Surface Polarization Drives Photoinduced Charge Separation at the

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Surface Polarization Drives Photoinduced Charge Separation at the P3HT / Water Interface Edoardo Mosconi, Paolo Salvatori, Maria Ilenia Saba, Alessandro Mattoni, Sebastiano Bellani, Francesco Bruni, Beatriz Santiago González, Maria Rosa Antognazza, Sergio Brovelli, Guglielmo Lanzani, Hong Li, Jean-Luc Bredas, and Filippo De Angelis ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00197 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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Surface Polarization Drives Photoinduced Charge Separation at the P3HT / Water Interface Edoardo Mosconi,a,b Paolo Salvatori,a,b Maria Ilenia Saba,c Alessandro Mattoni,c Sebastiano Bellani,d,e Francesco Brunif, Beatriz Santiago Gonzalez,f Maria Rosa Antognazza,d,* Sergio Brovelli,f Guglielmo Lanzani,d,e Hong Li,g Jean-Luc Brédas,h Filippo De Angelis a,b*

a

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, 06123, Perugia, Italy. b c

d

D3-Computation, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.

Istituto Officina dei Materiali CNR-IOM SLACS Cagliari, Cittadella Universitaria 09042, Monserrato (CA), Italy.

Center for Nano Science and Technology@Polimi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy. e

f

g

Politecnico di Milano, Dip.to di Fisica, P.zza L. Da Vinci 32, 20133 Milano, Italy.

Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, via Cozzi 55, IT20125 Milano, Italy

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400. h

Solar & Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology – KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia. Corresponding Authors: Maria Rosa Antognazza, Center for Nano Science and Technology@Polimi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy. E-mail: [email protected] Filippo De Angelis, Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNRISTM, Via Elce di Sotto 8, 06123, Perugia, Italy. E-mail: [email protected].

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Abstract. Hybrid devices employing organic semiconductors interfaced with an aqueous solution represent a new frontier in bioelectronics and energy applications. Understanding of the energetics and photoinduced processes occurring at the organic-water interface is fundamental for further progress. Here, we investigate the interfacial electronic structure of poly-3-hexylthiophene (P3HT) sandwiched between an Indium Tin Oxide (ITO) electrode and a liquid water electrolyte. The aqueous solution is found to polarize the polymer outermost layers, which together with the polymer p-(photo)doping by dissolved oxygen, localizes photogenerated electrons at the P3HT/water interface, while holes can be transferred to the ITO electrode. Under illumination, the polymer/water interface is negatively charged, attracting positive ions from the electrolyte solution and perturbing the ion distribution in the aqueous solution. The observed mechanism is of general character and could underlie the behavior of a variety of devices characterized by an organic/water interface, such as prosthetic devices for artificial vision and organic-based systems for photoelectrochemical applications. Graphics for Table of Contents

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Over the past two decades, the science and engineering of organic semiconducting materials has very rapidly advanced, leading to the demonstration and optimization of a range of functional devices, including organic light-emitting diodes (OLEDs), field-effect transistors, photodiodes, and photovoltaic solar cells. This evolution has been made possible by the understanding and exploitation of the peculiar features of organic π-conjugated macromolecules, such as their broad optical absorption, charge transfer ability, largely tunable optoelectronic properties, and the ease of processability by different deposition techniques. A number of organics-based applications ranging from photoelectrochemical water splitting devices,1-5 to electrolyte-gated field effect transistors,6-7 and bioelectronics8-14 have been developed taking inspiration from the prototypical organic solar cell architecture, Scheme 1a. Differently from solar cells, these devices share the use of liquid electrolytes at the interface with a photoactive polymer, Scheme 1b, typically P3HT. A fascinating application of organic optoelectronics is lightinduced neural stimulation, a forerunner of the artificial retinal prosthesis for visual restoration.10-13 The corresponding devices have a typically simple structure, consisting of an Indium Tin Oxide (ITO) substrate, an organic photoactive layer (poly-3-hexylthiophene, P3HT, or a P3HT/fullerene blend) and an aqueous electrolytic solution, in which the neuronal cells can grow and live. It has been recently demonstrated that such devices can promote neuronal stimulation under pulsed light illumination.12 It should be mentioned here that the actual mechanism behind cell stimulation in

vitro and in vivo is still far from being understood, with alternative mechanisms, such as thermallymediated photoexcitation,15 Faraday currents net flow, or occurrence of specific photochemical reactions, currently under investigation.

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Scheme 1. a) Schematics of an organic solar cell, with main photoinduced electron/hole (e-/h+) transfer pathways. The device consists of a transparent electrode, typically a conducting indium-tin oxide (ITO) substrate; the photoactive layer, typically a donor (polymer or small molecule) / acceptor (fullerene derivative) heterojunction, responsible for charge generation and transport; and a metal counter-electrode. b) Hybrid organic/aqueous electrolyte devices, highlighting the (e-/h+) pathways upon illumination and some possible interfacial phenomena, comprising both capacitive charge accumulation and net electron transfer reactions.

