**3. Results and discussion**

## **3.1 Characterization of flexible Cu(In,Ga)Se2 solar cells fabricated using lift-off process**

The *J-V* characteristics of the fabricated flexible solar cells are shown in Fig. 3. For comparison, the *J-V* characteristic of the standard solar cell is also shown. Solar cell parameters such as the short-circuit current density (*J*sc), the open-circuit voltage (*V*oc), the

Development of Flexible Cu(In,Ga)Se2 Thin Film Solar Cell by Lift-Off Process 409

1 cm

Al/NiCr grid

Active area

Standard

0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)

Fig. 3. Photo *J-V* curves of flexible solar cells using PTFE (red) and PI (blue) films. Photo *J-V* curve of standard solar cell without lift-off process (brack) is also shown for comparison.

Sample structure *Eff.* (%) *Jsc* (mA/cm2) *Voc* (V) *FF* (%) PI flexible 5.9 25.7 0.420 54.9 PTFE flexible 6.6 25.6 0.445 57.9 Standard 11.4 36.9 0.497 62.4 Table 2. Solar cell parameters obtaind from flexible solar cells using PI and PTFE films. Solar

cell parameters of standard solar cell are also shown for comparison.

PTFE

PI

Back electrode

40

30

20

Current density (mA/cm2)

10

0

Fig. 2. Photograph of flexible CIGS solar cells using PI film.

Fig. 1. Schematic illustration of fabrication procedure of flexible CIGS solar cell using lift-off process.

1 2 3 4

Mo SLG

Flexible film Silicone adh. Support SLG

Flexible film

Fig. 1. Schematic illustration of fabrication procedure of flexible CIGS solar cell using lift-off

CIGS

CdS i-ZnO

In2O3:Sn

5 6 7 8

Au Cond. epoxy

CIGS

CdS

In2O3:Sn i-ZnO

Au Cond. epoxy

Mo SLG

Flexible film Silicone adh. Support SLG

CIGS

CdS i-ZnO

In2O3:Sn

Al/NiCr

Au Cond. epoxy

CIGS

Au

Cond. epoxy

Adhesion

Flexible film Silicone adh. Support SLG

CIGS

Au

Mo SLG

Flexible film Silicone adh. Support SLG

CIGS

CdS

Au Cond. epoxy

CIGS

Mo SLG

Flexible film Silicone adh. Support SLG

> Mo SLG

Lift-off process

Flexible film Silicone adh. Support SLG

Separation

9 10

CIGS

CdS i-ZnO

In2O3:Sn

Au Cond. epoxy

process.

CIGS

Au Cond. epoxy

Fig. 2. Photograph of flexible CIGS solar cells using PI film.

Fig. 3. Photo *J-V* curves of flexible solar cells using PTFE (red) and PI (blue) films. Photo *J-V* curve of standard solar cell without lift-off process (brack) is also shown for comparison.


Table 2. Solar cell parameters obtaind from flexible solar cells using PI and PTFE films. Solar cell parameters of standard solar cell are also shown for comparison.

Development of Flexible Cu(In,Ga)Se2 Thin Film Solar Cell by Lift-Off Process 411

the lift-off process for the flexible solar cells. We speculate that the band gap profile of the inverted graded band gap structure is not beneficial for collecting the photogenerated carriers by long wavelength light. We conclude that the EQE reductions observed for the flexible solar cells are attributed to the influence of the inverted graded band gap structure. We describe an interesting point of our flexible solar cells as below. Different materials with different thermal tolerance temperatures are used as the flexible substrates of these flexible solar cells, as shown in Table 1. These flexible solar cells, however, show the similar

LBIC and optical microscope images of the flexible solar cell using the PTFE film are shown in Figs. 6(a) and 6(b), respectively. There is a low EQE region on the lower side of the solar cell from Fig. 6(a). This low EQE region corresponds approximately to the flexurelike region from a comparison between Figs. 6(a) and 6(b). This result therefore suggests that this flexure cause reduction of an EQE. LBIC and optical microscope images of the standard solar cell are shown in Figs. 6(c) and 6(d), respectively. In contrast, the LBIC and optical

6

Grid finger

5

4

3

Vertical axis (mm)

2

1

0

(a) (b) (c) (d)

Fig. 6. (a) LBIC and (b) optical microscope images of flexible solar cell using PI film. (c) LBIC and (d) optical microscope images of standard solar cell. Indicators of EQE intensity are

0 1.0 2.0 Horizontal axis (mm)

0.1 mm 0.1 mm

characteristics irrespective of the flexible film materials from Fig. 3 and Fig. 4.

Low EQE (arb. unit) High Low EQE (arb. unit) High

microscope images are uniform for the standard solar cell.

