Enhanced Efficiency of Dye-Sensitized Solar Cell by High Surface Area Anatase-TiO 2-Modified P 25 Paste

How to improve the conversion efficiency and the stability of dye-sensitized solar cells (DSSCs) are two major problems. Furthermore, reduction of the manufacturing cost of DSSCs and large-scale manufacture are also very important factors. As a raw material, commercial P25 would be used for large-scale manufacture of TiO 2 paste with simple preparation procedure. However, there are several drawbacks for P25 such as the low surface area of P25 powder and the poor connectivity among particles in film using tradition P25 paste without modification. In this paper, we introduced a simple modified method by adding high surface area anatase TiO 2 into pure P25 paste. The photoelectric conversion performances of DSSCs based on these photo-electrodes were tested. The results show that the open-circuit voltage, the fill factor and the energy conversion efficiency of the modified electrode were increased. It is found that the modified P25 films have fast electron transportation and a slow charge recombination. We conclude that through adding the anatase TiO 2 nanoparticles to the P25 paste with high surface area, it can not only improve the particles connectivity among inside the films, but also enhance the efficiency of DSSCs.


Introduction
Titanium dioxide (TiO 2 ) is a very important semiconductor; it has outstanding physicochemical properties, such as heat resistance, high refractive index, and wide band gap.TiO 2 nanocrystallites have been investigated widely in photocatalytic degradation, sensors, dye-sensitized solar cells (DSSCs) [1,2], and so on.With the shortage of fossil fuels and an increasing demand for energy, developing new types of cheap and environmentally friendly energy sources is becoming more and more important.Dye-sensitized solar cells (DSSCs) as a new-generation solar cell was developed in the early 1990s and it has attracted extensive attention attributed to its high conversion efficiency, low production cost, stable performance, and the simplicity of the process [3].DSSCs would be the most effective way of the utilization of solar energy and one of the most promising technologies [4].
DSSCs operate similarly as photosynthesis which takes place in leaves.DSSCs consist of mesoporous TiO 2 thin film photoanode, a nonaqueous electrolytic solution of the redox couple I − /I 3 − , and a platinized transparent conductive glass as a counter electrode.When the DSSCs are illuminated by sunlight, the dyed molecules will be excited by light absorption.Then the photon-generated electrons will be injected into mesoporous TiO 2 thin film.Simultaneously the oxidized dye molecules will accept electrons from I − in electrolyte to return to the ground state.The electrons in the mesoporous TiO 2 thin film will be collected by the transparent conductive oxide electrodes and flow through the external circuit to the counter electrode.To complete the reverse reaction of the redox couple, a counter electrode must be used to provide the electrons to I 3 − .Through these processes, solar energy is converted into electric energy [5].In order to realize future commercial production, extensive studies have been performed on each component of DSSCs, such as synthesis of TiO 2 film, types of dye sensitizer, the composition of electrolyte solution, and counter electrode.
The mesoporous TiO 2 thin film photoanode is one of the main components of DSSCs and it plays a key role in photovoltaic performances.It is well known that a photoelectrode with high efficiency should possess large surface area, high light-harvesting efficiency, rapid electron transport, and low electron recombination [3].It is well known that after modifications are always needed for TiO 2 film because of the insufficient connectivity between TiO 2 particles in the film.The poor connectivity would lead to low electron transport rate and high electron recombination hence reducing the conversion efficiency of DSSCs [6].Among many aftermodifications, TiCl 4 posttreatment is widely used.By this modification, an extra layer of TiO 2 will grow onto the surface of the former TiO 2 film to enhance the connectivity [7].Yu et al. adopted a similar method by modifying the conventional porous TiO 2 with a titanium organic sol [8].On the other hand, we need to design simple methods to prepare TiO 2 film for mass production.TiO 2 film via hydrothermal method [9] achieved the highest efficiency, while a several layers and steps are needed, which limits its large-scale manufacture.In this paper, we adopted a P25 paste for mass production and introduced a new method to modify it for enhancing the connectivity of particles.The preparation of the P25 paste via bead mill is simple and just one layer and one step are needed to prepare TiO 2 film, which can be easily used for the large-scale manufacture.Furthermore, the modification by adding high surface area anatase TiO 2 into pure P25 paste could cement the P25 nanoparticles together, which could enhance the connectivity of particles to improve the efficiency.The performances of DSSCs using modified paste were tested and compared to unmodified ones.It is showed that the open-circuit voltage, the fill factor, and the energy conversion efficiency of the modified electrode were increased.It is found that the modified P25 films have a rapid electron transport rate and a slow charge recombination rate.

