Long-term stability testing

Before investigating the durability of plastic DSC, it is important to test the stability of ITO/PEN in electrolyte and to check the sealing material and condition in the plastic cell. Figure 5-8(a) shows the variation of sheet resistance of ITO/PEN immersed in two electrolytes for 1000 h which are electrolyte 6 (EL 6 : 0.5 M PMII/0.05 M I₂ in MPN) and electrolyte 7 (EL 7 : 0.5 M LiI/0.05 M I₂ in MPN). The sheet resistance of ITO/PEN immersed in electrolyte I increases slightly from 11.2 to 12.2 ohm sq^-1; however, that in electrolyte II increases significantly from 11.1 to 18.6 ohm sq.^-1. The OM images of ITO/PEN surface are also showed in Figure 3b, which clearly observed by cracking phenomenon of ITO film after immersing in electrolyte II for 1000 h.

90

This indicates that the ITO/PEN film in the electrolyte containing LiI is unstable, implying a chemical etching reaction. [82]

Fig. 5-8. (a) The relationship between the sheet resistance of ITO/PEN and the immersion time in electrolyte at 60 ^oC. (electrolyte 6: 0.5 M PMII and 0.05 M I₂ in MPN; electrolyte 7: 0.5 M LiI and 0.05

M I₂ in MPN). (b) The OM images of ITO/PEN film after immersion in electrolyte II at 60 ^oC. (c) The sealing test cell and the weight loss of cell during thermal treatment at 60 ^oC in the dark.

91

A 60μm hot-melt type spacer was used to seal the test cell for preventing the

electrolyte from leaking. After heat-sealing at 125 ^oC for 10 sec, the electrolyte consisted of 0.8M PMII/0.1M I2/0.5M NMBI in MPN was injected in the spacer between two electrodes through the injection holes. The injection holes and edge of the test cell were then sealed with a UV glue and cured under UV light for 15 sec.

Figure 5-8(c) presents the test cell structure (inset image) and the weight loss data.

The weight loss was only -0.43 % after more than 2700 h at 60 ^oC in dark, indicating that the sealing condition was sufficient to avoid electrolyte leakage in plastic DSCs.

For light aging test, the performance of the devices was tested under continuous light irradiation (100 mW cm^-2) at 60 ^oC. It increases to a maximum initially then maintains at a steady state after 100 h as shown in Fig. 5-9. The apparent increase in J_SC and conversion efficiency may be due to improvement in electrolyte penetration into the mesoporous TiO₂ film, lowering of the TiO₂ conduction band boundary and activation of the Pt-coated counter electrode as reported previously. [83] Note that devices exhibit different conversion efficiency improvements with different cation iodides. In Fig. 5-9, J_SC and conversion efficiency of the device with electrolyte I increased significantly from 3.81 to 6.89 mA cm^-2 and from 1.85 % to 3.14 %, respectively, under 100 h continuous light irradiation. On the other hand, the device with electrolyte II achieves an efficiency value of 2.38 % from the initial value of 1.84

92

%, which JSC only slightly increases from 4.03 to 4.81 mA cm^-2 in Fig. 5-9. This improvement results from TBA⁺ on the TiO2 film surface that protected the voids in the dye-coated TiO2 film in turn blocked undesirable interfacial charge recombination and suppressed surface protonation. The gradual decrease in conversion efficiency of cell after 100 h in Fig. 5-9(b) suggests that the conformation, dye alignment, and intermolecular interactions of N719 on the surface of TiO2 film should change during the aging process.

Fig. 5-9. Photovoltaic parameters (J_SC, V_OC, FF and η) for plastic DSC with TBAI or PMII after visible light soaking (1 sun) at 60 ^oC. The TBAI electrolyte (EL 4) is 0.8 M TBAI, 0.1 M I₂, and 0.5 M NMBI

in MPN. The PMII electrolyte (EL 5) is 0.8 M PMII, 0.1 M I₂, and 0.5 M NMBI in MPN.

