Neutral, Polaron, and Bipolaron States in PEDOT Prepared by

Article pubs.acs.org/JPCC

Neutral, Polaron, and Bipolaron States in PEDOT Prepared by Photoelectrochemical Polymerization and the Effect on Charge Generation Mechanism in the Solid-State Dye-Sensitized Solar Cell Byung-wook Park,† Lei Yang,† Erik M. J. Johansson,*,† Nick Vlachopoulos,† Amani Chams,‡ Christian Perruchot,‡ Mohamed Jouini,‡ Gerrit Boschloo,† and Anders Hagfeldt† †

Department of Chemistry−Ångström Laboratory, Uppsala University, Box 523, SE 751 20 Uppsala, Sweden Laboratoire ITODYS, University Paris Diderot, Sorbonne Paris Cité, UMR 7086, 15 rue Jean Antoine de Baïf, F-75205 Paris 13, France

‡

S Supporting Information *

ABSTRACT: We investigate dye-sensitized solar cells (DSSCs) based on PEDOT as hole conductor and prepared by photoelectrochemical polymer deposition at different light intensities. We specifically investigate the effect of light intensity on the PEDOT polymer and in turn the efficiency of the solar cells. We find that the PEDOT prepared by this method is largely oxidized and contains significant amounts of polarons and bipolarons and only a small fraction of neutral PEDOT. Photoelectrochemical polymer deposition under low light intensity leads to a particularly low fraction of neutral PEDOT and a high fraction of bipolarons as measured in the UV−vis spectra. The solar cells based on PEDOT as a hole conductor prepared under these conditions are the most efficient with a higher power conversion efficiency, which can be explained by a longer electron lifetime, faster charge transport, and higher transparency of the PEDOT. Interestingly, we conclude that in this type of solid-state DSSCs the mechanism of dye regeneration occurs from PEDOT polarons that then form bipolarons, which is different from the mechanism of dye regeneration proposed in standard solid-state DSSCs. pores.21−35 To overcome this limitation, the photoelectrochemical polymer deposition (PEPD) technique is more useful than casting and related techniques.20 Indeed, the infiltration of monomer molecules in the pores of the dye-modified TiO2 helps the formation of the polymer inside the nanopores. There have been many attempts to apply PEDOT in ss-DSSCs; significant progress has been achieved using the PEPD technique by Yanagida et al.18,19,24,25 In the present work, one specific aspect of focused investigation is the PEPD technique under variable light intensity in regard to its effects on formation of PEDOT, solar energy conversion mechanism, and solar cell performance.

1. INTRODUCTION The dye sensitized solar cell (DSSC) is a promising alternative solar cell. The research field of DSSC has received attention partly because of announced remarkable photo conversion efficiency approaching 12% under 1 sun of air mass1,5 global tilt (AM 1.5G),1−4 the ease of fabrication processes, the use of earth abundant materials, and the possibility of various practical outdoor and indoor applications.1,5 The most common variant of DSSCs is based on mesoscopic nanocrystalline titanium dioxide (TiO2) and an electrolyte with a redox mediator.6−9 In the solid-state DSSC (ss-DSSC) the liquid electrolyte has been replaced by solid hole conductors (also called hole-transporting materials (HTMs)), which may give practical advantages.10−19 The most-represented HTMs for ss-DSSCs are 2,20,7,70tetrakis(N,N-di-p-methoxyphenylamine)-9,90-spirobifluorene (spiro-MeOTAD), 1 0 , 1 2 , 1 3 , 1 7 poly(3-hexylthiophene) (P3HT) 1 4 , 1 5 and poly(3,4-ethylenedioxythiophene) (PEDOT). 16,18,19 In addition to ss-DSSC applications, PEDOT has also been extensively used in other applied fields such as organic diodes, polymer thin film solar cells, and organic light-emitting diodes.20 Specifically, PEDOT has also been used in ss-DSSCs with high stability.17 In ss-DSSCs, the infiltration of the HTM in the nanoporous TiO2 may be difficult because of low polymer chain diffusion in the © 2013 American Chemical Society

