Chapter 3. High performance dye sensitized solar cells by adding titanate co-adsorbant
3.4 Result and Discussion
Figure 3.1 A schematic of TDT with dyes on a TiO2 surface.
Figure 3.1 shows a brief scheme of TDT assisted dyes on TiO2 surface. TDT is added in dye solution to be co-adsorbed onto TiO2. Phosphate functional group is also forming links with TiO2, therefore, dioctylphosphate from TDT and dyes are competing adsorption on the surface. Basically, pre-treating or post-treating of co-adsorbants does not effectively work as an intentional layer on the surface. If they are pre-treated, co-adsorbants will be replaced by dyes since the linkage between phosphi(o)nic acid and TiO2 surface is not covalently bound. Since most of co-adsorbants are bulky enough, they have difficulties penetrating through dyes to cover empty space.102, 106-108
When TDT is added to dye solution, the color of the solution has become darker, and this can be easily detected by naked eyes. The TDT concentration dependence is described in the figure 3.2.
Figure 3.3(a) shows UV-vis spectra changes after TDT addition to dye solution. Of course, low concentration of TDT is not effective, and high concentration causes aggregation and precipitation in a solution. Once 0.125 mM of TDT is added, the molar absorptivity of N719 dye is increased by ca.
30 % as shown in Figure 3.3(a).
Figure 3.2 Comparison of color changes depending on TDT concentration.
(a) N719 dye solution, (b) after TDT addition to N719 dye solution.
High molar absorptivity may mean that the conjugated dyes with TDT become more sensitive to light, which is advantageous for solar cell system. After coating of dyes and TDT, the absorption peak is completely changed as shown in Figure 3.3(b) with a decrease by more than 18 %. We also observed a slight shift of absorption peaks of N719 dye in solution and solid forms from 530 nm to 540 nm.91 TDT may interact with dyes in solution phase, and this can be transferred to the surface of TiO2 with prohibiting or retarding dyes-dimer formation. Therefore, the addition of co-adsorbant can reduce coating of dimers which are considered as wasting of photons.
The molecular conformations for a reference working electrode and TDT-assisted working electrode have been compared by diffuse reflectance infrared Fourier transform (DRIFT). Infrared spectroscopy is a strong tool to investigate any differences between electrodes prepared by conventional method and newly designed method. The middle set in Figure 3.4 is obtained from the sample using N719 only solution, and the upper set is from the sample using N719+TDT. For a comparison, TDT only electrode is also added as a bottom set. Although they are not identical to each other, most of the representative peaks which identify the states of the dye, appear to be very similar.
There are no noticeable peak shifts between the two spectra sets.
Figure 3.3 UV-vis spectra of DSSCs with and without TDT in (a) solution and (b) solidstate.
The peaks of interest appear at 2100 cm-1, 1541 cm-1, and 1368 cm-1, which are corresponding to – NC(S), aromatic carbon double bonds, and carbonyl groups (COO-), respectively.89-90, 109
The amount of coated dyes is compared for both electrodes by dissolving all the dyes from the TiO2
surface in 1 M NaOH solution. While the reference electrode shows 0.023 mmol/g, N719+TDT electrode shows only 0.0176 mmol/g, which is near 25 % decrease. Therefore, N719+TDT electrode has fewer amounts of N719 dyes, but shows competitive sensitivity to active sun light. This is reflected in solar cell performance.110
Figure 3.4 A comparison of DRIFT spectra sets obtained from reference (black line), TDT with dye (red line), and TDT (blue line).
According to JV curve as shown in Figure 3.5 (a) for cells prepared with reference electrode and N719+TDT electrode shows obvious changes. When the reference cell shows 8.2 % efficiency with 16.1 mA/cm2, 0.70 V, and 0.73 of fill factor (FF), the cell of interest shows 9.6 % efficiency with 17.2 mA/cm2, 0.75 V, and 0.74 of FF. Shunt resistances for each cells are quite different; 4.05 kΩ has been increased to 206 kΩ after TDT addition. Normally, shunt resistance increase helps preventing back electron flow to counter electrode from working electrode, thus higher cell performance. The solar cell performance improvement is mainly achieved from current increase.
As described earlier, we have found that less amount of dyes are adsorbed on TiO2 working electrode when TDT is assisted. It is very interesting that less amount of dyes can induce higher current densities. There could be a few factors the other co-adsorbants, higher sensitivity to light, and less dye-dimers which may cause waste of photons. This current density increases have been confirmed by IPCE as shown in Figure 3.5(b). As shown in the figure, the IPCE values are higher for TDT co-adsorbed sample in all rage of the wavelength.
Figure 3.5 (a) J–V curves for DSSCs with reference electrode and N719 with TDT electrode, (b) the values of IPCE spectra for reference and N719 with TDT cells.
Finally, electrochemical impedance spectroscopy (EIS) analysis has been adopted to investigate the interface of TiO2 and dyes. In Figure 3.6, EIS gives information on interface resistances such as a counter electrode/electrolyte interface, a dye and TiO2/electrolyte interface, and electrolyte diffusion in order from the left. The data have been analysed with an equivalent circuit111-112 as shown in the figure. As shown in the figure set, the fitting is well matched with the real data. The detail resistance values are summarized in the Table 1.While resistances for charge transfer at the counter electrode (the first arc) are almost the same, resistances for charge transfer at dye+TiO2/electrolyte interface (the second arc)93, 95 have been changed from 3 Ω to 3.3 Ω. Note that the first arc is used to normalize the
the resistance means that electron flows on the interface is retarded by TDT layer formation by about 10 %.
Figure 3.6 Electrochemical impedance spectra of DSSCs with reference and TDT cells.
Table 3.1 Summary of detailed resistance value for reference and TDT cells