Preparation of P25
Paste.The unmodified P25 paste (Sample 1) was prepared by a bead-mill method.5.6 g of P25 powder was ball-milled with 13 mL of EG for 24 h, and then 12.6 g of CA was added to the above mixture for another 24 h; the processes were done at room temperature [10].
The first modified P25 paste (Sample 2) was prepared by adding 0.56 g of anatase TiO 2 .5.04 g of P25 powder and 0.56 g of anatase TiO 2 (197 m 2 /g) powder were ball-milled with 13 mL of EG for 24 h, then 12.6 g CA was added to the mixture for another 24 h; the processes were done at room temperature [10].
The second modified P25 paste (Sample 3) was prepared by adding TiO 2 sol.The TiO 2 sol was prepared by a hydrothermal method.Typically, 10 mL of TTIP is mixed with 2.1 g acetic acid, and then using an ultrasonicator ultrasonicated for 15 min.The previous solutions were added drop-wise to 50 mL of distilled water, under vigorous stirring for 1 hour.Then 0.68 mL of nitric acid was added to the previous mixture solution and stirred for 6 h at 80 ∘ C. 10 mL of transparent sol was obtained by evaporation of the water.The preparation of the P25 paste modified with TiO 2 sol (Sample 3) was analogous to Sample 2. 0.56 g of anatase TiO 2 in Sample 2 was replaced by 2 mL of TiO 2 sol in Sample 3. The flow diagrams for synthesizing these P25 pastes are shown in Figure 1.

Preparation of Photoelectrodes and DSSCs.
The TiO 2 pastes were coated onto FTO substrates by doctor-blading, respectively, and then they were sintered at 500 ∘ C for 30 min.In order to ensure the TiO 2 mesoporous films with similar thickness, we pasted two layers of adhesive tapes on the edge of FTO; the film with a thickness of about 14.3 m was formed.The mesoporous TiO 2 photoelectrodes were preheated at 120 ∘ C for 30 min; after cooling down, the electrodes were washed with ethanol to remove the remaining small glass fragments.Afterwards, they were immersed in a 0.5 mM ethanolic N-719 solution for 24 h at room temperature to allow complete dye adsorption [11].After taking them out, the redundant dye molecules on the electrodes were washed away by ethanol for several times, and then the electrodes were dried with a hair dryer.The dye-adsorbed TiO 2 electrode was assembled into a cell holder, a drop of electrolyte solution was injected into the photoelectrode, and a clamp was used to press together the photoelectrode and a counter electrode (a platinum-sputtered F-SnO 2 glass).The electrolyte solution was composed of 0.05 M LiI, 0.03 M I 2 , 0.1 M 1-propy-3methylimidazolium iodide, 0.1 M GNCS, and 0.5 M 4-tertbutylpyridine in a mixed solvent of acetonitrile and PC (volume ratio: 1/1).A sandwich-type DSSC configuration was fabricated [6].

Results and Discussion
3.1.Photovoltaic Performances of DSSCs.The photocurrentvoltage characteristics of three DSSCs were measured by a CHI660C electrochemical Workstation (CH Instruments, Shanghai, China) at room temperature.Figure 2 shows the photocurrent density and photovoltage characteristics of three DSSCs.The resulting photovoltaic parameters derived from the - are shown in Table 1.
When we compare the photovoltaic performance parameters of the TiO 2 films modified by anatase TiO 2 or TiO 2 sol to the original P25 film without modification, as shown in Figure 2, we can observe that the open-circuit voltage ( oc ) and fill factor (FF) of Samples 2, 3 have been increased, and the short-circuit photocurrent density ( sc ) slightly reduced, but the energy conversion efficiency increased.The fill factor (FF) was calculated using (1) and the cells' overall energy conversion efficiency () was estimated by (2).In (1), the  max and  max are voltage and current density for maximum power output, respectively.In (2), the  in is the intensity of the incident light (82.9 mW/cm 2 ) [8]: The charge recombination can be estimated by the magnitude and onset of dark current.Recombination reaction happens between electron at conduction band of semiconductor and redox couple in electrolyte, which is described by the following [9]: Figure 2(b) shows the dark current-voltage characteristics of the photoanodes with and without modification.The dark current measurement suggested that the modified photoanodes had a slower photoelectrons recombination rate.It is noteworthy to see that the P25 photoelectrode modified with TiO 2 sol TiO 2 produces the lowest dark current, while the P25 photoelectrode without modification produces the highest dark current at the same potential about 0.6 V.These observations reflect a higher recombination at the P25 photoelectrode (Sample 1) than the P25 photoelectrode modified with anatase TiO 2 (Sample 2) and TiO 2 sol (Sample 3), which is supported by EIS measurements in Section 3.2.

