Imidazolium Anion-Type Ionic Liquid as a Crystal Growth Inhibitor. for Ionic Crystal/SWCNT based Solid-State Dye-Sensitized Solar Cell

Transcription

1 Imidazolium Anion-Type Ionic Liquid as a Crystal Growth Inhibitor for Ionic Crystal/SWCNT based Solid-State Dye-Sensitized Solar Cell Chuan-Pei Lee a, R.Vittal a, Po-Yen Chen a, and Kuo-Chuan Ho a,b, a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan * Corresponding author: Tel: ; Fax: kcho@ntu.edu.tw This manuscript was presented at the international conference on carbon nanostructured materials-cnano 09, October 4-8, 2009, the island of Santorini, Greece, and submitted for publication in Journal of Materials Chemistry.

2 Imidazolium Anion-Type Ionic Liquid as a Crystal Growth Inhibitor for Ionic Crystal/SWCNT based Solid-State Dye-Sensitized Solar Cell Abstract A solid organic ionic crystal, 1-ethyl-3-methylimidazolium iodide (EMII; mp: 79 o C), was employed as charge transfer intermediate (CTI) and sandwiched between the dye-sensitized porous TiO 2 and the Pt counter electrode to fabricate an all-solid-state dye-sensitized solar cell (DSSC). To further improve the cell efficiency, we utilized 1-methyl-3-propylimidazolium iodine (PMII), which can act simultaneously as a crystal growth inhibitor and a co-charge transfer intermediate. The co-cti exhibits a solid-like form until its content is increased to a weight ratio of 60%, and a smooth surface of the co-cti is observed in this case. In addition, single wall carbon nanotubes (SWCNT) were incorporated into the co-cti to create an extended electron transfer surface (EETS). This incorporation of SWCNT has enhanced the cell efficiency, apparently due to augmented electron transfer from the counter electrode to the CTI, resulting in reduced charge diffusion length. It is also established that the carbon material serves simultaneously both as charge transporter in the co-cti and as catalyst for the electrochemical reduction of I 3 -. We achieved a high efficient (3.82%) all-solid-state DSSC with this hybrid SWCNT-co-CTI, and the corresponding incident photo-to-current conversion efficiency (IPCE) has reached 50% at 520 nm. This cell 1

3 efficiency is higher than those reported in the literatures for all-solid-state DSSCs, where alkali iodide (efficiencies 2.4%, 1.5%) and imidazolium anion-type ionic crystal (efficiency 2.0%) were used as CTI. At-rest durability of the DSSC with the hybrid SWCNT-co-CTI was also studied and found to be far superior to that of a cell with an organic solvent electrolyte. Keywords: All-solid-state dye-sensitized solar cell; Charge transfer intermediate; Extended electron transfer surface; Long-term stability; Organic ionic crystal; Single wall carbon nanotube 1. Introduction Solar power is attractive due to the abundance and consistency of sunlight. Recently, dye-sensitized solar cells (DSSCs) have attracted much attention as highly efficient and low-cost alternatives to conventional silicon solar cells [1]. Typical DSSCs consist of three adjacent thin layers: a mesoporous oxide film, such as TiO 2 supported on a transparent conducting glass, dye molecules, such as ruthenium bipyridyl derivatives which are sensitive to visible light in the solar spectrum, and an organic liquid electrolyte containing iodide and triiodide ions as a redox mediator to reduce the oxidized dye molecules. These three layers are sandwiched together by a 2

4 second conducting glass covered with platinum [2]. However, presence of traditional organic liquid electrolytes in such devices has some problems such as a less long-term stability and a need for hermetic sealing. Therefore, solidification and quasi-solidification of DSSCs have been intensely studied with various approaches, such as using of p-type inorganic semiconductors [3-5], organic hole conducting materials [6-9], ionic gel electrolytes having a polymer or a gelator [10-13], and of ionic liquid (IL) electrolytes containing dispersed nano-components [14-19]. In these cases, imperfect filling of the dye-coated porous TiO 2 film by p-type inorganic semiconductors or polymers has resulted in poor efficiency for the cells. Moreover, the carrier diffusion length was limited in the case of conducting polymers due to their low conductivity. Although IL electrolytes solidified with nano-components can reduce leakages, it is not satisfactory, because they still contain some volatile components, e.g., 4-tert-butylpyridine: TBP or the sublimate, Iodine. Ikeda et al. have reported a clay-like conductive composite which contained only PACB particles and an ethyleneoxide-substituted imidazolium iodide; the corresponding solid-state DSSC showed a cell efficiency of 3.48% at 100 mw/cm 2 irradiation [20]. This paper presents the fabrication of high efficient solid state DSSCs with a hybrid SWCNT-co-CTI containing single wall carbon nanotubes (SWCNT), 1-ethyl-3-methylimidazolium iodide (EMII) and 1-methyl-3-propylimidazolium 3

