**2. Experimental**

#### **2.1 Flexible Cu(In,Ga)Se2 solar cell fabrication procedure**

A schematic illustration of the fabrication procedure of our flexible CIGS solar cell using the lift-off process is shown in Fig. 1 (Minemoto et al., 2010). A 0.8-m-thick Mo layer was deposited on an SLG substrate without intentional substrate heating by the radio frequency (RF) magnetron sputtering method. A 2.5-m-thick CIGS layer was deposited on the Mo/SLG substrate by the three-stage deposition process at the highest substrate temperature of approximately 550C (Contreras et al., 1994a; Negami et al., 2002). From energy dispersive x-ray spectrometry measurements, the Cu, In, Ga, and Se composition ratios of this CIGS layer were approxymately 23, 18, 8, and 51%, respectively. The [Cu]/[Ga+In] and [Ga]/[Ga+In] ratios of the CIGS layer were therefore calclated to be ~0.88 and ~0.31, respectively. After CIGS surface cleaning by a KCN solution, a 0.2-m-thick Au layer was deposited on the CIGS surface by a resistive evaporation method as a back electrode. The samples were annealed for 30 min at 250C in N2 ambient. Flexible films were bonded onto support SLG substrates with a silicone adhesion bond for preparation of the alternative substrates. These alternative substrates were also bonded onto the Au/CIGS/Mo/SLG structure with conductive epoxy glue. To dry the conductive epoxy glue, the samples were annealed on a hot plate at 100C for 10 min in the atmosphere. Then, the alternative-sub./epoxy/Au/CIGS stacked structures were detached from the primary Mo/SLG substrates by applying tensile strain. In this detachment, the CIGS layer was transferred to the alternative substrate side (Marrón et al., 2005). The lift-off flexible CIGS solar cells were fabricated using this peeled CIGS layer. After cleaning of the CIGS rear surface by a KCN solution, a 0.1-m-thick CdS layer was deposited on the CIGS rear surface by the chemical bath deposition method. 0.1-m-thick i-ZnO and 0.1-m-thick In2O3:Sn layers were deposited by the RF magnetron sputtering method. Al/NiCr grids were formed. Finally, the flexible CIGS solar cells using the lift-off process were completed by detaching the flexible films from the support SLG substrates. For comparison, we also prepared a standard solar cell where the lift-off process was not carried out (the Al/NiCr/In2O3:Sn/ZnO/CdS/CIGS/Mo/SLG structure). The properties of the films used in this study are summarized in Table 1. Figure 2 shows a photograph of the flexible solar cells using the PI film.


Table 1. Properties of PI, polytetrafluoroethylene (PTFE) and polyester films used in this study are summarized.