2. EXPERIMENTAL DETAILS 2.1. Procedure of Device Fabrication. A compact TiO2 layer was first deposited onto the surface of a precleaned FTO (Nippon sheet glass, 10 Ω cm−2) substrate by spray pyrolysis on a hot plate at 550 °C using an air brush at a distance of about 15 cm between the FTO conducting surface and the nozzle of the air brush; the thickness was controlled by the Received: July 1, 2013 Revised: September 29, 2013 Published: October 2, 2013 22484

dx.doi.org/10.1021/jp406493v | J. Phys. Chem. C 2013, 117, 22484−22491

The Journal of Physical Chemistry C

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output of an AM 1.5G solar simulator. In all cases, the total charges of the anodic photopolymerization were about 40 mC cm−2. This allows us to obtain the same amount of polymer deposited on working electrodes. For all experiments, an IviumStat potentiostat was used (Ivium Technologies, The Netherlands). The PEDOT-modified electrodes thus obtained underwent a post-treatment consisting of casting of 20 μL of solution containing 10 mM lithium bis-(trifluoromethaneslufone) imide (LiTFSI) and 180 mM 4-tert-butylpyridine (tBP) in acetonitrile. For UV−vis−NIR measurements, the film thickness of TiO2 was about 1.5 μm; this facilitates the measurements of the absorption spectra with high accuracy. In these samples, the polymerization charge density was 30 mC cm−2 and the light intensities were 0.5, 0.3, and 0.1 sun during PEPD. The spectra were measured approximately 24 h after the preparation of the samples. 2.3. Measurement of UV−Visible−NIR spectra. To measure UV−visible−NIR absorption spectra, sensitizermodified TiO2 electrodes and PEDOT photoelectrochemically deposited on working electrodes were prepared according to Procedure of Device Fabrication, and the measurements were performed on a Cary 5000 UV−vis−NIR spectrophotometer (VARIAN; photometric accuracy is 97%, supplied by KaironKem, France) were solubilized in acetonitrile as solvent containing 100 mM 1ethyl-3-methylimidazolium bis-(trifluoromethaneslufone) imide (EMITFSI) as electrolytic salt. The electrolyte was purged by nitrogen gas which prevents the oxidizing of PEDOT by oxygen during PEPD. A three-electrode one-compartment cell was used with a sensitizer-coated TiO2 on FTO as working electrode, a stainless steel plate as counter electrode, and an aqueous Ag/AgCl in 3 M NaCl as reference electrode. The dyecoated TiO2 working electrode was immersed into the monomer solution. This leads to the introduction of the monomer into the pores of the dye-coated TiO2. The PEDOT polymer was obtained by photoelectrochemical oxidation of bis-EDOT, leading to polymer deposition inside the nanopores of the dye-coated TiO2. The polymerization was achieved by applying a constant potential (+0.25 V versus Ag/AgCl) on the working electrode under bias light of varying intensity of 0.3 sun (300 W/m2), 0.1 sun (100 W/m2), and 0.01 sun (10 W/m2) by a white LED. According to Yanagida et. al, the potential for PEPD is optimized at +0.2 to +0.3 V, and this was used to achieve the highest solar cell efficiency of PEDOT-modified DSSCs.24 The onset of bis-EDOT oxidation potential has been addressed around 600 mV versus Ag/AgCl.19 The light intensity of the bias lamp was measured by a photo diode calibrated using the 22485



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6.1 mW cm−2 and modulation frequency of 9.3 Hz were used for the excitation blue LED. 2.7. Electron Lifetime and Transport Time. Electron lifetime and transport time as a function of light intensity were measured by the custom-made “toolbox setup” using a white LED (Luxeon Star 1W) as light source to provide the base light intensity. The transient voltage and current response of the cells were recorded using a 16-bit resolution digital acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems RS570) and a homemade electromagnetic switching system. A small square wave modulation ( > d) π L

where εr is the dielectric constant of the material (normally taken to approach 3 for organic semiconductors),44,56 μ the hole mobility, V the applied bias, and L the distance between two silver electrodes at which V is applied. The thickness of polymerized PEDOT is much lower than L (0.2 cm) and is not considered in the above expression. Fitting the J−V curves for each material with this expression (see Supporting Information) gives the hole mobility and conductivity listed in Table 2. Table 2. Hole Mobility in ss-DSSCs Based on PEDOT D35Sensitized TiO2 Modified Electrode under Different Light Intensities (0.3, 0.1, and 0.01 sun) light intensity during PEPD (sun)

hole mobility (cm (V s)−1)

conductivity (S cm−1)

0.3 0.1 0.01

4.0 × 10−5 6.0 × 10−5 1.3 × 10−4

1.0 × 10−3 3.5 × 10−3 7.6 × 10−3

This method has been shown to be applicable to many organic semiconductors.20,41,56 The plots used to calculate the hole mobility are included in Figure S9 in Supporting Information. From the results we can conclude that both hole mobility and conductivity are higher for the ss-DSSCs where PEDOT was prepared under low light intensity. Such an outcome is not surprising because the low light intensity preparation results in more bipolarons compared to when PEDOT is prepared at higher light intensities; the mobility and conductivity of PEDOT are usually considered to increase with an increasing number of bipolarons.45,57 We can therefore attribute the electron lifetime increases and the transport time decrease to the higher conductivity obtained at low light intensity PEPD. The higher conductivity of PEDOT for this type of electrode preparation also leads to an increase in the performance of the solar cell as described above.

