Chapter 5 Multiple electrophoresis deposited crackless TiO 2 photoanode for flexible
5.2 Photovoltaic characteristic of multiple electrophoresis deposition
For low-temperature fabricated DSCs especially by electrophoresis deposition (EPD), the volatile organic compounds such as isopropyl alcohol (IPA), methanol, or ethanol were the solvent of the binder-free suspension or paste. Cracks in the film would constantly occur after air-drying that degraded the device performance. We applied one-step and two-steps EPDs separately in this experiment to deposit P-90 TiO2
NPs onto ITO/PEN flexible substrates. For one-step EPD, we applied a constant
-2 for 5 mins; whereas, two-step EPD was performed twice
for 2.5 mins with the same conditions but via air-drying the sample between the steps.
The charge density in EPD stayed constant in these two processes.
Fig. 5-3. The microscopy pictures of electrophoretically deposited P-90 TiO2 film on ITO/PEN film by one-step process (a) and two-step process (b) before 100MPa compression.
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Table 5-1. . Dye-sensitized solar cell performances with one-step or two-step EPD preparation methods.
All photoanodes were compressed by 100 MPa pressure prior to device assembly.
Method
a. The film thickness is about 5 μm after 100MPa compression.
b. The electrolyte (EL 3) is 0.4 M LiI, 0.4 M TBAI, 0.04M I2 and 0.5 M NMBI in AN/MPN mixture (v/v=1:1).
c. The PV characteristics were measured under one sun irradiation (AM 1.5G, 100 mW cm-2).
Fig. 5-3 shows the surface morphology examined by an optical microscope, illustrating the interconnected P-90 TiO2 NPs thin film. Huge microcracks in the one-step process deposited films (Fig. 5-3 (a)) are wider and deeper than those of the two-steps ones (Fig. 5-3 (b)). These results suggest that the 2nd EPD could fill the cracks produced in the 1st EPD to form a better quality microstructure photoanode. To understand the role of thin films made by the one-step and two-step EPDs in DSC devices, three photoanodes for each method were prepared and assembled into DSC devices, whose performances are listed in Table 5-1. Because the same deposition
charges density of 6 mC cm-2 prepared all DSCs, we obtained a similar thickness of 5 μm with almost the same JSC and VOC of about 8.94 mA cm-2 and 0.784 V. However,
slightly improving the filling factor (FF) from 0.595 to 0.690 for the one made of two-step EPD may explain the fewer cracks of EPD film. In this plastic-based DSC with two-steps EPD photoanode, we achieved a conversion efficiency of 4.83%. The
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two-step EPD photoanode has better film quality to improve the DSC device efficiency.
In order to enhance the dye absorption and increase the thickness of the mesoporous photoanode made by EPD, we increased the total deposition charge density from 6 to 12 mC cm-2. Meanwhile, to clarify the effect of the device performance by different deposition rates, we varied the EPD current density from 20 to 5 μA cm-2. Table 5-2 provides the performance data of various DSCs. By increasing the applied EPD current density, the deposition thickness, as expected, increases from 5 to 20 μA cm-2. The higher applied EPD current density causes the higher internal EPD voltage (or electric field), thus, the TiO2 NPs in the solvent could easily overcome the gravity and friction force of the solvent. In contrast with the VOC, JSC increases with the TiO2-film thickness because the more photoanode surface area with the thicker film not only enhances the dye loading but also creates more inhomogeneous light intensity in the film, decreasing the effective Fermi level of TiO2 photoanode.
Table 5-2. Dye-sensitized solar cell performances with two-steps EPD preparation methods by various EPD currents and time.
a. The symbol L stands for the P-90 TiO2 film thickness after 100MPa pressure.
b. The electrolyte (EL 3) is 0.4 M LiI, 0.4 M TBAI, 0.04M I2 and 0.5 M NMBI in AN/MPN mixture (v/v=1:1).
c. The PV characteristics were measured under one sun irradiation (AM 1.5G, 100 mW cm-2).
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film, decreasing the effective Fermi level of TiO2 photoanode.
The DSCs with the slowest deposition rate (5 μA cm-2; 20 mins) have the highest filling factor of 0.71. This result suggests that the slower deposition rate, the better quality of TiO2 photoanode is - although the most efficient DSCs with a conversion efficiency of 5.13% was obtained using a two-step EPD with current density of 20 μA cm-2 for 5 mins in each step. The reason is because thicker TiO2 film provides a larger area for dye adsorption.
Fig. 5-4. The effect of different 2nd EPD TiO2 photoanode on J-V curve of DSC. The inset table shows the detail photovoltaic parameters under AM 1.5G one sun irradiation. The electrolyte (EL 3) is 0.4 M LiI, 0.4 M TBAI, 0.04M I2 and 0.5 M NMBI in AN/MPN mixture (v/v=1:1). a. The symbol L stands
for the TiO2 film thickness after 100MPa pressure.