93

To fairly evaluate device durability, the photovoltaic parameters of the devices at the steady state obtained after 100 h aging were used as a baseline. After continuous aging for 1000 h, the devices with electrolytes I and II still maintained 96.9 % and 72.3

% of the baseline efficiency measured at 100 h. This performance is better than that of previously reported. [82, 84] The major factor of degradation in the efficiency of the devices is due to a decrease of Voc (~ 0.13 V) that is caused by surface protonation under the accelerated aging test. [19, 85]

Fig. 5-10. EIS results of plastic DSCs with different iodides under one sun light soaking for (a)100, (b) 500, and (c) 1000 h. The data was measured at V_OC under AM 1.5 one-sun irradiation and fitted based

on the equivalent circuit model as shown in Fig 2-7. The TBAI electrolyte (EL 4) is 0.8 M TBAI, 0.1 M I₂, and 0.5 M NMBI in MPN. The PMII electrolyte (EL 5) is 0.8 M PMII, 0.1 M I₂, and 0.5 M

NMBI in MPN.

94

To understand the effects of different cation iodides, TBAI and PMII, on charge transportation and device durability, the EIS of the devices aged for 100, 500, and 1000 h were measured under open-circuit condition and illumination of 100 mW cm^-2. Figures 5-10(a) to (c) show the Nyquist plots of the impedance data.

Table 5-3. Parameters determined from fitting EIS data of plastic DSC with electrolytes containing TBAI and PMII, respectively.

a. The data was measured at V_OC under AM 1.5 one-sun irradiation and fitted based on the equivalent circuit model as shown in Fig 2-7.

b. The TBAI electrolyte (EL 4) is 0.8 M TBAI, 0.1 M I₂, and 0.5 M NMBI in MPN.

c. The PMII electrolyte (EL 5) is 0.8 M PMII, 0.1 M I₂, and 0.5 M NMBI in MPN.

The EIS-fitting data from these devices are listed in Table 5-3. The Rw values increase with light soaking time for electrolyte containing either TBAI or PMII that lowers the estimated electron diffusion coefficient (Deff) and shortens the diffusion length, Ln = L(Rk/Rw)^1/2, where L is the thickness of TiO2 film. However, the effective electron lifetime, τeff,increases with the light soaking time. It implies that

95

the recombination of electrons with triiodide at the interface of TiO2 NPs and the electrolyte has inhibited during the prolonged stability test resulting in a stable photocurrent output. Furthermore, the device with electrolyte I has a higher resistance of charge transfer at the Pt/electrolyte interface than that with electrolyte II as shown in Figure 5-10. This means that the electrolyte containing TBAI had a lower triiodide reduction rate at the Pt/electrolyte interface that leads to the lower FF value as shown in Fig. 5-9.

5.5 Summary

Electrophoretic deposition (EPD) at room temperature and compress treatment prepare TiO₂ thin films on flexible ITO/PEN substrates. The multiple EPDs filled up the cracks caused by drying the previous EPD film and the slow 2^nd deposition rate obtained a high conversion efficiency of 5.54%. EIS analyzed the great enhancement of the electron collection which improved the electron diffusion coefficient about 1 order of magnitude in crack-less multiple-EPD TiO₂ films. When the 100nm TiO₂ NPs was deposited on P-90 EPD film in a DSC, the device shows the best photovoltaic performance with an energy conversion efficiency of 6.63% due to the 100nm TiO₂ NPs efficiently scattered sunlight, especially in long wavelength region.

The durability of flexible devices with different cation iodides also has been

96

demonstrated. Under prolonged one-sun light-irradiation and 60 ^oC-thermal stress aging, our plastic DSC devices showed an initial improvement in performance of 96.9

% followed by an extended steady-state period of more than 1000h. The presence of TBAI in the electrolyte provides higher photocurrent and better durability. This improvement is a result of TBA⁺ on the TiO2 film surface, which sterically protects the voids in the dye-coated TiO2 film in turn blocks undesirable interfacial charge recombination to suppress surface protonation.

97

Chapter 6 Conclusions and