Electrochemical, spectroelectrochemical and surface photovoltage study of ambipolar C60-EDOT and C60-Carbazole based conducting polymers

Accepted Manuscript Electrochemical, spectroelectrochemical and surface photovoltage study of ambipolar C60-EDOT and C60-Carbazole based conducting polymers Daniel A. Heredia, Edwin J. Gonzalez Lopez, Edgardo N. Durantini, Javier Durantini, Thomas Dittrich, Jörg Rappich, Lorena Macor, Claudia Solis, Gustavo M. Morales, Miguel Gervaldo, Luis Otero PII:

S0013-4686(19)30789-3

DOI:

https://doi.org/10.1016/j.electacta.2019.04.103

Reference:

EA 34051

To appear in:

Electrochimica Acta

Received Date: 18 March 2019 Revised Date:

15 April 2019

Accepted Date: 16 April 2019

Please cite this article as: D.A. Heredia, E.J. Gonzalez Lopez, E.N. Durantini, J. Durantini, T. Dittrich, Jö. Rappich, L. Macor, C. Solis, G.M. Morales, M. Gervaldo, L. Otero, Electrochemical, spectroelectrochemical and surface photovoltage study of ambipolar C60-EDOT and C60Carbazole based conducting polymers, Electrochimica Acta (2019), doi: https://doi.org/10.1016/ j.electacta.2019.04.103. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract for:

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Electrochemical, Spectroelectrochemical and Surface Photovoltage Study of Ambipolar C60-EDOT and C60-Carbazole Based Conducting Polymers.

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Electrochemical, Spectroelectrochemical and Surface Photovoltage Study of Ambipolar C60-EDOT and C60-Carbazole Based Conducting Polymers.

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Daniel A. Heredia, Edwin J. Gonzalez Lopez, Edgardo N. Durantini

IDAS Departamento de Química, Universidad Nacional de Río Cuarto-CONICET Agencia Postal Nro. 3, X5804BYA Río Cuarto, Córdoba, Argentina.

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Javier Durantini, Thomas Dittrich, Jörg Rappich

Photovoltaik, 12489 Berlin, Germany.

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Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH, Institut fur Silizium-

Lorena Macor, Claudia Solis, Gustavo M. Morales, Miguel Gervaldo*, Luis Otero* IITEMA Departamento de Química, Universidad Nacional de Río Cuarto-CONICET

* Corresponding authors.

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Agencia Postal Nro. 3, X5804BYA Río Cuarto, Córdoba, Argentina.

E-mail address: [email protected]

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E-mail address: [email protected]

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Abstract Ethylenedioxythiophene (EDOT) and Carbazole (CBZ) based conducting polymers holding pendant C60 Buckminster fullerene were formed by electrochemical induced of

four

structurally

related

monomers.

Electrochemical

and

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dimerization

spectroelectrochemical characterization of the thin films clearly indicates the presence of EDOT and CBZ dimers as structural building blocks of the polymeric materials. Under

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positive applied potentials, all the synthesized conductive materials generate UV-visiblenear IR absorption bands that denote the presence of two oxidized states in the

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electrochemical generated dimers. Also, the four polymeric films showed reversible reduction of the C60 units. The capacity of the materials for the production of photoelectric effects was analyzed by surface photovoltage spectroscopy, showing that the illumination of C60-CBZ polymers causes preferential separation of photogenerated charge carriers in

Keywords

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the double-cable polymeric organic films.

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Electropolymerization; Fullerene; Double cable; Photoinduced charge separated states.

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1. Introduction The generation of molecular structures having bipolar conducting capabilities, where both, hole and electron transport can contribute to electronic conductivity, is of high

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interest for the development of organic based electronic and optoelectronic devices [1-3]. In this sense, the formation of photoinduced charge transfer states in the so called "double cable" systems was reported by first time by A. Cravino et al [4], demonstrating the

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potentiality of this kind of polymers for their application as active material in the development of single component organic solar cells. In the same way, during the last two

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decades, ambipolar organic materials have been profusely investigated, and a large number of structures have been synthetized and analyzed [5-9], being the generation of stable, efficient, and economical competitive single component solar cells one of the main objectives [1]. The architecture of single component polymeric solar cells is based on

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covalent linked electron donor and electron acceptor moieties, which are able to absorb sunlight and transport the photo-generated charge carriers to the contacts. In the usual mixed donor-acceptor (D-A) configurations, because of the effects of the different

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solubility and miscibility of the components that affect the material formation in a complex manner, it is often difficult to obtain suitable nanostructures for efficient device

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performance [10-13]. Contrarily, in single component architectures, the characteristic of the D-A interface can be wrought through the molecular engineering of the linker in the constituent organic material [1]. This can also provide strong stabilization of the device. Nguyen et al [14] recently reported the fabrication of single component solar cell devices that show no degradation in the power conversion efficiency after 100 h at 80 °C. The authors attributed this stability to the suppression of large phase segregation of donor and acceptor domains [14]. Although the maximum efficiency reached with single component 3

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solar cells has remained very low (∼ 1-2 %) for several years [1], a notable progress has been recently reached with double-cable polymers that provide impressive power conversion efficiencies up to 4.18% [15] and 5.58% [16]. The last case is the highest value

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reported so far for a single material device, whose performance is attributed by the authors to the well-packed and properly oriented polymer chains, obtained by an adequate material architecture and deposition methodology (electrospun nanofibers generation). Thus, the

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development of conjugated polymeric backbones with electron acceptor pendant residues is a very active area of research [1,5,6,8-10], because these structures combine three

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properties (light absorption, photo stimulated separated charge transfer generation and carrier transport) in a unique macromolecular material [1].

