Sinter NC coating
3.3 Photo-electrical analyses
3.3.2 The analyses of TiCl 4 processed DSSC
With sufficient understanding of the deviation, it is meaningful to discover the effect from the advanced treatment and structural modifications. Treatment of titanium tetrachloride aqueous solution on TiO2 nano-crystal has been confirmed to be able to improve the conversion efficiency of DSSC. Certainly, it is necessary to study the origin of the improvement, which may be helpful in the afterward works.
The experimental processes of TiCl4 treatment have been described in last section; samples with four different conditions were prepared: higher surface ratio;
higher surface ratio, treated by TiCl4; normal surface ratio and normal surface ratio, treated by TiCl4. The surface ratio was controlled by the addition of ethyl cellulose, which was used as the binder, wt% for the ones with higher surface ratio and 3.125wt% for the lower ones.
Fig.3.3.6 shows the dark current and photocurrent for the higher surface ratio devices. Comparing the blue line and the red one, it shows differences in both photocurrent and photovoltage. These two devices were with the same TiO2 thickness, but one was treated by TiCl4 (the red line) while the other one was not. Another TiCl4 treated device but with different thickness was also included in the plot and depicted smaller photocurrent with respect to the red one; while the photovoltages of these two were close, which was the standard characteristic for the devices with the same fabrication conditions but only with different thickness. It was revealed in the diagram that the photovoltage of this sample was also larger than the blue one. Based on the kinetic model of DSSC, the photovoltage arises from the resultant difference in chemical potential between the ionic electrolyte and the electron quasi Fermi level of titanium dioxide; since the ionic liquid used in these samples were the same and the chemical potential shifted when illumination was minor, the difference of Voc means the difference on the electron quasi Fermi levels and thus the excess electrons
Dye Sensitized Solar Cell
36
remained in the TiO2 when the dynamic equilibrium was reached. Therefore, a reasonable speculation for the effect induced by the treatment of TiCl4 is that the recombination was inhibited, so more excess electrons can be maintained with the same illumination and then the same amount of injection was achieved. Since the recombination process was inhibited, the recombination current at short circuit condition was, whence, reduced and lead to higher the photocurrent.
In order to confirm the speculation, the recombination processes of these devices were measured by the photovoltage transient method, and the results are shown in Fig.3.3.7(a). Obviously, the relaxation processes for the devices treated or non-treated by TiCl4 were rather distinct, and the former was much slower than the latter, which meant that the recombination rate difference was as the same as the previously predicted conclusion. The distributions of extracted recombination time constants at different quasi Fermi levels for each sample are shown in log scale in Fig.3.3.7(b).
Important information could be obtained from the result. The first was that the improvement of recombination rate was about an order. Next, the distribution of each one was near linear in log scale and the trends among the two were almost parallel.
Since the thickness was controlled the same for these two samples, the parallelism means the recombination mechanism, which should be assisted by the surface states that is the major recombination path in DSSC, is not changed.
Fig.3.3.8 is the result for devices with normal surface ratio. The improvements on both photocurrent and photovoltage can still be observed but were less when compared with high surface ratio conditions. The recombination characteristics of these samples are remained about an ordered difference and in the same range as before. It was wondered whether if any variation in electron transport in TiO2 nanocrystal was induced after the treatment of TiCl4. The photocurrent transient measurements for these two were conducted as shown Fig.3.3.9. (a). It seems no
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obvious deviation occured after TiCl4 treatment; both transient behaviors can be fitted well by a single exponential function, as Fig.3.3.9 (b) and (c), which is common for all cases of photocurrent transient with a small amount of injection carriers. A time constants could be extracted from the fitted curve, 80 ms for non-treated device and 83ms for TiCl4 treated one.
According to known results, the diffusivities of excess electrons in TiO2 nanocrystal are not always a constant one, but dependant on the photocurrent level. In order to discover this dependence of TiCl4 treated sample, combined photocurrent transients were measured under different background illumination intensities. In Fig.3.3.10 (a) and (b), current transient at different photocurrent levels are shown. As the first issue, the perturbation induced increments of current remained the same at low levels since the intensity of the perturbed light was constant and there was a large amount of dye molecules that can provide excited electrons when the background intensity was low and the excited dye molecules were less. On the other hand, when background intensity was raised, the increment of current by the perturbation can no more remain and then visibly decay. In spite of this variation, all the relaxation behaviors of photocurrent can be fitted by a single exponential function well as Fig.3.3.10 (c) – (h). The extracted diffusion time constant is collected in Fig.3.3.10 (i).
The increase of diffusion time with the decrease of current level was observed, and the time constant varies from 7ms to 16ms. The reason for longer diffusion time constant values could be related to the increase in depth of effective trapping sites of lower current level.
When the recombination and transport behaviors are understood, it seems the suggestion of the inhibition effect of TiCl4 treatment on recombination is valid, and it is necessary to know the origin of the cause. From DOS measurement (Fig.3.3.11), the relative intensity of surface DOS were extracted for both conditions, TiCl4 and
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non-treated. For the porosity of the film is unknown, the intensity was of a relative level, but it was still comparable between the data since the measurement conditions were the same. According to the results, suppression of intensity in deeper levels can be observed, which is thought to be the reason that recombination inhibition is from.