5 iodine (PMII), without the addition of iodine. A comparative study is made among the performances of the pertinent DSSCs, in comparison with those of the cells with bare ionic liquids. Stability tests prove the significance of these types of cells. This research assumes importance considering the 3.82% efficiencycy for the iodine free DSSC and its unfailing stability in its preliminary observations, because stability is one of the two essential criteria for a good DSSC, other being the efficiency. The advantage of this device with respect to the composite electrolyte lies also in the fact that both this ionic liquid and the carbon were considered to be environmentally friendly. The composite electrolyte thus deserves to find wide application in DSSCs. 2. Experimental Section Poly(ethylene glycol) (PEG, M.W. 20,000), 1-methyl-3-propyl imidazolium iodide (PMII), and 1-ethyl-3-methylimidazolium iodide (EMII) were obtained from Merk and TCI (Tokyo Chemical Industry CO., LTD.), respectively; tert-butyl alcohol (tba) was purchased from Acros; titanium(iv) isopropoxide (TTIP), acetonitrile (ACN), acetylacetone (AA), ethanol, neutral cleaner, and isopropyl alcohol (IPA) were obtained from Aldrich. The single wall carbon nanotubes (SWCNT), which was supplied from Sigma-Aldrich Inc.. The hybrid SWCNT-co-CTIs were prepared by mixing solid powder (SWCNT) 4

6 and the ILS mentioned above in a weight ratio of 1:8. At the same time, ACN was added to the composite to improve the mixing, and then removed on a hot plate with a temperature of 80 o C. As the organic solvent electrolyte a mixture of 0.1 M LiI, 0.6 M PMII, 0.05 M I 2, and 0.5 M TBP in gamma-butyrolactone (GBL, Fluka) was used. The commercial titanium dioxide (ST-21, 50 m 2 /g, 6 g, Ya Chung Industrial Co. Ltd., Taiwan) was thoroughly mixed with a solution of AA (500 µl) in DI-water (11 g). This was stirred for 3 days and 1.8 g of PEG was then added to the well-dispersed colloid solution. The final mixture was stirred for additional 2 days, and the TiO 2 paste was prepared. A fluorine-doped SnO 2 conducting glass (FTO, 15 Ω/sq., Solaronix S.A., Aubonne, Switzerland) was first cleaned with a neutral cleaner, and then washed with DI-water, acetone, and IPA, sequentially. The conducting surface of the FTO was treated with a solution of TTIP (0.084 g) in ethanol (10 ml) for obtaining a good mechanical contact between the conducting glass and TiO 2 film, as well as to isolate the conducting glass surface from the conductive composite charge transfer intermediate. A 10 µm-thick film of TiO 2 was coated by doctor blade method onto the treated conducting glass, which was patterned to contain pieces of cm 2 to be cut latter. The TiO 2 film was gradually heated to 450 o C in an oxygen atmosphere, and subsequently sintered at that temperature for 30 min. After sintering at 450 o C and 5

7 cooling to 80 o C, the TiO 2 electrode was immersed in a M solution of N719 (Solaronix S.A., Aubonne, Switzerland) in ACN and tba (volume ratio of 1:1) at room temperature for 24 h. After dye-absorption, a 25 µm-thick surlyn (SX , Solaronix S.A., Aubonne, Switzerland) was put on the dye-sensitized TiO 2 electrode and attached by heating. The hybrid SWCNT-co-CTI was then coated onto the dye-sensitized TiO 2 film at 80 o C to ensure that the ILs can penetrate well into the porous structure and remove the residual ACN. The TiO 2 electrode with the hybrid SWCNT-co-CTI was assembled with a platinum-sputtered conducting glass electrode (ITO, 10 Ω/sq.) by folders, and the edges were sealed by UV glue. The surface of the DSSCs was illuminated by a class A quality solar simulator (PEC-L11, AM1.5G, Peccell Technologies, Inc.). The incident light intensity (100mW/cm 2 ) was calibrated with a standard Si Cell (PECSI01, Peccell Technologies, Inc.). The photoelectrochemical characteristics of the DSSCs were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands). The thickness of TiO 2 film was determined using a surface profilometer (Sloan Dektak 3030). The incident photo-to-current conversion efficiency (IPCE) was obtained at short-circuit condition. The light source was emitted from a 450 W Xe lamp (Oriel Instrument, model 6266) equipped with a water-based IR filter (Oriel Instrument, 6

