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Influence of the preparation conditions of TiO2 electrodes on the performance of solid-state dye-sensitized solar cells with CuI as a hole collector
Solar Energy 81 (2007) 717–722 www.elsevier.com/locate/solener
Influence of the preparation conditions of TiO2 electrodes on the performance of solid-state dye-sensitized solar cells with CuI as a hole collector Li Yang a
a,*
, Zhengxi Zhang a, Shaohua Fang a, Xuhui Gao a, Masamichi Obata
b
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, No. 800 Dongchuan Road, MinHang District, Shanghai 200240, China b NGK Insulator, Ltd., 2-56 Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan Received 19 April 2005; received in revised form 17 February 2006; accepted 12 October 2006 Available online 10 November 2006 Communicated by: Associate Editor Arturo Morales-Acevedo
Abstract Solid-state dye-sensitized solar cells (DSSCs) were fabricated in which the thin p-CuI film acts as a hole collector. Influences of the different preparation methods, composition, aging time of the TiO2 pastes and sensitizing time on the performance of the cells were investigated. Different preparation routes for the TiO2 paste do not obviously affect the performance of the cells. The volume of water, acetic acid and 2-propanol contained in the TiO2 pastes and the amount of the TiO2 powder were determined. The efficiency of the cells remains nearly stable when the aging period of the TiO2 pastes is within one week. The favorable dying time is above 2 h. The cells having a favorable performance deliver a mean short-circuit photocurrent of 10.8 mA cm2 and mean open-circuit voltage of 0.61 V at 100 mW cm2 (1.5 AM). The mean fill factor and the mean efficiency of these cells are 0.55% and 3.7%, respectively. The short-circuit photocurrent rapidly decays after 3 h, and at the same time, the open-circuit voltage slowly decreases when the time increases, and then remains nearly stable after 24 h. 2006 Elsevier Ltd. All rights reserved. Keywords: Solid-state solar cells; Titanium dioxide; Dye sensitization; CuI; Preparation conditions
1. Introduction Dye-sensitized solar cells using a nano-crystalline metal oxide, dyes and organic liquid electrolytes have attracted much attention during the past decade due to their high photo-to-current conversion efficiency and low production cost (O’Reagan and Gra¨tzel, 1991; Nazeerudin et al., 1993, 2001; Smestad et al., 1994; Hagfeldt and Gra¨tzel, 1995, 2000; Murakoshi et al., 1995; Barbe´ et al., 1997; Sayama et al., 1998; Gra¨tzel, 2001; Hao et al., 2004). The electrolytes used in these cells are often composed of redox couples and volatile organic solvents, which lead to some *
Corresponding author. Tel.: +86 21 54748917; fax: +86 21 54741297. E-mail address: [email protected] (L. Yang).
0038-092X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2006.10.001
problems, such as the deterioration of cell performance during long-term operation and difficulty in sealing. Employing room-temperature ionic liquids as the solvent is considered one way to replace the conventional organic solvents (Papageorgiou et al., 1996; Matsumoto et al., 2001; Kubo et al., 2002; Wang et al., 2003, 2004; Kawano et al., 2004; Bach et al., 1998; Stathatos et al., 2002; Tennakone et al., 1995, 1996, 1997a,b, 1998a,b; Kumara et al., 2001; Senadeera et al., 2002; Bandara and Weerasinghe, 2005), whereas the high cost and easy leakage of the ionic liquids are serious problems for the development of such cells. The solidification of the electrolytes is expected to prevent electrolyte leakage, and solid-state dye-sensitized solar cells are quite promising for practical applications. Many studies have focused on replacing the liquid
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L. Yang et al. / Solar Energy 81 (2007) 717–722
Nomenclature Isc Voc
short-circuit photocurrent (mA/cm2) open-circuit voltage (V)
electrolytes with solid-state hole conductors (Bach et al., 1998), solvent-free polymer electrolytes (Stathatos et al., 2002) and p-type semiconductors (Tennakone et al., 1995, 1996, 1997a,b, 1998a,b; Kumara et al., 2001; Senadeera et al., 2002; Bandara and Weerasinghe, 2005). In the case of the p-type semiconductors, CuI (Tennakone et al., 1995, 1996, 1997a,b, 1998a,b), CuSCN (Tennakone et al., 1998a,b; Kumara et al., 2001), pentacene (PEN) (Senadeera et al., 2002) and NiO (Bandara and Weerasinghe, 2005) have been studied. In spite of much effort being expended in trying to improve the performance and stability of the solid-state dye-sensitized solar cells with p-type semiconductors by selecting the appropriate dyes (Tennakone et al., 1998a,b) or designing a new deposition technique of p-type semiconductors (Kumara et al., 2001) or using a p-type oxide semiconductor (Bandara and Weerasinghe, 2005), for example, Tennakone’s research group used ruthenium bipyridyl complexes as efficient sensitizers for solid-state photovoltaic cell and obtained a maximum efficiency of about 4.5% (Tennakone et al., 1998a,b), until now only a few studies regarding the preparation conditions of solid-state dye-sensitized solar cells have been reported. As is known to all, it is very important to understand and grasp the fabrication conditions of solar cells, especially for mass-production. In this study, we chose CuI as the hole collector, and fabricated solid-state dyesensitized solar cells according to the literatures reported by Tennakone’s research group (Tennakone et al., 1995, 1996, 1997a,b), and mainly investigated the effect of the preparation conditions of TiO2 electrodes on the performance of cells. Moreover, we also studied the stability of the cells.
