρIM), ρTOT, ρSUB, and ρIM being the electronic charge densities of the interface and the isolated TiO2 and IM, respectively. Green isosurfaces correspond to regions where ρdiff is positive, blue isosurfaces where it is negative, showing that charge density moves from blue to green regions (isovalue equal to ±2.02 × 10−2 electrons Å−3 for green/blue).

without further purification) from 30 mg mL−1 chlorobenzene solution was spin coated onto the substrates at 1000 rpm for 60 s. Spin coater rotation was activated 30 s after dispensing the solution onto the substrate in order to promote effective polymer infiltration inside the mesoporous TiO2 layer. In the case of samples without the interlayer, substrates were reheated at 200 °C for 30 min and then cooled down to 70 °C right before spin coating the P3HT, to enable residual water expulsion from the mesoporous TiO2 layer. Finally, ∼80 nm thick silver electrodes were thermally evaporated under high vacuum (10−6 mbar) onto the samples through a suitable shadow mask defining a device active area of ∼2 × 4 mm2. Current density−voltage (J−V) characteristics were measured by a Keithley model 2400 digital source-meter exposing the cell to a class AAA Newport Air Mass 1.5 Global (AM 1.5 G) full spectrum solar simulator. The power of incoming radiation was set at 100 mW cm−2 using a NREL calibrated silicon solar cell.

[101̅] and [010] directions, respectively, corresponding to a surface area of 11.37 × 10.45 Å . During geometry optimizations, the atoms in the bottom layer were fixed to their bulk positions. Test calculations with a surface of six atomic layers (108 atoms, 9.4 Å thick) did not show any significant difference. Molecular graphics have been generated by using the XCRYSDEN34 and VMD35 packages. 2.2. Experimental Methods. Solar cell devices were prepared as follows. Fluorine doped tin oxide (FTO) coated glass substrates (15 Ω per sq Pilkington) were etched with zinc powder and HCl (2.4 M) in water solution to define the required pattern for the device’s bottom electrode. Careful washing of the substrates following the etching step was performed with bidistilled water, acetone, and isopropanol. Oxygen plasma treatment was finally performed for 5 min to remove the last traces of organic residues. The substrates were then covered with a compact layer of TiO2 (∼100 nm-thick). Deposition of compact TiO2 was made by spray pyrolisis at 400 °C with oxygen as carrier gas, starting from a 1:10 by volume titanium diisopropoxide bis(acetylacetonate)/ethanol solution. A commercial Dyesol TiO2 paste (DSL 18NR-T), previously diluted in terpineol and ultrasonicated until complete mixing, was screen printed onto the compact TiO2 layer to get a mesoporous film of an average thickness of 800 nm. The substrates were then slowly heated to 550 °C (ramped over 1 1/2 h) and baked at this temperature for 30 min in air. After cooling, the substrates were soaked in TiCl4 solution (15 mM in water) and oven-baked for 1 h at 70 °C. After oven-baking, the substrates were rinsed with bidistilled water, dried in air, and baked again at 550 °C for 45 min. In the case of devices provided with 4MP(2MP) interlayer, after cooling down to ∼70 °C, the substrates were immersed in a saturated solution of 4MP(2MP) in chlorobenzene. In the case of devices with PYR(TBP) interlayer, after cooling down to ∼70 °C, the substrates were immersed in pure PYR(TBP). In both cases, substrate immersion was prolonged for several hours (about 20 h). All the substrates were then rinsed with pure chlorobenzene. P3HT (Mw = 20 000, purchased by Merck and used

