CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402160

Phenothiazinedioxide-Conjugated Sensitizers and a DualTEMPO/Iodide Redox Mediator for Dye-Sensitized Solar Cells Ryan Yeh-Yung Lin,[a, b] Te-Chun Chu,[b] Ping-Wei Chen,[b] Jen-Shyang Ni,[a] Pei-Chieh Shih,[b] Yung-Chung Chen,[a] Kuo-Chuan Ho,*[b, c] and Jiann T. Lin*[a] Metal-free dyes containing a phenothiazinedioxide entity in the spacer were synthesized. The best conversion efficiency (7.47 %) of the dye-sensitized solar cell (DSSC) by using new sensitizers with chenodeoxycholic acid as a co-adsorbent and the I/I3 electrolyte reached over 90 % of that of the standard N719-based cell (8.10 %). A new type of ionic liquid containing

the nitroxide radical (NOC) and iodide was successfully synthesized and applied to the DSSCs. If the I/I3 electrolyte was replaced with a dual redox electrolyte, that is, a TEMPO (2,2,6,6tetramethylpiperidin-1-oxyl) derivative with a dangling imidazolium iodide entity, the cell exhibited a high open-circuit voltage of 0.85 V and a cell efficiency of 8.36 %.

Introduction Dye-sensitized solar cells (DSSCs) are considered attractive renewable energy sources, as high solar energy to electricity conversion efficiency can be achieved with cheap materials and a simple fabrication process. Metal-free sensitizers of the donor–p conjugate–acceptor (D-p-A) type have received intense study, as they possess several advantages: flexibility in tuning the molecular architecture, high molar extinction coefficients, and low cost.[1] In 2007, Sun et al. reported a series of phenothiazine-based sensitizers[2a] that exhibited high cell performance. Later, Tian’s group also investigated phenothiazinebased sensitizers based on the following reasons: 1) The phenothiazine entity is an effective electron donor because of the presence of electron-rich sulfur and nitrogen heteroatoms. 2) The nonplanar phenothiazine entity can impede molecular aggregation and the formation of intermolecular excimers.[2b] Recently, Wong et al. reported a series of phenothiazine-based sensitizers exhibiting high cell performance with the highest power conversion efficiency reported to date (8.18 %) that even surpassed that of the reference cell based on N719

[a] R. Y.-Y. Lin, J.-S. Ni, Y.-C. Chen, Prof. J. T. Lin Institute of Chemistry Academia Sinica Nankang 11529, Taipei (Taiwan) a.edu.tw E-mail: [email protected] [b] R. Y.-Y. Lin, T.-C. Chu, P.-W. Chen, P.-C. Shih, Prof. K.-C. Ho Department of Chemical Engineering National Taiwan University Taipei 10617 (Taiwan) E-mail: [email protected] [c] Prof. K.-C. Ho Institute of Polymer Science and Engineering National Taiwan University Taipei 10617 (Taiwan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402160.

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(7.73 %) under 1 sun conditions (1 sun = 100 mW cm2).[2c] Several other phenothiazine-based sensitizers have also been successfully applied to DSSCs.[2d–f] In our earlier studies, we found that incorporation of an electron-withdrawing moiety in the conjugated spacer helped to redshift the absorption spectra, though charge trapping could accompany and deteriorate electron injection.[3] To our knowledge, no phenothiazinedioxide-based sensitizer has been reported for DSSCs. We therefore set out to develop sensitizers incorporating a phenothiazinedioxide entity in the conjugated spacer in the hope of redshifting the absorption spectra and retaining the good electron-donating ability of phenothiazine. The iodide/triiodide (I/I3) couple is the most common redox couple for DSSCs. However, some disadvantages limit the overall cell conversion efficiency:[4] 1) iodine is corrosive to the metal electrodes, which renders scaling up of DSSCs difficult; 2) the excessive driving force (over 600 mV) between the redox potential of I/I3 [  0.35 vs. normal hydrogen electrode (NHE)] and the sensitizer for dye regeneration leads to energy loss in DSSCs and a smaller photovoltage; 3) strong absorptivity of I/I3 in the visible-light region deteriorates the photocurrent owing to decreased absorption of the dye molecules. Therefore, new redox mediators as a substitute for I/I3 are also actively searched by many groups. These include cobaltII/III,[5] ferrocene/ferrocenium (Fc/Fc + ),[6] disulfide/thiolate,[7] and TEMPO/TEMPO + (TEMPO = 2,2,6,6-tetramethylpiperidin-1oxyl).[8] Notably, the cell performance of binary redox couples consisting of two physically mixed electrolytes may outrival that of either electrolyte.[9] An ionic liquid electrolyte with a dual redox couple, that is, nitroxide radical and iodide, was recently developed by us and represents an interesting alternative to the aforementioned binary redox couples.[10] The nitroxide radical can efficiently reduce I2C and avoid the formation of I3 through disproportionation of I2C. Therefore, the energy level of the nitroxide radical redox couple exhibits more posiChemSusChem 0000, 00, 1 – 10

