Impact on the performance of DSSCs

The influence of the complexation Na⁺ K⁺ and TBA⁺ on the performance DSSC based P1 and P7 was investigated and the parameter extracted from the corresponding devices are shown in Figure 5.10. The preparation and characterisation of these devices were similar to devices based on Li-containing electrolyte described above. Electrolyte A consists of 0.6 M 1-butyl-3-methylimidazolium iodide, 0.1 M I2 in 3-methoxypropionitrile solvent. Electrolyte BM, CM and DM contain 0.05, 0.125, and 0.25 M MClO4 (with M= Na, K, TBA), respectively in electrolyte A. The current voltage characteristics of the devices was measured in the standard condition (AM1.5 100 mW cm^-2 at 25°C)

0,00 0,05 0,10 0,15 0,20 0,25 360

390 420 450 480 510 0 2 4 6 8 10 12 14

Voc / mV

C oncentration of cations / M

N a⁺ ions containing electrolyte K⁺ ions containing electrolyte T BA⁺ ions containing electrolyte Li⁺ ions containing electrolyte A

Jsc / mA cm-2

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

300 350 400 450 500 0,0 0,4 0,8 1,2 1,6 2,0

Voc / mV

Concentration of cations / M B

Jsc / mA cm-2

Figure 5.10. Variations in Jsc and Voc of: (a) P1-based cells; (b) P7-based cells for different electrolyte type and concentrations.

Figure 5.10, shows that Jsc and Voc depend strongly on the nature and concentration of cations. For devices based on P1, Jsc increases but not linearly from 0.10 mA cm^-2 to 4.52 mA cm^-2 in Na⁺ ions containing electrolyte, from 0.10 mA cm^-2 to 2.80 mA cm^-2 in K⁺ ions containing electrolyte, and increase slightly from 0.10 mA cm^-2 to 0.49 mA cm^-2 in TBA⁺ ions containing electrolyte when the concentration of each cations types are changed from 0 to 0.25 M (Figure 5.10a). In contrast Voc decreases from 505 mV to 447 mV in Na⁺ ions containing electrolyte, from 505 mV to 448 mV in K⁺ ions containing electrolyte and from

505 mV to 470 mV in TBA⁺ ions containing electrolyte when the concentration in each cation types is changed from 0 to 0.25 M (Figure 5.10a).

Figure 5.10b shows the variation of Jsc and Voc in cells based on P7. From Figure 5.10b it can be seen that the variation of Jsc and Voc is similar to that observed in P1-based cells with different electrolyte type and concentration except the TBA-modified cell where Jsc decreases when [TBA⁺] is changed from 0 to 0.05M. Jsc generated by P7 based cells increases from 0.62, 1.27, 1.33 and to 1.57 mA cm^-2 with [Na⁺] and from 0.62, 1.00, 1.20 and 1.49 mA cm^-2 with [K⁺] when their concentration are varied from 0 to 0.05, to 0.125 and to 0.25 M, respectively. However, Jsc drops from 0.62 to 0.06, 0.09, 0.08 mA cm^-2 with TBA⁺ containing electrolyte when the concentration is changed from 0, 0.05, 0.125 and 0.25 M respectively.

Voc decreases in the different cell types as the concentration of the cation is increased. Voc

decreases from 505 to 447 mV in Na⁺ ions containing electrolyte, from 505 to 448 mV in K⁺ ions containing electrolyte and from 505 to 470 mV in Na⁺ ions containing electrolyte when the concentration of each cation in the electrolyte is varied from 0 to 0.25 M.

One can remark from the data (Fig.10a,b) that the parameters extracted from P1 and P7-based cells varied identically but not in the same order of magnitude. Devices based on Na⁺ and K⁺ containing electrolytes show considerable increase in Jsc with respect to devices based on TBA⁺ ions when the concentrations are increased. Globally, this enables the classification of the increase in Jsc in the order Li⁺ < Na⁺< K⁺ < TBA⁺ at each concentration.

It can be remarked that the smaller is cation size the higher is the photogenerated current. This is due to the fact that small cation can be adsorbed or intercalated easily on the TiO2 surface forming an thin electric double layer [265] resulting in higher concentration of I^- at the interface between TiO2 /dye where regeneration of the oxidized dye cation can be facilitated.

On the order hand intercalated small size cations screen well the injected electron and improve Jsc. It is worth noting that the essential advantage of the electrolytic screening process is its independence from recombination events. For example the redox potential of the Li⁺ ions [266,267] lies at -3 V (NHE), about 2 eV higher than the redox potential in the range of

> -1 V (NHE) of the electrons in the TiO2 network [262,267]. So Li⁺ like other cation types used in this work cannot react with injected electrons.

There have been a great number of studies involving the influence of the deposition and adsorption of various ions on the performance of dye-sensitized solar cells as well as the photocatalytic activity [262,263,268-274]. It is known that several cations existing in the electrolyte solution as the counteraction of I^- and I3- play important roles in the high-energy conversion efficiency. For example, the interaction of Li⁺ with the TiO2 surface enhances the

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.