FF g
fill factor efficiency (%)
Route I titanium tetraisopropoxide
acetic acid
2-propanol
water
TiO2 powder
TiO2 paste
Route II acetic acid
titanium tetraisopropoxide
water
TiO2 powder
TiO2 paste
2-propanol
Route III acetic acid
2-propanol
titanium tetraisopropoxide TiO2 powder
TiO2 paste
water
Route IV acetic acid TiO2 powder 2-propanol water titanium tetraisopropoxide
TiO2 paste
2. Experimental
2.3. Fabrication of solid-state photoelectrochemical cells
2.1. Materials
TiO2 pastes prepared by the above routes were painted on the conducting surface of the cleaned FTO glass plates placed on a hot plate heated to about 140 C, allowed to dry and sintered in air at about 500 C for 6 min. The process was repeated until a film of about 10 lm was obtained. The films were examined by SEM (JOEL, JSM-5600) in order to study the morphology. The TiO2 electrodes were immersed in a 1 · 104 M solution of an N3 dye in ethanol for 8 h, unless otherwise noted. Afterwards, the electrodes were washed with ethanol and dried in air. CuI was deposited on the N3 dye coated porous TiO2 films as described below. A solution of CuI was prepared by dissolving 0.12 g of CuI in 10 mL of acetonitrile. The
The TiO2 powder (P25) was donated by Mitsubishi Materials Corporation. The cis-di(thiocyanato)-N,Nbis(2,2 0 -bipyridil-4,4 0 -dicarboxylic acid) ruthenium (II) complex (N3 dye) was purchased from Kojima Chemical Reagents Inc. The fluorine-doped SnO2 layered (FTO) glass plates (1.5 · 1 cm2, sheet resistance = 12 X cm2) were purchased from Asahi Glass. All other chemicals were reagent grade and used as received. 2.2. Preparation of nanoporous TiO2 film TiO2 pastes were prepared by the following routes.
L. Yang et al. / Solar Energy 81 (2007) 717–722
N3 dye coated TiO2 plate was laid on a hot plate (surface temperature about 95 C) and the CuI solution was dropped on the dyed surface by a dropper, allowing the acetonitrile to evaporate until a layer of about 8 lm above the film surface was deposited. The counter electrode (a lightly platinized FTO glass plate) pressed onto the CuI surface to form an electrical contact. 2.4. Measurements of I–V characteristics I–V measurements were performed in air and at room temperature. The electrode size was 0.25 cm2. The photocurrent–voltage measurements were performed in a Bunkoh-Keiki CEP-25BX system under illumination with an AM 1.5 simulated sunlight (100 mW cm2). All measurements were carried out using 32 samples and the means were used as the results. 3. Results and discussion 3.1. Effect of different preparation routes of TiO2 pastes on the performance of the cells Table 1 shows that the different TiO2 pastes preparation routes do not obviously influence the performance of the cells. In other words, no obvious changes in the Table 1 Influence of TiO2 pastes preparation methods on the performance of the cells (water: 4 mL, acetic acid: 5.5 mL, 2-propanol: 14 mL, TiO2 powder: 0.5 g) Route
Isc (mA/cm2)
I II III IV
10.6 10.4 10.5 10.5
a
(0.7988a) (0.7802) (0.7998) (0.7976)
Voc (V)
FF
0.61 0.61 0.61 0.60
0.53 0.51 0.53 0.52
(0.0231) (0.0239) (0.0238) (0.0258)
g (%) (0.0322) (0.0380) (0.0399) (0.0383)
3.5 3.3 3.5 3.3
(0.3539) (0.3388) (0.3580) (0.3363)
Standard deviation.
719
performance of the cells have been observed when using the different routes. Therefore, the TiO2 pastes used in the cells discussed below will be prepared by route I. 3.2. Effect of composition of TiO2 pastes on the performance of the cells 3.2.1. Effect of volume of water From Table 2, we can see that Isc, Voc and g initially increase with the increase in the volume of water, reaching a maximum value at 3–4 mL of water, and then decrease. On the contrary, FF decreases with the increase in the water volume, reaching a minimum value at 3–5 mL of water, and then increases. Therefore, the volume of water contained in TiO2 pastes discussed below is 3–4 mL. 3.2.2. Effect of volume of acetic acid Table 3 shows that Isc, Voc, FF and g first increase with the increase in the volume of acetic acid, reaching a maximum value at a volume of 3–5.5 mL, and then decrease. As a result, the volume of acetic acid contained in the TiO2 pastes discussed below is considered to be 3–5.5 mL. 3.2.3. Effect of volume of 2-propanol Table 4 shows that Isc, Voc, FF and g initially increase, having a maximum value at a volume of 10–14 mL and then decrease when the volume of 2-propanol increases. Thus, the volume of the 2-propanol contained in the TiO2 pastes discussed below is 10–14 mL. 3.2.4. Effect of amount of TiO2 powder Table 5 shows that Isc and g initially increase with the increase in the amount of TiO2 powder, reaching a maximum value at 0.5–0.8 g of TiO2 powder and then decrease; on the other hand, Voc and FF remain nearly stable when the amount of the TiO2 powder increases, but Voc slightly decreases when the TiO2 powder amount exceeds 0.65 g. As