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CHEMSUSCHEM FULL PAPERS tive redox potential than I/I3 , and a higher open-circuit voltage (VOC) relative to that of a cell using I/I3 alone can be achieved. In addition to the I/I3 electrolyte, in this study we also tested our DSSCs with the new dual TEMPO/iodide redox couple (IL-I) to explore its wider applicability.

Results and Discussion Synthesis of the materials 3,7-Dibromo-10-hexyl-10H-phenothiazine and 3,7-dibromo-10hexyl-10H-pheno-thiazine 5,5-dioxide were synthesized by literature procedures.[2c, 11] The new sensitizers, LC1 to LC5, were synthesized as shown in Figure 1. Scheme 1 describes the synthetic pathways to the desired dyes. Palladium-catalyzed Stille coupling[12] of 3,7-dibromo-10-hexyl-10H-phenothiazine or 3,7dibromo-10-hexyl-10H-phenothiazine 5,5-dioxide with appropriate stannyl derivatives of the arylamine provided the formyl intermediates, which then underwent Knoevenagel condensation with cyanoacetic acid to afford the desired products. The detailed syntheses of the compounds are described in the Supporting Information.

www.chemsuschem.org New ionic liquid IL-I shown in Figure 1 was synthesized in moderate yield by following similar reported procedures,[10] as described in Scheme S1 (Supporting Information). Characterizations of electrolyte IL-I The UV/Vis absorption spectra, electrochemical properties, and rotating disk electrode experiments of the redox mediator are shown in Figures S1–S4. Although the concentration of I2 was 10 times less than that of IL-I and TEMPO, the UV/Vis absorption spectrum (Figure S1) of I2 is still much higher in intensity than that of either IL-I or TEMPO at approximately 500 nm. Thus, the optical losses are much smaller with the use of IL-I or TEMPO as the electrolyte as compared to those with the use of the I/I3 electrolyte. Figure S2 shows the cyclic voltammograms of IL-I, LiI, and TEMPO in 10.0 mm acetonitrile solution. Table S1 lists the redox potentials of these redox mediators. Three, two, and one redox peaks were observed for IL-I, LiI, and TEMPO, respectively (Figure S2). The two redox potentials observed for the iodide at 0.105 and 0.358 V vs. Ag/Ag + can be attributed to the I/I2C and I/I3 redox processes, respectively. The redox potential for TEMPO is 0.308 V vs. Ag/ Ag + . By comparison with LiI and TEMPO, the first redox peak of IL-I (0.105 V vs. Ag/Ag + ) corresponds to the first redox

Figure 1. Structures of the dyes and IL-I.

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Scheme 1. Synthetic pathways to the LC dyes.

