J. Phys. Chem. C 2008, 112, 11569–11574
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Effect of Doping Anions’ Structures on Poly(3,4-ethylenedioxythiophene) as Hole Conductors in Solid-State Dye-Sensitized Solar Cells Jiangbin Xia,*,† Naruhiko Masaki,† Monica Lira-Cantu,‡ Yukyeong Kim,† Kejian Jiang,† and Shozo Yanagida*,† Center for AdVanced Science and InnoVation, Osaka UniVersity, Suita, Osaka 565-0871, Japan, and Institut de Cie`ncia de Materials de Barcelona, Campus UAB Bellaterra E-08193, Spain ReceiVed: February 16, 2008; ReVised Manuscript ReceiVed: April 25, 2008
Poly(3,4-ethylenedioxythiophene) with different doping anions as hole conductors was investigated in solidstate dye-sensitized solar cells (DSC). The different perfluoroalkyl chains of bis(perfluorosulfonyl)imides (N(CnF2n+1SO2)2-) or perfluorosulfonates (CnF2n+1SO3-) were selected as doping anions in this study. Photoelectrochemical measurements reveal that the perfluoroalkyl chain in the doping anions plays an important role in the performance of PEDOT-based DSCs. This study presents a broader way to find PEDOT with good hole conductivity through changing doping anions’ structures. In addition, in the case of perfluorosulfonates, the appropriate perfluoroalkyl chain will improve the performance of the cell. Cyclic voltammetry (CV) and high oxidation potential stability testing indicate that N(CnF2n+1SO2)2- are more suitable as doping anions in PEDOT hole conductors in solid-state DSC. 1. Introduction Since the breakthrough in dye-sensitized solar cells (DSC) appeared in 19911 by the introduction of nanocrystalline TiO2, many scientists around the world have made great efforts to improve the performance of the DSC devices. Although significant advances in the performance of DSC have been made during the past few years, long-term stability testing is still a big challenge to practical use. Taking into consideration the practical application point, organic liquid electrolytes may not be a good choice because of their shortcomings such as solvent evaporation and iodine sublimation that result in high-temperature instability. Therefore, the replacement of I-/I3- redox is a trend in DSC research field. Taking into consideration the role of I-/I3- in DSC, we need to look for some hole-transporting materials or other appropriate redox mediators. So far, CuI or CuSCN2–4 based solidstate DSC show up to 3% conversion efficiency. On the other hand, Gra¨tzel et al. found an excellent low-molecular-weight organic hole conductor, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD),5–7 in 1998. Along with the further design of ruthenium dye (Ru-dye) and optimization, the latest record8 up to 5% has been obtained. Meanwhile, similar kinds of widely employed p-type conductive polymers, which can also serve as hole-transporting materials in solidstate DSC, have attracted the attention of many groups. For instance, our group first reported polypyrrole9 as a hole conductor to replace I-/I3- redox conductor in 1997. Subsequently, our group developed in situ photoelectropolymerization of 2,2′-bis(3,4-ethylenedioxythiophene) (bis-EDOT) to form poly(3,4-ethylenedioxythiophene) (PEDOT)10–14 in order to enhance the contact between dye molecule and hole conductor as well as pore filling of hole conductor in solid-state DSC. * Corresponding authors. Telephone and fax: +81-06-6879-7351 (S.Y.). E-mail:
[email protected] (J.X.);
[email protected] (S.Y.). † Center for Advanced Science and Innovation, Osaka University. ‡ Institut de Cie ` ncia de Materials de Barcelona.