Table 2 Influence of volume of water contained in TiO2 pastes on the performance of cells (acetic acid: 5.5 mL, 2-propanol: 14 mL, TiO2 powder: 0.5 g) Water (mL)
Isc (mA cm2)
Voc (V)
FF
1 2 3 4 5 6
0.8 8.8 10.6 10.6 7.7 0.6
0.46 0.58 0.61 0.61 0.59 0.57
0.87 0.56 0.51 0.53 0.49 0.87
(0.0635) (0.6787) (0.7802) (0.7988) (0.5897) (0.0425)
(0.0192) (0.0211) (0.0238) (0.0231) (0.0225) (0.0219)
g (%) (0.0527) (0.0379) (0.0340) (0.0322) (0.0312) (0.0580)
0.3 3.0 3.4 3.5 2.3 0.3
(0.0368) (0.3068) (0.3478) (0.3539) (0.2328) (0.0304)
Table 3 Influence of volume of acetic acid contained in TiO2 pastes on the performance of cells (water: 4 mL, 2-propanol: 14 mL, TiO2 powder: 0.5 g) Acetic acid (mL)
Isc (mA cm2)
Voc (V)
FF
1 3 5.5 8 11
9.1 9.5 10.6 9.2 3.1
0.63 0.65 0.61 0.57 0.51
0.52 0.54 0.53 0.53 0.50
(0.6902) (0.7199) (0.7988) (0.6993) (0.0234)
(0.0239) (0.0247) (0.0231) (0.0291) (0.0195)
g (%) (0.0312) (0.0330) (0.0322) (0.0328) (0.0309)
3.1 3.4 3.5 2.9 0.8
(0.3139) (0.3498) (0.3539) (0.2934) (0.0807)
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L. Yang et al. / Solar Energy 81 (2007) 717–722
Table 4 Influence of volume of 2-propanol contained in TiO2 pastes on the performance of cells (water: 4 mL, acetic acid: 5.5 mL, TiO2 powder: 0.5 g) 2-Propanol (mL)
Isc (mA cm2)
Voc (V)
FF
7 10 14 18 21
6.1 10.3 10.6 8.7 4.2
0.57 0.62 0.61 0.58 0.53
0.41 0.46 0.53 0.48 0.45
(0.4597) (0.7763) (0.7988) (0.6557) (0.3168)
(0.0215) (0.0235) (0.0231) (0.0216) (0.0202)
g (%) (0.0249) (0.0277) (0.0322) (0.0294) (0.0276)
1.4 3.0 3.5 2.5 1.0
(0.1418) (0.3096) (0.3539) (0.2529) (0.1018)
Table 5 Influence of amount of TiO2 powder contained in TiO2 pastes on the performance of cells (water: 4 mL, acetic acid: 5.5 mL, 2-propanol: 14 mL) TiO2 powder (g)
Isc (mA cm2)
Voc (V)
FF
0.3 0.5 0.65 0.8 1.3
7.8 10.6 10.8 10.7 7.9
0.61 0.61 0.61 0.60 0.59
0.53 0.53 0.55 0.55 0.55
(0.0238) (0.0231) (0.0231) (0.0223) (0.0229)
can be seen from this table, the amount of the TiO2 powder contained in the TiO2 pastes discussed below is 0.5–0.8 g. Based on these results, the TiO2 pastes used in the cells discussed below are composed of water, acetic acid, 2-propanol and TiO2 powder with a mass ratio of 1:1.43:2.77:0.16. 3.3. Effect of aging time of TiO2 paste on the efficiency of the cells The dependence of the cell efficiency on the aging time of the TiO2 pastes is shown in Fig. 1. The efficiency of the cells remains nearly constant with the increase of the aging time within one week, indicating that aged TiO2 pastes within one week are almost the same as the fresh paste for preparing the cells. 3.4. Effect of dying time of TiO2 surface on the efficiency of cells Fig. 2 shows the favorable time of contacting the N3 dye with the TiO2 surface. The N3 dye is continually adsorbed
g (%) (0.0392) (0.0322) (0.0335) (0.0395) (0.0383)
2.6 3.5 3.7 3.6 2.6
(0.2628) (0.3539) (0.3742) (0.3642) (0.2673)
4.0
3.5
η (%)
(0.5879) (0.7988) (0.8139) (0.8066) (0.5955)
3.0
2.5
2.0 0
1
2
3
4
5
6
7
8
9
hours Fig. 2. Influence of dying time on the efficiency of the cells.
on the TiO2 surface with the initial increase of the time, leading to an increased efficiency of the cells. The efficiency of the cells reaches a maximum value of about 3.7% when the dying time is extended to about 2 h, and remains nearly constant as the dying time continue to increase. 3.5. The performance of DSSC
4.0 3.5
η (%)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
2
4
6
8
10
12
Aging time (day) Fig. 1. Effect of aging time of TiO2 paste on the efficiency of the cells.
Fig. 3 shows one of the measured I–V curves for the DSSC samples. The short-circuit current, Isc, is 10.9 mA cm2, the open-circuit voltage, Voc, is 0.606 V and the fill factor, FF, is 0.55, which lead to a conversion efficiency of 3.6%. Fig. 4(a)–(c) shows the porous and high surface areas of the TiO2 film and CuI film and the cross section of the TiO2 electrode of this sample, respectively. The performance of a number of DSSC samples, which have been prepared according to above discussion, was studied in order to confirm its repeatability, and the results are shown in Table 6. The mean short-circuit photocurrent, open-circuit voltage, fill factor and efficiency are 10.8 mA cm2, 0.61 V, 0.55, 3.7%, respectively.