A full understanding of the photo-electrochemical processes taking place at the organic/electrolyte interface is a crucial step to optimize all related devices. Limited work has been carried out on the characterization of the P3HT/water interface, in view of understanding the operating principles of organic-based water splitting devices.1, 5 Interfacial acid-base reactions at the P3HT surface have been proposed to drive photogenerated electrons from the bulk of the polymer to the surface, where hydrogen or oxygen reduction reactions occur.2,

16-17

Beyond these studies,

mainstream research was devoted to the investigation of the role of water impurities on the degradation of the organic layer, to test and improve the device temporal stability.18-20 We explore at various levels of complexity and on various spatial scales the P3HT/water interface, to probe the electronic modifications occurring to the polymer under illumination, as revealed by dedicated experiments. Notice that we are not simulating the carrier dynamics across the 4 ACS Paragon Plus Environment

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P3HT/water interface, rather we aim at understanding how the presence of the water environment affects the polymer energy levels. We have implemented different models of the P3HT/water system, spanning various space and time scales, generated by room temperature classical molecular dynamics (MD) simulations21 in which P3HT was represented by two interdigitated stacks of five chains embedded into a liquid water environment, Figure 1. Each polymer stack was represented by a unit cell of six thiophene units periodically replicated along the direction [010] or [100] directions, Figure 1 and Figure S1 in Supporting Information, thus representing a polymer chain of infinite length. We first focus on a “face-on” configuration of the polymer, exposing the (010) surface (corresponding to the thiophene planes) to the water solution. The interaction between water and P3HT is generally not specific, though thiophene can act as a hydrogen bonding π-acceptor.22 We observe that water molecules are preferentially oriented with hydrogen atoms (Hw) pointing towards the electron-rich thiophene rings, with average Hw-S and Hw-C distances of 2.2 Å and 2.7 Å respectively, see inset to Figure 1a-1c and Table S1 in Supporting Information. To investigate the average electronic structure of the polymer in contact with the liquid water environment under thermal conditions, we extracted 30 snapshot geometries sampled over the last 15 ps of the MD trajectory and calculated the interface electronic structure by Density Functional Theory (DFT) employing periodic boundary conditions, to simulate the infinite polymer chain. To take into account the asymmetric effect of a single sided P3HT/water interface, we alternately removed the water layer above or under the polymer, retaining a ~7 Å-thick layer of water molecules for electronic-structure calculations, Figure 1a. The overall water effect on the P3HT stack is a polarization of the polymer surface induced by the presence of the water molecules, shifting the electronic levels of the outermost polymer layer towards lower energies (i.e., towards more negative values vs. the vacuum level). This effect is consistent with the involvement of thiophene π-electrons in hydrogen bonds with the surrounding water environment. In Figure 1b-d, we report the local density of states in the P3HT band-gap region scanned along the direction 5 ACS Paragon Plus Environment

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perpendicular to the polymer stack, mediated over the investigated geometries, along with the reference starting structure. A clear bending of both the polymer HOMOs and LUMOs towards lower energies is observed, which is particularly marked at the polymer layers exposed to the water interface, independent of the investigated polymer/water side, see Figure S6 and S7, Supporting Information. The P3HT LUMO (yellow/red isosurface in Figure 1b-d) is localized on the polymer layer exposed to water, while the HOMOs (cyan/red isosurface in Figure 1b) lie on the inner polymer layers. This electronic structure variation is insensitive to variation in the DFT functional, whereby use of a hybrid functional shifts the P3HT energy levels and opens the P3HT band-gap, correcting the underestimate typical of the employed DFT, still delivering a quantitatively similar picture of the interface state bending, see Table S2 in Supporting Information. Notably, the P3HT/water interface exposing the (100) surface 23-25 shows a similar behavior. MD simulations show that a few water molecules are able to penetrate in the space between the alkyl chains, orienting the hydrogen atoms towards the thiophene sulfur atoms, at an average distance of ~2.6 Å, see Table S1 in Supporting Information. Even if the interactions between the water molecules and the polymer backbone are weaker compared to the (010) interface, a polarization of the polymer stack facing the water solution can still be observed, with the LUMO localized on the water exposed surface and the HOMO localized on the opposite side, Figure S6 in Supporting Information.