Recently, polyaniline15 or polydiacetylenes16 were employed as organic hole conductors in solid-state DSC. As far as organic hole conductors are concerned, the hole mobility or hole conductivity is a characteristic feature which plays a decisive role not only in bulk heterojection polymer solar cells17–22 but also in the organic light emission diode (OLED) field23 because of the necessary balance of holetransporting properties with that of the electron-transport material and their function. Triggered by recent progress in pursuing the improvement of the hole mobility for bulk heterojunction polymer solar cells especially in (1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61) (PCBM)-poly(3-hexylthiophene) (P3HT) systems,20–22 recently we examined typical doping anions24 with the intention of tuning the hole conductivity of PEDOT in solid-state DSC. We found that bistrifluoromethanesulfonylimide (TFSI- or N(CF2SO2)2-) and CF3SO3are more superior than the commonly used ClO4- as doping anions in the conversion efficiency of PEDOT-based solid-state DSC. In this paper, we further continue to investigate the effect of the perfluoroalkyl chain on the doping anions as mentioned above. Moreover, we examine the stability of these PEDOT hole conductors from an electrochemical point of view as well as quantum chemical calculations. These experiments will offer more fundamental and comprehensive information on PEDOTbased solid-state DSC. 2. Experimental Section 2,2′-Bis(3,4-ethylenedioxythiophene) (bis-EDOT) dimer was purchased from Azuma. LiCF3SO3 was purchased from Aldrich. Li(C4F9SO3), Li(n-C8F17SO3), lithium bistrifluoromethanesulfonylimide (LiN(CF3SO2)2), LiN(C3F7SO2)2, and LiN(C4F9SO2)2 were purchased from Wako Chemical Co. Ltd. LiN(C2F5SO2)2 was purchased from Kishida Chemical Co. Ltd. Other common reagents and organic solvents were purchased from Wako Chemical Co. Ltd. The chemical structures of bis-EDOT, PEDOT, and different counteranions are shown in Scheme 1. Nanoporous TiO2 electrodes were prepared on an FTO (Nippon Sheet Glass, SnO2:F, 10 ohms/square) from colloidal
10.1021/jp801878a CCC: $40.75 2008 American Chemical Society Published on Web 07/03/2008
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SCHEME 1: Chemical Structures of (a) bis-EDOT and PEDOT; (b) Counteranions N(CF3SO2)2-, N(C2F5SO2)2-, N(C3F7SO2)2-, and N(C4F9SO2)2-; and (c) Counteranions CF3SO3-, C4F9SO3-, and C8F17SO3-
Nanoxide-T paste (Solaronix) by doctor-blade techniques. The films were annealed at 450 °C for 30 min in air. The resulting TiO2 films (thickness is around 5.5 µm, measured by a profiler, Sloan, Dektak3) were cut into pieces. Then, the electrodes were immersed into 3.0 × 10-4 M cis-bis(isothiocyanato)(2,2′bipyridyl-4,4′-dicarboxylato)(2,2′-bipyridyl-4,4′-dinonyl)ruthenium(II) (known as Z-907, Solaronix) in acetonitrile/tert-butanol (1:1) for 18 h. Acetonitrile solution of 0.1 M lithium salts were prepared before use. Dyed TiO2 films served as working electrodes with a platinum foil counter electrode while a Ag/AgCl was used as reference electrode (BAS 100B/W electrochemical system). An 0.01 M solution of bis-EDOT in supporting electrolyte was used for the in situ photoelectropolymerization or electropolymerization. The in situ photopolymerization was achieved by applying the constant potential (+0.2 V vs Ag/AgCl) under light irradiation of a 500 W Xe lamp (22 mW cm-2, λ > 520 nm) for 30 min. After in situ photoelectropolymerization, the resulting TiO2/dye/PEDOT electrode was rinsed using ethanol and dried and then 1 drop of BMIm-TFSI (1-butyl-3-methylimidazolium bistrifluoromethanesulfonylimide) with 0.2 M TBP (4-tert-butylpyridine) and 0.2 M Li(CF3SO2)2 was added on the surface. The sandwich-type devices were finished by clipping a counter electrode of gold sputtered on FTO substrate (FTO/ Au). Typical areas of the electrodes were around 0.30 cm2. The photoelectrochemical properties of the DSC were studied by recording the current-voltage (I-V) characteristics of the unsealed type cell under illumination of AM1.5 (1 Sun; 100
mW/cm2, calibrated by a secondary standard c-Si cell) using a solar simulator (Yamashita Denso, YSS-80). The data were obtained from the average of at least three examples. To investigate the electrochemical properties on a PEDOT hole conductor in DSCs, cyclic voltammetry (CV) was carried out in three-electrode measurements in 0.1 M Li(CF3SO2)2 in acetonitrile; a 1 cm2 area of the PEDOT/FTO served as working electrodes. A platinum foil and Ag/AgCl served as a counter electrode and reference electrode, respectively (BAS 100B/W electrochemical system). The deposited 15 mC/cm2 PEDOT films by electropolymerization were used in CV experiments. Meanwhile, these PEDOT films with different doping anions were exposed to series potentials above the half-wave oxidation potentials of the PEDOT (E1/2, p) for 2 min. After each potential exposure, a cyclic voltammogram was then carried out by scanning rate about 100 mV/s between -1.4 and 0.6 V vs Ag/ AgCl. The film thicknesses of PEDOT and TiO2 were measured with a surface profilometer (Dektak 3 from Sloan Tech.). The detailed calculation procedure is according to the literature.25 The initial structures of the molecular model were optimized using ab initio molecular orbital theory with Spartan ʼ06. Single-point energy calculations were employed at the HF/6311+G**//HF/6-31G* level. 3. Results and Discussion 3.1. Photovoltaic Performance with Different Doping Anions. Figure 1 shows the photocurrent-voltage characteristics for PEDOT/DSC fabricated by the different doping anions, and
Anion Doping in PEDOT-Based DSC
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Figure 2. Relationship between carbon number of doping anions and conversion efficiency of solid-state DSC.