L. Yang et al. / Solar Energy 81 (2007) 717–722
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Table 6 Mean value of performance of DSSCs Performance 2
Isc (mA cm ) Voc (V) FF g (%)
Mean value
Standard deviation
10.8 0.61 0.55 3.7
0.8139 0.0231 0.0335 0.3742
0.8
12
0.7
10
3.6. Effect of time on the performance of the cells
0.5
6
0.4 0.3
4
0.2 2
The main problem of the solid-state dye-sensitized solar cells with p-CuI as the hole collector is their instability (Tennakone et al., 1995, 1997a,b). The dependence of the short-circuit photocurrent and open-circuit voltage on the elapsed time is given in Fig. 5. The photocurrent remains nearly stable for about 3 h, and then rapidly decays. Deterioration of the CuI film is responsible for this phenomenon (Tennakone et al., 1995). In addition, the dye rapidly degrades in the presence of oxygen, moisture and UV light due to the photocatalytic activity of TiO2 (Tennakone et al., 1997a,b), which also decreases the photocurrents. The open-circuit voltage
V oc (V)
Fig. 3. I–V characteristics of a solid-state dye-sensitized solar cell under 100 mW cm2 illumination.
I sc (mA cm-2)
0.6 8
0
0.1 0
10
20
30
40
50
60
70
0.0 80
Time (hour) Fig. 5. Dependence of short-circuit photocurrent and open-circuit voltage on elapsed time.
slowly initial decreases with the increasing time, and then remains nearly constant after 24 h. The decrease in the open-circuit voltage is about 3–4%. Therefore, how to maintain the stability of these cells is a future research topic.
Fig. 4. Scanning electron micrograph showing surface of TiO2 film (a), CuI film (b) and cross section of the TiO2 electrode treated with N3 dye (c).
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L. Yang et al. / Solar Energy 81 (2007) 717–722
4. Conclusions The influences of the preparation conditions of TiO2 electrodes on the performance of solid-state dye-sensitized solar cells have been studied. The following conclusions can be made: (1) The different preparation methods of the TiO2 pastes do not influence the performance of the cells. (2) The mass ratio of water, acetic acid, 2-propanol and TiO2 powder contained in the TiO2 pastes was determined to be 1:1.43:2.77:0.16. (3) The efficiency of cells undergoes no obvious change with the increase in the aging time of the TiO2 pastes within one week. (4) The favorable dying time is above 2 h, and the photoenergy conversion efficiency of cells can reach a value of about 3.7%. (5) The open-circuit voltage slowly decreases when the time increases, and then remains nearly stable; on the other hand, the photocurrent rapidly decays after 3 h.
Acknowledgements This work is financially supported by the National Key Project of China for Basic Research under Grant No. 2006CB202600, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References Bach, U., Lupo, D., Comte, P., et al., 1998. Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 395, 583–585. Bandara, J., Weerasinghe, H., 2005. Solid-state dye-sensitized solar cell with p-type NiO as a hole collector. Sol. Energy Mater. Sol. Cells 85, 385–390. Barbe´, C.J., Arendes, F., Comte, P., et al., 1997. Nanocrystalline titanium oxide electrodes for photovoltaic applications. J. Am. Ceram. Soc. 80, 3157–3171. Gra¨tzel, M., 2001. Photoelectrochemical cells. Nature 414, 338–344. Hagfeldt, A., Gra¨tzel, M., 1995. Light-induced redox reactions in nanocrystalline systems. Chem. Rev. 95, 49–68. Hagfeldt, A., Gra¨tzel, M., 2000. Molecular photovoltanics. Acc. Chem. Res. 33, 269–277. Hao, S., Wu, J.H., Fan, L.Q., et al., 2004. The influence of acid treatment of TiO2 porous film electrode on photoelectric performance of dyesensitized solar cell. Solar Energy 76, 745–750. Kawano, R., Matsui, H., Matsuyama, C., et al., 2004. High performance dye-sensitized solar cells using ionic liquids as their electrolytes. J. Photochem. Photobiol. A: Chem. 164, 87–92. Kubo, W., Kitamura, T., Hanabusa, K., et al., 2002. Quasi-solid-state dye-sensitized solar cells using room temperature molten salts and a low molecular weight gelator. Chem. Commun., 374–375.
