**4. Conclusion**

414 Solar Cells – Thin-Film Technologies

The *J-V* characteristics of the flexible solar cells are shown in Fig. 9. The results of the standard solar cells without the lift-off process are also shown in Fig. 9. Solar cell parameters obtained from the *J-V* characteristics are summarized in Table 3. All parameters of the ZnO/CdS solar cell is higher than those of the (Zn,Mg)O solar cell for the standard solar cells. On the other hand, although there are the differences in the window layer/ buffer layer structures for the

ZnO/CdS standard

0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)

EQE spectra of these solar cells are shown in Fig. 9. EQEs of the (Zn,Mg)O standard solar cell are higher than those of the ZnO/CdS standard solar cell in the region from 300 to 480 nm, because the band gap of (Zn0.83,Mg0.17)O is higher than those of CdS and ZnO (Minemoto et al., 2000). These high EQEs in this region is therefore attributed to a low transmission loss of the short wavelength light. Moreover, the tendency of this result is also observed for the flexible solar cells. We found that the (Zn,Mg)O window layer structure was effective for reducing a transmission loss of the short wavelength light even in our

Sample structure *Eff.* (%) *Jsc* (mA/cm2) *Voc* (V) *FF* (%) (Zn,Mg)O flexible 1.0 14.8 0.231 30.5 ZnO/CdS flexible 1.0 14.8 0.227 30.2 (Zn,Mg)O standard 8.3 32.4 0.465 54.9 ZnO/CdS standard 13.7 34.9 0.562 70.0 Table 3. Summary of solar cell parameters obtained from flexible solar cells using (Zn,Mg)O window layer and conventinal ZnO window/CdS buffer layers. For comparison, solar cell parametaers otained from standard solar cells using (Zn,Mg)O window layer and ZnO

flexible solar cells, these flexible solar cells show the similar properties.

(Zn,Mg)O standard

ZnO/CdS flexible

(Zn,Mg)O flexible

Fig. 9. Photo *J-V* curves of flexible solar cells using (Zn,Mg)O window layer and conventional ZnO window/CdS buffer layers. Photo *J-V* curves of standard solar cells

40

30

20

Current density (mA/cm2)

10

0

without lift-off process are also shown for comparison.

window/CdS buffer layers are also summarized.

flexible solar cells.

After we described the review of the lift-off process, we also described the advantages of the lift-off process in the flexible CIGS solar cell fabrication. We developed the fabrication procedure of the flexible CIGS solar cells using the lift-off process. The characteristics of the flexible solar cells were shown compared to the standard solar cell. Although the conversion efficiencies of the flexible solar cells using the lift-off process are an approximately half conversion efficiency of the standard solar cell, the flexible solar cells showed the similar characteristics irrespective of the substrate materials. Moreover, we attempted the concept of a Cd-free solar cell. We found that the choice of back electrode materials is a crucial problem rather than the window layer/buffer layer structure. We expect that the lift-off process further advances through our results.