Figure 1. Current density vs voltage curves for DSC employing different photoelectrochemically deposited PEDOT hole conductors with anions: (a) bis(perfluorosulfonyl)imides and (b) perfluorosulfonates. TiO2 layer thickness is 5.5 µm.
their parameters are listed in Table 1. In addition, the relationship between carbon numbers of doping anions and the conversion efficiency is shown in Figure 2. It is obvious that PEDOT doped with bis(perfluorosulfonyl)imides shows better performance than that doped with perfluorosulfonates. We attribute this to the better hole conductivity or hole mobility of PEDOT doped with bis(perfluorosulfonyl)imides than doped with the latter. This behavior is consistent with our previous study.24 However, as shown in Figure 2, the influences of perfluoroalkyl chain length in doping anions are quite different. In the case of bis(perfluorosulfonyl)imide systems, the conversion efficiencies of bis(perfluorosulfonyl)imides are comparable in the range of 2.7-2.8% when the carbon number is C2, C4, and C6. Along with the increase of carbon number (C8), the conversion efficiency decreases drastically to 2% because of the poorer Voc and fill factor as well as Jsc. Compared with the
bis(perfluorosulfonyl)imides, in the perfluorosulfonates C4 shows the best conversion efficiency of about 2.5%. Along with the increase of carbon number (C8), the conversion efficiency also shows a decrease to 1.4%. Therefore, the excessively long perfluoroalkyl chain in the doping anions will play a negative role in the photoconversion processes. As already discussed in our previous paper,24,26 the solvation behaviors of imides and methanesulfonates are quite different because of the feature hard/soft relationship. In the case of bis(perfluorosulfonyl)imides, there is no big difference among these highly delocalized anions in the range C2-C6, resulting in similar performance in solid-state DSC. However, compared with CF3SO3-, along with the introduction of the perfluoroalkyl chain, more charge is delocalized on SO3- of C4F9SO3-, which will induce preferred stacking through a transverse PEDOT ring, resulting in better conductivity. This may explain the varying tendency of the carbon number change in the performance of solid-state DSC. In order to deeply understand the influence of the anions’ structure on the properties of PEDOT, we carried out quantum calculations in the following section. Interestingly, the order of dark current in the N(CnF2n+1SO2)2series is N(C4F9SO2)2- > N(CF3SO2)2- > N(C3F7SO2)2- ∼ N(C2F5SO2)2-, which means that moderate perfluoroalkyl chain length can suppress electron leakage more effectively. 3.2. Comparison of Conductivity of PEDOT Hole Conductor. As shown in Table 2, the conductivity sequence of bis(perfluorosulfonyl)imides is N(CF3SO2)2- > N(C2F5SO2)2∼ N(C3F7SO2)2- > N(C4F9SO2)2- and the sequence in perfluorosulfonates is C4F9SO3- > CF3SO3- > C8F9SO3-. These sequences are well consistent with the tendency of the cell’s performance. It is clear that all cell performances show a linear dependence on log σ as shown in Figure 3. We noted that PEDOT-N(C4F9SO2)2- and PEDOTC8F17SO3- show the highest dark currents among these cells (see Figure 1). These might be due to the effect of these longest
