Article pubs.acs.org/JPCC
Poly(3,4-ethylenedioxythiophene) Hole-Transporting Material Generated by Photoelectrochemical Polymerization in Aqueous and Organic Medium for All-Solid-State Dye-Sensitized Solar Cells Jinbao Zhang,†,# Lei Yang,†,# Yang Shen,† Byung-Wook Park,† Yan Hao,† Erik M. J. Johansson,† Gerrit Boschloo,† Lars Kloo,‡ Erik Gabrielsson,∥ Licheng Sun,∥ Adel Jarboui,§ Christian Perruchot,§ Mohamed Jouini,§ Nick Vlachopoulos,*,† and Anders Hagfeldt†,§,⊥ †
Department of Chemistry-Ångström Laboratory, Uppsala University, 75120 Uppsala, Sweden Applied Physical Chemistry, Department of Chemistry, KTH-Royal Institute of Technology, Teknikringen 30, SE-10044 Stockholm, Sweden § Université Paris Diderot Paris 7, Sorbonne Paris Cité, ITODYS UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France ∥ Organic Chemistry, Department of Chemistry, KTH-Royal Institute of Technology, Teknikringen 30, SE-10044 Stockholm, Sweden ⊥ School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea ‡
S Supporting Information *
ABSTRACT: We applied organic donor-π-acceptor (D-π-A) sensitizers for photoelectrochemical polymerization (PEP) because of their appropriate energy levels and high light absorption. The polymerized conducting polymer PEDOT was used as hole conductor in all-solid-state dye-sensitized solar cells (ssDSCs). By combination of the D-π-A sensitizers and the generated PEDOT from PEP of bis-EDOT in acetonitrile, the resulting device showed an average power conversion efficiency of 5.6%. Furthermore, the PEP in aqueous micellar electrolytic medium was also employed because of the ability to decrease oxidation potential of the precursor, thereby making the polymerization process easier. The latter method is a cost-effective and environmentally friendly approach. Using as hole conductor the so-obtained PEDOT from PEP of bis-EDOT in aqueous electrolyte, the devices exhibited impressive power conversion efficiency of 5.2%. To compare the properties of the generated polymer from bis-EDOT in these two PEP methods, electron lifetime, photoinduced absorption (PIA) spectra, and UV−vis−NIR spectra were measured. The results showed that PEDOT from organic PEP exhibits a delocalized conformation with high conductivity and a smooth and compact morphology; a rough morphology with high porosity and polymer structure of relatively shorter chains was assumed to be obtained from aqueous PEP. Therefore, better dye regeneration but faster charge recombination was observed in the device based on PEDOT from aqueous PEP of bis-EDOT. Subsequently, to extend the aqueous PEP approach in consideration of the ability to decrease the oxidation potential of the precursor, the easily available precursor EDOT was for the first time used for PEP in aqueous medium in a variant of the aforementioned procedure, and the device based on the so-obtained PEDOT shows a more than 70-fold increase in efficiency, 3.0%, over that based on the polymer generated from EDOT by PEP in organic media. It was demonstrated that aqueous micellar PEP with EDOT as monomer is an efficient strategy for generation of conducting polymer hole-transporting materials.
1. INTRODUCTION Dye-sensitized solar cells (DSCs) have exhibited great potential as energy conversion devices for providing a sustainable electricity source from the sun because of their low fabrication costs, good stability, and promising conversion efficiency. The first fundamental studies on efficient high surface area TiO2 electrode sensitization were made in the 1980s and then followed by the seminal work in 1991 by O’ Regan and Grätzel on the first efficient electricity-producing DSCs based on colloidal TiO2 layers.1−3 DSCs consist of an n-type electronic © 2014 American Chemical Society
conductor, typically porous semiconductor titanium dioxide, sensitized by a monolayer of sensitizer and filled with a chargetransport medium, either an electrolyte containing a redox mediator, in liquid or solid polymer electrolyte-based DSCs, or Special Issue: Michael Grätzel Festschrift Received: December 20, 2013 Revised: April 8, 2014 Published: April 21, 2014 16591