3. RESULTS AND DISCUSSION 3.1. Adhesion of the IM on the Metaloxide Surface. To investigate the behavior of the different IMs, we first performed atomistic simulations of the molecule−substrate interaction in the framework of DFT. For each IM, we calculated the minimum-energy configuration of the isolated molecule adsorbed on the ideal TiO2 anatase (101) surface (see Figure 2a). All molecules efficiently bind to the surface by forming an effective Ti−N bond of ∼0.2 nm involving the active lone pair of the nitrogen atom and a 5-fold coordinated titanium atom of the substrate (marked in violet in Figure 2a). The adsorption energy Eads, computed by subtracting the energies of the IMs and the surface from that of the interacting system, is ∼1.2−1.3 eV for PYR, 4MP, and TBP and 1.0 eV for 2MP. Note that the formation of stable Ti−N bonds has been already reported in literature for N-containing heterocycles36,37 and for a broad class of organic dyes on TiO2.38 The lower Eads of 2MP is explained by the interaction of its thiol group with the substrate. As detailed below, only for the 2MP derivative there is an additional S−H−O hydrogen bond with the surface, that 13896



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Figure 3. (a) Projected density of states of the IM/TiO2 systems (continuous lines for interacting IM−TiO2, dashed lines for the separated components); the electronic eigenvalues have been aligned by using the 1s level of an He atom inserted as a reference in all the supercells.7 (b) Correlation between the TiO2 band edge shift ΔE and the molecular dipole component μz orthogonal to the substrate.

this is not the case for 4MP since an increase in both VOC and JSC have been reported on mesoporous TiO2.6 Therefore, the computed ground-state electronic properties alone do not permit a clear understanding of the higher power conversion efficiency measured in the presence of the 4MP interlayer in ref 6. There must be other physical properties responsible for the above observations; for example, an increase of the polymer/ substrate interface area was observed in the presence of 4MP.6 To clarify this point, given the known capability of these molecules to form self-assembled monolayers on metallic substrates,16,17 we extended the analysis of the single molecule to the case of monolayers. In fact, a high density of molecules on TiO2 anatase surface is expected due to the large computed binding energies (∼1.0 eV). In principle, all 5-fold coordinated titanium atoms can be passivated. In this ideal regime the density of molecules is as large as 8.4 μmol m−2, a value comparable with typical dye loadings.42 In order to investigate the monolayers and their thermodynamical stability, we used MPMD in combination with DFT. First of all, a molecular interlayer is obtained by replicating the molecules over all possible adsorption sites of the surface. The resulting systems are then fully optimized by a sequence of atomic relaxations followed by a 100 ps long annealing at 1 K. The same procedure is repeated for all IMs in order to compare the energetics of the different interlayers. Overall, as a consequence of the different adhesion configurations of the single molecule, we found three possible regimes, schematically depicted in Figure 4: (i) ordered monolayers, in the case of the planar PYR and 4MP molecules. As a result of the π → π intermolecular interaction, we observe the spontaneous formation of a homogeneous monolayer with ordered zigzag pattern, which remains stable at room temperature; (ii) partially ordered monolayers, like for TBP. Because of the long-range interaction between the butyl group and the substrate, the adsorbed molecule is slightly tilted (see Figure 2). In addition, the steric repulsion of the butyl groups (extending in the out-of-plane molecule by about 2.2 Å, more than half the distance between two adsorption sites on the surface, ∼1.9 Å) gives rise to the formation of partially covered layers, with a lower molecular density; (iii) disordered monolayers, as observed for 2MP. Though this molecule is planar like PYR and 4MP, the contemporary formation of a Ti−N bond and a S−H−O hydrogen bond gives rise to an anchoring to the surface that tilts the molecule (cf. Figure 2), thus hindering the formation of a homogeneous ordered monolayer. Moreover, the smaller adhesion energy makes possible molecule detachment and disordered films.

