**3.2 Development of Cd-free flexible Cu(In,Ga)Se2 solar cells**

412 Solar Cells – Thin-Film Technologies

Mo SLG

1 2 3 4

5 6 7 8

Al/NiCr

In2O3: Sn In2O3: Sn (Zn,Mg)O

Polyester film Silicone adh. Support SLG

CIGS

Ni Cond. epoxy

Mo SLG

Polyester film Silicone adh. Support SLG

Separation

CIGS

In2O3: Sn

Ni Cond. epoxy

CIGS

Ni

Cond. epoxy

Adhesion

Polyester film Silicone adh. Support SLG

CIGS

Ni

Mo SLG

Polyester film Silicone adh. Support SLG

> Mo SLG

(Zn,Mg)O

In2O3: Sn

CIGS

Ni Cond. epoxy

Polyester film Silicone adh.

9

Adhesion

Lift-off process

CIGS

Ni

Mo SLG

Cond. epoxy (Zn,Mg)O

(Zn,Mg)O

CIGS

Ni Cond. epoxy

Polyester film Silicone adh. Support SLG

10

(Zn0.83,Mg0.17)O window layer and lift-off process.

Fig. 7. Schematic illustration of fabrication procedure of flexible CIGS solar cell using

Polyester film Silicone adh.

(Zn,Mg)O

In2O3: Sn

CIGS

Ni Cond. epoxy

CIGS

We developed a new Cd-free flexible CIGS solar cell using a (Zn,Mg)O window layer. The fabrication procedure is shown in Fig. 7. This process is basically similar to Fig. 1. We deposited a 0.1-m-thick (Zn0.83,Mg0.17)O window layer in stead of the ZnO window/CdS buffer layers. The RF magnetron cosputtering method using ZnO and MgO targets was used as the deposition technique (Minemoto et al., 2000, 2001). We also deposited a 0.2-mthick Ni layer by the resistive evaporation method as the back electrode in stead of the Au layer. In this subsection, a 55-m-thick polyester film was used as a flexible substrate. Interestingly, when the flexible solar cell using the polyester film was separated from the support SLG substrate, the detachment occurred not at the support SLG/polyester interface but at the polyester/epoxy interface due to the weaker adhesion at the polyester/epoxy interface. After the substrate-free structure was once, the polyester film was therefore bonded onto the rear surface of the solar cell with a silicone adhesion bond. The photograph of the flexible solar cells fabricated via the above procedure is shown in Fig. 8. We also prepared not only the flexible solar cells using the conventional ZnO window/CdS buffer layers but also the solar cells without the lift-off process for comparison.

Fig. 8. Photograph of flexible solar cells using polyester film. Left solar cells are Cd-free solar cells using (Zn,Mg)O window layer. Right solar cells consist of conventional ZnO window/CdS buffer layers structure.

Development of Flexible Cu(In,Ga)Se2 Thin Film Solar Cell by Lift-Off Process 415

(Zn,Mg)O flexible

ZnO/CdS flexible

Fig. 10. EQE spectra of flexible solar cells using (Zn,Mg)O window layer (red) and conventional ZnO window/CdS buffer layers (blue). EQE spectra of standard solar cells using (Zn,Mg)O window layer (dark red) and ZnO window/CdS buffer layers (dark blue)

Here, we discuss why these flexible solar cells showed the similar solar cell parameters. In this subsection, we used Ni in stead of Au as a back electrode material. In subsection 3.1, the ZnO/CdS flexible solar cells with the Au back electrode showed a conversion efficiency of ~6%. We think that the Ni back electrode may limit performance of these solar cells. We therefore speculate that the Ni atoms, which diffused into the CIGS layer from the back side

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

This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through a Grant-in-Aid for Young Scientists (B). The authors are

due to the low temperature annealing, behave as recombination centers for electrons.

ZnO/CdS

standard

400 600 800 1000 1200 Wavelength (nm)

1.0

0.8

(Zn,Mg)O

standard

0.6

External quantum efficiency

0.4

0.2

0

process further advances through our results.

are also shown for comparison.

**4. Conclusion** 

**5. Acknowledgment** 

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

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 without lift-off process are also shown for comparison.

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 flexible solar cells.


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 window/CdS buffer layers are also summarized.

Fig. 10. EQE spectra of flexible solar cells using (Zn,Mg)O window layer (red) and conventional ZnO window/CdS buffer layers (blue). EQE spectra of standard solar cells using (Zn,Mg)O window layer (dark red) and ZnO window/CdS buffer layers (dark blue) are also shown for comparison.

Here, we discuss why these flexible solar cells showed the similar solar cell parameters. In this subsection, we used Ni in stead of Au as a back electrode material. In subsection 3.1, the ZnO/CdS flexible solar cells with the Au back electrode showed a conversion efficiency of ~6%. We think that the Ni back electrode may limit performance of these solar cells. We therefore speculate that the Ni atoms, which diffused into the CIGS layer from the back side due to the low temperature annealing, behave as recombination centers for electrons.