8 model 6123NS), and focused through a monochromator (Oriel Instrument, model 74100) onto the photovoltaic cell under test. The monochromator was incremented through the visible spectrum to generate the IPCE (λ) as defined in bleow, IPCE (λ) = 1240(J SC /λψ) where λ is the wavelength, J SC is short-circuit photocurrent (ma/cm 2 ) recorded with a potentiostat/galvanostat, and ψ is the incident radiative flux (W/m 2 ) measured with a optical detector (Oriel Instrument, model 71580) and power meter (Oriel Instrument, model 70310). This curve can be derived from the measured absorption spectrum of the photosensitizer for comparison. 3. Results and Discussion An IL usually has favorable properties from the viewpoint of a DSSC, such as negligible vapor pressure, high thermal stability, wide electrochemical potential window, and high ionic conductivity [21-25]. Carbon nanotubes (CNTs) are remarkable materials, which are being widely studied because of their extraordinary electronic and mechanical properties. Considering these aspects of ILs, an incombustible and non-volatile charge transfer intermediate comprising with EMII and SWCNT was incorporated into DSSCs for this study (Scheme 1). We expected that this IL would allow perfect contact between the surface of dye-coating porous 7

9 TiO 2 and the extended electron transfer material (EETM), SWCNT [18], and would function as a charge transfer intermediate. Two ILs, EMII and PMII with different viscosities were used as charge transporting intermediates (co-cti) of DSSCs. The EMII was blended with different weight percentage of PMII to form co- charge transporting intermediators. As shown in Fig. 1, the co-cti exhibits a solid-like form until its content is increased to a weight ratio of 60 %, and a smooth surface of the co-cti is observed in this case (Fig. 2). In addition, single wall carbon nanotubes (SWCNT) were incorporated into the co-cti to create an extended electron transfer surface. This incorporation of SWCNT has enhanced the cell efficiency, apparently due to augmented electron transfer from the counter electrode to the CTI, resulting in reduced charge diffusion length. We achieved a high efficient (3.82 %; Fig. 5; Table 1) all-solid-state DSSC with this hybrid SWCNT-co-CTI, and the corresponding incident photo-to-current conversion efficiency (IPCE) has reached 50 % at 520 nm as shown in the inset of Fig. 5. This cell efficiency is higher than those reported in the literatures for all-solid-state DSSCs, where alkali iodide (efficiencies 2.4 %, 1.5 %) and imidazolium anion-type ionic crystal (efficiency 2.0 %) were used as CTI. In contrast to that reported in the literature, no 4-tert-butyl-pyridine (TBP; bp: 197 o C) was added into the hybrid SWCNT-co-CTI in our study. The advantage of this device with respect to the 8

10 composite electrolyte lies also in the fact that both this ionic liquid and the carbon were considered to be environmentally friendly. The composite electrolyte thus deserves to find wide application in DSSCs. It is well-known that I 2 exists in the electrolyte containing iodide in the form of polyiodides such as I 3 - or I 5 - (eq 1). An efficient transport of iodide and triiodide in the electrolyte is necessary for good performance of the conventional DSSC because the oxidized state of the dye (dye + ) should be regenerated by I - ions efficiently after the electrons from the excited state of the dye are injected into the conduction band of TiO 2 under illumination (eq 2). Moreover, the electrons accumulated at the counter electrode by the external circuit will lead to concentration overpotentials for the electrolyte at the counter electrode and to loss of energy of the DSSC, if the electrons are not transferred to I 3 - efficiently (eq 3). The reactions are as follows: I - + I 2 I I 2 I 5 - (in carrier mediator) (1) 3I - + 2dye + I dye (at dye-sensitized TiO 2 /carrier mediator) (2) I e - 3I - (at EETS/carrier mediator) (3) We emphasize that the cell works at high efficiency without the addition of iodide to the composite electrolyte in our study. Moreover, we also found that incorporation of I 2 is not necessary to drive our device [20], and even detrimental in our case as can be seen in Table 1. Table 1 shows the photovoltaic performances of 9

11 the DSSCs with PMII/CB composite electrolyte containing various amount of I 2 (wt%). These results demonstrate that the cell works best with an iodine-free composite electrolyte. We suggest that the iodide anion based ILs can provide sufficient I - for the regeneration of the oxidized dye under illumination (eq 2); I - in turn oxidizes to I 3 - [18, 26], which can be reduced back to I - at the EETS (eq 3). Increasing content of I 2 increases the locally high concentration of polyiodides in the porous dye-coated TiO 2 matrix. It facilitates recombination of injected conduction band electrons with polyiodides, and increases the dark current as shown in Fig. 4. Furthermore, the increasing content of I 2 also leads to enhanced light absorption even in the visible range by the carrier mediator existing in the porous dye-coated TiO 2 matrix. This decreases the light-harvesting of dye molecules [27]. Therefore, both V OC and J SC show decreases with the increases in the wt% of I 2. As expected, the solid-state DSSCs showed excellent durability compared with that of the cell with organic liquid electrolyte. Fig. 6 (a) shows the at-rest durability data of the DSSCs with hybrid SWCNT-co-CTI and organic liquid electrolyte. In this experiment, all cells were sealed by surlyn and UV glue, simultaneously. The cell efficiency was measured once per day after storing it under dark at room temperature. Efficiencies were normalized to the average value of the preceding five days, because we had noticed that the cell efficiency reaches a stable value at room temperature in 10