Optical properties process of LiI (0.358 V vs. Ag/Ag + ), the second redox peak (0.257 V vs. Ag/Ag + ) can be attributed to the redox process of TEMPO, and the third redox peak (0.450 V vs. Ag/Ag + ) can be The data relevant to the optical absorption spectra of the dyes ascribed to the second redox process of LiI. Figure S3 shows in THF are collected in Table 1. The dyes have one or two the energy levels of different electrolytes and summarizes the major absorption bands in the range from 330 to 500 nm (Figworking mechanism of IL-I in the DSSCs. The energy level of ure 2 a). The band below 400 nm is attributed to the localized the NOC/N=O + redox couple (0.247 V vs. Ag/Ag + ) lies bep–p* transition of the conjugated molecules. The band in the tween that of I/I3 (0.095 V vs. Ag/Ag + ) and that of I/ longer wavelength region (480–600 nm) is ascribed to the inI2C(0.450 V vs. Ag/Ag + ). Therefore, formation of I3 can be suptramolecular charge-transfer transition with delocalized p–p* pressed owing to interception of I2C by NOC.[9c] The standard transition character. LC1 has a significantly higher maximum heterogeneous electron-transfer rate constants (k0) and diffuabsorption (labs) value than the other dyes. Likely, the electronsion coefficients (D0) for the NO radical of IL-I, the iodide of IL-I, TEMPO, and the iodide of LiI were calTable 1. Electro-optical parameters of the dyes. culated from Figure S4, and the data are summarized in Table S1. The improved value of k0 for the NO E1/2(ox)[b] HOMO/LUMO E0–0[c] E0–0*[d] Dye labs [nm] lem[a] radical on IL-I implies decreased resistance of the 4 1 1 [a] [nm] [mV] [eV] [eV] [V] (e [10 m cm ]) electrolyte reduction at the counter electrode, and LC1 332 (3.47), 446 (1.81) 608 282 5.08/2.76 2.32 1.71 both the short-circuit current density (JSC) and the LC2 340 (3.33), 418 (3.82) 510 534 5.33/2.68 2.65 1.36 DSSC performance are thus improved. Similar to our LC3 378 (sh.), 416 (5.78) 518 453 5.25/2.55 2.71 1.32 LC4 354 (3.22), 416 (4.01) 538 280 5.08/2.64 2.44 1.59 previous observation,[10] the D0 value of the iodide of LC5 3.52 (2.83), 424 (4.08) 543 273 5.07/2.50 2.58 1.45 IL-I is close to that of the iodide of LiI, and the D0 value of the NO radical of IL-I is slightly higher than [a] Recorded in THF solution at 298 K; the absorption coefficient e is added in parentheses. [b] Recorded in THF solution. Eox = 1/2(Epa+Epc), DEp = EpaEpc, in which Epa and that of TEMPO, which implies higher reactivity of Epc are the anodic and cathodic peak potentials, respectively. Oxidation potential reelectrolyte IL-I. ported was adjusted to the potential of ferrocene, which was used as an internal reference. The values in parentheses are the peak separation of the cathodic and anodic waves. Scan rate = 100 mV s1. [c] The band gap, E0–0, was derived from the intersection of the absorption and emission spectra. [d] E0–0* is the excited-state oxidation potential vs. NHE.

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www.chemsuschem.org grams are shown in Figure S5. All of the dyes exhibit one or two reversible oxidation waves. The first oxidation wave in LC1--LC5 is mainly due to the removal of an electron from the triphenylamine segment (LC2–LC5), which is consistent with the theoretical computations (see below). The oxidation potential of LC2 is higher than that of LC1; this again is proof of the electron-deficient nature of the “SO2” entity, which weakens the electronic interaction between the two mutually trans nitrogen atoms. The presence of an electron-donating alkoxy substituent in LC4 and LC5 apparently increases the electron density of the arylamine. The oxidation potential of LC5 is lower than that of LC4, and this is attributable to the fact that thienothiophene has more electrons than thiophene. The excited-state potential (E0–0*, 1.32 to 1.71 V vs. NHE), estimated from the difference in the first oxidation potential in the ground state and the zero–zero excitation energy (E0–0) of the dyes, is more negative than the conduction-band edge of the TiO2 electrode (0.5 V vs. NHE), and this ensures that there is enough driving force for electron injection into the conduction band of TiO2. The oxidation potentials of the dyes (0.973– 1.242 V vs. NHE) are more positive than the oxidation potential of the I/I3redox couple (0.4 V vs. NHE), which is indicative of favorable dye regeneration. Theoretical approach