Kumara, G.R.R.A., Konno, A., Senadeera, G.K.R., et al., 2001. Dyesensitized solar cell with the hole collector p-CuSCN deposited from a solution in n-propyl sulphide. Sol. Energy Mater. Sol. Cells 69, 195– 199. Matsumoto, H., Matsuda, T., Tsuda, T., et al., 2001. The application of room temperature molten salt with low viscosity to the electrolyte for dye-sensitized solar cell. Chem. Lett., 26–27. Murakoshi, K., Kano, G., Wada, Y., et al., 1995. Importance of binding states between photo-sensitizing molecules and the TiO2 surface for efficiency in a dye-sensitized solar cell. J. Electroanal. Chem. 396, 27– 34. Nazeeruddin, M.K., Pe´chy, P., Renouard, T., et al., 2001. Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells. J. Am. Chem. Soc. 123, 1613–1624. Nazeerudin, M.K., Kay, A., Rodicio, I., et al., 1993. Conversion of light to electricity by cis-X2bis(2,2 0 -bipyridyl-4,4 0 -dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl, Br, I, CN, and SCN) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 115, 6382–6390. O’Reagan, B., Gra¨tzel, M., 1991. A low-cost, high-efficiency solar cell based on dye- sensitized colloidal TiO2 films. Nature 353, 737–739. Papageorgiou, N., Athanassov, Y., Armand, M., et al., 1996. The performance and stability of ambient temperature molten salts for solar cell applications. J. Electrochem. Soc. 143, 3099–3108. Sayama, K., Sugihara, H., Arakawa, H., 1998. Photoelectrochemical properties of a porous Nb2O5 electrode sensitized by a ruthenium dye. Chem. Mater. 10, 3825–3832. Senadeera, G.K.R., Jayaweera, P.V.V., Perera, V.P.S., et al., 2002. Solidstate dye-sensitized photocell based on pentacene as a hole collector. Sol. Energy Mater. Sol. Cells 73, 103–108. Smestad, G., Bignozzi, C., Argazzi, R., 1994. Testing of dye sensitized TiO2 solar cells I: Experimental photocurrent output and conversion efficiencies. Sol. Energy Mater. Sol. Cells 32, 259–272. Stathatos, E., Lianos, P., Lavrencic-Stangar, U., et al., 2002. A highperformance solid-state dye-sensitized photoelectrochemical cell employing a nanocomposite gel electrolyte made by the sol–gel route. Adv. Mater. 14, 354–357. Tennakone, K., Kumara, G.R.R.A., Kumarasinghe, A.R., et al., 1995. A dye-sensitized nano-porous solid-state photovoltaic cell. Semicond. Sci. Technol. 10, 1689–1693. Tennakone, K., Kumara, G.R.R.A., Wijayantha, K.G.U., 1996. The suppression of the recombination of photogenerated carries in a dyesensitized nano-porous solid-state photovoltaic cell. Semicond. Sci. Technol. 11, 1737–1739. Tennakone, K., Kumara, G.R.R.A., Kottegoda, I.R.M., et al., 1997a. The photostability of dye-sensitized solid state photovoltaic cells: factors determining the stability of the pigment in a nanoporous nTiO2/cyaniding/p-CuI cell. Semicond. Sci. Technol. 12, 128–132. Tennakone, K., Kumara, G.R.R.A., Wijayantha, K.G.U., et al., 1997b. A nanoporous solid-state photovoltaic cell sensitized with copper chlorophyllin. J. Photochem. Photobiol. A: Chem. 108, 175–177. Tennakone, K., Kumara, G.R.R.A., Kottegoda, I.R.M., et al., 1998a. Sensitization of nano-porous films of TiO2 with santalin (red sandalwood pigment) and construction of dye-sensitized solid-state photovoltaic cells. J. Photochem. Photobiol. A: Chem. 117, 137–142. Tennakone, K., Kumara, G.R.R.A., Kottegoda, I.R.M., et al., 1998b. A solid-state photovoltaic cell sensitized with a ruthenium bipyridyl complex. J. Phys. D: Appl. Phys. 31, 1492–1496. Wang, P., Zakeeruddin, S.M., Comte, P., et al., 2003. Gelation of ionic liquid-based electrolytes with silica nanoparticles for quasi-solid-state dye-sensitized solar cells. J. Am. Chem. Soc. 125, 1166–1167. Wang, P., Zakeeruddin, S.M., Humphry-Baker, R., et al., 2004. A binary ionic liquid electrolyte to achieve P7% power conversion efficiencies in dye-sensitized solar cells. Chem. Mater. 16, 2694–2696.
Influence of the preparation conditions of TiO2 electrodes on the performance of solid-state dye-sensitized solar cells with CuI as a hole collector Li Yang a
a,*
, Zhengxi Zhang a, Shaohua Fang a, Xuhui Gao a, Masamichi Obata
b
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, No. 800 Dongchuan Road, MinHang District, Shanghai 200240, China b NGK Insulator, Ltd., 2-56 Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan Received 19 April 2005; received in revised form 17 February 2006; accepted 12 October 2006 Available online 10 November 2006 Communicated by: Associate Editor Arturo Morales-Acevedo
Abstract Solid-state dye-sensitized solar cells (DSSCs) were fabricated in which the thin p-CuI film acts as a hole collector. Influences of the different preparation methods, composition, aging time of the TiO2 pastes and sensitizing time on the performance of the cells were investigated. Different preparation routes for the TiO2 paste do not obviously affect the performance of the cells. The volume of water, acetic acid and 2-propanol contained in the TiO2 pastes and the amount of the TiO2 powder were determined. The efficiency of the cells remains nearly stable when the aging period of the TiO2 pastes is within one week. The favorable dying time is above 2 h. The cells having a favorable performance deliver a mean short-circuit photocurrent of 10.8 mA cm2 and mean open-circuit voltage of 0.61 V at 100 mW cm2 (1.5 AM). The mean fill factor and the mean efficiency of these cells are 0.55% and 3.7%, respectively. The short-circuit photocurrent rapidly decays after 3 h, and at the same time, the open-circuit voltage slowly decreases when the time increases, and then remains nearly stable after 24 h. 2006 Elsevier Ltd. All rights reserved. Keywords: Solid-state solar cells; Titanium dioxide; Dye sensitization; CuI; Preparation conditions
1. Introduction Dye-sensitized solar cells using a nano-crystalline metal oxide, dyes and organic liquid electrolytes have attracted much attention during the past decade due to their high photo-to-current conversion efficiency and low production cost (O’Reagan and Gra¨tzel, 1991; Nazeerudin et al., 1993, 2001; Smestad et al., 1994; Hagfeldt and Gra¨tzel, 1995, 2000; Murakoshi et al., 1995; Barbe´ et al., 1997; Sayama et al., 1998; Gra¨tzel, 2001; Hao et al., 2004). The electrolytes used in these cells are often composed of redox couples and volatile organic solvents, which lead to some *
Corresponding author. Tel.: +86 21 54748917; fax: +86 21 54741297. E-mail address: [email protected] (L. Yang).