weakens the covalent Ti−N bond. The S−H−O interaction is furthermore responsible for a different molecular orientation, which is not perperdicular to the substrate as observed for PYR and 4MP (cf. Figure 2a). Also, the TBP is tilted due to the long-range interactions between the butyl group and the metaloxide surface. The different properties of the molecules are further studied by calculating the spatial distribution of the electronic charge density. In particular, Figure 2b reports for all systems the charge redistribution ρdiff upon formation of the hybrid interface. This quantity, obtained by subtracting the electronic charge densities of the isolated substrate ρSUB and of the interface modifier ρIM from that of the interacting system ρTOT (ρdiff = ρTOT − ρSUB − ρIM),7 is correlated to the charge redistribution induced by the molecule−surface interaction. Because of the larger electron affinity of titania, a fraction of the molecular electronic density moves toward the surface, thus contributing to the Ti−N bond. Consistently, the sums of the partial Löwdin charges of the PYR, 4MP, and TBP bound molecules are found to decrease after adhesion. The only exception is the 2MP molecule, whose net charge transfer to the substrate almost vanishes. Once more, we attribute the different behavior of 2MP to the S−H−O hydrogen bond where the O → S−H charge-displacement balances the charge transfer associated to the Ti−N bond. In agreement with the above results, as shown by the projected densities of states reported in Figure 3a, after anchoring to the substrate the HOMO and LUMO levels of each molecule are lowered with the only exception of the 2MP. The interaction between the molecule and the substrate is known to affect the metaloxide energy levels13,36,37,39,40 and has been explained in terms of molecular dipoles. For the cases here investigated, the molecular dipole points away from the substrate, and it upshifts the band edge of TiO2. In line with previous studies,36,37 we found a good correlation between the unpward shift of the TiO2 band edge (about 0.22, 0.45, 0.46, and 0.55 eV) and the dipole component orthogonal to the substrate (1.2, 2.2, 2.3, and 2.9 D) for 2MP, PYR, 4MP, and TBP, respectively (see Figure 3b). 3.2. Self-Assembled Monolayers of IMs. Transferring the above molecular information on the expected macroscopic behavior of the PV device, under the hypothesis that the polymer electronic levels are not strongly affected by the IM, it is expected an increase in the device’s open circuit voltage VOC in the order TBP, PYR, 4MP, and 2MP. This should also lead to a decrease in the generated photocurrent JSC, due to a reduction in the electron transfer driving force.1,41 However, 13897



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simulations by a subsequent DFT full geometry optimization. From one side these additional DFT calculations confirm the zigzag patterns of the PYR and 4MP monolayers; at the same time they also provide accurate information on the energetic stability of the monolayers. More specifically, if N is the number of molecules in the monolayer, we calculate the binding energy Ebind of the film as the difference between the adsorption energy of the monolayer Eml and that of the same number of single bound molecules Eads: Ebind = Eml − NEads. By definition, this quantity is positive when the monolayer is more stable than the single bound molecules, as a result of attractive interactions between neighboring molecules. Notably, we obtain different results between PYR and 4MP, namely, Ebind = 0.1 eV/mol (0.0 eV/ mol) in the 4MP (PYR) case. This reveals an energetic driving force for assembling molecules into monolayers that occurs only for the 4MP molecule and that can be therefore attributed to the thiol−thiol interaction within neighbors. 3.3. Interaction of the Polymer with the Functionalized Surface. The different morphologies of the interlayers are expected to have an impact on the interaction of the substrate with the polymeric component of the solar cell. We clarify this point by analyzing via MPMD the interaction and mobility of a single P3HT molecule on the different substrates (see Figure 4). We considered a P3HT oligomer formed by eight monomers saturated by methyl groups since previous investigations have shown that the energy per monomer of the P3HT octamer is close to the limit of an infinite chain.22 In real systems, the organic phase is an aggregation of many molecules on top of the surface. However, the interaction of the single polymer chain with the substrate is a key mechanism that deserves an extensive analysis.22,26 For all substrates, we found

Figure 4. Ternary systems P3HT/IM/TiO2 with molecular interlayers formed by the different IMs considered. Ordered monolayers are found for PYR and 4MP, a partially ordered monolayer for TBP, and a disordered one for 2MP. For clarity, all hydrogen atoms have been omitted and the carbon atoms of P3HT are marked in white to distinguish them from those of the interlayer.