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with proper energy levels and light absorption ability because of the fact that π-units in the D-π-A structure are used to control the HOMO−LUMO gap, which can determine the photon absorption edge.30 Beyond that, another way to engender efficient PEP is to use a precursor with a proper onset oxidation potential. Initially, pyrrole was used as precursor; however, this lead to a nontransparent polymer.31,32 Recently, 3,4-ethylenediopyrrole (EDOP) has been reported for the first time as a precursor of PEP by combination with D-π-A-based sensitizers, which gives 3.0% efficiency by efficient suppression of charge recombination.21 bis-EDOT (2,3-dihydro-5-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)thieno[3,4-b][1,4]dioxine)23−25,33−35 has been mostly used as precursor for PEP in organic media because of its low oxidation potential compared to the redox potentials of the dyes; bis-EDOT is also preferred because of its higher reactivity and solubility as compared to that of the alternative triethylenedioxythiophene (tri-EDOT). In this work, bis-EDOT was employed as precursor of PEDOT, and by combination of D-π-A-based sensitizers and PEDOT HTM, the devices show excellent photovoltaic performance, with an average power conversion efficiency of 5.6%. Moreover, to have a lower electrochemical polymerization potential of the precursor, PEP in aqueous micellar electrolyte was employed;29 additional advantages in aqueous medium are low cost and environmental compatibility.36−42 At present, by using aqueous PEP of the precursor bis-EDOT, the generated PEDOT-based devices with a D-π-A sensitizer show the impressive photoconversion efficiency of 5.2%. In addition, to extend the aqueous PEP approach, the oneunit precursor EDOT was employed for the first time as precursor for in situ PEP to generate PEDOT instead of bisEDOT because EDOT can be oxidized at lower potential when water is used as solvent, compared with that in organic medium. In this work both an organic and an aqueous PEP medium were investigated. The devices based on PEDOT obtained from EDOT as precursor and D-π-A sensitizer in these two media exhibited power conversion efficiency of 0.04% and 3.0%, respectively. The high efficiency of the device based on PEP of EDOT in aqueous solvent may result from a more efficient polymerization reaction in this medium.
a solid hole-transporting material (HTM) in solid-state DSCs (ssDSCs).4−6 Charge separation can occur at the interface by injection of electrons from the photoexcited sensitizer into the conduction band of the semiconductor oxide.7 Subsequently, the sensitizer in the oxidized form can be regenerated by the surrounding hole transporter. Therefore, the charge-transport medium is one of the most critical components in DSCs, affecting the rate of dye regeneration, determining the photovoltage of DSCs system, and therefore playing a key role in the photoconversion efficiency. Several types of redox couple-based liquid DSCs have shown above 10% energy conversion efficiency.8−10 However, liquid DSCs may suffer from electrolyte leakage, electrode corrosion, and long-term stability. Therefore, solid-state hole-transporting materials, replacing the liquid electrolyte, have attracted a lot of attention.11−13 Spiro-OMeTAD has been one of most popular and favorable HTMs because of good solubility in common solvents and easy fabrication process by spin coating.14−16 However, it has low conductivity (10−4 cm2 V−1 s−1), high cost, and poor infiltration in metal oxide pores, thereby limiting the large-scale application of ssDSCs.17 This has motivated different research groups to search for new alternative HTMs.18−21 Among the various options, conjugated polymers have been widely investigated as potential HTMs in ssDSCs because they have high conductivity, great thermal stability, and switchable optical and redox properties.22 In this respect poly(3, 4-ethylenedioxythiophene), namely, PEDOT, has been the most outstanding representative polymer hole conductor. Initially, a rather poor photoconversion efficiency was obtained using the so-called RuN719 dye because of strong interfacial electron recombination.23 When the doping anions of PEDOT were optimized, a great enhancement of photovoltaic performance was obtained by using the anion TFSI− as dopant and, initially, Ru-Z907 as sensitizer.24 In further studies, to suppress the high interfacial charge recombination and control polymer penetration, an organic indoline-type dye, D149, was efficiently used with PEDOT as HTM in ssDSCs, and the corresponding device showed a promising efficiency of 6.1%.25 Because of the low solubility of PEDOT and to improve its contact as HTM with dye molecules, in situ photoelectrochemical polymerization (PEP) is the most common method of generating a PEDOT film at the dye/TiO2 electrode interface.21,26−28 During PEP, irradiation by light can excite the sensitizer, and the oxidized dye molecules can work as active sites to initiate and maintain the polymerization reaction. Photogenerated holes on the highest occupied molecular orbital (HOMO) level of dyes can oxidize the precursor and trigger oxidized precursor coupling. Hence, the choice of both employed precursor and dye sensitizer significantly influences the polymerization process. At first glance, one of the critical requirements for the dye sensitizers would be that they should have a more positive redox potential (E°Dye+/Dye) than the onset oxidation potential of the precursor (Eonset) because this potential difference could be considered as a kind of driving force of PEP, determining the rate of polymerization process and the properties of the polymer produced inside the TiO2 pores. Another consideration of dye sensitizers is that there is often a trade-off between a wide absorption spectra range and a high extinction coefficient, both of which affect the ability to utilize the light. In this work, different donor-π-acceptor (D-πA)-based sensitizers were applied to polymerize suitable precursors to get a regular polymer29 by choosing the dyes