Several polymers generated by electrochemical polymerization from monomers holding carbazole [17,18], triphenylamine [19], 3,4-ethylenedioxythiophene [20], and

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thiophene [21] with pendant acceptor moieties have been synthesized and characterized as components in electronic devices. In this way, thin films holding molecular heterojunctions can be formed over transparent base contacts in just one single step. On the other hand,

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fullerenes are a class of molecular structure that holds very remarkable electronic and optical properties [22-24]. Fullerenes can accommodate up to six electrons in their

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delocalized π cloud, and possess considerable electron mobility in condensed phase arrangements [10,25]. The examples of the applications of fullerene in organic electronic devices are nearly endless [26-31], frequently being part of double cable polymeric structures [1,4-6,8,9,21,32-34]. In this work we report the formation of ambipolar conducting polymers generated by electrochemical oxidation of 3,4-ethylenedioxythiophene (C60-2EDOT and C60-4EDOT,

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Fig. 1) holding pendant C60 Buckminster fullerenes moieties. The electrochemical and spectroelectrochemical characterization of the films clearly indicates the presence of EDOT dimers as hole conducting material, meanwhile the electrochemical activity of the C60

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moieties as electron acceptor is demonstrated. Also, carbazole containing monomers (C602CBZ and C60-4CBZ, Fig. 1) were electropolymerized [35-37] and the capacity for the production of photoelectric effects of both, EDOT and CBZ based polymers, was analyzed

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by modulated and time resolved surface photovoltage spectroscopy, showing that the CBZ

2. Experimental section 2.1. Synthesis

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based double cable polymers are able to produce photoinduced charge separated states.

The strategy explored for the preparation of fullerene C60 adducts bearing EDOT

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and carbazole moieties was based on a Bingel-type reaction (Fig. 1) [38]. Precursors 2EDOT and 2CBZ were prepared by esterification reaction of the corresponding commercial alcohols. Hydroxymethyl EDOT 1 and carbazole-ethanol 2 were treated with

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malonyl dichloride in the presence of pyridine at room temperature, yielding the malonates 2EDOT and 2CBZ in 77% and 70% yields, respectively.

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Bingel reactions of 2EDOT and 2CBZ with fullerene C60 were carried out under

conventional conditions [38]: the malonates were allowed to react with fullerene C60, CBr4, and DBU in toluene at room temperature for 7 h. TLC analysis of both reactions revealed the formation of two adducts together with the unreacted fullerene C60 (first-eluted brown band). The other two respective TLC spots, with a lower retardation factors (Rf), are also brown under visible light. After a simple purification and isolation of each individual cycloadducts by flash chromatography, using toluene to remove the unreacted fullerene and 5

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then a polarity gradient with toluene/EtOAc, their UV/Vis spectra disclosed that two different cycloadducts had been obtained. These seconds-eluted brown bands were identified as the expected monoadducts C60-2EDOT and C60-2CBZ in 23% and 33% yields,

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respectively. The structure and purity of these monoadducts were unambiguously confirmed by analysis of their 1H NMR and 13C NMR spectra. Monoadducts C60-2EDOT and C60-2CBZ (Fig. 1) were obtained as single regioisomers. C60-2CBZ spectra were

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compared with previously published literature data [35,36]. On the other hand, for the thirdeluted brown bands, HRMS measurements disclosed that bisadducts C60-4EDOT and C60-

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4CBZ (Fig. 1) were formed. The lower Rf is in accordance with a higher functionalization grade.

The 1H NMR spectra of the electropolimerizable C60-2EDOT, C60-2CBZ, C604EDOT and C60-4CBZ monomers are similar to their respective precursors (2EDOT and

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2CBZ), but shifted downfield by the electron-withdrawing effect of fullerene C60. The proton spectra exhibited all the malonate proton signals but, as expected, the malonate methylene protons were missing in mono and bisadducts. From the analysis of

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C NMR

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spectra, it can be confirmed the incorporation of malonate moieties to fullerene C60. Carbon spectra combined the diagnostic resonances of fullerene C60 and the signals of the

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respective malonates.

Finally, we can determine that it is possible to favor the monoadduct product using

an excess of fullerene C60 and working in higher dilution. On the other hand, when the ratio malonate/fullerene C60 is increased, the yield of bisadduct is enhanced. The benefits of this synthetic route for the preparation of the electroactive monomers include the experimental straightforwardness, scalability and reproducibility. Furthermore, with inexpensive starting materials and commercially available standard reagents, the syntheses were achieved in two 6

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steps with a good overall yield (taking into account that unreacted fullerene C60 can be reused).

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2.1.1. General Dichloromethane (DCM) was dried at reflux over P2O5 (7 h) followed by distillation. Toluene was dried from sodium/benzophenone ketyl. All other reagents were used as received. The progress of the reactions was followed and controlled by silica gel

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thin-layer chromatography (TLC) analysis. TLC Analtech, DE, USA, plates 250 mm. Flash

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column chromatographies were carried out with cyclohexane/DCM or EtOAc/toluene mixtures. Silica gel 60 (0.040–0.063 mm, 230–400 mesh) from Merck (Darmstadt, Germany) was used for flash column chromatography. The NMR spectroscopic data were provided by the Instituto de Química Rosario-CONICET. The spectra were recorded on an FT-NMR Bruker Avance 300 spectrometer. The NMR spectra were acquired at 300 MHz 13

C. Mass spectra were measured on a Bruker Micro-TOF-QII

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for 1H and 75 MHz for

spectrometer (Bruker Daltonics, MA, USA) equipped with an ESI source (ESI-MS).

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2.1.2. Synthesis of 2EDOT

To a stirred solution of hydroxymethyl EDOT (231 mg, 1.34 mmol) and pyridine

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(177 µL, 2.24 mmol) in DCM (3 mL) was added dropwise a solution of malonyl dichloride (62 µL, 0.64 mmol) in DCM (2 mL) under argon atmosphere. The reaction mixture was refluxed overnight. Then, it was allowed to cool to room temperature and diluted with DCM (10 mL). The resulting solution was washed sequentially with an HCl solution (0.5 M), saturated Na2CO3 solution and brine. Finally, the organic layer was dried with Na2SO4, and concentrated under reduced pressure. Chromatography of the residue furnished 2EDOT (184 mg, 70%) as a yellow-pale oil. 1H NMR (300 MHz, CDCl3) δ 6.35 (s, 4H), 4.44-4.34 7

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(m, 6H), 4.22 (d, J = 11.4 Hz, 2H), 4.12-3.96 (m, 2H), 3.50 (s, 2H).