Figure 2. (a) Absorption spectra of the dyes in THF. (b) Absorption spectra of the LC dyes on a TiO2 thin film.

deficient nature of “SO2” jeopardizes the electron-donating power of the arylamine, as electron-deficient “SO2” and the alkyl amino entity of the 10-hexyl-5,5-dioxido-10H-phenothiazine moiety reside at mutual ortho positions in LC2–LC5. Compared with LC4, LC5 exhibits a more prominent redshift in the absorption spectra because the length of conjugation in thienothiophene is greater than that in thiophene.[13] The labs value of LC4 is comparable to that of LC2 despite the presence of the better electron-donating arylamine. Possibly, the relatively weaker S0 !S1 transition band is embedded in the S0 !S2 transition band, which has negligible contribution from the triarylamine (see the section entitled Theoretical Approach). The long wavelength edge of the absorption spectra of the dyes adsorbed on TiO2 (Figure 2 b) exhibit a prominent redshift with the tail extending beyond 600 nm; this indicates significant Jaggregation of the dyes.[14] Electrochemical properties Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods were used to investigate the electrochemical characteristics of the dyes. The relevant electrochemical data are presented in Table 1 and representative cyclic voltammo 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

To gain insight into the correlation between the geometrical and the electronic properties of the LC dyes, we performed quantum chemistry computations on the organic sensitizers. The results of the theoretical computations are summarized in Table S2. Selected frontier orbitals of the dyes are shown in Figure S6. Comparison of the optimized structure of LC1 with that of LC2 indicates that there is no significant difference in the dihedral angle between the phenothiazine (or phenothiazinedioxide) moiety and the neighboring phenyl (LC1: 35.28; LC2: 34.58) or thienyl (LC1: 22.18; LC2: 20.48) entity (Figure S7). The HOMO orbitals in these compounds are mainly composed of the arylamine, with small contributions from the phenothiazine (or phenothiazinedioxide), and the HOMO1 orbitals are mainly composed of the phenothiazine (or phenothiazinedioxide) extending to the acceptor, with small contribution from the arylamine. In contrast, the LUMO orbitals mainly consist of the acceptor extending to part of the phenothiazine (or phenothiazinedioxide). There is prominent charge-transfer character in both the S0 !S1 and S0 !S2 transitions. The S0 !S2 transition is nearly from the HOMO1 to the LUMO, and the p–p* transition character is more evident in LC2–LC5 owing to a smaller contribution from the arylamine to their HOMO1 orbitals, which is in accordance with the absorption spectra (Note: the absorption band of the longest wavelength consists of both the S0 !S1 and S0 !S2 transitions on the basis of the computation results). The Mulliken charges for the S1 and S2 states were calculated from the time-dependent density functional theory (TDDFT) results. Differences in the Mulliken charges between the excited state and the ground state were grouped into several segments (Figure S8), arylamino (TPA), thiophene ring (T1), and ChemSusChem 0000, 00, 1 – 10

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phenothiazinedioxide (Ptz-SO2); phenothiazine (Ptz) and thiophene (T2); or thienothiophene ring (TT) and 2-cyanoacrylic acid (Ac), to estimate the extent of charge separation upon excitation (Table S2). Figure S6 displays the changes in Mulliken charges of all dyes for the S0 !S1 and S0 !S2 transitions. There is significant charge transfer from the arylamine to the 2-cyanoacrylic acid for the S0 !S1 transition. It is also evident that Ptz or Ptz-SO2 plays a more important role than the arylamine in charge transfer for the S0 !S2 transition. Photovoltaic devices The photovoltaic performance statistics under 1 sun (AM 1.5) illumination are collected in Table 2. The photocurrent–voltage (J–V) curves and the incident photon-to-current conversion ef-

Table 2. DSSCs performance parameters of the dyes.[a] Dye

VOC [V]