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Figure 1. Structural interface model used in MD simulations (a) and electronic structure (b-d) of P3HT in contact with water under thermal conditions for the (010) polymer slab. In panel (a) the regions with full colors represent the subsystems employed for electronic structure calculations. The inset of panel (a) shows a zoom of the highlighted interface region, evidencing the P3HT/water hydrogen bonding pattern. Panel (b) shows a plot of the local density of states around the polymer band-gap, scanned along the interface normal, indicated as the z axis, averaged over the MD simulation. Panel (c) reports the interface electronic structure of panel (b) with a different perspective, along with the isodensity plot of relevant molecular orbitals (HOMO, yellow and purple colors; LUMO, red and blue colors) obtained from the first production snapshot (d). Atom 7 ACS Paragon Plus Environment

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labels: Green=C; Yellow=S; Red=O; White=H. The energy scale has been shifted to match the P3HT HOMO at 0.0 eV. Notice that a P3HT band-gap of ∼1.4 eV is calculated at the DFT level of theory employed here, which increases to ∼2.5 eV when using hybrid DFT, see Table S2 in Supporting Information.

To simulate the ITO/P3HT/water assembly, characterizing the full device assembly, we considered an ITO slab exposing the (222) surface. This model was extensively investigated by Brédas and co-workers, 26-27 and it was shown to reproduce the ITO work function and its variation with surface preparation and cleaning treatments, see Supporting Information. The interface between ITO and organic semiconductors has been widely investigated from an experimental standpoint,28 while to our knowledge no computational modeling of the ITO/P3HT interface has been reported. A simplified interface model was employed in this case, still based on a sixthiophene unit cell periodically repeated representing P3HT, Figure 2. A stack of eight polymer units was employed, with ethyl groups replacing the P3HT hexyl chains. The length of the alkyl side chain was shown to exert a minimal variation on the electronic properties of P3HT oligomers, amounting at most to a 0.01-0.03 eV HOMO and LUMO shifts.29 A further simplification introduced to model the ITO/P3HT/water interface regards the description of the water environment, which was mimicked by 12 water molecules, Figure 2a and Supporting Information. This simplification provides qualitatively similar results in terms of energy levels bending to those in the full model extracted from MD simulations, see Table S3 in Supporting Information, allowing us to effectively deal with the complex system describing the triple ITO/P3HT/water interface. On this same model we further evaluated the combined effect of water and molecular oxygen, Figure 2a. The calculated ITO/P3HT interface geometry, optimized by periodic DFT calculations including dispersion corrections, see Supporting Information, is consistent with previous work on the binding of P3HT to metal-oxide surfaces, showing that the polymer preferentially exposes the (010) surface to the planar oxide surface,30 due to a stronger electrostatic adhesion.21

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The P3HT HOMOs (LUMOs) show an upward (downward) bending at the ITO/polymer interface, Figure 2b. The HOMOs shift towards higher (i.e., less negative vs. the vacuum) energies when approaching the ITO surface, in agreement with experimental data,31 indicating a favorable energy landscape for hole injection into ITO. The HOMO (yellow and purple isosurface in Figure 2d) shows a significant delocalization between the inner polymer layers and the conducting oxide. The P3HT unoccupied molecular orbitals present a significant bending toward lower energies at the ITO interface; this is consistent with the collection of photogenerated electrons by ITO, which is observed in organic solar cells not employing an electron-blocking layer at the ITO/P3HT interface.28 This electronic structure picture thus accounts for a role of ITO acting per se as either electron or hole acceptor. Concerning the P3HT/water interface, a marked downshift, of about 0.2 eV, of both the P3HT HOMOs and LUMOs is observed also in this case, due to the polarization induced by water molecules on the outermost layer of the P3HT, as seen for the isolated P3HT/water interface in Figure 1, suggesting that photogenerated electrons can preferentially localize on the P3HT layers in contact with the water solution, see the LUMO (red and blue isosurface in Figure 2d). HOMOs and LUMOs downshift is found to involved mainly the P3HT layer in contact to the water. This suggests that the band banding is located at the interface and the region where the efficient charge extraction is taking place extends for a few nanometers. The ITO/P3HT/water interface can thus induce spatial separation of photogenerated carriers, leading, upon illumination, to an accumulation of electrons at the polymer/water interface due to the aforementioned surface polarization. Holes can be transferred to ITO, assisted by the favorable electronic-level alignment. The presence of molecular oxygen at the polymer/water interface adds new electron acceptor states localized on the oxygen atoms (see LUMO plot in Figure 2d) just below the polymer unoccupied states, Supporting Information.

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Figure 2. Structural model of the P3HT polymer in an ITO/P3HT/water/O2 triple interface (a). The O2 molecule is marked by a red circle. In (b) we report a plot of the local density of states for the summed ITO, P3HT and O2 components in the P3HT band-gap region, scanned along the z axis of 10 ACS Paragon Plus Environment

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the system, i.e. the ITO surface normal. In (c) the local DOS is reported along a different view. Panel (d) shows the isodensity plot of relevant molecular orbitals. The energy scale has been shifted to match the P3HT HOMO to 0.0 eV. The ITO density of states has been divided by five in the graphical representation. Atom labels: Green=C; Yellow=S; Red=O; White=H. Grey= In; Orange = Sn.