The above analysis shows that small details of the molecular structure, such as the thiol group position in the pyridine ring, can induce strong differences in the interlayer morphology. Only PYR and 4MP give rise to high density stable films. We further validate this prediction obtained by means of MPMD

Figure 5. (a) Polymer trajectories on different substrates; the circles give a qualitative measure of the area swept by the polymer. (b) Distribution of the polymer orientation during dynamics on different substrates; (from bottom to top, TiO2, 2MP, TBP, PYR, and 4MP). 13898



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K) P3HT in order to avoid any pore filling issues (unpublished data). In Figure 6, we report the device parameters retrieved

that the polymer−substrate adhesion energy is larger than the polymer−polymer interaction energy (by at least 0.2 eV per monomer). When deposited on the substrate, therefore, the polymer is expected to increase the interface area with the substrate in order to minimize the total energy. The kinetics of this process will depend on the polymer mobility on the substrate, and the largest interface area is expected when the mobility of the single P3HT molecule is maximum. According to the above arguments, our idealized model of a single P3HT octamer diffusing on the different substrates can be seen as a physical probe, which gives insights on the effective interface area. Our calculations show that, at room temperature, the polymer binds to the substrate giving rise in all cases to a face-on configuration that is preserved during a 2 ns long thermal annealing. During the same annealing time, the polymer diffuses on the substrate making possible to study its diffusivity by analyzing its mean square displacement. In Figure 5a, we report the calculated trajectories of the polymer’s center of mass, representing its translational degree of freedom. The circles containing the trajectories are a clear and simple indication of the polymer’s mobility, which can be correlated to the structural order of the interlayers: it is larger for the ordered PYR and 4MP monolayers, it is intermediate for the partially ordered TBP monolayer, and it is small for the disordered 2MP monolayer and the naked TiO2 surface. Additional information on the diffusion mechanisms can be obtained by the analysis of the polymer rotational degrees of freedom. In particular, we focus on the polymer’s backbone orientation choosing the [100] crystallographic direction of the surface as the initial orientation and reference for angles. For the different IMs, the distribution η(θ) of the polymer orientations during the annealing process is reported in Figure 5b. η(θ) is found to be larger for 4MP and PYR surfaces and smaller for TBP, 2MP, and the bare TiO2 surfaces. Again, we find a correlation between the polymer’s angular mobility and the degree of order of the interlayer. In the case of 4MP, in addition, η(θ) is found to exhibit deviations to negative angles that are not observed for PYR. The smaller energy barrier for polymer migration in the presence of an ordered interlayer can be interpreted in terms of the different local charges and the corresponding electrostatic energy landscape seen by the polymer. The bare TiO2 surface shows positive and negative ions alternate at different heights, thus modulating the local electrostatic landscape. When the metaloxide surface is homogeneously functionalized the polymer backbone is spatially separated from TiO2 by about 1 nm, and the substrate results atomically flat with much smaller atomic partial charges and a corresponding smoother electrostatic energy landscape. Accordingly, the polymer easily migrates on the surface, yielding a larger polymer/substrate interface area. This is consistent with the experimental findings based on positron annihilation spectroscopy reported in ref 6 for P3HT on TiO2 and IM/TiO2. Summarizing these results, we infer that the ease of migration of the polymer on ordered 4MP, together with the spontaneous assembling of 4MP interlayer, is expected to yield a larger effective donor−acceptor interface for this specific interlayer. 3.4. Performances of P3HT/IM/TiO2 Solar Cells. To better rationalize the theoretical findings, P3HT/TiO2 solar cells presenting different IMs have been tested. In order to nail down the interface phenomena with respect to the bulk properties of the mesoporous structure, 800 nm thick TiO2 mesoporous films were infiltrated by low molecular weight (20

Figure 6. Electrical performances of P3HT/IM/TiO2 based solar cells with different molecular interlayers.