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C NMR (75 MHz,

CDCl3) δ 165.7, 141.1, 140.8, 100.2, 100.1, 71.1, 65.3, 63.3, 40.9. ESI-MS [m/z] 413.0366

2.1.3. Synthesis of C60-2EDOT and C60-4EDOT

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[M+H]+ (412.0287 calculated for C17H16O8S2).

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU, 99 µL, 0.66 mmol) was added dropwise to a vigorously stirred solution of 2EDOT (135 mg, 0.33 mmol), CBr4 (109 mg, 0.33

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mmol) and fullerene C60 (236 mg, 0.33 mmol) in anhydrous toluene (40 mL). The resulting

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mixture was stirred at room temperature for 7 h under argon atmosphere. After evaporation of the solvent in vacuum, the residue was chromatographed to give C60-2EDOT (84 mg, 23%) and C60-4EDOT (69 mg, 14%), as brown solids. C60-2EDOT: 1H NMR (300 MHz, CDCl3) δ 6.37 (s, 4H), 4.81-4.53 (m, 6H), 4.38-4.11 (m, 4H). 13C NMR (75 MHz, CDCl3) δ 163.1, 145.3, 145.2, 145.0, 144.7, 143.9, 143.1, 142.2, 141.8, 141.0, 141.0, 140.6, 139.2,

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100.5, 100.4, 71.1, 71.0, 65.3, 64.8, 64.7. ESI-MS [m/z] 1131.0209 [M+H]+ (1130.0130 calculated for C77H14O8S2). C60-4EDOT: 1H NMR (300 MHz, CDCl3) δ 6.49-6.23 (m, 8H), 4.77-4.41 (m, 12H), 4.37-4.00 (m, 8H). 13C NMR (75 MHz, CDCl3) δ 163.0, 162.9, 146.5,

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145.5, 144.7, 144.5, 143.6, 142.6, 142.3, 141.0, 140.6, 100.4, 71.1, 71.0, 65.3, 64.7. ESI-

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MS [m/z] 1541.0339 [M+H]+ (1540.0260 calculated for C94H28O16S4). 2.1.4. Synthesis of 2CBZ The synthesis was performed as described above for 2EDOT using a solution of 9H-

carbazole-9-ethanol (250 mg, 1.19 mmol) and pyridine (153 µL, 1.89 mmol) in DCM (3 mL) and malonyl dichloride (53 µL, 0.54 mmol) in DCM (2 mL). Chromatography of the residue furnished 2CBZ (203 mg, 77%) as a white solid. The spectroscopic data of the product were in full agreement with the literature [25].

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2.1.5. Synthesis of C60-2CBZ and C60-4CBZ The synthetic procedure was as described above for C60-2EDOT and C60-EDOT

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using DBU (87 µL, 0.57 mmol), 2CBZ (140 mg, 0.28 mmol), CBr4 (95 mg, 0.28 mmol) and fullerene C60 (202 mg, 0.28 mmol) in anhydrous toluene (35 mL). The residue was chromatographed to give C60-2CBZ (135 mg, 33%) and C60-4CBZ (88 mg, 18%), as brown

literature [35].

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2.2. Instrumentation and Measurements

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solids. In both cases, the spectroscopic data of the products were in full agreement with the

All electrochemical experiments were performed with a potentiostat-galvanostat Autolab (Electrochemical Instruments) using a three-electrode configuration cell, consisting of a Pt wire as the working electrode, a Pt loop served as the counter electrode,

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and a silver wire employed as the quasi-reference electrode. Electrochemical behavior of the monomers was investigated by cyclic voltammetry (CV) in a dry DCM deoxygenated solution containing 0.5 mM of the corresponding monomer and 0.1 M of

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tetrabutylammonium hexafluorophosphate (TBAPF6) as support electrolyte, at room temperature. Ferrocene was employed as internal standard. The experimental potentials

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were subsequently calibrated versus Fc+/Fc using the formal potential of this redox couple versus Saturated Calomel Electrode (SCE). The electropolymerization of the C60-EDOT and C60-CBZ monomers was also carried out on Indium-Tin Oxide (ITO, Delta. Tech. 8–12 Ω/square) semi-transparent electrodes. The amount of electroactive material that remains as a film over the electrode surface after different number of polymerization cycles was evaluated as proportional to the polymer oxidation charge. This charge was calculated by

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integration of current of the corresponding CV obtained at 100 mV/s, in a solution containing only supporting electrolyte. UV–vis absorption spectra were measured with a Hewlett Packard UV–Vis diode

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array spectrophotometer. In situ spectroelectrochemical measurements of the polymeric films were carried out in a homemade cell. ITO coated glasses covered with the organic polymers were used as working electrode, a platinum wire as a counter electrode and a

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silver wire as a quasi-reference electrode. The cell was placed in the optical path of the sample light beam. An ITO electrode without polymer was used as blank for absorption

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background correction.

Charge separated states in double cable electro-polymers were studied by modulated surface photovoltage (SPV) spectroscopy and transient SPV [39]. In both methods, the light-induced change of the contact potential difference (CPD) is coupled out with a fixed

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capacitor which is formed between the sample surface and a transparent reference electrode (SnO2:F) by a mica spacer [40]. The SPV reflects the amount of photogenerated charge carriers separated in space and the distance between the positive and negative charge

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centers.