0038-092X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2006.10.001
problems, such as the deterioration of cell performance during long-term operation and difficulty in sealing. Employing room-temperature ionic liquids as the solvent is considered one way to replace the conventional organic solvents (Papageorgiou et al., 1996; Matsumoto et al., 2001; Kubo et al., 2002; Wang et al., 2003, 2004; Kawano et al., 2004; Bach et al., 1998; Stathatos et al., 2002; Tennakone et al., 1995, 1996, 1997a,b, 1998a,b; Kumara et al., 2001; Senadeera et al., 2002; Bandara and Weerasinghe, 2005), whereas the high cost and easy leakage of the ionic liquids are serious problems for the development of such cells. The solidification of the electrolytes is expected to prevent electrolyte leakage, and solid-state dye-sensitized solar cells are quite promising for practical applications. Many studies have focused on replacing the liquid
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L. Yang et al. / Solar Energy 81 (2007) 717–722
Nomenclature Isc Voc
short-circuit photocurrent (mA/cm2) open-circuit voltage (V)
electrolytes with solid-state hole conductors (Bach et al., 1998), solvent-free polymer electrolytes (Stathatos et al., 2002) and p-type semiconductors (Tennakone et al., 1995, 1996, 1997a,b, 1998a,b; Kumara et al., 2001; Senadeera et al., 2002; Bandara and Weerasinghe, 2005). In the case of the p-type semiconductors, CuI (Tennakone et al., 1995, 1996, 1997a,b, 1998a,b), CuSCN (Tennakone et al., 1998a,b; Kumara et al., 2001), pentacene (PEN) (Senadeera et al., 2002) and NiO (Bandara and Weerasinghe, 2005) have been studied. In spite of much effort being expended in trying to improve the performance and stability of the solid-state dye-sensitized solar cells with p-type semiconductors by selecting the appropriate dyes (Tennakone et al., 1998a,b) or designing a new deposition technique of p-type semiconductors (Kumara et al., 2001) or using a p-type oxide semiconductor (Bandara and Weerasinghe, 2005), for example, Tennakone’s research group used ruthenium bipyridyl complexes as efficient sensitizers for solid-state photovoltaic cell and obtained a maximum efficiency of about 4.5% (Tennakone et al., 1998a,b), until now only a few studies regarding the preparation conditions of solid-state dye-sensitized solar cells have been reported. As is known to all, it is very important to understand and grasp the fabrication conditions of solar cells, especially for mass-production. In this study, we chose CuI as the hole collector, and fabricated solid-state dyesensitized solar cells according to the literatures reported by Tennakone’s research group (Tennakone et al., 1995, 1996, 1997a,b), and mainly investigated the effect of the preparation conditions of TiO2 electrodes on the performance of cells. Moreover, we also studied the stability of the cells.
FF g
fill factor efficiency (%)
Route I titanium tetraisopropoxide
acetic acid
2-propanol
water
TiO2 powder
TiO2 paste
Route II acetic acid
titanium tetraisopropoxide
water
TiO2 powder
TiO2 paste
2-propanol
Route III acetic acid
2-propanol
titanium tetraisopropoxide TiO2 powder
TiO2 paste
water
Route IV acetic acid TiO2 powder 2-propanol water titanium tetraisopropoxide
TiO2 paste
2. Experimental
2.3. Fabrication of solid-state photoelectrochemical cells
2.1. Materials
TiO2 pastes prepared by the above routes were painted on the conducting surface of the cleaned FTO glass plates placed on a hot plate heated to about 140 C, allowed to dry and sintered in air at about 500 C for 6 min. The process was repeated until a film of about 10 lm was obtained. The films were examined by SEM (JOEL, JSM-5600) in order to study the morphology. The TiO2 electrodes were immersed in a 1 · 104 M solution of an N3 dye in ethanol for 8 h, unless otherwise noted. Afterwards, the electrodes were washed with ethanol and dried in air. CuI was deposited on the N3 dye coated porous TiO2 films as described below. A solution of CuI was prepared by dissolving 0.12 g of CuI in 10 mL of acetonitrile. The
The TiO2 powder (P25) was donated by Mitsubishi Materials Corporation. The cis-di(thiocyanato)-N,Nbis(2,2 0 -bipyridil-4,4 0 -dicarboxylic acid) ruthenium (II) complex (N3 dye) was purchased from Kojima Chemical Reagents Inc. The fluorine-doped SnO2 layered (FTO) glass plates (1.5 · 1 cm2, sheet resistance = 12 X cm2) were purchased from Asahi Glass. All other chemicals were reagent grade and used as received. 2.2. Preparation of nanoporous TiO2 film TiO2 pastes were prepared by the following routes.