JSC [mA cm2]

FF

h [%]

Rct2[b] [W]

Dye loading [mol cm2]

LC1 LC2 LC3 LC4 LC5 N719

0.65 0.69 0.66 0.69 0.57 0.71

11.75 13.86 12.20 14.29 10.29 17.95

0.64 0.67 0.68 0.69 0.67 0.64

4.86 6.37 5.44 6.80 3.94 8.10

29.41 23.39 26.77 20.28 40.48 –

2.18  107 4.72  107 4.71  107 4.07  107 3.86  107 –

[a] Experiments were conducted by using TiO2 photoelectrodes with approximately 12 mm thickness and 0.16 cm2 working area on the FTO (15 W sq1) substrates. [b] Rct2 = The heterogeneous electron transfer resistance at the TiO2/dye/electrolyte interface. Figure 3. (a) J–V curves of DSSCs based on the dyes. (b) IPCE spectra of DSSCs based on the dyes.

ficiency (IPCE) of the cells are plotted in Figure 3 a, b. The devices exhibited moderate to good power conversion efficiency ranging from 4.86 to 6.80 %. LC4 showed the best cell efficiency, which reached approximately 60 % of that of the standard cell based on N719 (8.10 %). The value of JSC for LC1 is lower than that for LC2 and can be attributed to several factors: 1) the cell of LC1 is the least efficient in dark current suppression (see below and the electrochemical impedance measurements) and 2) LC1 has a lower adsorbed dye density than LC2. LC3 has the highest molar extinction coefficient among the LC dyes and has a dye density that is similar to that of LC2. However, its cell efficiency is inferior to that of LC2 owing to the fact that it is less effective in suppressing dark current (see below). The loading of the dye and the absorption spectra of LC4 and LC2 are comparable. However, the cell with LC4 has a higher IPCE than the cell with LC2 throughout the absorption range, which leads to a higher JSC value. As aforementioned, because of the two hexyloxy substituents in LC4, it is slightly better at light harvesting than LC2. Though LC4 is more efficient in suppressing dark current than LC2 (see below and the electrochemical impedance measurements), a higher VOC value is not observed. Possibly, dye aggregation deteriorates electron injection and blurs the effect of dark current suppression. As chenodeoxycholic acid (CDCA) was added as a co-adsorbent, the cell with LC4 indeed exhibited a higher value of VOC than the cell with LC2 (see below). The cell with  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

LC5 showed the lowest efficiency of all the cells, and this can be attributed to two major reasons: 1) it is the least efficient in dark current suppression and 2) electron transport is less efficient than that in other cells. The dark currents and the electrochemical impedance spectra (EIS) of DSSCs in the dark are shown in Figures 3 a and 4 a, respectively. The radius of the intermediate frequency semicircle in the Nyquist plot represents the charge transfer between the TiO2 surface and the electrolyte. The resistance towards the dark current decreases in the order LC4  LC2 > LC3 > LC1 > LC5, which is consistent with the trend of the dark current (Figure 3 a) and the observed VOC values (Table 2). LC4 can suppress dark current better than LC2, and this may be attributed to the presence of the two hexyloxy substituents in the former; no such effect can be found in LC5. Possibly, the longer conjugated skeleton in LC5 leaves more void space for the electrolytes to penetrate. The somewhat lower dye loading may also lead to higher charge recombination. Electrochemical impedance under illumination of 100 mW cm2 was also measured under open-circuit conditions, and the Nyquist plots of the DSSCs with different dyes are shown in Figure 4 b. The radius of the intermediate frequency semicircle (102–105 Hz) in the Nyquist plot will reflect electron-transport resistance, and a smaller radius implies that there is less efficient electronChemSusChem 0000, 00, 1 – 10

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Figure 4. (a) Electrochemical impedance spectra (Nyquist plots) of DSSCs for the dyes measured in the dark. (b) Electrochemical impedance spectra (Nyquist plots) of DSSCs based on the dyes measured under AM 1.5 illumination.