2. EXPERIMENTAL SECTION 2.1. Chemicals Used. All chemicals were purchased from Sigma-Aldrich unless otherwise indicated. Bis-EDOT (K192, Kairon Chemicals, France) was used for photoelectrochemical polymerization. Dyes D35 and LEG4 were supplied by Dyenamo AB (Sweden).43,44 Z907 was purchased from Dyesol (Australia). 2.2. Device Fabrication. The fluorine-doped SnO2 (FTO, 15 Ω-square) substrates were etched with zinc powder and HCl (concentration 3 M) to form the desired electrode pattern. The substrates were cleaned in an ultrasonic bath for 30 min in the following order: deionized water, acetone, and ethanol. A compact layer of TiO2, intended to block the recombination current at the FTO support, was prepared on cleaned FTO substrate by spray pyrolysis according to the published procedure.45 To generate the porous, dye-adsorbing TiO2 layer, the colloidal TiO2 paste was doctor-bladed on the compact layer surface then sintered by a stepwise heating process: 180 °C for 10 min, 320 °C for 10 min, 400 °C for 10 min, and 450 °C for 30 min in an oven (Nabertherm Controller P320) in air atmosphere to form the nanoporous TiO2 film. After the sintering was complete, the films were 16592
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Figure 1. Chemical structures of different sensitizers (a) Z907, (b) D35, and (c) LEG4.
TiO2 electrode, the same current density of 8 μA cm−2 was applied for 2000 s; and for the nonaqueous polymerization with the dye Ru-Z907/TiO2, 6 μA cm−2 and 3000 s. For the aqueous micellar polymerization of bis-EDOT with dye LEG4/TiO2 and dye D35/TiO2, the respective conditions were 6 μA cm−2 and 2000 s; for aqueous micellar polymerizations of EDOT with dye LEG4/TiO2 and dye D35/TiO2, 6 μA cm−2 and 2500 s were used, respectively. The electrode potential was monitored against the reference electrode so that it did not overstep a stipulated positive potential limit, 0.6 V/SHE (standard hydrogen electrode), to prevent both the overoxidation of the formed conducting polymer and its generation at sites where the underlayer could be damaged; in the latter case the FTO substrate could be directly exposed to the electrolyte. After the electrolyte was rinsed off, the films were washed using ethanol and then dried by N2 flow. 2.4. Electrochemical Measurement. A three-electrode electrochemical cell was used for the cyclic voltammetry experiments. The supporting electrolyte for electrochemical measurements was either (a) organic electrolyte, 0.1 M LiN(CF3SO2)2 in acetonitrile, or (b) aqueous electrolyte, 0.1 M LiN(CF3SO2)2 and 50 mM Triton X-100 in water. In all cases, stainless steel was used as the counter electrode with an area of 3 cm2. The reference electrode was in all cases aqueous Ag/AgCl/3 M NaCl; a salt bridge electrolyte was interposed between the working and reference electrode, containing 0.1 M LiN(CF3SO2)2 in water or acetonitrile, depending on the working electrode solvent. For experiments in organic electrolyte, the reference electrode was calibrated with ferrocene in 1 M KCl; the literature value of 0.624 V/SHE was used for conversion of potential values.46 For the experiments in aqueous medium, the reference electrode was calibrated with ferricyanide in the same electrolyte, for the redox potential of which the literature value of 0.471 V/SHE (or 0.266 V/SCE, versus the standard calomel electrode) was used for conversion of potential values.47 All potentials are reported versus SHE. For measurements of the oxidation potentials of bis-EDOT in (a) organic and (b) aqueous solution, a glassy carbon disk (d = 3 mm) was used as working electrode, which was carefully polished with 0.05 μm alumina slurry before every experiment. For measurements of dye redox potentials, the dye was chemisorbed at mesoporous TiO2/FTO electrodes (area of 1 cm2) without blocking compact underlayer; dye/TiO2/FTO served as the working electrode for redox potential determination. 2.5. Solar Cell Characterization. Current−voltage characteristics were measured under 100 mW cm−2 (AM 1.5G illumination) using a Newport solar simulator (model 91160)