Figure 6. Electron lifetimes (a) and charge transport times (b) of ssDSSCs based on PEDOT D35- sensitized TiO2 modified electrode polymerized under different light intensity values (0.3, 0.1, and 0.01 sun).

D35-sensitized TiO2 electrode modified by PEDOT prepared under different light intensity values (0.3, 0.1, and 0.01 sun). The sample prepared at low light intensity during PEPD showed longer electron lifetimes and faster charge transport times compared to the samples prepared at higher light intensity. The charge transport time was similar to the values previously reported for other types of solid-state sensitizer solar cells.26,54 The longer electron lifetime can explain the higher open-circuit voltage measured for the solar cell with PEDOT prepared under low light intensity, and it may also to some extent explain the higher current in the J−V measurement. 3.5. Hole Mobility and Conductivity of PEDOT. After charge regeneration, the hole must be efficiently transported from D35+ to the silver cathode via the PEDOT layer without recombining with electrons in the TiO2. If the conductivity of the PEDOT is fast, the hole can efficiently be transported away from the electron in the TiO2, resulting in the avoidance of recombination and the increase in electron lifetime. High conductivity would at the same time decrease the measured transport time for the device and decrease the recombination of charges within the device. Therefore, the mobility and the conductivity of the PEDOT in the solar cells were measured to investigate their relevance in regard to the longer electron lifetime for the solar cell based on PEDOT prepared under lower light intensity.

4. CONCLUSIONS ss-DSSCs were prepared using dye-sensitized TiO2 as a substrate to photopolymerize in situ bis-EDOT and to obtain PEDOT layers under different light intensity values. As established by UV−vis−NIR measurement, the PEDOT deposited by PEPD under low light intensity led to the formation of a small amount of neutral species and a large amount of bipolaron-containing species when compared to the PEDOT prepared by PEPD at high light intensity. The large amount of oxidized polymer that may be obtained under low light intensity conditions may possibly be due to the more regular PEDOT of long chains formed under these conditions, which can easily oxidize to form polarons and bipolarons. Contrarily, the less oxidized polymer obtained under high light conditions may be attributed to less regular PEDOT composed of fewer long chains (oligomers) which are more difficult to oxidize. 22489