transport resistance. The electron-transport resistance decreases in the order LC5 > LC1 > LC3 > LC2 > LC4. Except for LC5, the phenothiazinedioxide-based dyes have more efficient dark current suppression and electron transport than the phenothiazine-based dye (LC1). The role of the “SO2” entity, however, needs further study. Charge extraction method and intensity-modulated photovoltage spectroscopy characterization The VOC value is determined by the position of the edge of the conduction band (ECB) and the electron density in the TiO2 film.[15] The relative shift in the conduction band of TiO2 was estimated by means of the charge extraction method (CEM), as shown in Figure 5 a. The cell with LC5 has the lowest dn (electron density) value among all the cells at the same VOC, which indicates that it experiences the most upward shift in the conduction-band edge of TiO2. Therefore, the lowest VOC value of the LC5-based cell largely stems from inefficient dark current suppression by LC5 (see below and the Supporting Information). Notably, “SO2”-free LC1 also has a relatively high TiO2 conduction-band edge. The electron lifetime (t) was derived as a function of VOC by using intensity-modulated photovoltage spectroscopy (IMVS),  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 5. (a) VOC as a function of electron density for DSSCs sensitized with LC dyes. (b) Electron lifetime as a function of VOC for DSSCs sensitized with LC dyes.

as shown in Figure 5 b. The trend is also consistent with the VOC and EIS data in the dark (see below). Though LC5 shows the most upward shift in the conduction-band edge of TiO2, its smallest t value is in accordance with the least efficient electron transport and/or dark current suppression of the cell. Co-adsorbed DSSCs The co-adsorbent approach has been widely used to improve cell efficiency by antiaggregation of the dye molecules and dark current suppression.[16] As a result, both JSC and VOC can be improved. In view of the significant dye aggregation of the LC dyes on the TiO2 surface, CDCA[17] was used as the co-adsorbent of the LC dyes. Suppression of dye aggregation was supported by a blueshift in the absorption spectra of the LC dyes on the TiO2 film upon the addition of CDCA (Figure 6 a, inset). Cell performance parameters measured with co-adsorbent are shown in Table 3, and the corresponding J–V curves and IPCE spectra are shown in Figure 6 a, b. There are increased JSC and VOC values for all the LC dyes after the addition of CDCA. Among them, LC4 exhibited the highest cell performance [power conversion efficiency (h) = 7.47 %, JSC = 15.08 mA cm2, VOC = 0.73 V, fill factor (FF) = 0.68], and the efficiency reached 92 % of that of the N719-based standard cell. Notably, the phenothiazinedioxide dyes showed better cell performance than the phenothiazine dye (i.e., LC1). ChemSusChem 0000, 00, 1 – 10

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Figure 7. (a) J–V curves with IL-I. (b) IPCE spectra of DSSCs based on dyes with the IL-I redox couple. 1 mm CDCA was added into the dye solutions.

Figure 6. (a) J–V curves of LC/CDCA DSSCs based on co-adsorbed approaches. Inset: the absorption spectra of LC/CDCA on a TiO2 thin film. (b) IPCE spectra of LC/CDCA DSSCs based on co-adsorbed approaches.

Table 4. Photovoltaic electrolytes.[a]

Table 3. Performance parameters of co-adsorbed DSSCs.[a]

parameters

of

the

DSSCs

by

using

IL-I

Dye

VOC [V]

JSC [mA cm2]

FF

h [%]

Dye

VOC [V]

JSC [mA cm2]

FF

h [%]

LC1 LC2 LC3 LC4 LC5

0.68 0.70 0.70 0.73 0.67

12.50 14.33 14.53 15.08 14.06

0.62 0.71 0.70 0.68 0.66

5.31 7.04 7.05 7.47 6.23

LC1 LC2 LC3 LC4 LC5

0.82 0.83 0.85 0.85 0.83

11.63 13.56 13.81 13.90 12.69

0.68 0.70 0.69 0.71 0.69

6.49 7.88 8.06 8.36 7.33

[a] Experiments were conducted by using TiO2 photoelectrodes with a thickness of approximately 12 mm and a working area of approximately 0.16 cm2 on the FTO (15 W sq1) substrates. For the co-adsorbed solar cells, 1 mm CDCA was added into the dye solutions.