There are two possible speculations of the suppression of surface DOS: first one is that surface state inhibition by Ti bound to surface oxygen site, and thus the second is the effective bonding site increases and bound dye increases. Advanced exploration could be tried to identify these assumptions. Some advanced test methods might be applied to verify the cause of the phenomenon.
Besides, based on available observations, it may be concluded that the transport of excess electrons was mainly inside the nanocrystal since it was slightly affected by the inhibition of density of surface states. Even though the electrical field was limited by the geometrical boundary conditions, the finitely induced band bending of TiO2 and coupling between bipyridyl and TiO2 was sufficient to collect excess electrons inside nanocrystalline bulk, and thus caused higher transport ability than originally expected ones.
Dye Sensitized Solar Cell
(a) darkcurrent and (b) photocurrent to voltage
characteristics of DSSC fabricated on different substrates (1) ITO 30 Ω / □ (2) FTO 15 Ω / □ (3) ITO(2) 100 Ω / □
100 150
Re(Z) (ohm)
Fig.3.3.2 EIS of DSSC with different substrate
(1) ITO 30 Ω / □ (2) FTO 15 Ω / □ (3) ITO(2) 100 Ω / □
Dye Sensitized Solar Cell
Fig.3.3.3 (a) darkcurrent and (b) photocurrent to voltage
characteristics of DSSC fabricated by different paste solvent (1)alpha-terpineol and (2) poly ethylene glycol
(a)
(b)
Fig.3.3.4
EIS of DSSC by different paste solvent
(1)alpha-terpineol and (2) poly ethylene glycol
0 200 400 600 800
Dye Sensitized Solar Cell
(a) dark current and photocurrent to voltage characteristics, (b) EIS, and (c) EIS phase plot of DSSCs using different electrolytic solvent
(1)PC propylene carbonate and (2)MPN methoxypropionitrile.
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Fig.3.3.6
(a) current to voltage characteristic under different light intensities (b) EIS plot at 100mW/cm2 and 50mW/cm2 and
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0 200 400 600 800
-1600 -1400 -1200 -1000 -800 -600 -400 -200 0 200 400 600
5um non treated dark current 4.5um TiCl4 30mins dark current 5um TiCl4 30mins dark current 5um non treated photocurrent 4.5um TiCl4 30mins photocurrent 5um TiCl4 30mins photocurrent
current (A/cm2 )
voltage (mV)
Fig.3.3.7 photocurrent and dark current to voltage
properties of DSSC of highly transparent bottom layer with (1)TiCl4 treated 5um, (2)TiCl4 treated 4.5um, and
(3)non-treated 5um
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0 5 10
0.0 0.2 0.4
photo voltage (V)
time (sec)
5m TiCl4 30mins 5m non treated
0.2 0.3
0.1 1
5m non-treated 5m TiCl4 30mins
recombination tiime (sec)
open circuit voltage (mV)
Fig.3.3.8
(a)relaxation behaviors and (b)extracted recombination constants of DSSCs of
(1)TiCl4 treated (2)non-treated (b)
(a)
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0 -200 -400 -600 -800
-5500 -5000 -4500 -4000 -3500 -3000 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 3000
voltage (mV) current (uA/cm2 )
8um P25 dark current 8um P25 photocurrent 8um P25 TiCl4 dark current 8um P25 TiCl4 photocurrent
Fig.3.3.9 dark current and photocurrent to voltage properties of DSSC (1)with TiCl4 and (2)without TiCl4 treatment on lower transparency bottom layers
Dye Sensitized Solar Cell Equation: y = A1*exp(-x/t1) + y0 Weighting: Equation: y = A1*exp(-x/t1) + y0 Weighting:
(a) current transient with and without TiCl4 treatment and (b) (c) fitted by single exponential function
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0.00 0.04 0.08
-0.0005 0.0000
photocurrent (A/cm2 )
time (sec)
0.00 0.04 0.08
-0.00205 -0.00200 -0.00195 -0.00150 -0.00145 -0.00140 -0.00135 -0.00130 -0.00125 -0.00120
photocurrent (A/cm2 )
time (sec)
(a)
(b)
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0.00 0.02 0.04 0.06 0.08
-0.00008 Equation: y = A1*exp(-x/t1) + y0 Weighting:
0.00 0.02 0.04 0.06 0.08
-0.00010 Equation: y = A1*exp(-x/t1) + y0 Weighting:
0.00 0.02 0.04 0.06 0.08
-0.00022 Equation: y = A1*exp(-x/t1) + y0 Weighting:
0.00 0.02 0.04 0.06 0.08
-0.00036 Equation: y = A1*exp(-x/t1) + y0 Weighting:
0.00 0.02 0.04 0.06 0.08
-0.00136 Equation: y = A1*exp(-x/t1) + y0 Weighting:
0.00 0.02 0.04 0.06 0.08
-0.00206 Equation: y = A1*exp(-x/t1) + y0 Weighting:
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Current (mA/cm2) Diffusion time (sec)
2.00E-05 0.0167
Fig.3.3.11 photocurrent transient of TiCl4 treated DSSC at varied background intensities
(a)(b) the relative background levels
(c)(d)(e)(f)(g)(h) single exponential fitted results (i) extracted diffusion time constants
(i)
Table 3.3.1 extracted diffusion time constants at different current levels
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200 400 600 800 1000
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.8 5um P25
5m P25 TiCl4
open circuit voltage (mV)
1/V
Fig.3.3.12
Extracted surface DOS versus energy levels of DSSC of
(1) TiCl4 treated (2) non-treated
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References
[1] M. Grätzel. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 4 2003, 145.
