In situ lithium ions coordination in DSSCs

In this section the influence of the coordination site on the performance of DSSCs based on P1 and P7 will be discussed. P1 and P7 are used as sensitizer in this work because the former presents more metal ions coordination sites than the latter. Both have the same adsorption mode on the surface of TiO2, which could predict the same behaviour of anchored groups towards cations contained in the electrolyte. To carry out this investigation solutions with different lithium concentrations were used. The electrolyte composition is described as follows: Electrolyte A contains 0.6 M 1-butyl-3-methylimidazolium iodide, 0.1 M I2 in 3-methoxypropionitrile solvent. Electrolyte B, C and D contain 0.05, 0.125, and 0.25 M LiClO4, respectively in electrolyte A. The current voltage characteristics of the devices was measured under standard conditions (AM1.5 100mW cm^-2 at 25°C)

5.3.1. Impact of lithium ion coordination on the performance of DSSCs

The influence of lithium ions coordination sites on the performances of DSSCs based on P1 and P7 is investigated and their J-V characteristics is measured under standard

conditions. It is known that the dipole moment of organic molecule [122,159,173,261] on the surface of TiO2 and the nature of the electrolyte cations [262-264] change the conduction band energy level considerably. Since in this section we aim to scrutinize the TiO2 CB shifting through the generated Voc of each device, the effect of the dipole moment of each dye on TiO2

has to be avoided. The J-V characteristics of modified devices are shown in Figure 5.6.

0,0 -0,2 -0,4 -0,6 -0,8 0

2 4 6 8 10

A)

Voltage / V

Current density / mA.cm-2

electrolyte A electrolyte B electrolyte C electrolyte D

0,0 -0,1 -0,2 -0,3 -0,4 -0,5 -0,6 -0,4

0,0 0,4 0,8 1,2 1,6 2,0

B)

Current density / mAcm-2

Voltage / V

electrolyte A electrolyte B electrolyte C electrolyte D

0,00 0,05 0,10 0,15 0,20 0,25 320

360 400 440 480 520

0 2 4 6 8 10 C)

Voc / mV Jsc / mA cm-2

Concentration of Li⁺ / M

0,00 0,05 0,10 0,15 0,20 0,25 250

300 350 400 450 500 550

0,0 0,4 0,8 1,2 1,6 D) 2,0

Jsc / mA cm-2

Voc / V

Concentration of Li⁺ / M

Figure 5.6. Current-voltage characteristics of devices based on P1 dye (A) and P7 dye (B) at different concentrations of lithium ions; the variations in Jsc and Voc of P1-based cells (C), P7-based cells (D) with electrolytes containing different Li⁺ concentrations in mol/L.

Table 5.2. Photovoltaic parameters of DSSCs based on P1 and P7 dyes with different electrolytes A, B (A + 0.05 M Li⁺), C (A + 0.125 M Li⁺) and D (A + 0.25 M Li⁺) under AM 1.5 sunlight (100 mWcm^-2).

Sensitizer Electrolyte Voc/mV Jsc/mAcm^-2 FF Vmax/mV Jmax/mAcm^-2 IPCE/ % η/ % P1 A 505 0.09 0.587 390 0.07 14.5 0.03

B 403 6.98 0.588 270 6.11 63.7 1.65 C 375 6.68 0.566 260 5.18 53.7 1.42 D 369 7.68 0.521 240 6.17 63.3 1.48 P7 A 491 0.62 0.489 330 0.450 22.1 0.15

B 345 1.20 0.567 240 0.974 38 0.23 C 324 1.33 0.529 230 0.990 34.7 0.23 D 304 1.75 0.498 200 0.264 43.7 0.27

From Figure 5.6, one can see that the device characteristics depend on the Li⁺ ion concentration. The parameters of modified cells are summarized in Table 5.2. For cells incorporating P7, Jsc increases gradually from 0.62 mA cm^-2 to 1.20, 1.33 and 1.75 mA cm^-2 when the concentration of Li⁺ ions is changed from 0 M to 0.05 M, 0.125 M and 0.25 M, respectively. However, this is counterbalanced by a decrease in Voc from491, 345, 324 and 304 mV, respectively.

Beside P7-based cells, P1-based cells show the same tendency. Jsc increases strikingly from 0.09 to 6.98 mA cm^-2 when the Li⁺ ion concentration is changed in the device from 0 to 0.05 M after what it increases slightly until 7.68 mA cm^-2 with a Li⁺ ion concentration of 0.25 M. In contrast, Voc decreases from 505 to 403, 375 and 369 mV when the Li⁺ ion concentration is increased from 0 M to 0.05 M, 0.125 M and 0.25 M, respectively. The increase in Jsc with Li⁺ ions concentration is likely due to a high screen effect generated by lithium to the photogenerated electron-hole pair (oxidized dye and injected electron).