SPV transients were excited at a wavelength of 450 nm (duration timer of the laser

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pulses 3 ns at full width of half maximum, intensity about 1 mJ/cm²) and recorded with a logarithmic readout [39]. The repetition rate of the laser pulses was set to 1 Hz and 10 transients were averaged for the measurement of 1 SPV transient. The AFM characterization was performed with an Agilent 5500 SPM microscope

(Agilent Technologies, Inc.) working in acoustic AC mode and equipped with a 9µm x 9µm

scanner,

using

aluminum

backside-coated

silicon

probes

(MikroMasch,

HQ:NSC14/AL BS) with a typical tip radius of ∼ 8 nm and a spring constant ∼ 5 N/m, 10

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vibrating just under their fundamental resonance frequency (∼ 160 kHz). Prior to being analyzed, the electrodes were washed with solvent to remove remaining electrolyte and low-weight molecular species. The experiments were realized in stationary dry-air

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atmosphere and the images were treated and analyzed using Gwyddion, open source software. The thicknesses of the electro films were measured by using the scratch-and-scan method, which consists in the mechanical removal of a sharp line of the film and

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subsequent imaging of the surface by AFM (see Fig. S1 for more details).

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3. Results and discussion 3.1. Electrochemistry

Fig. 2a-b show cyclic voltammograms at platinum electrodes of C60-2EDOT and C60-4EDOT monomers. As it can be seen the oxidation of both monomers present one irreversible wave at around 1.35 V and two cathodic peaks, associated to the formation of

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the EDOT radical cation and to the radical anion (black lines) and dianion (red lines) of the C60 units, respectively. For both monomers after the first anodic peak, in the reverse scan a cathodic peak is detected at around 0.45 V (Fig. 2a-b, black lines). On the other hand, C60-

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2CBZ and C60-4CBZ monomers present two cathodic peaks at around -0.65 (Fig. 2c-d red

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lines) and -1.05 V (Fig. 2c-d black lines), and two anodic peaks at about 1.30 and 1.45 V (Fig. 2c-d black lines). The first and second cathodic peaks are related to the formation of the radical anion and dianion of the C60, while the anodic peaks are attributed to the formation of the radical cation and dication of the CBZ units [41,42]. It can also be seen that the current of the first anodic peak for C60-4EDOT and C60-4CBZ monomers are bigger than the current of the first peak observed for C60-2EDOT and C60-2CBZ (related to C60 reduction current), which is consistent with the fact that C60-2EDOT and C60-2CBZ present 11

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two EDOT and CBZ units respectively, while C60-4EDOT and C60-4CBZ present four EDOT and CBZ units residues able to be oxidized. When the applied potential is continuously cycled in the range where the side chain

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moieties are oxidized (EDOT in C60-2EDOT and C60-4EDOT; CBZ in C60-2CBZ and C604CBZ) the process conducts to an increase in the oxidation-reduction currents with every new cycle in all cases (Fig. 3a-d). In the case of EDOT functionalized fullerenes in the

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second scan a new redox wave with an anodic peak at 0.50V and a cathodic peak at 0.45 V appears (Fig. 3a-b, blue lines), showing that the coupling product (probably diEDOT) is

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oxidized at lower potentials than the monomer [43-45]. Repetitive cycling until the foot of the first oxidation process conducts to increases in the oxidation reduction currents with every new cycle (black lines), indicating the formation of an electroactive film on the electrode surface. On the other hand, in the CBZ functionalized fullerenes the first anodic

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peak is irreversible and in the reverse scan two cathodic peaks are observed at around 0.90 and 1.15 V (clearly seen in Fig. 2c-d, red lines). In the second scan towards positive potentials two new oxidation waves are observed; one of these at more negative potentials

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than the first anodic peak, and the second one superimposed to the first anodic peak (Fig. 3c-d, blue lines). This indicates that the oxidation product (probably dicarbazole (DCBZ))

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is easily oxidized than the CBZ units present in both monomers [41]. Also, as in the case of C60-EDOT monomers, continuous cycling of C60-2CBZ or C60-4CBZ conducts to increases in the oxidation reduction currents with every new cycle, indicating the formation of conducting films on the electrode surfaces, in fully agreement with the results reported by Sun et al [35-37] (Fig. 3c-d, black lines). Fig. 4a-d show the electrochemical responses of the films in a solution containing only supporting electrolyte (free of monomers). The peak currents of the redox systems in 12

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the four cases exhibit a linear relation with the applied potential scan rate, in agreement with the expected for the presence of electroactive products adsorbed on the base electrode surface. C60-2EDOT-film and C60-4EDOT-film present similar electrochemical responses

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which are characterized by a broad oxidation that starts at around 0.20 V and that extends as a broad wave to ∼1.00 V. Moreover, in scans scan towards positive potentials at low scan rates (25 mV/s) two anodic peaks are clearly observed at similar applied potentials for

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both films (around 0.45 and 0.65 V) (insets in Fig. 4a-b). On the negative range a cathodic peak is observed, which occurs at potentials similar than those detected for the monomers

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in solution and associated to the reduction of the C60 units, presents in the films [21,35,46]. It is confirmed by absence of the reduction peaks in this potential range in the cyclic voltammogram of the film formed by electropolymerization of 2EDOT (see Fig. S4a in SI). It is also observed a much bigger anodic peak current (related to the cathodic peak current)

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for C60-4EDOT-film than C60-2EDOT-film, indicating that C60-4EDOT-film presents more oxidizable redox centers that C60-2EDOT-film. Likewise, as it can be seen in Fig. 4c-d, in both films formed by CBZ functionalized fullerenes the response of both electrodes are

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characterized by three bell shaped redox systems, which consist of two anodic and one cathodic peaks. It is also observed that the magnitudes of the anodic and cathodic peak

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currents are similar for C60-2CBZ-film, while in C60-4CBZ-film the anodic peak current is bigger than the cathodic peak current, in concordance with the number of CBZ units present in each monomer. Moreover, the cathodic peaks observed in the films are located at similar potentials than those observed for the monomers in solution, confirming that the C60 units are still present in the films and that retain their electroactivities [21,35,46]. Likewise, in the case of EDOT derivatives, electropolymerized 2CBZ films do not exhibit reduction

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processes, confirming that the reduction process observed for C60-2CBZ-film and C604CBZ-film are associated to the presence of the fullerene moieties in the films (see Fig. S4b in SI).