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The cracks produced in the 1st EPD film could be filled in the 2nd EPD and the larger applied current density provides the faster deposition rate. We therefore further prepared the DSCs with three deposition rates for the 2nd TiO2 EPD, i.e., (20 μA cm-2; 5 mins), (10 μA cm-2; 10 mins), and (5 μA cm-2; 20 mins), but with the same 1st EPD condition (20 μA cm-2; 5 mins). Figure 5-4 shows the current density–voltage (J–V) curves of these three devices. The inset table summarizes the corresponding thickness (L) of TiO2 film after 100 MPa pressure, short-circuit current (JSC),
open-circuit voltage (VOC), fill factor (FF), and solar-to-electricity conversion efficiency (η).
Although we maintained the same product of current density and deposition time in the three samples, the results reveal quite large difference in film thickness. The
slowest deposition rate for a 4.9 μm thick device and the fastest deposition rate for 10.1μm one indicates that the cracks produced after drying the 1st EPD film may have
been filled up more at the slow EPD rate than at the fast rate; therefore the thickness of TiO2 film is not linearly proportionate to the 2nd EPD rate. With increasing photoanode thickness from 4.9 to 10.1 μm, Voc decreases from 0.781 V to 0.723V, whereas, JSC mainly ascribed to the enlargement of the surface area for dye adsorption.
The fastest deposited (thickest) DSC is only 1 mA cm-2 larger than that of the slowest (thinnest) deposited one. We acquired maximal FF of 0.721 and a conversion
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efficiency of 5.54% for the DSC fabricated with the slowest 2nd deposition rate (5 μA cm-2).
Fig. 5-5. The Nyquist plots of DSC device with the different 2nd EPD TiO2 photoanode. The data was measured at VOC under AM 1.5 one-sun irradiation and fitted based on the equivalent circuit model as shown in Fig 2-7. The inset table listed the detail fitting parameters of photoanode. The electrolyte
(EL 3) is 0.4 M LiI, 0.4 M TBAI, 0.04M I2 and 0.5 M NMBI in AN/MPN mixture (v/v=1:1).
To investigate the interfacial charge transfer processes occurring in the each component of DSCs, the electrochemical impedance spectroscopy (EIS), such as the photoanode, electrolyte, and Pt counter electrode [66, 67], has been used widely.
Figure 5-5 compares the Nyquist plots of the DSCs with different 2nd EPD rates which were analyzed and fitted with the well-known transmission-line impedance model (Fig.
2-7) which is discussed in Chapter 2 and also used in the research of this thesis [39, 80]. The 1st semicircle (in the kHz range) typically stands for the behavior of the Pt
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counter electrode, and the 2nd semicircle, determined by only one parameter, (ωk/ωd) = (RK/RW), represents the impedances related to charge-transfer processes in the TiO2
photoanode [67].
The inset table of Fig. 5-5 summarizes the fitting results, suggesting that the charge-transfer resistance (RW) decreases by about 1 order of magnitude by reducing the 2nd EPD rate of the TiO2 film from 20 μA cm-2 to 5 μA cm-2. The slower 2nd EPD rate provides better fill-in for the cracks formed during drying of the 1st EPD film for more efficient electron transport pathway in the TiO2 photoanode. The effective electron lifetime (τeff), the lifetime of electrons being recombined, and back-injection into the electrolyte is inversely proportional to the fitting peak frequency (ωmax) of the 2nd semicircle in the case of RK >> RW. The effective electron diffusion time in TiO2 photoanode (1/ωd), given as ωd = ωk/(RK/RW), decreases from 118.8 to 18 ms as the 2nd EPD rate decreases. The effective electron diffusion coefficient in the photoanode (Deff) is calculated using the relation: Deff = (RK/RW)(L2/τeff), where L is the thickness of photoanode. An efficient Deff of 2.18x10-5 cm2 s-1 was obtained from the DSC made with the slowest 2nd EPD rate at 5 μA cm-2. The electron diffusion length expresses the competition between the collection and the recombination of electrons. The effective diffusion length (Ln) can be expressed as Ln=(Deff τeff)0.5. As shown in Fig.
5-5, the obtained Ln/L increases about three times by decreasing the 2nd EPD rate from
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20 μA cm-2 to 5 μA cm-2. All the fitting parameters from EIS analysis indicate that the DSC devices with the higher quality TiO2 film deposited by the slower deposition rate (5 μA cm-2) have more efficient electron transport in the photoanode to achieve a higher conversion efficiency of 5.54%.