DSSCs using electrolyte IL-I The J–V curves and the IPCE spectra of the DSSCs with the use of electrolyte IL-I measured under AM 1.5G illumination are shown in Figure 7 a, b. The corresponding photovoltaic parameters are given in Table 4. For IL-I, NOBF4 was used as an additive to help the oxidation of NOC to N=O + . As aforementioned, I2-free IL-I will not compete with the dyes for light har 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

[a] Experiments were conducted by using TiO2 photoelectrodes with a thickness of approximately 12 mm and a working area of approximately 0.16 cm2 on the FTO (15 W sq1) substrates. 1 mm CDCA was added into the dye solutions.

vesting. Moreover, the VOC values of DSSCs based on the LC dyes were boosted to above 800 mV. The DSSC with LC4 showed the highest power conversion efficiency (h) of 8.36 %, with VOC = 847 mV, JSC = 13.90 mA cm2, and FF = 0.71. Therefore, there is successful interception of I2C by the NOC radical, and the NOC/N=O + couple instead of the I/I3 redox pair plays a role in dye regeneration. DSSCs with the I/I3 electrolyte (Table 3) had slightly higher JSC values than those with IL-I. ChemSusChem 0000, 00, 1 – 10

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CHEMSUSCHEM FULL PAPERS A decrease in the dye regeneration rate because of the decreased potential gap between IL-I and the dye may result in a decreased forward electron-transfer rate.[10, 18] However, comparatively weak absorption in the visible-light region and a higher value of k0 of IL-I render its JSC value close to that of DSSCs with the iodide electrolyte. With the use of IL-I as the electrolyte, we also found that the phenothiazinedioxide dyes showed better cell performance than the phenothiazine dye (i.e., LC1).

Conclusions In summary, we synthesized metal-free organic sensitizers containing phenothiazinedioxide between the donor and the acceptor. DSSCs based on these sensitizers exhibited efficiencies between 3.94 and 6.80 % with the standard I/I3 electrolyte under AM 1.5 illumination. Upon adding chenodeoxycholic acid as the co-adsorbent, the cell performance of the DSSCs improved, and the highest cell performance reached approximately 92 % of that of the N719-based standard cell. By using IL-I as a dual redox couple, the open-circuit voltage was boosted to over 0.80 V, which is at least 100 mV higher than that of DSSCs with the standard iodide electrolyte. The cell efficiency (8.36 %) of LC4 was further improved and surpassed that of the N719-based standard cell.

Experimental Section Materials and instrumentation All solvents used were purified by standard procedures or purged with nitrogen before use. 1H NMR spectra were recorded with a Bruker 400 MHz spectrometer. Absorption spectra were recorded with a Dynamica DB-20 UV/Vis spectrophotometer. All chromatographic separations were performed on silica gel (60 m, 230– 400 mesh). Mass spectra (FAB) were recorded with a VG70–250S mass spectrometer. Elemental analyses were performed with a PerkinElmer 2400 CHN analyzer.

