Investigation of the potential drop in the devices

electron transfer from the adsorbed sensitized dye to the conduction band in TiO2 and also the electron transfer from I^- to the oxidized dye, leading to a high photocurrent. Kelly et al. [146], after studying the effect of various cations types on the charge separation in a hybrid TiO2/dye system found that the quantum yield for interfacial charge injection from vibrationally hot molecular excited states to TiO2 is decreases in the order Ca²⁺ > Sr²⁺~ Ba²⁺ > Li⁺ > Na⁺ >K⁺ ≥ Rb⁺~ C^s+ ~ TBA⁺> neat CH3CN. Grätzel et al. [273] also reported an increased of photocurrent in the order of Mg²⁺ > Li⁺ > Na⁺ and attributed that to the charge density of the metal ions which are found to be potential-determining. Some evidence was found that the adsorption of metal ions is responsible for the positive shift of the flat band potential [146,262,263,273] at low concentrations, while intercalation close to the electrode surface may be important at higher concentrations [262,264].

While Jsc is increased in devices when the concentration of cations in the electrolyte is augmented, the open circuit voltage, Voc, in contrast decreases but not linearly. As observed in devices based on Li⁺ ions containing electrolyte (Figure 5.6c,d and Table 5.2) Voc decreases in each cell type but tend to reach a plateau at high cation concentration. For each type of cation Voc decreases when the concentration is increased in DSSCs in the order Li⁺ > Na⁺> K⁺ >

TBA⁺. Such decrease in Voc as a function of cation size is due to the fact that small sized cations are easily intercalated into the nanoporous hybrid TiO2/dye system, wherease bulky cations like TBA⁺ hardly penetrates the space between the adsorbed dye molecule and the surface of TiO2. This adsorption of cation shifts the conduction band edge of TiO2 resulting in the decrease of Voc. Fitzmaurice et al. [262,263] reported that the addition of 1 mM NaClO4, LiClO4, and Mg(ClO4)2 lead to slight positive shifts vs NHE of 0.07, 0.06, and 0.02 V in the flat band potential. As observed in this work each device type show decrease in Voc when the concentration of cation is increased. However, the amplitude of the decrease in P7-based cells cell is not similar than in P1-based cells. In the following sub-section the difference in potential drop in both devices types will be scrutinized.

0,00 0,05 0,10 0,15 0,20 0,25 -100

-80 -60 -40 -20

0 c N a-P 1-cells

K -P 1-cells T B A -P 1-cells N a-P 7-cells K -P 7-cells T B A -P 7-cells

V oc

diff / mV

C oncentration of cations / M

Figure 5.11. The potential drop (V_oc^diff) in P1 and P7-based cells at different concentration of Na⁺ K⁺ TBA⁺ in M.^*

∗ The potential difference has been calculated following the same calculation route as described in the case of Li⁺ containing electrolyte.

Table 5.3. Summary of the values of potential drop Vocdiff for P1 and P7-based cell at each cations concentration

Potential drop (V_oc^diff) [M⁺]/M

Na⁺-P1-cell Na⁺-P7-cell K⁺-P1-cell K⁺-P7-cell TBA⁺-P1-cell TBA⁺-P7-cell

0 0 0 0 0 0 0

0.05 - 66 - 80 - 46 - 71 - 39 - 57

0.125 - 67 - 91 - 55 - 74 - 49 - 58

0.25 - 70 - 92 - 57 - 75 - 52 - 60

From Figure 5.11 and Table 5.3, it can be seen that the potentialdrop in each device decreases and reaches a plateau at a cation concentration lying between 0.125 M and 0.25 M.

However, the amplitude of drop differs in each case. For example, in P1 cells in conjunction with Na⁺, K⁺ and TBA⁺, V_oc^diffdrops in the order Na⁺ > K⁺ > TBA⁺ for a given cation concentration. The same tendency is observed in P7-based cells where V_oc^diffdrops in the same order Na⁺ > K⁺ > TBA⁺ for a given cation concentration. Globally, one can remark that the

drops are less pronounced in P7-based cells (using Na⁺, K⁺, TBA⁺)(Fig. 5.10b) with respect to the devices containing Li⁺ ions. As already mentioned the reason is simple. The small cations are more readily adsorbed than the cation having larger size, and in return the TiO2 CB is shifted to the more positive side, which reduces the Voc.

However, when one looks closely how the drop in potential varies in each modified cell type, one remarks that for each cation concentration the drop in potential is more pronounced in P7-based cells than in P1-based cells. For example for [Na⁺] = 0.25 M, the potential drop in P1-based cells is V_oc^diff= -70 mV whereas in P7-based it is -92 mV.

Moreover, for [K⁺] = 0.25 M, the potential drop in P1-based cells V_oc^diff= -57 mV whereas in P7-based it is -75 mV. The difference could be attributed to the cation complexation sites existing in dye P1. Like it was observed in the lithium modified cells discussed above, in P7 based cells, Na⁺ and K⁺ ions are free to access the TiO2 surface with respect to P1-based cells.

In fact, oxygen in P1 is likely to coordinate to Na⁺ and K⁺ present in the electrolyte, inhibiting them from contacting the surface of TiO2. This explains that the decrease in the Voc is smaller for P1 as compared to P7.

Kuang et al. [92,258] reported a similar observation. By using dye K51 containing triethylene oxide methyl ether (TEOME) the latter behaving like potential ion coordination sites, they found an invariance in the potential of their device. They attributed the potential invariance of the cell to the inhibition of Li⁺ adsorption to the TiO2 surface by the coordination of Li⁺ to the TEOME groups of the K51 sensitizer.

However, the drop of the potential in devices containing TBA⁺ though behaving qualitatively like other cation types is quantitatively different. The decrease in Voc in P1 and P7-based cells is smaller than in devices based on other cations. At [TBA⁺]=0.25 M,

diff

Voc = -52 mV in P1-based cells whereas V_oc^diff= -60 mV in P7-based ones. These values are the lowest with respect to that obtained with other cation types. The difference could be due to the fact that bulky TBA⁺ can hardly coordinate to oxygen of complexation site in P1 and P7 sensitizer. Moreover, TBA⁺ hardly penetrates the space between the adsorbed dye molecule and the surface of TiO2. It could remain only at a certain distance of the TiO2 surface forming some cloud of positive charges, inducing just a slight positive shift of the TiO2 CB. This could have as consequence a poor electron injection because of the poor overlap between dye excited level and TiO2 CB [146].