L. Yang et al. / Solar Energy 81 (2007) 717–722
N3 dye coated TiO2 plate was laid on a hot plate (surface temperature about 95 C) and the CuI solution was dropped on the dyed surface by a dropper, allowing the acetonitrile to evaporate until a layer of about 8 lm above the film surface was deposited. The counter electrode (a lightly platinized FTO glass plate) pressed onto the CuI surface to form an electrical contact. 2.4. Measurements of I–V characteristics I–V measurements were performed in air and at room temperature. The electrode size was 0.25 cm2. The photocurrent–voltage measurements were performed in a Bunkoh-Keiki CEP-25BX system under illumination with an AM 1.5 simulated sunlight (100 mW cm2). All measurements were carried out using 32 samples and the means were used as the results. 3. Results and discussion 3.1. Effect of different preparation routes of TiO2 pastes on the performance of the cells Table 1 shows that the different TiO2 pastes preparation routes do not obviously influence the performance of the cells. In other words, no obvious changes in the Table 1 Influence of TiO2 pastes preparation methods on the performance of the cells (water: 4 mL, acetic acid: 5.5 mL, 2-propanol: 14 mL, TiO2 powder: 0.5 g) Route
Isc (mA/cm2)
I II III IV
10.6 10.4 10.5 10.5
a
(0.7988a) (0.7802) (0.7998) (0.7976)
Voc (V)
FF
0.61 0.61 0.61 0.60
0.53 0.51 0.53 0.52
(0.0231) (0.0239) (0.0238) (0.0258)
g (%) (0.0322) (0.0380) (0.0399) (0.0383)
3.5 3.3 3.5 3.3
(0.3539) (0.3388) (0.3580) (0.3363)
Standard deviation.
719
performance of the cells have been observed when using the different routes. Therefore, the TiO2 pastes used in the cells discussed below will be prepared by route I. 3.2. Effect of composition of TiO2 pastes on the performance of the cells 3.2.1. Effect of volume of water From Table 2, we can see that Isc, Voc and g initially increase with the increase in the volume of water, reaching a maximum value at 3–4 mL of water, and then decrease. On the contrary, FF decreases with the increase in the water volume, reaching a minimum value at 3–5 mL of water, and then increases. Therefore, the volume of water contained in TiO2 pastes discussed below is 3–4 mL. 3.2.2. Effect of volume of acetic acid Table 3 shows that Isc, Voc, FF and g first increase with the increase in the volume of acetic acid, reaching a maximum value at a volume of 3–5.5 mL, and then decrease. As a result, the volume of acetic acid contained in the TiO2 pastes discussed below is considered to be 3–5.5 mL. 3.2.3. Effect of volume of 2-propanol Table 4 shows that Isc, Voc, FF and g initially increase, having a maximum value at a volume of 10–14 mL and then decrease when the volume of 2-propanol increases. Thus, the volume of the 2-propanol contained in the TiO2 pastes discussed below is 10–14 mL. 3.2.4. Effect of amount of TiO2 powder Table 5 shows that Isc and g initially increase with the increase in the amount of TiO2 powder, reaching a maximum value at 0.5–0.8 g of TiO2 powder and then decrease; on the other hand, Voc and FF remain nearly stable when the amount of the TiO2 powder increases, but Voc slightly decreases when the TiO2 powder amount exceeds 0.65 g. As
Table 2 Influence of volume of water contained in TiO2 pastes on the performance of cells (acetic acid: 5.5 mL, 2-propanol: 14 mL, TiO2 powder: 0.5 g) Water (mL)
Isc (mA cm2)
Voc (V)
FF
1 2 3 4 5 6
0.8 8.8 10.6 10.6 7.7 0.6
0.46 0.58 0.61 0.61 0.59 0.57
0.87 0.56 0.51 0.53 0.49 0.87
(0.0635) (0.6787) (0.7802) (0.7988) (0.5897) (0.0425)
(0.0192) (0.0211) (0.0238) (0.0231) (0.0225) (0.0219)
g (%) (0.0527) (0.0379) (0.0340) (0.0322) (0.0312) (0.0580)
0.3 3.0 3.4 3.5 2.3 0.3
(0.0368) (0.3068) (0.3478) (0.3539) (0.2328) (0.0304)
Table 3 Influence of volume of acetic acid contained in TiO2 pastes on the performance of cells (water: 4 mL, 2-propanol: 14 mL, TiO2 powder: 0.5 g) Acetic acid (mL)
Isc (mA cm2)
Voc (V)
FF
1 3 5.5 8 11
9.1 9.5 10.6 9.2 3.1
0.63 0.65 0.61 0.57 0.51
0.52 0.54 0.53 0.53 0.50
(0.6902) (0.7199) (0.7988) (0.6993) (0.0234)
(0.0239) (0.0247) (0.0231) (0.0291) (0.0195)
g (%) (0.0312) (0.0330) (0.0322) (0.0328) (0.0309)
3.1 3.4 3.5 2.9 0.8
(0.3139) (0.3498) (0.3539) (0.2934) (0.0807)
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Table 4 Influence of volume of 2-propanol contained in TiO2 pastes on the performance of cells (water: 4 mL, acetic acid: 5.5 mL, TiO2 powder: 0.5 g) 2-Propanol (mL)
Isc (mA cm2)
Voc (V)
FF
7 10 14 18 21
6.1 10.3 10.6 8.7 4.2
0.57 0.62 0.61 0.58 0.53
0.41 0.46 0.53 0.48 0.45
(0.4597) (0.7763) (0.7988) (0.6557) (0.3168)
(0.0215) (0.0235) (0.0231) (0.0216) (0.0202)
g (%) (0.0249) (0.0277) (0.0322) (0.0294) (0.0276)
1.4 3.0 3.5 2.5 1.0
(0.1418) (0.3096) (0.3539) (0.2529) (0.1018)
Table 5 Influence of amount of TiO2 powder contained in TiO2 pastes on the performance of cells (water: 4 mL, acetic acid: 5.5 mL, 2-propanol: 14 mL) TiO2 powder (g)
Isc (mA cm2)
Voc (V)
FF
0.3 0.5 0.65 0.8 1.3
7.8 10.6 10.8 10.7 7.9
0.61 0.61 0.61 0.60 0.59
0.53 0.53 0.55 0.55 0.55