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The film formations and their electrochemical responses can be explained over the basis of the electrochemical characteristics of the constituent moieties. It is kwon that EDOT and its derivatives generate unstable radical cations which react to form EDOT

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dimers, being these easily oxidized than EDOT monomer. The EDOT dimers, trimers and oligomers can also undergo radical coupling forming PEDOT chains. Then, and after

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multisweep cycles in the oxidation range, these dimers and monomers can form films constituted of oligomeric and polymeric materials over the electrode surfaces [43,44,47]. On the other hand, it is known that CBZ is oxidized forming an unstable radical cation, which reacts with another radical cation through the 3-3’ positions, to form dicarbazole

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(DCBZ). DCBZ is more easily oxidized than CBZ [41]. It has also been observed that these dicarbazoles are adsorbed on the electrode surface forming polymeric films during continuous potential cycling, because oligomers of increasing chain length accumulate at

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the electrode surface and at certain length they are no longer soluble in the solution and then precipitate on the substrate [48,49]. These films presented two oxidation processes

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attributed to the formation of the radical cation and dication species of dicarbazole [48,49]. The amount of electroactive deposited material (which is proportional to the

oxidation charge of the film, obtained by integration of the voltammetric response in a solution containing only supporting electrolyte), film thicknesses, and optical absorbance can be easily controlled by changing the number of the voltammetric cycles, keeping all the others experimental variables constant (monomer concentration, solvent, support electrolyte, electrode material, temperature) [18,19,35]. Fig. 5 a-d show the variation of 14

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oxidation charge, film thickness (measured by AFM, vide infra), and light absorbance as function of the number of CV scans applied in the deposition processes for C60-2EDOTfilm, C60-4EDOT-film, C60-2CBZ-film and, C60-4CBZ-film, respectively. The oxidation

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charge, thickness, and absorbance of the four polymeric films present a linear correlation with the number of CV cycles in the studied range. In the case of CBZ functionalized fullerene compounds, C60-4CBZ-film grows around two times faster than its counterpart

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C60-2CBZ-film (∼7 and ∼13 nm/cycle respectively), indicating that the reactivity for film formation (radical cation coupling, deposition and growing) of the CBZ moieties are

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similar in both compounds [35], while C60-2EDOT-film and C60-4EDOT-film present a similar growing rate (∼10 and ∼9 nm/cycle respectively). Also, the film oxidation charge and light absorbance of C60-#EDOT-films (Figs. 5a-b) grow at similar rate, indicating that in the case of the monomer that holds four EDOT dimerizable centers, the coupling rate or

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the deposition process are less effective in comparison with C60-2EDOT monomer under

3.2. AFM

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the present experimental conditions.

AFM measurements were conducted to examine the resulting surface topography of

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the electro-synthesized films. Fig. 6a-d show representative AFM images of the films electrodeposited on ITO coated glass substrates after seven voltammetric cycles. Images are presented in the tridimensional (3D) height mode to emphasize morphology differences between film surfaces. The images are representative of the entire sample surfaces and can be described as a random distribution of globular features, where big grains are indeed an association of several smaller grains, as shown in Fig. S2. The most evident difference is

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observed between the surface height distribution of C60-4EDOT-film and C60-4CBZ-film (Fig. 6b and 6d), and C60-2EDOT-film and C60-2CBZ-film (Fig. 6a and 6c). The surface height distribution for C60-4EDOT-film and C60-4CBZ-film are narrower than C60-2EDOT-

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film and C60-2CBZ-film. In accordance with these last observations, the calculated mean square roughness (Sq) of C60-4EDOT-film and C60-4CBZ-film are also smaller (2.3 nm for the C60-4EDOT-film and 3.8 nm for the C60-4CBZ-film) than C60-2EDOT-film and C60-

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2CBZ-film (8.0 nm for the C60-2EDOT-film and 9.4 nm for the C60-2CBZ-film). The analysis of the surface topography shows that the C60-2EDOT-film and C60-2CBZ-film are

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rougher than C60-4EDOT-film and C60-4CBZ-film. This behavior could be associated to differences in the thickness between films. For the same number of voltammetric cycles the C60-4EDOT-film and C60-4CBZ-film are thicker than C60-2EDOT-film and C60-2CBZ-film (see Fig. 5) However, an analysis of the films with different thickness demonstrates that the

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surface morphology depends exclusively of the utilized monomer, and not on the number of voltammetric cycles (see Fig. S3).

3.3. Spectroelectrochemical characterization of the films

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Spectroelectrochemical studies were carried out in order to obtain information about the redox processes that undergoes the four polymeric films and to confirm the

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polymerization mechanism. C60-2EDOT and C60-4EDOT monomers in DCM solution show the typical fullerene transition bands at around 260 and 325 nm (insets in Fig. 7a-b) [24,34], while C60-2EDOT-film and C60-4EDOT-film present a wide absorption band that starts at about 700 nm and extends to the IR region (which could be due to partial oxidation of the films), and also a band in the UV region with a shoulder at 460 nm (Fig. 7a-b), that could be attributed to the π- π* transition of an extended conjugated system. These absorption bands confer a light brown coloration to the films formed over ITO electrodes. 16

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Fig. 8a-b show the difference absorption spectra at different applied potentials of C602EDOT-film and C60-4EDOT-film. It is clearly seen that when the films start to get oxidized the band at ∼ 460 nm is bleached, and at the same time a new broad band that start

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at ∼ 550 nm and that extends to 900 nm appears. At more positive potentials the 460 nm band is completely bleached and a new band in the IR region (λmax ∼ 970 nm) appears, reaching its maximum value when the film is fully oxidized (1.20 V). The optical changes

SC

produced by the different applied potentials in the principal absorption traces are shown in Fig. 8c-d. At around 0.25 V, when the films start to get oxidized the 460 nm trace decreases

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and at the same time the trace at 740 nm starts to increase. Further oxidation of the film conducts to a growth of the IR trace (970 nm) and the biggest change is observed at 1.20 V when the film is fully oxidized. Also, the 460 nm trace rises in the range of applied potentials of 0.8 to 1.2 V, showing that the second oxidation state of the polymers presents

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a second absorption band centered at around this wavelength. When the applied potential is swept in the back direction the traces show opposite changes until they reach their initial values. Contrarily to PEDOT and other PEDOT-based polymer films, where a band in the

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580-650 nm range (neutral state) and a second one that extends to the IR zone (oxidized state) have been observed [45,47], C60-2EDOT-film and C60-4EDOT-film present bands at

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450 nm (neutral state), 740 nm (semioxidized state), and 970 nm (fully oxidized state). Also PEDOT and similar based PEDOT polymers have shown only two redox states (undoped and doped) which have been associated to the mentioned absorption bands, while the electro and spectroelectrochemical studies show that C60-2EDOT-film and C60-4EDOTfilm present three different redox states. Thus, the experimental results are consistent with the presence of EDOT dimers in the films.