EIS and OCVD Analyses.
In order to confirm the speculation that modification is beneficial to retard electron recombination and facilitate electron transport, electrochemical impedance spectroscopy (EIS) measurements were carried out to investigate the kinetics of the electrochemical processes Hz at the open circuit, respectively.Figure 3(a) shows the Nyquist plots which exhibit two semicircles, including a large semicircle at low frequencies and a small one at high frequencies; the plots were fitted with an equivalent circuit [12] (shown in the inset of Figure 3(a)).The equivalent circuit consisted of  1 which is ascribed to the sheet resistance of FTO and contacting resistance,  2 which is ascribed to the contact resistance between the FTO and TiO 2 film and the charge transfer resistance at the interfaces of the redox electrolyte/Pt counter electrode, and  3 which is ascribed to the charge transport resistance of the accumulation/transport of the injected electrons within TiO 2 film and the charge transport resistance at the TiO 2 /redox electrolyte interfaces.CPE1 and CPE2 are constant phase elements of the capacitance corresponding to  2 and  3 , respectively [4].As shown in Table 2, the paste modified with TiO 2 sol exhibits the lowest values of  3 among the three types of DSSCs, which implies the most efficient charge-transfer process at the interface of TiO 2 /electrolyte interfaces and across TiO 2 film.By adding small particles into the paste, it could cement the TiO 2 nanoparticles together so the electron pathways would be widened [8].Consequently, the overall efficiency would be increased by the enhanced electron transfer efficiency.The Bode phase plots of EIS spectra, as shown in Figure 3(b), display the frequency peaks of the charge transfer process at different interfaces of three different DSSCs.The characteristic low frequency peak ( max ) is located at 5.486 Hz for Sample 1, 4.542 Hz for Sample 2 and Sample 3. Table 2 shows the electron lifetime for recombination (  ) of DSSC.The electron lifetime in DSSCs is determined by the characteristic frequency peak in the low frequency ( max ) according to the following [13]: We can notice that the DSSCs' electron lifetimes using the modified pastes were longer than that without modification, which implies that the modified pastes could improve the electron lifetime.The longest electron lifetime is due to  The open-circuit voltage decay (OCVD) technique was employed for investigating the electron lifetime (  ), which is conducted by turning off the illumination on DSSC in a steady state and monitoring the subsequent decay of  oc .The electron lifetime was calculated by the following equation, where   is the Boltzmann constant,  is the absolute temperature, and  is the positive elementary charge [14,15]: lifetime spectra of these samples.As shown in Figure 4(b), we could note that the variation trend of the electron lifetime measured by OCVD is the same as the EIS results.The electron having longer electron lifetime and faster transfer rate would be a result of additional electron pathways in the TiO 2 film [16].As a result, the TiO 2 pastes modified with high surface anatase TiO 2 could improve the performance of the TiO 2 films in DSSCs.

Conclusion
In summary, we have successfully fabricated three kinds of modified P25 pastes and investigated them in DSSCs photoanodes.We employed high surface area anatase TiO 2 to improve the connectivity among the TiO 2 particles.The photoelectric measurement results showed that the modified cells have low charge transport resistance and long electron lifetime; it is assigned that the rapid electrons transport rate delays the process of their recombination.The photoelectric measurement results showed that open-circuit voltage ( oc ), the fill factor (FF), and the energy conversion efficiency all have remarkably increased.We conclude that through the addition of extra anatase TiO 2 in P25 paste, the electron pathways inside TiO 2 film would be enlarged, which would result in high photoelectric parameters.In brief, we introduce a simple and effective strategy, which provides a good way to improve the overall energy conversion efficiency of DSSCs.

Table 2 :
Parameters obtained by fitting the impedance spectra of DSSCs using the equivalent circuit in Figure3.