Kelly et al. attributed such cation dependence electron injection yield to a thermodynamic effect, in which cation adsorption induce a positive shift of TiO2 acceptor states, resulting in more favourable energetics for electron injection [146]. Many authors have done the same remark. By increasing lithium concentration in electrolyte Kuang et al.

[92,258] observed an increase in Jsc followed by a decrease in Voc. They attributed the change in Jsc and Voc to the lowering of the TiO2 conduction band level in the presence of Li⁺ ions.

However, surface adsorption of Li⁺ ions on TiO2 has been revealed to reduce the recombination of injected electrons and the mediator [92]. In fact, the electron injected into the TiO2 network and the negative charges of the mediator are screened effectively over a diameter of the ionic cloud due to Li⁺ ions. This could explain the increase of Jsc with Li⁺ ion concentration.

5.3.2. Influence on Li-coordination on the open-circuit photovoltage drop

In the aim to investigate the influence of the coordination site of dye molecules on the performance of DSSCs, the potential drop^* for both P1 and P7-based cells at different lithium concentration were measured. P1 and P7-based cells show a drop in Voc when the lithium concentration increases (Fig.5.6c,d). Figure 5.7 shows the difference in Voc plotted against lithium-ion concentration for devices containing P1 and P7.

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

-250 -200 -150 -100 -50 0 50

V oc

diff / mV

Concentration of Li⁺ / M

P1-cells P7-cells

Figure 5.7. The potential drop in P1 and P7-based cells at different Li⁺ concentration in M.^*

∗ The potential difference has been calculated by making the difference between the Voc value at each [Li⁺] and Voc obtained with electrolyte A considered as the electrolyte without lithium (where [Li⁺]=0).

( ) ( )

diff

oc oc M 0 oc M

V =V ⎡⎣ ⁺⎤⎦= −V ⎡⎣ ⁺⎤⎦=a

From Figure 5.7, it can be seen that P1 and P7-based cells show the same behaviour.

Voc drops from 0, -146, -0.167 to -0.184 mV in P7-based cells and from 0, -0.102, -0.130 to -0.136 mV in P1-based cells when [Li⁺] is changed from 0, 0.05, 0.125 to 0.25 M, respectively. Globally, Voc drops with lithium ion concentration and exhibits a plateau at high lithium ion concentration. However, the Voc drop in P7-based cells is higher than that of P1-based cells. This strongly suggests that the lithium in P7-P1-based cell is free to access the TiO2

surface. Hence, its adsorption onto TiO2 induces a positive shift of the TiO2 CB. In P1-based cells a real difference is observed. A kind of in-situ Li complexation is observed in device based on P1with respect to devices based on P7. In P1 based cells, a high concentration of ions is held by the dye molecules and thus causes an increasingly strong “charge screening”

effect [258]. Moreover, at [Li⁺]=0.25M a considerable drop of Voc is observed because the coordinating capacity of the P1 sensitizer has been exceeded and Li⁺ remains free to adsorb to the TiO2 surface lowering the conduction-band level and hence the Voc.

Kuang et al. [92] have investigated the consequence of surface adsorption of Li ions on the charge recombination rate and the position of the quasi-Fermi level. By using a transient-voltage-decay technique, they found that in the presence of low concentrations of Li⁺ no real impact is observed but at [Li⁺]=250mM the recombination lifetime was 13 ms for the dye K60 containing lithium-coordination sites. Comparing this finding to that obtained with Z907Na (non-ion-coordinating sensitizer) [258] where in the presence of only 50 mM Li ions the transient lifetime is changed from 5.5 ms to 10.4 ms), they concluded that the complexation by an ion-coordinating sensitizer influences the recombination process.

However, at high concentrations of Li ions the charge recombination process is slower leading to high Jsc. This suggests that in P1 the charge recombination rate is reduced [92] and the open-circuit voltage is increased since the coordinated ions are prevented from adsorbing to the TiO2 surface and the potential is positively shifted. According to these observations, it is confirmed that, the O-atom of phenoxy group, the carbonyl groups of anchor and amide groups are likely to coordinate to Li⁺ present in the electrolyte, preventing Li⁺ from contacting TiO2. This explains the fact that the decrease in Voc is smaller for P1-based cells with respect to P7-based cells. Li⁺ scavenging by coordination to P1 dye is corroborated by the FTIR spectroscopy data shown above.