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On the other hand, C60-2CBZ and C60-4CBZ monomers in DCM solution present the typical C60 band with maximum at 326 nm, and three more bands at 259, 293, and 340 nm, attributed to absorption of the CBZ moieties (Inset Fig. 7c-d) [35]. C60-2CBZ-film and

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C60-4CBZ-film in the neutral state show one absorption peak at 305nm with a shoulder at ∼ 430 nm, attributed to π-π* transitions of DCBZ units [50,51], and present a light brown coloration. When the films start to oxidize two new bands appear, which are clearly

SC

observed in the absorption spectra of the films plotted as ∆Abs at different applied potentials (Fig. 9a-b); one with maximum at 425 nm and another one that extend from 600

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nm to the IR region, which keep growing until the first oxidation peak is reached. When the films are further oxidized (second oxidation peaks, see Fig. 4c-d) the last mentioned band stop growing and a new band centered at around 710 nm appears, reaching a maximum value at 1.50 V of applied potential. Fig. 9c-d shows the changes in the principal absorption

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wavelengths as function of the applied potential for C60-2CBZ-film and C60-4CBZ-film respectively. As it can be seen in both cases the bands at 425 and 1060 nm start to increase at around 0.80 V and reach their maximum at the potential of the first oxidation peak. At

EP

more positive potentials the traces at 710 nm start growing until the second oxidation peaks is reached (1.50 V). In the reverse scan the traces match again the reduction reverse peaks

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and finally recover their initial values, showing the stability of the films. The spectroelectrochemical changes observed are fully consistent with the presence of DCBZ moieties in the films generated by coupling of two CBZ radical cations [48,49]. Several CBZ containing polymers have the apparition of a band at around 420 nm and another more broad band that extends to the IR in the first oxidized state, and a band in the 700-800 nm range in the second oxidized state, being these bands associated to the formation of the

18

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radical cation of DCBZ (polaron) and to the formation of the diradical of DCBZ (bipolaron) respectively. in

the

case

of

EDOT

derivatives,

electrochemical

and

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Therefore,

spectroelectrochemical data are consistent with the presence of EDOT dimers in the polymeric materials, although the formation of trimers and tetramers is also possible, but less probable [52,53]. It should be noted that C60-2EDOT-film and C60-4EDOT-film

SC

present higher oxidation potential peaks (∼ 0.50 V) compared to PEDOT and similar

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PEDOT related polymers (-0.10 V vs SCE) [45]. The results are in agreement with other proposed polymeric structures formed by EDOT dimers [52,53], which presented absorption bands associated to three redox states, similar to those observed by us in C602EDOT-film and C60-4EDOT-film. Therefore, the generation of dimers and the consequent formation of polymeric structures where the C60 units are connected by these EDOT dimers

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are proposed (see Fig. S5a-b in SI). Also, a similar analysis can be made about the chemical structure of C60-2CBZ-film and C60-4CBZ-film. The electrogenerated polymers present two redox systems (radical cation and dication of DCBZ), typical of DCBZ unit oxidations. The

EP

electrochemical and spectroelectrochemical data are in agreement with the results already

AC C

reported [35,36], showing that the CBZ units are bounded by the 3-3’positions [49,54], as it is schematized in Fig. S5c-d in SI. The conductivity of the formed organic polymeric films is assumed that occurs by a

“hopping” mechanism where inter-chain coupling between polymer chains plays a key factor in the film conductivity [55-57]. The π-conjugation is limited to dimers and the charge hopping between the dimeric moieties in different redox states is responsible of the

19

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hole transport mechanism. In the reduced state a similar phenomenon can be proposed, where the redox processes in the C60 moieties are involved. 3.4. Surface Photovoltage

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The in-phase (x-signals) and phase-shifted by 90° (y-signals) spectral dependence of light-modulated SPV for C60-2CBZ and C60-4CBZ films are shown in Fig. 10a-b. Incidentally, the sign of the in-phase SPV signal is positive when photo-generated electrons

SC

are preferentially separated towards the internal surface. The x-signals reach a maximum (0) and the y-signals are nearly 0 (maximum) if the duration times of charge separation and

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relaxation are much shorter (longer) than the modulation period.

For the C60-2CBZ-film, the sign of the x-signals was positive, which denotes that photo-generated electrons were separated towards the internal surface. In contrast, the sign of the x-signals was negative for the C60-4CBZ-film, i.e photo-generated electrons were

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separated towards the external surface. Surprisingly, EDOT-based electro-polymers (C602EDOT-film and C60-4EDOT-film) showed only very weak or practically no SPV signals under the present experimental conditions, i.e. the EDOT dimers did not form efficient

EP

charge separated states in the polymer.