TABLE 1: Photovoltaic Properties of the DSCs doping ions bis(perfluorosulfonyl)imides
perfluorosulfonates
-
N(CF3SO2)2 N(C2F5SO2)2N(C3F7SO2)2N(C4F9SO2)2CF3SO3C4F9SO3C8F17SO3-
Voc (mV)
Jsc (mA/cm2)
FF
η (%)
750 ( 50 825 ( 30 730 ( 60 545 ( 30 635 ( 5 700 ( 35 580 ( 50
5.3 ( 0.6 4.7 ( 0.3 4.8 ( 0.3 5.3 ( 0.8 4.8 ( 0.4 4.7 ( 0.5 3.7 ( 0.3
0.73 ( 0.02 0.74 ( 0.02 0.76 ( 0.02 0.68 ( 0.02 0.71 ( 0.01 0.73 ( 0.01 0.67 ( 0.04
2.9 ( 0.2 2.8 ( 0.1 2.7 ( 0.2 1.9 ( 0.1 2.2 ( 0.1 2.5 ( 0.2 1.4 ( 0.2
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TABLE 2: Relationship between Cells’ Conversion Efficiency with Conductivity of Different PEDOT Hole Conductors and in Situ Photoelectropolymerization Charge doping ions
charge of PEP (mC/cm2)
conductivity (S/cm)
η (%)
N(CF3SO2)2N(C2F5SO2)2N(C3F7SO2)2N(C4F9SO2)2CF3SO3C4F9SO3C8F17SO3-
15.8 13.6 14.3 9.0 10.7 11.7 8.6
130 108 106 25 86 91 7.5
2.9 ( 0.2 2.8 ( 0.1 2.7 ( 0.2 1.9 ( 0.1 2.2 ( 0.1 2.5 ( 0.2 1.4 ( 0.2
perfluoroalkyl chains on the PEDOT molecular organization at the dye/PEDOT interface or accidental penetration of redox species at some parts of the FTO/blocking layer/nano-TiO2. The latter may relate to the size and morphology of the anions, as is well known for tert-butylpyridine (TBP). According to the conductivity data of these PEDOT films as shown in Table 2, once the carbon number reaches a maximum, the longest perfluoroalkyl chains in anions have a negative effect on conductivity as well as on conversion efficiency. It is noted that the total polymerization charge of C8 in bis(perfluorosulfonyl)imides is lower around 35% than shorter perfluoroalkyl chains, which means that the longest perfluoroalkyl anion might have a drastic influence on the mechanism of PEDOT growth. A similar phenomenon is observed as well in perfluorosulfonates. All these facts indicate that the doping anions’ structures have a remarkable influence on the conductivity of PEDOT as well as on the performance of solid-state DSC. Meanwhile, we cannot exclude the factor that a long perfluoroalkyl chain such as N(C4F9SO2)2- or C8F17SO3- serves as a kind of surfactant which would alter the PEDOT growth mechanism in supporting electrolyte during in situ photoelectropolymerization processes. Here we briefly discuss the role of doping anions in a PEDOT hole conductor. Previous study27 indicates that bc-layers are separated by counterions along the a-axis. In general, PEDOT becomes more conducting as the crystallinity increases. We can imagine that these doping anions are distributed homogeneously between liner oligomer PEDOT rings. Therefore, the bulkier in size an anion is, the larger distance that exists between interactive PEDOT rings, which will induce the poorest π-π stacking between PEDOT rings leading to the lowest hole conductivity. Further quantum calculations of the stacking structures need to be done to deeply understand this. At this moment, it is
Figure 3. Linear dependence of conductivity with conversion efficiency of the cells.