Experimental data based on transient photovoltage, transient photocurrent and current-voltage characteristics upon illumination support the picture coming from the electronic structure calculations. ITO/P3HT samples were fabricated by spin coating (approx. thickness, 130 nm; see SI section for all fabrication and characterization details) and illuminated by using a collimated LED source (OPTOLED), peaking at 530 nm, at a photoexcitation density of 65 mW/cm2, impinging from the ITO side. Devices were positioned within a quartz electrochemical cell, in correspondence to a planar window to avoid optical aberrations, and immersed for the two/thirds of their surface in a Sodium Chloride (NaCl, 0.2M) solution in ultrapure water, used as the electrolyte. Measurements were carried out at room temperature and ambient pressure, without removing the oxygen dissolved in the solution (measured in about 7 mg/L), unless otherwise specified. We employed a threeelectrode configuration, as shown in Figure 3a. The current flowing through the working electrode (WE, the ITO/P3HT device in our case) is properly counter-balanced at the counter-electrode (CE, a platinum wire in the present experiments) through the feedback circuit of the potentiostat. The potential of the WE is then measured against a third electrode, the reference electrode (RE, saturated Ag/AgCl), whose potential is fixed, Figure 3a. In the case of photovoltage measurements, the transient signal generated at the WE is measured as the potential necessary to keep the system in open-circuit condition (I = 0). In the photocurrent measurements and current-voltage characteristics, a potential difference (0.05 V vs. Ag/AgCl in the case of photocurrent; in the range -0.6 V ÷ 0.3 V vs Ag/AgCl for I-V curves) is applied between the RE and the WE, see also Supporting 11 ACS Paragon Plus Environment

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Information. Observation of negative current signal and of positive photovoltage indicates preferential migration of electrons towards the electrolyte, and of holes towards the CE. Conversely, positive current and negative photovoltage indicate preferential accumulation of holes at the solid/liquid interface, and migration of electrons towards the CE.

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Figure 3. Experimental characterization of the ITO/P3HT/water solution device. (a) Illustration of the three-electrode electrochemical cell for photovoltage and photocurrent measurements. In a photovoltage measurement, the transient signal generated at the ITO/P3HT Working Electrode (WE) is measured as the potential necessary to keep the system in open-circuit condition, I = 0. In a photocurrent measurement, fixed potential value(s) is (are) applied between the WE and the Ag/AgCl Reference Electrode (RE). (b) Transient photovoltage, black line, and transient photocurrent, red line, measurements. 1s after illumination onset, the negative photocurrent and the positive photovoltage signs both indicate preferential migration of electrons towards the P3HT/water interface. (c) I-V characteristics for the device under illumination (solid line) and in dark condition (dashed line).

Figure 3b shows the photovoltage and the photocurrent transient dynamics. The initial, positive peak in the photocurrent (red solid line) is shortly followed by a negative plateau, stable over tens of seconds. Correspondingly, the initially negative photovoltage signal (black solid line), after less than 1s, turns toward increasingly positive values. The initial behavior can be attributed to a capacitive charging of the ITO/P3HT interface due to electrons accumulation at the ITO, leading to negative photovoltage. Over a sub-second temporal scale, the photocurrent and the photovoltage signals reverse their sign, according to a preferential accumulation of electrons at the interface with the electrolyte.32 The negative charging of the polymer at the interface with water is in full agreement with the surface polarization effects suggested by the quantum chemical calculation. Electrons migrating towards the interface give rise to net charge transfer phenomena, promoting oxygen reduction and/or hydrogen evolution reactions, as documented by the occurrence of a negative, constant signal in the photocurrent dynamics, not compatible with a purely capacitive effect (which instead would imply an exponential decrease). The I-V characteristics in steady state conditions, Figure 3c, confirms this picture, showing a photocathodic behavior of the P3HT electrode, supported by electron transfer reactions promoted at the P3HT/electrolyte interface. Accordingly, the observation of hydrogen and oxygen reduction reactions upon steady state visible light illumination has been recently reported. In particular, in the presence of dissolved oxygen into the electrolyte, the photo-catalytic oxygen reduction processes are predominant respect to hydrogen evolution. Upon oxygen removal, the photocurrent, reduced by approximately one order of 13 ACS Paragon Plus Environment