In order to analyze the origin of the observed photovoltaic effects in C60-CBZ

AC C

polymers, films of pure electropolymerized 2CBZ films were prepared (see Fig. 1) under the same conditions and the electrochemical response was similar to the one observed in Fig. 4c-d, lacking only the reduction processes associated to the presence of the fullerene moieties (see Fig. S4b in SI). Under modulated illumination, 2CBZ polymeric films did not show remarkable modulated SPV signals (Fig. 10a-b dashed lines), demonstrating that the presence of the electron acceptor C60 moieties is essential for the generation of photoinduced charge transfer states in the double cable electropolymers. However, C60-2CBZ 20

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and C60-4CBZ electropolymers showed different signs in the SPV signals, even for samples of different absorbances and thus different thicknesses. From a structural point of view, the main difference between C60-2CBZ and C60-

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4CBZ polymers is the fact that the former one is believed as formed mainly by linear chains, meanwhile C60-4CBZ can contain a more cross-linked and branched structure [3537]. As remark, the phase angle (arctan of the ratio between the y- and x-signals) of the

SC

modulated SPV spectra remained constant over the whole spectral range, indicating that charge separation, transport and recombination mechanism were independent of the photon

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energy in both cases [39,58]. For the small signals case, the PV amplitudes are proportional to the light intensity. The spectra of the SPV amplitudes of the C60-2CBZ and C60-4CBZ films were normalized to the photon flux and the corresponding spectra are shown in Fig. 10c-d. The SPV spectra matched well with the light absorption spectra of the

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electrochemical generated films, indicating that the double cable polymers containing CBZ dimers and C60 pendant moieties are able to produce photoinduced charge separated states. Fig.11a shows SPV transients for layers of 2CBZ deposited with 1, 3 and 5 CV

EP

polymerization cycles (thickness 9.8, 23.7 and 31.4 nm, respectively). The transients were negative over the whole range and set on within the duration time of the laser pulse. The

AC C

SPV values were of the same order at the short times (variation by up to 40%), i.e. initial charge separation was very similar for all three layers and can be attributed to dissociation of excitons at the ITO/2CBZ interface followed by a preferential escape of electrons towards the external surface. The transient of the layer deposited with 1 cycle decayed monotonously and could be fitted with two stretched exponential functions with time constants of about 0.8 µs and 3 ms and the corresponding stretching parameters of 0.22 and 0.25, respectively. In contrast, 21

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the SPV signals increased in time for the other layers and reached the maximum at 3 µs and 60 µs for deposition with 3 and 5 CV cycles, respectively. The increase of the SPV signal in time was caused by continued diffusion of electrons into the 2CBZ layer [58,59]. The

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increase of the time at which the maximum was reached was caused by an increase of the maximum distance between the centers of negative and positive charge with increasing number of CV polymerization cycles. The decay within 0.1 – 1 s was limited by the

SC

repetition rate of the laser pulses.

Fig.11b shows SPV transients for layers of C60-2CBZ deposited with 1, 3 and 5 CV

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cycles (thickness 9.7, 27.7 and 41.8 nm respectively). The transients were positive over the whole range and the SPV signals and were of the same order of magnitude at short times for the layers deposited with 3 and 5 CV cycles. Therefore, initial charge separation can be attributed to dissociation of excitons at the ITO/C60-2CBZ interface followed by a

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preferential escape of holes towards the external surface, i.e. electrons were trapped at the first layer of C60 molecules at the interface with ITO. The times at which were reached the maxima of SPV signals were 5 and 8 µs for the layers deposited with 3 and 5 CV cycles,

EP

respectively. This means that, in contrast to the 2CBZ layers and despite the increase of the layer thickness, the maximum distance between the centers of negative and positive charge

AC C

did increase only little with increasing number of CV cycles. The reason for this is charging of the layer due to incomplete relaxation of the separated charge within the repetition time of laser pulses.

For the C60-2CBZ layer deposited with 1 cycle, a tiny negative SPV signal was

observed within the first 10 ns and changed to positive after about 15 ns after switching on the laser pulse. This gives evidence for competition between CBZ and C60 moieties regarding the preferential trapping of holes at CBZ or electrons at C60 at the interface with 22

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ITO after dissociation of excitons. For more than one cycle of film formation, the successive polarization scans can led to a partial re-ordering of trapping sites at the interface with ITO so that the SPV transients were dominated by transport of holes for the

RI PT

layer deposited with 3 and 5 CV cycles.

Fig. 11c shows SPV transients for layers of C60-4CBZ deposited with 1, 3 and 5 CV cycles (thickness 40.7, 93.3 and 112.6 nm respectively). The transients were negative over

SC

the whole range and the SPV signals were of the similar order of magnitude at short times for the layers deposited with 3 and 5 CV cycles. Therefore, initial charge separation can be

M AN U

attributed to dissociation of excitons at the ITO/C60-4CBZ interface followed by a preferential escape of electrons towards the external surface, i.e., in contrast to C60-2CBZ layers, holes were mainly trapped at the first layer of CBZ moieties at the interface with ITO. The times at which were reached the maxima of SPV signals were 130 and 80 µs for

TE D

the layers deposited with 3 and 5 CV cycles, respectively, i.e. the SPV transient was shorter for the thicker layer. This gives evidence for massive trapping of electrons in the polymer dendrites blocking partially the transport of electrons. For the layer deposited with 1 cycle,

EP

the SPV signal was positive at the beginning of the transient and changed to negative at about 50 µs. Furthermore, the maximum negative SPV signal was reached at about 8 ms.

AC C

Therefore, both electrons and holes were transported within the C60-4CBZ layer deposited with 1 CV cycle whereas the hole transport dominated the SPV transient at short times and the electron transport at long times, i.e. electrons were trapped at C60 moieties for much longer times than holes at CBZ moieties in C60-4CBZ layers. In summary, CBZ and C60 moieties in the electrogenerated films act as traps for holes and electrons, respectively. Under successive electropolymerization CV cycles, charge is injected at the contacts and can cause a partial re-orientation of moieties at the 23

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interface with ITO which can lead to a change of initial charge separation. Furthermore, the formation of preferred percolation paths in the bulk by local alignment of moieties takes place under the successive polarization cycles and causes the build-up of dendritic

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structures. The behavior of C60-2CBZ and C60-4CBZ polymers is qualitatively different at the interface with ITO. After polarization cycles, initial charge separation was dominated by trapping of holes near the ITO interface with 2CBZ and C60-2CBZ electropolymers. In

SC

contrast, initial charge separation was dominated by trapping of electrons at the ITO interface with C60-4CBZ electropolymer. It seems that this effect is caused by the formation

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of ladder structures in the C60-4CBZ electropolymer.