Figure 4. Cyclic voltammetry of different PEDOT films: (a) bis(perfluorosulfonyl)imides and (b) perfluorosulfonates. Supporting electrolyte: 0.1 M LiN(CF3SO2)2 in acetonitrile.
beyond the scope of this paper. On the other hand, the key point for the high performance in solid-state DSC is the necessity of close contact between the hole conductor and the Ru-dye. Therefore, we need to think about the distance between the Rudye and the PEDOT ring during the design of novel Ru-dyes. 3.3. Cyclic Voltammetry Behavior of PEDOT Film. The influence of the perfluoroalkyl chain on the electrochemical behavior of doped PEDOT films (deposited charge 15 mC/cm2) was investigated in a 0.1 M LiN(CF3SO2)2 acetonitrile solution. As shown in Figure 4, all PEDOT films exhibit an oxidation peak at 0.3 V with a redox peak around -0.5 V. It seems that the doping anions almost do not change the electrochemical behavior in the electrolyte or in the solid-state DSC. In the case of bis(perfluorosulfonyl)imides, the bulkiest in size N(C4F9SO2)2- gives the lowest redox current with 0.35 mA/ cm2 while the others show higher and comparable current values around 0.5 mA/cm2. However, in the case of perfluorosulfonates as shown in Figure 4b, CF3SO3- exhibits the highest redox current around 0.5 mA/cm2 due to its smallest anion while C4F9SO3- and C8F17SO3- show lower redox currents of about 0.3 mA/cm2. Interestingly, most PEDOT films doped with N(CnF2n+1SO2)2- give higher redox currents compared with those doped with CnF2n+1SO3-. Taking into consideration that the reduction current is related to the anion release and Li+ insertion,28,29 such a phenomenon can be attributed to the more flexible anion exchange or dedoping processes of N(CnF2n+1SO2)2-. 3.4. Stability of the PEDOT Hole Conductor. As far as we know, PEDOT is one of the most stable polymers so far.30 Figure 5 shows the relative stabilities of all PEDOT films against the oxidation potential. The relative electroactivity of each
Anion Doping in PEDOT-Based DSC
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11573 TABLE 3: Calculated HOMO and LUMO Energy Levels (eV), Chemical Hardness, and Dipole Moments Obtained by ab Initio MO Methods (HF/6-311+G**//HF/6-31G*)
Figure 5. Comparison of stability overoxidation for galvanostatically deposited film (15 mC/cm2) of PEDOT based doping anions of (a) bis(perfluorosulfonyl)imides and (b) perfluorosulfonates.
polymer is determined by the ratio of the total charge to the integrated charge obtained from freshly prepared ones.31,32 As shown in Figure 5, most PEDOT films can keep 70-90% of initial charge in the range of 0-1.2 V oxidation potential over E1/2,p, while the values of Qmax(E)/Qmax(initial) decrease drastically in the range of 1.3-1.6 V. It seems that bis(perfluorosulfonyl)imides (N(CnF2n+1SO2)2-) demonstrate better stability compared with the perfluorosulfonate series. The alkyl lengths have almost no effect on stability of the former system. However, along with the increase of the carbon number in perfluorosulfonates (CnF2n+1SO3-), the stability gets poorer. That is, PEDOT-C8F17SO3- shows the poorest stability for its lowest onset degradation voltage about 0.8 V. At this point, N(CnF2n+1SO2)2- might be the best candidate in the PEDOT series in solid-state DSC as a hole conductor. In addition, it is noted that PEDOT-N(C4F9SO2)2- shows great improvement once it is exposed at 1.2 V over E1/2,p, whose integrated charge is about 60% improvement compared with the initial value. This special behavior might be due to the bulkier size of N(C4F9SO2)2-. At higher applied potential, PEDOT-N(C4F9SO2)2- would be activated to release N(C4F9SO2)2- much more compared with those at lower applied potential or freshly made film. Taking into consideration the working condition in solid-state DSC, PEDOT is stable enough to serve as a hole conductor from the viewpoint of electrochemistry. 3.5. Ab Initio Molecular Orbital (MO) Calculations. In order to further understand the effect of doped anions on PEDOT hole conductor, ab initio MO methods (HF/6-311+G**//HF/ 6-31G*) were carried out to obtain their thermal dynamic
anions
EHOMO (eV)
ELUMO (eV)
chemical hardness (η′)
dipole moment (D)
N(CF3SO2)2N(C2F5SO2)2N(C3F7SO2)2N(C4F9SO2)2CF3SO3C4F9SO3C8F17SO3-
-8.37 -8.59 -8.74 -8.76 -7.37 -7.56 -7.60
4.78 4.57 4.38 4.10 5.30 4.28 3.51
6.58 6.58 6.56 6.43 6.34 5.92 5.56
0.02 1.40 1.00 1.58 4.79 11.57 23.2