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magnitude, is attributed to hydrogen reduction reactions (see Refs. 1, 16, and Supporting Information, Figure S12). In any case, the activation of redox reactions at the interface with the electrolyte becomes predominant over the concomitant electron injection at the ITO contact only after a few hundreds of milliseconds, explaining the sign reversal dynamics. Further support to the proposed negative charging of the polymer surface exposed to water comes from measurements of the Zeta-potential of a colloidal suspension of large polythiophene nanoparticles (diameter ranging from 100 to 500 nm) under photoexcitation. Even though these structures are dimensionally between individual polymer chains and the solid, the detection of a negative Zeta-potential of ca -40 mV, largely independent from the nanoparticles size, is fully consistent with our theoretical prediction, see Figure S10 in Supporting Information. The sign and magnitude of the Zeta-potential measured for P3HT/water systems is remarkably similar to the classic behavior of TiO2 photoelectrodes in contact with aqueous solutions,33 associated to surface acid-base equilibria which also shift the electrode flatband potential as a function of pH, and in general to the behavior of metal oxide photocatalysts for water splitting.34 Finally, in order to locally probe the charge state of the polymer surface in direct contact with water, we exploited the ratiometric sensing ability of so-called dot-in-bulk (DiB) nanocrystals (NCs) (Figure 4a). These systems consist of a small CdSe core (3 nm diameter) over coated with an ultra-thick shell CdS leading to a total diameter of ~20 nm. As a result of the polytypic crystal structure of the CdS shell and the electrostatic repulsion between core and shell holes, upon photoexcitation DiB-NCs support two types of excitons.35-36 Core-excitons emit in the red spectral region and are tightly confined in the center of the NC, which makes them essentially insensible to the NC environment and to surface states. Shell excitons, on the other hand, are green-emitting, Coulombically bound states, that can sample the NC surfaces and are therefore strongly affected by surface charges and by the NC chemical surroundings. This property has been recently exploited in ratiometric sensing experiments where the core photoluminescence (PL) acted as the internal 14 ACS Paragon Plus Environment

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reference for the surface sensitive shell emission, Figure 4a.37 A representative PL spectrum of DiB NCs on glass is shown in Figure 4b. Importantly for our study, in these systems, quenching of the shell PL is due exclusively to nonradiative transfer of photogenerated shell electrons, while the few picoseconds residence time of photogenerated holes in the shell makes them unaffected by surface states and chemical agents. As a result, DiB NCs are particularly suitable for ratiometric detection of electron withdrawing agents and to optically probe the acidity of the NC environment, without incurring in possible ambiguities due to the coexistence of electron and hole trapping processes that instead characterize conventional core-only NCs.37

Figure 4. Ratiometric sensing of the local acidity of the ITO/P3HT/water interface using DIB NCs. (a) Schematic representation of DiB-NCs deposited on ITO/P3HT substrate immersed in water. Under UV illumination, negative polarization of the P3HT surface leads to local acidification of the NC environment. On the right hand side a CdSe/CdS DiB NC is depicted together with the characteristic energy diagram and the frontier levels of P3HT in dry conditions (dark red lines). Trapping of photogenerated shell electrons leads to selective quenching of the shell luminescence while the core emission is unaffected. (b). Absorption and photoluminescence spectra of DiB NCs 15 ACS Paragon Plus Environment

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(core radius 1.5 nm, shell thickness 5.8 nm) showing the characteristic two-color emission under cw excitation (excitation wavelength 400 nm) due to radiative recombination of core (650 nm) and shell (510 nm) excitons. (c) Core (red) and shell (green) emission intensity as a function of the excitation time for DiB NCs deposited on glass in dry conditions (lines) and immersed in water (circles). (d) Time evolution of the core (red tones) and shell (green tones) emission intensity for DiB NCs on ITO/P3HT immersed into water (circles), deoxygenated water and oxygen-enriched water (the water is deionized in all cases). In all conditions, the core emission is constant over time while the shell luminescence drops to ca. 30% in 500 s due to progressive formation of electron poor environment.

In our experiments, we deposited DiB-NCs on the P3HT polymer film and checked the stability of both the green and red PL in air. The stability of both emissions has been further confirmed depositing the NCs on glass and collecting the PL spectra over time both in air and in water; no quenching is observed in either conditions, Figure 4c. The shell/core PL ratio of the NCs on the dry P3HT film in is slightly lower than for DiB NCs on glass, which accounts for the initial mild electron withdrawing effect of P3HT, in agreement with the alignment of the frontier levels of the polymer and the NCs in dry conditions, Figure 4a and Figure S11 in Supporting Information. Upon adding water in the absence of photoexcitation, the polarization of the frontier states of the outermost P3HT layer enhances electron transfer from the DiB to the polymer, leading to an initial dimming of the shell PL whilst the core emission remains constant. Although already indicative of enhanced electron harvesting ability of the polarized polymer in water, this process is superimposed to another dramatic effect, possibly even more relevant for our discussion. Specifically, in time and under UV photoexcitation, the shell PL undergoes a progressive decay ultimately reaching ~30% of its initial value. Concomitantly, the core PL intensity remains constant thereby further confirming the photochemical stability of the NCs during the experiment. The dramatic drop of the shell PL points to the gradual formation of an electron accepting (or depleted) NC environment under illumination. Since one possible origin of this effect could be the gradual evolution of radical oxygen close to the film surfaces efficiently scavenging photogenerated NC electrons, we repeated the sensing experiments in identical excitation conditions but changing the oxygen concentration of the deionized water solution. As shown in Figure 4d, the ratiometric response of the DiB-NCs in the 16 ACS Paragon Plus Environment

