Impact of UV pretreatment TiO 2 on the performance of DSSCs

0 1 2 3 4 5 0

8 10 12 14 16 18

P7 loaded [10-9 Mol cm-2 ]

UV exposure time [H]

P7 loaded pretreated TiO

2

Figure 6.3 Variation of the quantity of P7 dye adsorbed as a function of UV exposure time of TiO2 film.

The figure 6.3 depicts the evolution of the quantity of adsorbed dyes P7 as a function of UV exposure time of TiO2. One can see that the quantity of adsorbed dyes increases with the exposure time and tends to reach a plateau at exposure time range between 2 and 4 hours where 16.65×10^-9 and 16.12×10^-9 mol/cm² of P7 is chemisorbed, respectively. The increase in adsorption after an exposure time from 0 to 4 hours is like due to the presence of more adsorption sites, which leads to the adsorption of P7 dye. From a thermodynamic point of view, the surfaces consisting of unsaturated atoms are energetically unstable and therefore are surface active sites. In our laboratory, it was really difficult with the help of (ATR-FTIR) to scrutinize the mechanism and the different steps of adsorption of P7 onto the hydrophilic TiO2

surface, because that technique was not very sensitive to explore the OH group on TiO2

surface. By using MIR-IRAS, Hirose et al. [288] successfully observed adsorption of N719 on OH site of TiO2 surface. They observed the disappearance of OH groups after adsorption of N719 and concluded that UV pretreatment of TiO2 enhances N719 chemisorption.

(electrolyte 2). The devices are characterized under solar simulated white light AM1.5 (86 mW / cm²).

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

1,0 0,0 -1,0 -2,0 -3,0

Untreated Li-cell Pretreated Li-cell Untreated TBA-cell Pretreated TBA-cell

a)

Current density / mA cm2

Voltage / V

400 450 500 550 600 650 700

0 5 10 15 20 25

b) Untreated TiO₂ Li-cell

Pretreated TiO₂ Li-cell Untreated TiO₂ TBA-cell Pretreated TiO₂ TBA-cell

IPCE / %

Wavelength / nm

Figure 6.4: a) J-V characteristics of devices based on UV pretreated TiO2 films and P1:

untreated Li-cell (red filled circle); 2h UV pretreated Li-cell (red open circle); untreated TBA-cell (blue filled square) and 2h UV pretreated TiO2 TBA-cell (blue open square). b) corresponding photocurrent action spectra: untreated Li-cell (red filled circle); 2h UV pretreated Li-cell (red opened circle); untreated TBA-cell (blue filled square) and 2h UV pretreated TiO2

TBA-cell (blue opened square). TiO2 thickness ~ 4µm; Incident light intensity 86 mW.cm^-2

Table 1. Summary of current voltage characteristics before and after UV pretreatment of TiO2

film for DSSCs based on Li⁺ and TBA⁺ containing electrolyte Time

(hour) Voc (mV) Jsc (mA/cm²) FF η (%) IPCE (%)

TBA- Cell

Li- Cell

TBA- Cell

Li- Cell

TBA- Cell

Li- Cell

TBA- Cell

Li- Cell

TBA- Cell

Li- Cell 0 627 531 0.46 1.37 0.63 0.60 0.22 0.5 6.5 12.3 2 638 590 0.84 2.82 0.59 0.63 0.36 1.2 9.0 21.1

The devices made with untreated TiO2 film in conjunction with electrolyte 1 have an overall energy-conversion efficiency (η) of 0.5 %. The short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) are 1.37 mA cm^-2, 531 mV and 0.601 respectively.

Devices made with 2h UV pretreated TiO2 film generate an efficiency of 1.2 % for Jsc, Voc

and FF of 2.82 mA cm^-2, 590 mV and 0.631 respectively (Fig. 6.4a). This corresponds to more than two times improvement in efficiency after UV-pretreatement of TiO2. The incident photon to electron conversion efficiencies (IPCE) of the untreated and pretreated titanium oxide devices were 11.9 % and 21 % respectively, which corresponds to an overall improvement of 72 % in quantum efficiency (Fig. 6.4b).

It is well known that Li⁺ is a potential-determining ion for TiO2 because it specifically adsorbs or intercalates into the TiO2 [302,303].The additional positive surface charge from adsorbed Li⁺ shiftsthe TiO2 conduction band (CB) edge to a more positive potential (thereby enabling more facile electron injection from some adsorbed dyes). In the aim to evaluate the Li⁺ ions effects and to be sure that the performance of the device is not the consequence of its adsorption but rather of UV pretreatment, electrolyte 2 was also tested. As depicted in the Figure 6.4a, the short circuit current density increases from 0.46 mA cm^-2 to 0.84 mA cm^-2 for untreated and pretreated of TiO2 films respectively. The complete characteristics are summarized in Table 1. No decrease in the Voc for both Li-cells and TBA-cells is observed which allowed to conclude that the CB edge of TiO2 is not significantly affected by the treatment and that all the mechanisms occur only on its surface. The improvement in Jsc and quantum efficiency is consistent with the fact that UV-pretreatment generates more OH adsorption sites on TiO2 surface for P1 and increases electron injected density in the TiO2

conduction bandunderillumination. Furthermore, Voc in TBA-cells is higher than that of Li-cells. The reason is simple. Because of its large size and the non reactive alkyl groups on its periphery, TBA⁺ does not adsorb on the TiO2 surface nor does it react chemically with it. But the small Jsc obtained, with respect to Li⁺ containing electrolyte is a direct consequence of interaction of Li⁺ with TiO2, which favours electron injection. Furthermore, both charges, i.e., the electron injected into the TiO2 network and the positive counter charge left behind on the dye molecule are screened [267] effectively over a diameter of the ionic cloud due to 0.3 M LiI, i.e., over a few nm with respect to TBA⁺ ions diameter. On the other hand small size cations by screening efficiently photoinjected electrons in the CB, prevent their recombination with triiodide contained in the electrolyte [258,304]. The weak adsorption of TBA⁺ onto TiO2

surface can be the main cause for the low photocurrents generated by TBA-cells with respect to Li-cells [47,187].

A curious fact is the variation of Voc after treatment (Table1). Voc is increased from 531 to 590 mV for Li-cells with untreated and treated TiO2, respectively. The same trend is observed in TBA-cells where Voc increased from 627mV and 638 mV for untreated and

treated devices, respectively. This behaviour is particularly strange, since it is known that the exposure of TiO2 to UV light provokes shifting of the Fermi level to more positive value [187]. One would expect a decrease in potential in the UV pretreated TiO2 devices but the opposite occurs. This means that the whole process occurs only on the surface of TiO2. Recently, Hirose et al. [288], also observed an improvement of the performance of DSSCs based on UV pretreated TiO2 sensitized with N719 dye without significant change in Voc.

The possible cause of such improvement in performance of DSSCs has been attributed to the OH generation which enhances the chemisorption of N719 on the TiO2 surface and to the photocleaning effect by the photocatalytic reaction.

However, a deeper study is required to better understand how bridging O-atom behaves during chemisorption process of acid anhydride onto TiO2 surface. According to Ref.

[296] newly surface-introduced OH species seem to be metastable and tend to disappear in the dark under air and restore the initial surface state hydroxylation. It could be very interesting to know how the generated OH during chemisorption of P1 or P7 really behaves. However, from these preliminaries results, one can say that the UV pretreatment of titanium oxide film improves the performance of DSSCs based on perylene derivatives independently of the nature of counter ions in the electrolyte.