[2] M. Grätzel. Prog. Photovolt: Res. Appl. 2006, 14, 429.
[3] M.K. Nazeeruddin; I. Kay; A. Rodicio; R. Humphrey-Baker; E. Muller; M. Gratzel; J.
Am. Chem. Soc., 1993, 115, 6382
[4] Y. CHIBA et al., Japanese Journal of Applied Physics Vol. 45, No. 25, 2006, pp.
L638–L640
[5] S. Hao et al. Solar Energy 80 (2006) 209–214
[6] Fantacci, S.; De Angelis F.; Selloni. A. J. Am. Chem. Soc. (Article) 2003, 125(14), 4381.
[7] Cahen et al. J. Phys. Chem. B 2000, 104, No. 9.
[8] Hagfeldt, A.; Lindquist, S. E.; Gra¨tzel, M. Sol. Energy Mater. Sol. Cells 1994, 32, 245.
[9] Benko¨, G.; Kallioinen, J.; Korppi-Tommola, J. E. I.; Yartsev, A. P.; Sundstro¨m, V. J.
Am. Chem. Soc. 2002, 124, 489.
[10] Tachibana, Y.; Moser, J. E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem.
1996, 100, 20056.
[11] Asbury, J. B.; Ellingson, R. J.; Gosh, H. N.; Ferrere, S.; Nozik, A. J.; Lian, T. J. Phys.
Chem. B 1999, 103, 3110.
[12] Kallioinen, J.; Benko¨, G.; Myllyperkio¨, P.; Khriachtchev, L.; Skårman,B.; Wallenberg, R.; Tuomikoski, M.; Korppi-Tommola, J.; Sundstro¨m, V.; Yartsev, A. P. J. Phys. Chem.
B 2004, 108, 6365.
[13] Jani Kallioinen; Ga´bor Benko1; Villy Sundstro1m; Jouko E. I. Korppi-Tommola;
Arkady P. Yartsev. J. Phys. Chem. B 2002, 106, 4396.
[14] Tachibana, Y.; Nazeeruddin, M. K.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. R. Chem. Phys.
2002, 285, 127.
[15] Bernard Wenger; Michael Gra¨ tzel; Jacques-E. Moser. J. Am. Chem. Soc. 2005, 9, 127,
Dye Sensitized Solar Cell
52
NO.35, 12151.
[16] Pelet, S.; Gra¨tzel, M.; Moser, J. E. J. Phys. Chem. B 2003, 107, 3215.
[17] A. Vittadini; A. Selloni; F. P. Rotzinger; M. Grätzel. Phys. Rev. Lett. 1998, 81, 14.
[18] Filippo De Angelis; Simona Fantacci; Annabella Selloni; Michael Gra1tzel; Mohammed K. Nazeeruddin, Nano Lett. 2007, Vol. 7, No. 10.
[19] Dung Duonghong; Jeremy Ramsden; Michael Gratzel, J. Am. Chem. Soc. 1982, Vol. 104, No. 11.
[20] Michael Gratzel and Arthur J. Frank, J. Phys. Chem. 1982, 86, 2964.
[21] S. Ito et al. Solar Energy Materials & Solar Cells 2003, 76, 3.
[22] W. John Albery* and Philip N. Bartlett* J. Electrochem. Soc.1984, Vol.131, No.2, 315.
[23] T. Hoshikawa; M. Yamada; R. Kikuchi; K. Eguchi, Journal of The Electrochemical Society 2005, 152, E68-E73.
[24] A.J. Frank et al. J. Phys. Chem. B 2000, 104, 3930. /Coordination Chemistry Reviews 2004, 248, 1165.
[25] S. Nakade; Y. Saito; W. Kubo; T. Kitamura; Y. Wada; S. Yanagida, J. Phys. Chem. B 2003, 107, 8607.
[26] Akira Usami and Hajime Ozaki, J. Phys. Chem. B 2001, 105, 4577.
[27] Saif A. Haque; Yasuhiro Tachibana; David R. Klug; James R. Durrant, J. Phys. Chem. B 1998, 102, 1745.
[28] N. Kopidakis; N. R. Neale; K. Zhu; J. van de Lagemaat; A. J. Frank, App. Phys. Lett.
2005, 87, 202106.
[29] Nazeeruddin et al. J. Am. Chem. Soc. 2001, Vol. 123, No. 8.