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(7) Kashif, M. K.; Axelson, J. C.; Duffy, N. W.; Forsyth, C. M.; Chang, C. J.; Long, J. R.; Spiccia, L.; Bach, U. A New Direction in DyeSensitized Solar Cells Redox Mediator Development: In Situ FineTuning of the Cobalt(II)/(III) Redox Potential through Lewis Base Interactions. J. Am. Chem. Soc. 2012, 134, 16646−16653. (8) Yum, J.-H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.; Bessho, T.; Marchioro, A.; Ghadiri, E.; Nazeeruddin, Md. K.; Grätzel, M.; et al. A Cobalt Complex Redox Shuttle for Dye-Sensitized Solar Cells with High Open-Circuit Potentials. Nat. Commun. 2012, 3, 631. (9) Daeneke, T.; Kwon, T.-H.; Holmes, A. B.; Duffy, N. W.; Bach, U.; Spiccia, L. High-Efficiency Dye-Sensitized Solar Cells with Ferrocene-Based Electrolytes. Nat. Chem. 2011, 3, 211−215. (10) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissörtel, F.; Salbeck, J.; Spreitzer, H.; Grätzel, M. Solid-State Dye-Sensitized Mesoporous TiO2 Solar Cells with High Photon-to-Electron Conversion Efficiencies. Nature 1998, 395, 583−585. (11) O’Regan, B.; Lenzmann, F.; Muis, R.; Wienke, J. Solar Cell Fabricated with Pressure-Treated P25-TiO2 and CuSCN: Analysis of Pore Filling and IV Characteristics. Chem. Mater. 2002, 14, 5023− 5029. (12) Cai, N.; Moon, S.-J.; Cevey-Ha, L.; Moehl, T.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. An Organic D-π-A Dye for Record Efficiency Solid-State Sensitized Heterojunction Solar Cells. Nano Lett. 2011, 11, 1452−1456. (13) Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N.L.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. Tris(2-(1H-pyrazol-1yl)pyridine)cobalt(III) as p-Type Dopant for Organic Semiconductors and Its Application in Highly Efficient Solid-State Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 18042−18045. (14) Abrusci, A.; Sai Santosh Kumar, R.; Al-Hashimi, M.; Heeney, M.; Petrozza, A.; Snaith, H. J. Influence of Ion Induced Local Coulomb Field and Polarity on Charge Generation and Efficiency in Poly(3-Hexylthiophene)-Based Solid-State Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2011, 21, 2571−2579. (15) Chang, J. A.; Rhee, J. H.; Im, S. H.; Lee, Y. H.; Kim, H.-J.; Seok, S. I.; Nazeeruddin, Md. K.; Grätzel, M. High-Performance Nanostructured Inorganic−Organic Heterojunction Solar Cells. Nano Lett. 2010, 10, 2609−2612. (16) Koh, J. K.; Kim, J.; Kim, B.; Kim, J. H.; Kim, E. Highly Efficient, Iodine-Free Dye-Sensitized Solar Cells with Solid-State Synthesis of Conducting Polymers. Adv. Mater. 2011, 23, 1641−1646. (17) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Yum, J.-H.; Grtäzel, M.; Park, N.-G.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (18) Manseki, K.; Jarernboon, W.; Youhai, Y.; Jiang, K.-J.; Suzuki, K.; Masaki, N.; Kim, Y.; Xia, J.; Yanagida, S. Solid-State Dye-Sensitized Solar Cells Fabricated by Coupling Photoelectrochemically Deposited Poly(3,4-ethylenedioxythiophene) (PEDOT) with Silver-Paint on Cathode. Chem. Commun. 2011, 47, 3120−3122. (19) Xia, J.; Masaki, N.; Lira-Cantu, M.; Kim, Y.; Jiang, K.; Yan-agida, S. Influence of Doped Anions on Poly(3,4-ethylenedioxythiophene) as Hole Conductors for Iodine-Free Solid-State Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 1258−1263. (20) Elschner, A.; Kirchmeyer, S.; Lövenich, W.; Merker, U.; Reuter, K. PEDOT: Principles and Applications of an Intrinsically Conductive Polymer; CRC Press: Boca Raton, FL, 2011; 167−244. (21) Ding, I.-K.; Tétreault, N.; Jérémie, B.; Hardin, B. E.; Smith, E. H.; Rosenthal, S. J.; Sauvage, F.; Grätzel, M.; McGehee, M. D. PoreFilling of Spiro-OMeTAD in Solid-State Dye Sensitized Solar Cells: Quantification, Mechanism, and Consequences for Device Performance. Adv. Funct. Mater. 2009, 19, 2431−2436. (22) Docampo, P.; Hey, A.; Guldin, S.; Gunning, R.; Steiner, U.; Snaith, H. J. Pore Filling of Spiro-OMeTAD in Solid-State DyeSensitized Solar Cells Determined Via Optical Reflectometry. Adv. Funct. Mater. 2012, 5010−5019. (23) Snaith, H. J.; Humphry-Baker, R.; Chen, P.; Cesar, I.; Zakeeruddin, S. M.; Gratzel, M. Charge Collection and Pore Filling

PIA spectra of the different samples showed that the dye molecules are regenerated by PEDOT. For the sample with PEDOT polymerized at low light intensity, the PIA spectrum also indicated that the number of polarons in PEDOT decreases while the number of bipolarons in PEDOT increases. A mechanism is therefore suggested where the polarons in PEDOT regenerate the dye, resulting in bipolaron formation in PEDOT. The solar cell with PEDOT prepared at low light intensity showed the highest photovoltage and photocurrent, in agreement with the higher IPCE observed for this device. The measured electron lifetime was also high for this solar cell, and the transport time was fast, which explains the higher efficiency. On the basis of conductivity and mobility measurements of the PEDOT layer in the solar cells, it was concluded that both conductivity and mobility were higher for the solar cell with PEDOT prepared under low light intensity. This result was used to explain the faster charge transport and longer lifetime of the charges compared to the solar cells with PEDOT prepared at high light intensity as well as, in combination with the improved light transmission due to less neutral PEDOT, the higher efficiency of this resulting solar cell.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental data on photoelectrical polymerization, absorption spectra, photoinduced absorption spectra, solar cell performance, and conductivity measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Uppsala University, Department of Chemistry−Ångström Laboratory, Box 523, SE 751 20 Uppsala, Sweden. Phone: 0046184713663. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the Swedish Energy Agency, the Swedish Research Council (VR), the STandUP for Energy program, and the Göran Gustafsson Foundation. This work was also financially supported by PHCDalen Swedish/French bilateral program (26224RH).

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REFERENCES

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Neutral, Polaron, and Bipolaron States in PEDOT Prepared by

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