6

5

4

3

Vertical axis (mm)

2

1

0

0 1.0 2.0 Horizontal axis (mm)

shown next to LBIC images.

conversion efficiency (*Eff.*), and the fill factor (*FF*) are summarized in Table 2. The conversion efficiencies of the flexible solar cells are an approximately half conversion efficiency of the standard solar cell. EQE spectra of these solar cells are shown in Fig. 4. EQEs of the flexible solar cells remarkably decrease in the long wavelength region from 700 to 1200 nm compared to the standard solar cell. We discuss this cause as below.

Fig. 4. EQE spectra of flexible solar cells using PTFE (red) and PI films (blue). EQE spectrum of standard solar cell without lift-off process (black) is also shown for comparison. EQE spectra of flexible solar cells are similar irrespective of substrate materials.

As shown in Fig. 5(a), the band gap profile of the standard solar cell consists of the graded band gap structure because of the three-stage deposition process. The diffusion length of electrons generated by the long wavelength light near the back electrode is improved due to the quasi-electric field in which the CIGS layer forms (Contreras et al., 1994b). The graded band gap structure is therefore beneficial for collecting the photogenerated carriers. On the other hand, as shown in Fig. 5(b), the band gap profile of the CIGS layer is inverted due to

Fig. 5. Schematic illustrations of band gap profiles of CIGS layers. CIGS absorber layers with (a) double geraded band gap and (b) inverted double graded band gap structures are shown.

conversion efficiency (*Eff.*), and the fill factor (*FF*) are summarized in Table 2. The conversion efficiencies of the flexible solar cells are an approximately half conversion efficiency of the standard solar cell. EQE spectra of these solar cells are shown in Fig. 4. EQEs of the flexible solar cells remarkably decrease in the long wavelength region from 700

PI

400 600 800 1000 1200

PTFE

Standard

Surface Rear

h Photogenerated + hole

e-

Photogenerated electron

Conduction

band

surface

Valence band

Wavelength (nm)

Fig. 4. EQE spectra of flexible solar cells using PTFE (red) and PI films (blue). EQE spectrum of standard solar cell without lift-off process (black) is also shown for comparison. EQE

As shown in Fig. 5(a), the band gap profile of the standard solar cell consists of the graded band gap structure because of the three-stage deposition process. The diffusion length of electrons generated by the long wavelength light near the back electrode is improved due to the quasi-electric field in which the CIGS layer forms (Contreras et al., 1994b). The graded band gap structure is therefore beneficial for collecting the photogenerated carriers. On the other hand, as shown in Fig. 5(b), the band gap profile of the CIGS layer is inverted due to

spectra of flexible solar cells are similar irrespective of substrate materials.

Rear surface

> Valence band

Conduction band

(a) (b)

Fig. 5. Schematic illustrations of band gap profiles of CIGS layers. CIGS absorber layers with (a) double geraded band gap and (b) inverted double graded band gap structures are shown.

to 1200 nm compared to the standard solar cell. We discuss this cause as below.

1.0

0.8

0.6

External quantum efficiency

0.4

0.2

0

e-

Photogenerated electron

Surface

h Photogenerated + hole

the lift-off process for the flexible solar cells. We speculate that the band gap profile of the inverted graded band gap structure is not beneficial for collecting the photogenerated carriers by long wavelength light. We conclude that the EQE reductions observed for the flexible solar cells are attributed to the influence of the inverted graded band gap structure.

We describe an interesting point of our flexible solar cells as below. Different materials with different thermal tolerance temperatures are used as the flexible substrates of these flexible solar cells, as shown in Table 1. These flexible solar cells, however, show the similar characteristics irrespective of the flexible film materials from Fig. 3 and Fig. 4.

LBIC and optical microscope images of the flexible solar cell using the PTFE film are shown in Figs. 6(a) and 6(b), respectively. There is a low EQE region on the lower side of the solar cell from Fig. 6(a). This low EQE region corresponds approximately to the flexurelike region from a comparison between Figs. 6(a) and 6(b). This result therefore suggests that this flexure cause reduction of an EQE. LBIC and optical microscope images of the standard solar cell are shown in Figs. 6(c) and 6(d), respectively. In contrast, the LBIC and optical microscope images are uniform for the standard solar cell.

Fig. 6. (a) LBIC and (b) optical microscope images of flexible solar cell using PI film. (c) LBIC and (d) optical microscope images of standard solar cell. Indicators of EQE intensity are shown next to LBIC images.

Development of Flexible Cu(In,Ga)Se2 Thin Film Solar Cell by Lift-Off Process 413

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

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

layers but also the solar cells without the lift-off process for comparison.

(Zn,Mg)O flexible ZnO/CdS flexible

solar cells solar cells

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.

Conventional

Fig. 7. Schematic illustration of fabrication procedure of flexible CIGS solar cell using (Zn0.83,Mg0.17)O window layer and lift-off process.