from a statistic investigation over 10 samples. In the Supporting Information, we report the J−V characteristic of one selected sample for each type of device, which responds to the average behavior observed. An inspection of Figure 6 shows that the order of the open circuit voltages VOC follows that of the computed shift of the TiO2 band edge and, according to Figure 3, that of the order of the interface dipoles. More specifically, devices made with the TBP IM display, with respect to the control sample, a strong VOC enhancement at the price of the extracted photocurrent JSC, which is the typical behavior 13899



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expected in the presence of a dipole at the interface.1,5 Notably, in the case of both PYR and 4MP IMs, while the VOC increases, the JSC also increases. This is suggesting that not only the energetic of the interface plays a role in the optimization of the device but also the simple morphology of a few molecular layers can be of paramount importance. In fact, the enhancement of the device photocurrent, together with an overall improvement of the device performances, correlates well with the degree of order of the interface. As extensively discussed in the previous sections, the PYR interlayer, and even more the 4MP one, allow for an easier migration of the polymer on the surface, which is mainly driven by a better self-organization of the molecule monolayer and by the nature of the functional group exposed to the polymer. This is expected, first of all, to yield a larger effective donor−acceptor interface, which will definitely improve the charge generation in the devices. Then, it will lead the polymer chain to assume an energetically more favorable molecular conformation, i.e., planar, face-on with respect to the surface, which supports the formation of a crystalline lamellar structure in the polymeric phase (UV−vis spectra taken on bilayer samples with only 10 nm thick polymer phase demonstrate the presence of a major disordered phase at the naked interface and the formation of a P3HT crystalline phase in the presence of the 4MP interlayer). As largely demonstrated by different studies on fully organic solar cells, the ordered crystalline phase will definitely have beneficial effects on the exciton diffusion length and on the local charge mobility at the interface, which will result in an enhancement in charge generation and reduction in charge recombination, respectively, as demonstrated macroscopically by the improvement in the solar cell photocurrent.

AUTHOR INFORMATION

Corresponding Author

*(A.M.) E-mail: [email protected]. Phone: +39 070 6754868. Fax: +39 070 6754892. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by the Italian Institute of Technology under Project Seed ″POLYPHEMO″, Regione Autonoma della Sardegna under L. R. 7/2007 (CRP-249078), MIUR Under PON 2007-2013 (Project NETERGIT), and Consiglio Nazionale delle Ricerche (Project RADIUS). We acknowledge computational support by IIT Platform ″Computation″, and CINECA. A.P. research has received funding from the European Union Seventh Framework Programme [FP7/ 2007-2013] under grant agreement 316494.

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4. CONCLUSIONS In conclusion, we reported a combined experimental and multiscale computational study to investigate the effect of different pyridine-based functionalizers of the TiO2/P3HT interface. By means of density functional theory calculations, we computed the electronic structure and adsorption energy of the IMs, both isolated and self-assembled, on a TiO2 anatase (101) surface. Small details in the molecular structure of the functionalizer imply different adhesion configurations on the metaloxide surface, inducing strong differences in the morphology of the corresponding self-assembled monolayer. Model potential molecular dynamics simulations have been employed to study the translational and rotational diffusion of a P3HT oligomer on the naked and functionalized TiO2 surfaces. Overall, we found that the ground-state electronic properties of the single molecule/TiO2 system can mainly lead to beneficial effects on the device photovoltage. Even so, only a fine-tuning at the atomistic scale of the supermolecular interactions at the interface can realistically lead to an overall improvement of the macroscopic device performances (i.e., power conversion efficiency). Moreover, among ordered interlayers, we identify the beneficial effect of thiol groups on the surface, with respect to hydrogens or other terminating chemical groups. S−H terminating groups are able to stabilize homogeneous and ordered films and increase polymer mobility on the inorganic surface, thus yielding a larger polymer/substrate interface area.

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S Supporting Information *

J−V curves of hybrid TiO2/P3HT solar cells. This material is available free of charge via the Internet at http://pubs.acs.org. 13900



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Role of Molecular Thermodynamical Processes at Functionalized

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