Article pubs.acs.org/JPCC
Conductive PEDOT Covalently Bound to Transparent FTO Electrodes Stefano Carli,*,† Laura Casarin,† Giacomo Bergamini,‡ Stefano Caramori,† and Carlo Alberto Bignozzi*,† †
Department of Chemistry and Pharmaceutical Sciences, University of Ferrara, Via Fossato di Mortara 17-27, 44121 Ferrara, Italy Department of Chemistry G. Ciamician, University of Bologna, Via Selmi 2, Bologna 40126, Italy
‡
S Supporting Information *
ABSTRACT: A new 3,4-ethylenedioxythiophene (EDOT) monomer derivatized with aminopropyl-triethoxysilane (APTES-EDOT) was prepared via a simple two step reaction in high yield. The new monomer can be firmly grafted to the fluorine−tin-oxide (FTO) conductive glass, where the irreversible electro-oxidation of surface bound APTES-EDOT, in the presence of unsubstituted EDOT monomers in solution, triggers the cationic polymerization of EDOT, resulting in the incorporation of PEDOT chains into APTES-EDOT. As a result, the modified PEDOT film (Si-PEDOT) is covalently bound to the FTO surface and easily withstands mechanical stresses that are critical for the adhesion of regular PEDOT. When tested with Co(III)/(II) redox shuttles, electrodeposited Si-PEDOT films showed decreased charge transfer and mass transport resistances with respect to both platinum and conventional PEDOT films, leading to enhanced relative efficiencies (≈10%) when employed as counter electrode in transparent dye sensitized solar cells.
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INTRODUCTION Poly(3,4-ethylenedioxythiophene) (PEDOT) was first synthesized by Jonas et al., from Bayer AG Laboratories in the late 1980s.1 Since then, the intrinsically conductive polymer PEDOT has been intensively studied for the past three decades due to its high conductivity (up to 300 S cm−1) and excellent stability.2 PEDOT has found many applications in organic lightemitting diodes,3 electrochromic windows,4 sensors5 and organic solar cells.6 In the oxidized doped state PEDOT has good transparency in the UV−vis region, helping its adoption as counter-electrode material in dye sensitized solar cells (DSSCs) in alternative to the most widely used platinum.7,8 Recently, we have examined the performance of various types of PEDOT-coated fluorine-doped tin oxide (FTO) conductive glasses as counter electrodes in DSSCs, containing Co(III)/(II) complexes as electron mediators, trying to clarify the key factors affecting cell performances.9 It was concluded that the use of PEDOT electrodes in association with Co(III)/(II) couples in this type of solar devices produces enhanced photocurrents, with respect to Pt and Au counter electrodes, because of a decreased mass transport resistance arising from a larger electroactive area of the polymer-based material, which was produced through potentiostatic electropolymerization of 3,4ethylenedioxythiophene (EDOT). One of the main problems for the practical application of PEDOT as a counter electrode is, however, the weak adhesion to the conductive glass of the solar device, which may result in the loss of catalytic material covering the electrode, affecting the long-term operation of the solar device. In order to circumvent this problem, which may become crucial in the dimensional scaling up of the counter electrodes for large area (>10 cm2) © 2014 American Chemical Society
solar devices, we have prepared a new alkoxysilane substituted EDOT monomer (Figure 1), which can be firmly grafted to the
Figure 1. Structure of the silanized monomer EDOT-APTES (1).
FTO conductive glass. The irreversible electro-oxidation of surface bound EDOT-APTES, in the presence of unsubstituted EDOT monomers in solution, triggers the cationic polymerization of EDOT, resulting in the nucleation and growth of PEDOT on the modified FTO surface, finally leading to a strongly adherent conducting polymeric film with good electrocatalytic properties with respect to the Co(III)/(II) couple. We report here on the preparation of the new EDOT monomer and on the morphological and electrochemical characterization of PEDOT films electrochemically grown on Special Issue: Michael Grätzel Festschrift Received: December 30, 2013 Revised: February 25, 2014 Published: March 13, 2014 16782
dx.doi.org/10.1021/jp412758g | J. Phys. Chem. C 2014, 118, 16782−16790
The Journal of Physical Chemistry C
Article
distance between cofacially assembled FTO and Ti electrodes was ca. 2.5 mm. PEDOT depositions were carried out by either potentiostatic (step potential program: 0 V, 5 s; 0.2 V, 5 s; 0.5 V, 5 s; 1.6 V, 30 s) or potentiodynamic (2 subsequent scans in the potential range 0−1.7 V at a scan rate of 50 mV s−1) procedures by using a 10 −2 M EDOT/0.1 M LiClO 4 acetonitrile solution. The freshly deposited PEDOT electrodes were carefully washed with acetonitrile to remove residual oligomers weakly interacting with the surface. Solar Cell Fabrication. Mesoporous titania films (ca. 6 μm thick) were prepared by blading a commercial colloidal TiO2 paste on FTO electrodes, which were left to dry under a gentle warm air stream before sintering at 450 °C for 30 min. The resulting transparent films were immersed in a 0.4 M TiCl4 solution for 12 h, rinsed with water and fired at 450 °C for 30 min. Finally, the photoanodes were immersed in a 0.2 mM solution of MK2 in a 1:1 toluene/acetonitrile mixture for 24 h. Solar cells were assembled in open configuration by holding the two electrodes together with metallic clamps and by using a 25 μm thick Surlyn frame as sealer. The active cell area was 0.25 cm2. A similar configuration was used for symmetrical dummy cells consisting of two identical PEDOT electrodes working, respectively, as anode and cathode, separated by a ca. 100 μm thick parafilm frame. Electrolyte Formulation. The electrolytes used in the dummy cells consisted either of 0.05 M Co(bpy) 32+ and 0.05 M Co(bpy) 33+ or of the less soluble 0.03 M Co(dtb)32+ and 0.03 M Co(dtb)33+ in the presence of 0.1 M LiClO4 in acetonitrile. Two different types of acetonitrile-based electrolytes (type A and B) were evaluated in DSSCs. Type A electrolyte consisted of Co(bpy)33+/2+: 0.18M, Co(II)/0.028M,Co(III)/0.1 M Li(CF3SO3)/0.2 TBP (TBP = 4-tert-butylpyridine). Type B electrolyte was based on Co(dtb)33+/2+: 0.18 M, Co(II)/ saturated Co(III),/0.1 M Li(CF3SO3)/0.2 TBP, in acetonitrile. Counter Electrode Characterization in Dummy Cells. The catalytic properties of the counter electrodes (CEs) were investigated in thin layer (100 μm) dummy cells, with a geometric active area of 0.25 cm2, by linear sweep voltammetry (LSV) at 5 mV s−1 and by electrochemical impedance spectroscopy (EIS), by superimposing a sinusoidal 10 mV perturbation at a fixed DC potential (0 mV, or where specified, at 0.05 and 0.1 V). Impedance data were analyzed using commercially available ZSimpWin 3.21 software and were fitted in terms of the equivalent electric circuits shown in Scheme 3 with relative errors