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three experimental configurations is essentially identical, which rules out oxygen as major player, although it might still affect the speed of the reaction. Most importantly, the whole body of ratiometric sensing analysis supports the picture of progressive acidification of the solution in close proximity to the P3HT film under illumination due to accumulation of bulk electrons on the polymer surface progressively charge balanced by the formation of an electron poor (acidic) layer of polarized water molecules. To sum up, three different experiments support our theoretical prediction, independently showing that our simulations capture the correct picture of the photo-electrochemical process occurring upon exposure of a P3HT film to water. Our results are further corroborated by recent experiments on astrocytes grown on the polymer surface that were tentatively interpreted invoking acidification of the cell/polymer interface.38 Last, it is worth noting that a local change in pH could trigger a re-adjustment of the protein conformation in the cleft and thus affect the cell properties through a photo-chemical induced mechanical effect.39 We now proceed to investigate how the proposed behavior of the ITO/P3HT/water interface may explain the functioning of a retinal prosthesis device. In making this step, we do the bold assumption that the neuronal cell response is only related to a perturbation of the ionic equilibrium at the cellular membrane level, i.e. that the neuronal response is mediated by a change in the local composition of the electrolyte environment. Furthermore we neglect the true nature of the cleft. This space separating the polymer surface and the cell membrane is certainly consisting of layers of adhesion protein and trans membrane proteins that could modulate the contact dynamics. Albeit the neuronal response is certainly a much more complex phenomenon than our simplified model can simulate, here we wish to evaluate whether the proposed model and interfacial electronic structure may account for a redistribution of potentially active species in a biological environment, i.e. alkali cations, upon illumination of the investigated ITO/P3HT/water assembly. In this discussion, the effect of dissolved oxygen is noteworthy, due to its role in photo-activated doping processes of the 17 ACS Paragon Plus Environment

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polymer.32 We thus evaluated the binding of a hydrated Na+ cation to the neutral and reduced P3HT/water interface, including molecular oxygen. This investigation has been carried out using reduced P3HT cluster models made by four thiophene units with methyl groups replacing the P3HT hexyl side chains, where the interface with the electrolytic solution is described via inclusion of four water molecules, and consecutively one oxygen molecule, plus one Na+ cation with a hydration shell of six water molecules, Figure S9 in Supporting Information. The effect of the bulk water environment was further included here by a surrounding polarizable continuum solvation model.40 These cluster models have been shown to give consistent results concerning the effect of water molecules on the surface in comparison to the previously presented MD approach, see Table S3 in Supporting Information. We further assessed the convergence of the calculated binding energies for the employed cluster models with respect to the number of thiophene units employed for geometry optimizations (i.e. the oligomer length) and the number of polymers stacks included in the model. In both cases, the calculated binding energies are largely independent from the employed model, see Table S4 in Supporting Information, so we discuss results obtained for a four-thiophene/eight stack model hereafter. The formation of a reversible P3HT-O2 charge transfer complexes has been demonstrated to take place in air,41-42 which can be further stabilized in the presence of moisture due to the formation of O2(H2O)n clusters.18 The reversible formation of the P3HT+ : 3O2- molecular complex was also confirmed in polymer thin films directly exposed to aqueous solutions, in samples exposed to the concomitant action of oxygen and visible light. Interestingly, it was demonstrated that the direct contact with water does not determine specific degradation phenomena; conversely, it even turns out to reduce the effect, possibly due to a decreased content of oxygen in water respect to air. Here, we calculate an interaction energy of 0.15 eV for the P3HT/O2 complex, which nicely compares with the experimental value of 0.11 eV.41 In line with the results shown previously for the ITO/P3HT/water/O2 system, we note that, upon interaction with O2, two acceptor states are created 18 ACS Paragon Plus Environment