stability and chemical hardness (η′) as well as dipole moments of various doping anions as shown in Table 3. Previous study33 has already revealed that the 6-311+G** basis set is a more accurate description of the MOs. Our result is well consistent with those of refs 33 and 34. According to Koopman’s theorem, Pearson35 showed that 2η′ is equal to the gap between the HOMO and LUMO levels (for closed-shell molecules). In addition, such an increased HOMO-LUMO gap is linked to the thermodynamic stability of the chemical species.36,37 As shown in Table 3, the calculated order of chemical hardness of these anions is N(CF3SO2)2- ∼ N(C2F5SO2)2- ∼ N(C3F7SO2)2- >N(C4F9SO2)2- > CF3SO3- > C4F9SO3- > C8F9SO3-. That is, the bis(perfluorosulfonyl)imides are more stable than the perfluorosulfonates. On the other hand, the perfluoroalkyl chain has almost no influence on the chemical hardness of bis(perfluorosulfonyl)imides while perfluorosulfonates show lower values of η′ along with the increment of perfluoroalkyl chain, which means that C8F9SO3- would be the most unstable doping anion in PEDOT films. Combining with the stability test against the oxidation potential in the above section, such a tendency is in good agreement with the experimental order as shown in Figure 5. It seems that the perfluoroalkyl chain length has a considerable effect on the dipole moments of perfluorosulfonates while it has almost no effect on the bis(perfluorosulfonyl)imides. We attribute this phenomenon to the different symmetries between these two systems. 4. Conclusions As a kind of promising candidate for the replacement of I-/ I3 redox in solid-state DSC, the properties of a hole conductor like PEDOT play a decisive role. The selection of doping anions and their different perfluoroalkyl chains will lead to extraordinary characteristic PEDOTs, which will be reflected in the performance of solid-state DSC. In the case of perfluorosulfonates, the appropriate perfluoroalkyl chain will improve the performance of the cell. However, the longer perfluoroalkyl chain the doping anions contain, the poorer conversion efficiency the solid-state DSCs can get. This phenomenon might be due to the unfavorable orientation of the PEDOT ring and excessive distance between PEDOT and the center of the Ru-dye induced by the long perfluoroalkyl chain. The various tendencies between bis(perfluorosulfonyl)imides or perfluorosulfonates would be attributed to the different solvation behaviors of the anions in acetonitrile. All these facts once more reveal that the hole conductivity or hole mobility plays a decisive role in the PEDOT solid-state DSC owing to the anion properties and their structures.38 Electrochemical behavior and stability testing at overoxidation potential as well as quantum calculations reveal that bis(perfluorosulfonyl)imides are excellent doping anions in PEDOT -
11574 J. Phys. Chem. C, Vol. 112, No. 30, 2008 hole conductor. This study helped us to understand more deeply the interaction between the PEDOT hole conductor and the Rudye in solid-state DSC. Acknowledgment. This research was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Tennakone, K.; Kumara, G. R. A.; Kottegoda, I. R. M.; Wijayantha, K. G. U.; Perera, V. P. S. J. Phys. D: Appl. Phys. 1998, 31, 1492. (3) O’Regan, B.; Schwartz, D. T.; Zakeeruddin, S. M.; Gra¨tzel, M. AdV. Mater. 2000, 12, 1263. (4) Kumara, G. R. A.; Konno, A.; Shiratsuchi, K.; Tsukahara, J.; Tennakone, K. Chem. Mater. 2002, 14, 954. (5) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583. (6) Kru¨ger, J.; Plass, R.; Gra¨tzel, M.; Matthieu, H. Appl. Phys. Lett. 2002, 81, 367. (7) Karthikeyan, C. S.; Wietasch, H.; Thelakkat, M. AdV. Mater. 2007, 19, 1091. (8) Snaith, H. J.; Moule, A. J.; Klein, C.; Meerholz, K.; Friend, R. H.; Gratzel, M. Nano Lett. 2007, 7, 3372. (9) Murakoshi, K.; Kogure, R.; Wada, Y.; Yanagida, S. Chem. Lett. 1997, 26, 471. (10) Saito, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. Synth. Met. 2002, 131, 185. (11) Saito, Y.; Fukuri, N.; Senadeera, G. K. R.; Kitamura, T.; Wada, Y.; Yanagida, S. Electrochem. Commun. 2004, 6, 71. (12) Fukuri, N.; Saito, Y.; Kubo, W.; Senadeera, G. K. R.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Electrochem. Soc. 2004, 151, A1745. (13) Fukuri, N.; Masaki, N.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2006, 110, 25251. (14) Mozer, A. J.; Jiang, K. J.; Wada, Y.; Masaki, N.; Mori, S. N.; Yanagida, S. Appl. Phys. Lett. 2006, 89, 043509. (15) Tan, S. X.; Zhai, J.; Wan, M. X.; Meng, Q. B.; Li, Y. L.; Jiang, L.; Zhu, D. B. J. Phys. Chem. B 2004, 108, 18693. (16) Wang, Y. P.; Yang, K.; Kim, S. C.; Nagarajan, R.; Samuelson, L. A.; Kumar, J. Chem. Mater. 2006, 18, 4215. (17) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (18) Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 85, 5757.