4. Conclusions

Four structurally related monomers containing electron donor (EDOT or CBZ) and

TE D

electron acceptor (C60 Buckminster fullerene) moieties were synthesized in two steps by a methodology that allows to obtain them with good yields, using inexpensive and commercially available starting materials. Electrochemical oxidation of the monomers

EP

induces the formation ambipolar polymers as films on Pt and ITO electrodes. The cyclic voltammetry methodology permits a fine control of the amount of deposited active material

AC C

and film thickness. The electrochemical and spectroelectrochemical characterization of the polymers allows to propose the formation of EDOT and CBZ dimers by coupling of the corresponding radical cations. These dimers are the structural building blocks of the polymeric materials that also contain electroactive pendant C60 units. Under positive applied potentials, all the synthesized conductive materials generate UV-visible-near IR absorption bands that are in agreement with the formation of two oxidized states in the electrochemical generated dimers. Thus, the four polymeric films can be positively and 24

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negatively charged by oxidation of the EDOT or CBZ dimeric moieties and by electrochemical reduction of the C60 fullerene units respectively. The surface photovoltage spectroscopy analysis demonstrates that under monochromatic illumination C60-CBZ

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polymers are able to produce electron-hole preferential separation in the photoelectrodes. The SPV measurements showed that the initial charge separation depends very sensitively to the configuration of CBZ and C60 moieties at the interface with ITO were excitons

SC

dissociate. The initial charge separation was dominated by trapping of holes at CBZ units at the ITO interface with 2CBZ and C60-4CBZ electropolymers, while the initial charge

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separation was dominated by trapping of electrons at C60 moieties at the ITO interface with C60-2CBZ films. Acknowledgements

Authors are grateful to Secretaría de Ciencia y Técnica, Universidad Nacional de Río

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Cuarto (Secyt-UNRC), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Agencia Nacional de Promoción Científica y Tecnológica (ANPCYT) of Argentina for financial support. The authors would like to thank Mg. Pablo Duché (IQUIR

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References

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- NMR Service) for data collection.

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Fig. 1. Synthetic steps and Chemical structures of: 1, 2, 2EDOT, 2CBZ, C60-2EDOT, C60-4EDOT, C60-

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2CBZ, and C60-4CBZ molecules. Reagents and conditions: a) malonyl dichloride, pyridine, DCM, reflux, 10 h (2EDOT, 70 %); b) fullerene C60, DBU, CBr4, toluene, room temp., 6 h (C60-2EDOT 23 %, C60-4EDOT 14 %); c) malonyl dichloride, pyridine, DCM, reflux, 10 h (2CBZ, 77 %); d) fullerene C60,

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DBU, CBr4, toluene, room temp., 6 h (C60-2CBZ 33 %, C60-4CBZ 18 %).

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Fig. 2. First anodic and cathodic scans of: a) C60-2EDOT, b) C60-4EDOT, c) C60-2CBZ, and d) C604CBZ monomers at different inversion potential ranges. Pt electrode, scan rate 100 mV/s.

Fig. 3. Ten consecutive cyclic voltammograms of: a) C60-2EDOT, b) C60-4EDOT, c) C60-2CBZ, and d)

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C60-4CBZ monomers. Pt electrode, scan rate 100 mV/s. Red lines: first cycle, blue lines: second cycle.

Fig. 4. Cyclic voltammograms of: a) C60-2EDOT-film, b) C60-4EDOT-film, c) C60-2CBZ-film, and d)

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C60-4CBZ-film, deposited on a Pt electrode, in a solution containing only supporting electrolyte. Scan rate 100 mV/s. Inset: Cyclic voltammograms of: a) C60-2EDOT-film, b) C60-4EDOT-film, deposited on

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a Pt electrode at 25 mV/s, in a solution containing only supporting electrolyte.

Fig. 5. Oxidation charge, thickness, and Absorbance vs number of polymerization cycles of: a) C602EDOT-film, b) C60-4EDOT-film, c) C60-2CBZ-film, and d) C60-4CBZ-film, deposited on ITO electrode.

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Fig. 7. Absorption spectra of: a) C60-2EDOT-film, b) C60-4EDOT-film, c) C60-2CBZ-film, and d) C604CBZ-film, deposited on an ITO electrode. Insets: Absorption spectra of the corresponding monomers

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Fig. 8. Difference spectra of: a) C60-2EDOT-film, b) C60-4EDOT-film. Absorption traces of: c) C60-

the applied potential. Scan rate 20 mV/s.

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2EDOT-film, d) C60-4EDOT-film, deposited on ITO electrode at selected wavelengths as function of

Fig. 9. Difference spectra of: a) C60-2CBZ-film, and d) C60-4CBZ-film. Absorption traces of: c) C60-

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2CBZ-film, and d) C60-4CBZ-film, deposited on ITO electrode at selected wavelengths as function of the applied potential. Scan rate 20 mV/s.

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Fig 10. SPV (surface photovoltage) spectra of electropolymerized films of a) C60-2CBZ-film, b) C604CBZ-film. SPV spectra of 2CBZ (red and black dotted lines). SPV spectra photovoltage amplitude,

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normalized to the photon flux of c) C60-2CBZ-film, d) C60-4CBZ-film. Film where obtained by five CV cycles under the experimental conditions described in the experimental section.

Fig 11. SPV transients excited at 450 nm for samples of 2CBZ a), C60-2CBZ b) and C60-4CBZ c) deposited with 1, 3 and 5 cycles (black, red and blue lines, respectively). The arrows mark the onset of the laser pulses.

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Highlights for:

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Electrochemical, Spectroelectrochemical and Surface Photovoltage Study of Ambipolar C60-EDOT and C60-Carbazole Based Conducting Polymers.

Four C60-based donor-acceptor monomers are electropolymerized on Pt and ITO.

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The polymers can oxidized and reduced as part of an ambipolar structure.

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The presence of EDOT or DCBZ dimers in the polymer structures is confirmed.

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Photoinduced charge separation and charge migration are observed in the films.

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Photovoltage spectra fully match the absorption spectra of the polymeric films.

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