12 about 5 days. Although the overall power conversion efficiency of the solid-state DSSC has increased by about 2.6%, the overall power conversion efficiency of the DSSC with organic liquid electrolyte has decreased by about 30% after more than 1,000 h. Further, we assembled two fresh DSSCs, one with PMII/PACB composite electrolyte and the other with an organic liquid electrolyte, stored them in an oven at 70 o C, and then measured their cell efficiencies at different periods of time. Efficiencies were normalized to those of the first day, because according to our experience the efficiencies usually reach stable values at 70 o C within 1 day. As shown in Fig. 6 (b), the DSSC with the PMII/PACB-composite electrolyte shows an extraordinary durability even through it was stored at the temperature of 70 o C. The cell with organic liquid electrolyte lost its efficiency virtually in no time, notwithstanding the fact that the boiling point of the organic solvent, GBL is 204 o C. Thus, these results proved the unfailing stability of the cell under our investigation both at room temperature and at 70 o C, though its efficiency is somewhat lesser than those of other quasi solid-state cells with lesser stabilities. 4. Conclusion Efficient solid state DSSC was developed using a hybrid SWCNT-co-CTI and without the addition of iodine. A high efficiency (3.82%) solid-state DSSC was 11

13 achieved with a hybrid SWCNT-co-CTI containing 1-ethyl-3-methylimidazolium iodide (EMII), PMII (1-methyl-3-propylimidazolium iodine) and SWCNT under AM1.5 full sunlight. It is established that the carbon material in the composite electrolyte serves simultaneously as a charge transporter in the electrolyte and as the catalyst for electrochemical reduction of I 3 -. Finally, the stability of DSSC with the hybrid SWCNT-co-CTI is proved to be far superior to that with an organic solvent-based electrolyte. The DSSC with the hybrid SWCNT-co-CTI showed an excellent durability both at room temperature and at 70 o C. Acknowledgements This work was supported in part by the National Research Council of Taiwan, Republic of China, under grant NSC M Some of the instruments used in this study were made available through the financial support of the Academia Sinica, Taipei, Taiwan, Republic of China, under grant AS-97-TP-A08. 12

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19 Scheme Scheme 1: Schematic illustration for the charge-transport processs in the DSSC with SWCNT-co-CTI. List of Tables Table 1: Photovoltaic performances of the DSSCs with SWCNT-co-CTI, containing various amount of I 2 (wt%), measured at 100 mw/cm 2 light intensity. Figure Captions Fig. 1 The picture of the co-cti with different weight percent of PMII. Fig. 2 SEM images of (a) Bare EMII, (b) co-cti after recrystallization by treating at 90 oc; EMII/PMII: 40/60 (weight ratio). Fig. 3 SEM side view of the hybrid SWCNT -co-cti. Fig. 4 Photovoltaic performances of the DSSCs with SWCNT-co-CTI, containing various amount of I 2 (wt%), measured under dark. Fig. 5 The photocurrent-voltage characteristics of the DSSC with hybrid SWCNT- co-cti as CTI. Inset shows the incidient photo-to-current conversion efficiency of this cell. Fig. 6 At-rest durability data of the DSSCs with hybrid SWCNT-co-CTI and with an organic liquid electrolyte. 18

20 Counter electrode e - e - Carbon e - TiO 2 e - Dye EMI+ e - I - FTO hν Step 1: 3I - + 2dye + Step 2: I e - I dye (at dye-sensitized TiO 2 /carrier mediator) 3I - (at EETS/carrier mediator) Scheme 1 19

21 Table 1 Iodine (wt %) V OC (mv) J SC (ma/cm 2 ) η (%) FF

22 0 wt% 20 wt% 40 wt% 60 wt% Fig. 1 21

23 (a) (a) Fig. 2 22

24 Counter electrode CTI layer Fig. 3 23

25 Dark current density (ma/cm 2 ) wt% I wt% I wt% I wt% I 2 12 wt% I Voltage (mv) Fig. 4 24

26 9.0 Photocurrent density (ma/cm 2 ) IPCE (%) Wavelength (nm) Voltage (mv) Fig. 5 25

27 (a) Normalized efficiency At-rest durability (stored at room temperature) Organic solvent electrolyte Hybrid SWCNT-co-CTI Time (day) (b) Normalized effieiency At-rest durability (stored at 70 o C) Organic solvent electrolyte Hybrid SWCNT-co-CTI Time (day) Fig. 6 26