Cell fabrication of the DSSCs For the DSSCs, the TiO2 film consisted of a transparent layer and a scattering layer with a thickness of 12 and 5 mm, respectively, as measured by a profilometer (Dektak3, Veeco/Sloan Instruments Inc., USA). The TiO2 electrode with a 0.16 cm2 geometric area was immersed in a THF solution containing 3  104 m organic sensitizer or in acetonitrile/tert-butyl alcohol (1:1 v/v) containing cis-di(thiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719, Solaronix S.A., Switzerland, 3  104 m) overnight. For the co-adsorbed solar cells, chenodeoxycholic acid (CDCA) was added into the dye solutions at a concentration of 1 mm. Platinized fluorine-doped tin oxide (FTO) was used as a counter electrode and was controlled to have an active area of 0.16 cm2 by adhered polyester tape with a thickness of 60 mm. The standard electrolyte was composed of 0.1 m lithium iodide (LiI), 0.05 m iodine (I2), 0.6 m 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and 0.1 m guanidine thiocyanate (GuSCN) dissolved in acetonitrile. The dual TEMPO/iodide redox mediator (IL-I) electrolyte was made of 0.4 m IL-I and 0.4 m NOBF4 in acetonitrile. Photoelectrochemical  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org characterization of the solar cells was performed by using an Oriel Class A solar simulator (Oriel 91195A, Newport Corp.). For the TiO2 colloid solution, the TiO2 precursor was prepared by the following sol–gel processes. Titanium(IV) tetraisopropoxide (TTIP; 72 mL) was added to 0.1 m nitric acid (430 mL in water) with constant stirring, and the mixture was heated at 85 8C for 8 h. After cooling the mixture down to room temperature, the resultant colloid was transferred to an autoclave (PARR 4540, U.S.A.) and heated at 240 8C for 12 h to allow the TiO2 particles to grow uniformly (  20 nm in size). The TiO2 colloid was then concentrated until the TiO2 content was 8 % by weight. The TiO2 paste for the transparent layer was prepared by adding 25 wt % (with respect to the TiO2) of polyethylene glycol to the above solution to control the pore diameters and to prevent the film from cracking during drying. The TiO2 paste for the scattering layer was prepared from the same batch of the TiO2 colloid by adding 50 wt % (with respect to the TiO2) of PT-501A (15 m2 g1, 100 nm, 99.74 %, Ya Chung Industrial Co. Ltd., Taiwan). The sizes of the TiO2 particle were approximately 400 nm. The two kinds of TiO2 pastes were then subjected to fabrication of the photoanode.

Characterization of the DSSCs Photocurrent–voltage characteristics of the DSSCs were recorded with a potentiostat/galvanostat (CHI650B, CH Instruments, Inc., USA) at a light intensity of 1.0 sun calibrated by an Oriel reference solar cell (Oriel 91150, Newport Corp.). Incident photo-to-current conversion efficiency (IPCE) curves were obtained under short-circuit conditions. The light source was a class A quality solar simulator (PEC-L11, AM1.5G, Peccell Technologies, Inc.); light was focused through a monochromator (Oriel Instrument, model 74100) onto the photovoltaic cell and measured with an optical detector (Oriel Instrument, model 71580) and power meter (Oriel Instrument, model 70310).

Quantum chemistry computations The structures of the molecules were optimized by using B3LYP/631G*. For each molecule, a number of possible conformations were examined, and the one with the lowest energy was used. For the excited state, we employed time-dependent density functional theory (TDDFT) with the B3LYP functional. All of them were performed with Q-Chem 4.0 software.[19] A number of reports exist in which TDDFT was employed to characterize excited states with charge-transfer character.[20] In some cases, underestimation of the excitation energies was seen.[20, 21] Therefore, in the present work, we used TDDFT to characterize the extent of the charge shift and to avoid drawing conclusions from the excitation energy.

Acknowledgements We acknowledge the support of the National Taiwan University, the Academia Sinica (AS) and National Science Council (NSC) Taiwan), and the Instrumental Center of Institute of Chemistry (AS). Keywords: energy conversion · ionic liquids · metal-free · sensitizers · solar cells

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FULL PAPERS R. Y.-Y. Lin, T.-C. Chu, P.-W. Chen, J.-S. Ni, P.-C. Shih, Y.-C. Chen, K.-C. Ho,* J. T. Lin* && – && Phenothiazinedioxide-Conjugated Sensitizers and a Dual-TEMPO/Iodide Redox Mediator for Dye-Sensitized Solar Cells

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

An electric disposition: Metal-free dyes based on an arylamine donor and an acceptor containing a phenothiazinedioxide unit in the spacer have been designed and synthesized. Dye-sensitized solar cells based on phenothiazinedioxide using a chenodeoxycholic acid coadsorbent and a dual 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)/iodide electrolyte exhibit high light-to-electricity conversion efficiencies.

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