(0.0238) (0.0231) (0.0231) (0.0223) (0.0229)
can be seen from this table, the amount of the TiO2 powder contained in the TiO2 pastes discussed below is 0.5–0.8 g. Based on these results, the TiO2 pastes used in the cells discussed below are composed of water, acetic acid, 2-propanol and TiO2 powder with a mass ratio of 1:1.43:2.77:0.16. 3.3. Effect of aging time of TiO2 paste on the efficiency of the cells The dependence of the cell efficiency on the aging time of the TiO2 pastes is shown in Fig. 1. The efficiency of the cells remains nearly constant with the increase of the aging time within one week, indicating that aged TiO2 pastes within one week are almost the same as the fresh paste for preparing the cells. 3.4. Effect of dying time of TiO2 surface on the efficiency of cells Fig. 2 shows the favorable time of contacting the N3 dye with the TiO2 surface. The N3 dye is continually adsorbed
g (%) (0.0392) (0.0322) (0.0335) (0.0395) (0.0383)
2.6 3.5 3.7 3.6 2.6
(0.2628) (0.3539) (0.3742) (0.3642) (0.2673)
4.0
3.5
η (%)
(0.5879) (0.7988) (0.8139) (0.8066) (0.5955)
3.0
2.5
2.0 0
1
2
3
4
5
6
7
8
9
hours Fig. 2. Influence of dying time on the efficiency of the cells.
on the TiO2 surface with the initial increase of the time, leading to an increased efficiency of the cells. The efficiency of the cells reaches a maximum value of about 3.7% when the dying time is extended to about 2 h, and remains nearly constant as the dying time continue to increase. 3.5. The performance of DSSC
4.0 3.5
η (%)
3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
2
4
6
8
10
12
Aging time (day) Fig. 1. Effect of aging time of TiO2 paste on the efficiency of the cells.
Fig. 3 shows one of the measured I–V curves for the DSSC samples. The short-circuit current, Isc, is 10.9 mA cm2, the open-circuit voltage, Voc, is 0.606 V and the fill factor, FF, is 0.55, which lead to a conversion efficiency of 3.6%. Fig. 4(a)–(c) shows the porous and high surface areas of the TiO2 film and CuI film and the cross section of the TiO2 electrode of this sample, respectively. The performance of a number of DSSC samples, which have been prepared according to above discussion, was studied in order to confirm its repeatability, and the results are shown in Table 6. The mean short-circuit photocurrent, open-circuit voltage, fill factor and efficiency are 10.8 mA cm2, 0.61 V, 0.55, 3.7%, respectively.
L. Yang et al. / Solar Energy 81 (2007) 717–722
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Table 6 Mean value of performance of DSSCs Performance 2
Isc (mA cm ) Voc (V) FF g (%)
Mean value
Standard deviation
10.8 0.61 0.55 3.7
0.8139 0.0231 0.0335 0.3742
0.8
12
0.7
10
3.6. Effect of time on the performance of the cells
0.5
6
0.4 0.3
4
0.2 2
The main problem of the solid-state dye-sensitized solar cells with p-CuI as the hole collector is their instability (Tennakone et al., 1995, 1997a,b). The dependence of the short-circuit photocurrent and open-circuit voltage on the elapsed time is given in Fig. 5. The photocurrent remains nearly stable for about 3 h, and then rapidly decays. Deterioration of the CuI film is responsible for this phenomenon (Tennakone et al., 1995). In addition, the dye rapidly degrades in the presence of oxygen, moisture and UV light due to the photocatalytic activity of TiO2 (Tennakone et al., 1997a,b), which also decreases the photocurrents. The open-circuit voltage
V oc (V)
Fig. 3. I–V characteristics of a solid-state dye-sensitized solar cell under 100 mW cm2 illumination.
I sc (mA cm-2)
0.6 8
0
0.1 0
10
20
30
40
50
60
70
0.0 80
Time (hour) Fig. 5. Dependence of short-circuit photocurrent and open-circuit voltage on elapsed time.
slowly initial decreases with the increasing time, and then remains nearly constant after 24 h. The decrease in the open-circuit voltage is about 3–4%. Therefore, how to maintain the stability of these cells is a future research topic.
Fig. 4. Scanning electron micrograph showing surface of TiO2 film (a), CuI film (b) and cross section of the TiO2 electrode treated with N3 dye (c).
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4. Conclusions The influences of the preparation conditions of TiO2 electrodes on the performance of solid-state dye-sensitized solar cells have been studied. The following conclusions can be made: (1) The different preparation methods of the TiO2 pastes do not influence the performance of the cells. (2) The mass ratio of water, acetic acid, 2-propanol and TiO2 powder contained in the TiO2 pastes was determined to be 1:1.43:2.77:0.16. (3) The efficiency of cells undergoes no obvious change with the increase in the aging time of the TiO2 pastes within one week. (4) The favorable dying time is above 2 h, and the photoenergy conversion efficiency of cells can reach a value of about 3.7%. (5) The open-circuit voltage slowly decreases when the time increases, and then remains nearly stable; on the other hand, the photocurrent rapidly decays after 3 h.
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