Xia et al. (19) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924. (20) Mihailetchi, V. D.; Xie, H. X.; Boer, B. D.; Koster, L. J. A.; Blom, P. W. M. AdV. Funct. Mater. 2006, 16, 699. (21) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (22) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 12, 85. (23) Culligan, S. W.; Chen, A. C.-A.; Wallace, J. U.; Klubek, K. P.; Tang, C. W.; Chen, S. H. AdV. Funct. Mater. 2006, 16, 1481. (24) Xia, J.; Masaki, N.; Lira-Cantu, M.; Kim, Y.; Jiang, K.; Yanagida, S. J. Am. Chem. Soc. 2008, 130, 1258. (25) Ue, M.; Murakami, A.; Nakamura, S. J. Electrochem. Soc. 2002, 149, A1572. (26) Salomon, M. J. Solution Chem. 1993, 22, 715. (27) Kim, T. Y.; Park, C. M.; Kim, J. E.; Suh, K. S. Synth. Met. 2005, 149, 169. (28) Skaarup, S.; Bay, L.; Vidanapathirana, K.; Thybo, S.; Tofte, P.; West, K. Solid State Ionics 2003, 159, 143. (29) Xia, J.; Masaki, N.; Jiang, K.; Yanagida, S. J. Mater. Chem. 2007, 17, 2845. (30) Groenendaal, L. B.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481. (31) Schottland, P.; Zong, K.; Gaupp, C. L.; Thompson, B. C.; Thomas, C. A.; Giurgiu, I.; Hickman, R.; Abbound, K. A.; Reynolds, J. R. Macromolecules 2000, 33, 7051. (32) Sarac, A. S.; So¨nmez, G.; Cebeci, F. C. J. Appl. Electrochem. 2003, 33, 295. (33) Johansson, P.; Tegenfeldt, J.; Lindgren, J. J. Phys. Chem. A 2000, 104, 954. (34) Benrarah, D.; Arnaud, R.; Sanchez, J.-Y. Electrochim. Acta 1995, 40, 2437. (35) Pearson, R. G. J. Chem. Educ. 1987, 64, 561. (36) Passo, R. G. J. Org. Chem. 1989, 54, 1423. (37) Zhao, Z.; Passo, R. G. J. Am. Chem. Soc. 1989, 111, 7371. (38) At present, we believe that HOMO/LUMO levels of different doped PEDOTs show very similar values. It is widely accepted that Voc of the devices is determined by the difference between the quasi-Fermi level of TiO2 and the HOMO level of PEDOT. However, the interface properties would result in different Vocʼs of the devices because the conditions of interfaces change the electron density, that is, the quasi-Fermi level of TiO2. As discussed in the paper, the doped anion with the longest alkyl chain (N(C4F9SO2)2-, or C8F9SO3-) would change the growth speed of PEDOT and integrated charges during in situ photoelectrochemical polymerization processes, which would greatly change the conditions of the interface between PEDOT and dye molecules.
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