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in the P3HT band gap, due to the lowest unoccupied orbitals of the oxygen molecule lying ∼1.0 eV below the P3HT LUMO, see Table S5 and Figure S9 in Supporting Information. The formation of these new levels may result in the p-doping of the polymer, as mentioned above, due to the electron trapping at these states,43 to give superoxide anions (O2-) and free holes in the polymer layer.18, 42 Notice that the interaction of molecular oxygen with P3HT is also predicted to be involved in the initial stages of the polymer oxidative damage, with the formation of stable oxidation products, although with fairly large activation energies.20 The binding energies of the hydrated Na+ cation are shown in Table 1. Two possible conditions, with and without the formation of the P3HT/O2 complexes at the water interface, were considered. Furthermore, all these models have been investigated in their neutral state and after adding one electron to the system, to simulate the possible conditions obtained under illumination. This model is based on the notion that following photon absorption in P3HT, a long-lived population of electrons and holes (negative and positive polarons) is formed, and assuming that the photogenerated hole is transferred towards the ITO substrate. Table 1. Binding energy (eV) of one hydrated Na+ cation to P3HT in the presence of water and oxygen, in their neutral and 1-electron reduced states.

neutral

reduced

P3HT /water 1.59

1.83

P3HT/water/ O2 1.58

2.40

For the neutral models, representative of the conditions in the dark, we find essentially no differences in the binding energies between the model incorporating or not the oxygen molecule (~1.6 eV). When one electron is added to the system, which simulates the accumulation of 19 ACS Paragon Plus Environment

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photogenerated electrons at the P3HT/water interface due to the

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aforementioned interface

polarization, the binding energies increase to ∼1.8 eV for the interface with water and to 2.4 eV for the water-oxygen interface. This difference is due to the electrostatic interaction between the Na+ cation and the added electron, which in one case is delocalized within the first polymer layer, while in the other it is trapped on the O2 molecule at the interface, Figure S9 in Supporting Information. Clearly, the presence of a P3HT/water/O2 complex at the polymer surface can amplify the driving force for drawing alkali cations from the electrolyte solution, although it is not strictly needed for the system to operate. We have investigated the electronic structure and photoelectrochemical behavior of the ITO/P3HT/aqueous electrolyte interface, widely employed in bio sensors, water-gated transistors, photoelectrochemical cells, or bio-polymer interfaces for optical stimulation of cell activity and artificial retinal prosthesis. Our simulations describe the energetics of the polymer frontier orbitals, at the interface with the conducting oxide and liquid water. Our findings suggest three phenomena that could take place at the illuminated interfaces: 1) The ITO/P3HT interface, similarly to what happens in organic photovoltaic devices, may support both electron and hole injections from the polymer to the conductive oxide, assisted by a favorable energy-level alignment, which is confirmed by experimental observations. 2) The outermost P3HT layer can be polarized by a water environment, inducing the down-shift of the polymer energy levels. Upon charge illumination, photogenerated electrons are preferentially localized at the polymer/water interface. 3) The addition of molecular oxygen on the hydrated outermost polymer layer is energetically favorable, leading to the formation of electron trapping sites at the interface with the electrolyte. The inferred electronic structure agrees with a number of optoelectronic characterization experiments, in particular those of relevance for the photoelectrochemical applications of these 20 ACS Paragon Plus Environment

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devices. Furthermore, we observe that, under illumination, the polymer surface could attract cations from the solution, e.g., Na+, with a sizable light on / light off selectivity. The consequent variation of the ionic equilibrium in the solution representing the extracellular environment suggests a possible coupling mechanism between the photoexcited polymer and the cell membrane, leading to light-induced neuronal stimulation. Alternatively red-ox reaction at the polymer/liquid interface, supported by the aforementioned polarization, could activate chemical species that in vivo are involved in the signal photo-transduction cascade. Our description of the cell photoexcitation mechanism is admittedly vastly constrained by the model approximations and simplifications. Further studies and investigations are required to comprehend the complexity of the actual system. We believe, however, that our approach represents a key first step into the characterization of organic/electrolyte interfaces that play a crucial role in bio-electronics and bio-photonics applications. Acknowledgements PS, EM, FDA, AM and MIS acknowledge CompuNet, Istituto Italiano di Tecnologia, for funding this research. AM and MIS thank Regione Autonoma della Sardegna (CRP CRP-24978 and Premialità 2014) and CINECA (Project ISCRA-SOAP). SB, GL, and MRA acknowledge the financial support from the European Community through projects FP7-PEOPLE-212-ITN 316832 (OLIMPIA) and the FET Collaborative Project n. 309223 (PHOCS), from the Telethon – Italy foundation (grants GGP12033 and GGP14022), and from Fondazione Cariplo (grant ID 20130738). JLB acknowledges generous support from King Abdullah University of Science and Technology and ONR-Global (Grant N62909-15-1-2003). The authors acknowledge Wan Ki Bae for the synthesis of the dual-emitting CdSe/CdS heterostructures. Supporting Information. Description of the computational models and details, experimental details, statistical analysis of the MD geometries, Figures and Tables concerning the polymer 21 ACS Paragon Plus Environment