treated with 40 mM TiCl4 aqueous solution at 70 °C for 30 min; in this way an overlayer is formed, increasing the roughness factor and therefore the dye uptake. After subsequent rinsing with deionized water and ethanol, the film was sintered again by following the aforementioned procedure. After being cooled to 90 °C, the hot TiO2 film was immersed into the dye solution for 18 h. The dyes used for this purpose were the ruthenium dye Ru-Z907 (0.2 mM in a mixture of acetonitrile and tert-butyl alcohol, v/v 1:1) and two metal-free organic dyes: D35 (0.2 mM in ethanol) and LEG4 (0.2 mM in a mixture of acetonitrile and tert-butyl alcohol, v/v 1:1). The dye structures are shown in Figure 1. The films were rinsed in ethanol and dried by a N2 gas flow. The dye-sensitized TiO2 layer was covered by the hole-transporting material PEDOT by means of PEP. After polymerization, the electrode was carefully washed and then underwent a post-treatment procedure by one drop of solution consisting of 0.02 M LiN(CF3SO2)2 and 0.2 M 4-tert-butylpyridine (TBP) in acetonitrile. Finally, after removal of remaining acetonitrile by drying, a 200 nm thick Ag (SigmaAldrich; ≥ 99.99% trace metals basis) contact layer, forming the dark counter electrode of the device, was deposited onto the PEDOT organic semiconductor layer, covering the dyesensitized TiO2 electrode, by thermal evaporation in a vacuum chamber (Leica EM MED020) with a base pressure of about 10−5 mbar to complete the device. 2.3. Photoelectrochemical Polymerization (PEP). A three-electrode cell was employed for PEP. The FTO substrates supporting the dye-sensitized TiO2 layers were used as working electrodes. The counter electrode and reference electrode were stainless steel and aqueous Ag/AgCl (3 M NaCl), respectively. Two alternative electrolytes for polymerization of bis-EDOT were chosen: (a) organic electrolyte, with 10 mM bis-EDOT and 0.1 M LiN(CF3SO2)2 in acetonitrile, and (b) aqueous micellar electrolyte, a saturated solution of bis-EDOT (solubility below 1 mM) in an aqueous solution containing 2% v/v acetonitrile, 0.1 M LiN(CF3SO2)2, and 50 mM TritonX-100 surfactant as solubilization medium. For the nonaqueous PEP of EDOT, a solution of 10 mM EDOT and 0.1 M LiN(CF3SO2)2 in acetonitrile was used. For the aqueous PEP, an aqueous solution containing 10 mM EDOT, 50 mM TritonX-100. and 100 mM LiN(CF3SO2)2 was used. The white light was provided by a LED lamp (ELFA Distrelec, Light Injector, white, 6000 K-6°, 3 W, 4.5 V, 700 mA). The illumination was from the back (glass) side of the FTO/TiO2 layer. For the nonaqueous polymerization of bis-EDOT with the dye LEG4/TiO2 and dye D35/TiO2 electrodes, a current density of 8 μA cm−2 was applied for 1500 s; for the nonaqueous polymerization of EDOT with the dye D35/ 16593
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The cyclic voltammetry curve of bis-EDOT in acetonitrilebased electrolyte is shown in Figure SI-1a of the Supporting Information. The onset oxidation potential of bis-EDOT at 10−2 M is 0.9 V/SHE. Cyclic voltammetry curves in organic medium of three dyes, LEG4, D35, and Ru-Z907, on TiO2 film were also investigated, as shown in Figure SI-1d,e of the Supporting Information. The redox potentials for LEG4 and D35 are 1.06 V/SHE and 1.16 V/SHE, respectively. For Z907, the redox potential is 1.05 V/SHE, as shown in Figure SI-1d of the Supporting Information. The redox potential of each dye is higher than the onset oxidation potential of the bis-EDOT electrochemical polymerization at a conductive substrate in an acetonitrile-based electrolyte, which demonstrates that these three dyes can be used for PEP of bis-EDOT in an organic electrolyte. The onset oxidation potential of bis-EDOT ( 2) precursor units, after its oxidation (HPmH)+• (step 5), there are different ways to continue coupling. One possibility is to couple with (HPH)+•, resulting in an oligomer with (m + 1) units (step 5.1). Another possibility is to couple with another oligomer (HPqH)+• so that longer chains HPm+qH are created (step 5.2). For the PEP in organic medium, the concentration of precursor is 10 mM. Therefore, a sufficiently high number of precursor molecules are available in the pores to come into contact with dye molecules. After illumination, a large amount of (HPH)+• is created during the photo-oxidation process on the surface of the dye/TiO2 working electrode. As a result, upon further polymerization, the oligomer species (HPmH)+• has more chances to couple with a unit of oxidized precursor (HPH)+•. Therefore, this process is easy to proceed because of the high concentration of precursor, so that a regular, smooth, ordered and, finally, highly doped polymer with longer chains is formed. In contrast, in an aqueous electrolyte, micelles are introduced into the system by adding a surfactant, e.g., TritonX-100, to enhance the precursor solubility. However, the concentration of precursor is usually much lower than that in organic medium (