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(17) Fumagalli, F.; Bellani, S.; Schreier, M.; Leonardi, S.; Rojas, H. C.; Ghadirzadeh, A.; Tullii, G.; Savoini, A.; Marra, G.; Meda, L., et al. Hybrid organic-inorganic H2-evolving photocathodes: understanding the route towards high performance organic photoelectrochemical water splitting. J. Mater. Chem. A 2016, 4, 2178-2187. (18) Zhuo, J.-M.; Zhao, L.-H.; Png, R.-Q.; Wong, L.-Y.; Chia, P.-J.; Tang, J.-C.; Sivaramakrishnan, S.; Zhou, M.; Ou, E. C. W.; Chua, S.-J., et al. Direct Spectroscopic Evidence for a Photodoping Mechanism in Polythiophene and Poly(bithiophene-alt-thienothiophene) Organic Semiconductor Thin Films Involving Oxygen and Sorbed Moisture. Adv. Mater. 2009, 21, 47474752. (19) Norrman, K.; Gevorgyan, S. A.; Krebs, F. C. Water-Induced Degradation of Polymer Solar Cells Studied by H218O Labeling. ACS Appl. Mater. Interfaces 2009, 1, 102-112. (20) Volonakis, G.; Tsetseris, L.; Logothetidis, S. Impurity-related effects in poly(3hexylthiophene) crystals. Phys. Chem. Chem. Phys. 2014, 16, 25557-25563. (21) Saba, M. I.; Mattoni, A. Effect of Thermodynamics and Curvature on the Crystallinity of P3HT Thin Films on ZnO: Insights from Atomistic Simulations. J. Phys. Chem. C 2014, 118, 46874694. (22) Cooke, S. A.; Corlett, G. K.; Legon, A. C. Rotational spectrum of thiophene···HCl. Does thiophene act as an aromatic π-type electron donor or an n-type electron donor in hydrogen-bond formation? J. Chem. Soc., Faraday Trans. 1998, 94, 1565-1570. (23) Porrazzo, R.; Bellani, S.; Luzio, A.; Bertarelli, C.; Lanzani, G.; Caironi, M.; Antognazza, M. R. Field-effect and capacitive properties of water-gated transistors based on polythiophene derivatives. APL Mater. 2015, 3, 014905. (24) Bellani, S.; Porro, M.; Caddeo, C.; Saba, M. I.; Miranda, P. B.; Mattoni, A.; Lanzani, G.; Antognazza, M. R. The study of polythiophene/water interfaces by sum-frequency generation spectroscopy and molecular dynamics simulations. J. Mater. Chem. B 2015, 3, 6429-6438. (25) Yimer, Y. Y.; Yang, B.; Bhatta, R. S.; Tsige, M. Interfacial and wetting properties of poly(3hexylthiophene)–water systems. Chem. Phys. Lett. 2015, 635, 139-145. (26) Li, H.; Paramonov, P.; Brédas, J.-L. Theoretical study of the surface modification of indium tin oxide with trifluorophenyl phosphonic acid molecules: impact of coverage density and binding geometry. J. Mater. Chem. 2010, 20, 2630-2637. (27) Li, H.; Winget, P.; Brédas, J.-L. Transparent Conducting Oxides of Relevance to Organic Electronics: Electronic Structures of Their Interfaces with Organic Layers. Chem. Mater. 2014, 26, 631-646. (28) Hains, A. W.; Liu, J.; Martinson, A. B. F.; Irwin, M. D.; Marks, T. J. Anode Interfacial Tuning via Electron-Blocking/Hole-Transport Layers and Indium Tin Oxide Surface Treatment in Bulk-Heterojunction Organic Photovoltaic Cells. Adv. Funct. Mater. 2010, 20, 595-606. (29) Oliveira, E. F.; Lavarda, F. C. Effect of the length of alkyl side chains in the electronic structure of conjugated polymers. Mat. Res. 2014, 17, 1369-1374. (30) Noori, K.; Giustino, F. Ideal Energy-Level Alignment at the ZnO/P3HT Photovoltaic Interface. Adv. Funct. Mater. 2012, 22, 5089-5095. (31) Schneider, M.; Wagenpfahl, A.; Deibel, C.; Dyakonov, V.; Schöll, A.; Reinert, F. Band bending at the P3HT/ITO interface studied by photoelectron spectroscopy. Org. Electron. 2014, 15, 1552-1556. (32) Bellani, S.; Fazzi, D.; Bruno, P.; Giussani, E.; Canesi, E. V.; Lanzani, G.; Antognazza, M. R. Reversible P3HT/Oxygen Charge Transfer Complex Identification in Thin Films Exposed to Direct Contact with Water. J. Phys. Chem. C 2014, 118, 6291-6299. (33) Enright, B.; Redmond, G.; Fitzmaurice, D. Spectroscopic determination of flatband potentials for polycrystalline TiO2 electrodes in mixed solvent systems. J. Phys. Chem. 1994, 98, 6195-6200. (34) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253-278. 23 ACS Paragon Plus Environment

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