**1. Introduction**

404 Solar Cells – Thin-Film Technologies

Nair, P.K., Nair, M.T.S., Fernandez, A., & Ocampo, M. (1989). Prospects of chemically

Nakada, T., & Kunioka, A. (1999). Direct evidence of Cd diffusion into Cu.In,Ga.Se2 thin

Oladeji, I.0., & Chow, L. (1997). Optimization of Chemical Bath Deposited Cadmium Sulfide

Ortega-Borges, R., & Lincot, D. (1993). Mechanism of chemical bath deposition of cadmium

Palatnik, L.S., & Sorokin, V.K. (1978). *Materialovedenie v mikroelektronike*, Energia, Moscov Qiu, S. N., Lam, W. W., Qiu, C. X., & Shih, I. (1997). ZnO/CdS/CuInSe2 photovoltaic cells

Rau, U., & Scmidt, M. (2001). Electronic Properties of ZnO/CdS/Cu(In, Ga)Se2 Solar Cells - Reddy, T.A., Gordon, J.M., &. de Silva, I.P.D. (1987). Mira: A one-repetitive day method for

Reynolds, D.C., Leies, G., Antes, L.L., & Marburger, R.E. (1954). Photovoltaic effect in

Rincon, M.E., Sanchez, M., Olea, A., Ayala, I., & Nair, P.K. (1998). Photoelectrochemical

Romeo, N., Bosio, A., & Canevari, V. (2003). The role of CdS preparation method in the

Romeo, N., Bosio, A., Tedeschi, R., & Canevari, V. (2000). Back contacts to CSS CdS/CdTe solar cells and stability of performances. *Thin Solid Films*, No.361-362, pp. 327–329 Rothwarf, A. (1982). *Proc. 16th IEEE Photovoltaic Specialists Conf.*, San Diego, 1982, CA, IEEE,

Savadogo, O., & Mandal, K.C. (1993). Low-cost technique for preparing n-Sb2S3/p-Si

Savadogo, O., & Mandal, K.C. (1994). Low cost schottky barrier solar cells fabricated on

Svechnikov, S.V., & Kaganovich, E.B. (1980). CdSxSe1−x photosensitive films: Preparation,

Sze, S. M. (1981). *Physics of semiconductor devices (2nd edn.)*, John Wiley & Sons Inc., New York Tiwari, S., & Tiwari, S. (2006). Development of CdS based stable thin film photo electrochemical solar cells. *Solar Energy Materials & Solar Cells*, Vol.90, pp. 1621–1628

Tuttle T.R., Contreras M.A., et al. (1995). *Proc. SPIE*, Vol.2531, SPIE, Bellingham, pp. 194

Wolf, R. (Ed). (1975). *Cadmium Sulphede Solar Cells.* Applied Solid State Sciences, vol. 5, Nev

Vossen J.L., & Kern W. (1978). *Thin Film Processes*, Academic Press, New York Wagner, S. (1975). Epitaxy in solar cells. *Journal of Crystal Growth*, No.31, pp. 113–121

Thin Films. *J. Electrochem. Soc.,* Vol.144, No.7, pp. 2342-2346

cadmium sulfide. *Physical Review*, No.96, pp. 533–534

*Energy Conversion*, 11–18 May 2003, Osaka, Japan, pp. 469–470

heterojunction solar cells. Appl. Phys. Lett., No.63, pp. 228

Soubane, D., Ihlal, A., & Nouet, G., (2007). *M. J. Condensed Matter*, Vol.9**,** pp. 32-35 Stevenson, R. (2008). First Solar: Quest for \$1 Watt. *Spectrum*, Vol.45, No.8, pp. 22–33

*Mater. & Solar Cells*, No.52, pp. 399–411

No.22, pp. 829–836

Vol.113/114, pp. 764-767

pp. 2444-2446

3464-3473

pp. 123–133

New York, pp. 791

*Soc.*, No.141, pp. 2871

pp. 41–54

York

deposited metal chalcogenide thin films for solar control applications. *J. Phys. D*,

films during chemical-bath deposition process of CdS films. *Appl. Phys. Lett*, Vol**.74**,

sulfide thin films in the ammonia-thiourea system. *J. Electrochem. Soc.*, No.140**,** pp.

fabricated using chemical bath deposited CdS buffer layer. *Appl. Surf. Sci.*,

predicting the long-term performance of solar energy systems. *Sol. Energy*, No.39,

behavior of thin CdS, coupled CdS/CdSe semiconductor thin films. *Solar Energy* 

performance of CdTe/CdS thin film solar cell. *3rd World Conference on Photovoltaic* 

CdSe and Sb2S3 films chemically deposited with silicotungstic acid. *J. Electrochem.* 

properties and use for photodetectors in optoelectronics. *Thin Solid Films*, No. 66,

Clean energy resources as an alternative to fossil fuels has been required. Photovoltaics is the most promising among renewable energy technologies. On the other hand, the cost of the electrical energy generated by the solar cells was higher than that generated by fossil fuels. The cost reduction of the solar cell is therefore required.

Since high-conversion efficiencies have been demonstrated for solar cells using GaAs substrates in 1977 (Kamath et al., 1977; Woodall et al., 1977), a critical problem is how to reduce power generation cost. The characters required to solar cells strongly depend on its applications. In particular, thin film solar cells are promising for terrestrial applications, because thin film solar cells are more advantageous than bulk type solar cells in terms of consumption of raw materials. Konagai et al. fabricated the thin film solar cells on a single crystalline GaAs substrate by the liquid phase epitaxy mehtod, and focused on the reuse of GaAs substrates by detaching these thin film solar cells from the GaAs substrates (Konagai et al., 1978). Konagai et al. named this separation technique the Peeled Film Technology (PFT). This is the invention of the lift-off method in solar cell development. A specific explanation of the PFT is as follows. An Al1-xGaxAs layer was introduced between the thin film solar cell and the GaAs substrate as a release layer. The thin film solar cell was separated from the GaAs substrate by etching the Al1-xGaxAs layer by the HF solution, because Al1-xGaxAs was readily dissolved by the HF solution compared to GaAs. Since a chemical technique was mainly used for the peeling, this method is defined as a chemical lift-off process. Recently, this has been researched as the epitaxial lift-off (ELO) method (Geelen et al., 1997; Schemer et al., 2000, 2005a; Voncken et al., 2002; Yablonovitch et al., 1987).

On the other hand, the cleavage of lateral epitaxial films for transfer (CLEFT) process, where the thin film was mechanically peeled, was developed as a transfer method of a single crystalline GaAs thin film (McClelland et al., 1980). A specific explanation of the CLEFT process is as follows. A photoresist was applied to a surface of a GaAs substrate. The photoresist was patterned with equally-spaced stripe openings by standard photolithographic techniques. Next, a GaAs layer was grown on this patterned substrate surface. In this case, a GaAs layer was grown on only the openings of the photoresist. The lateral growth of a GaAs layer occurs during the GaAs deposition. A single crystalline GaAs layer is therefore formed on the photoresist. Alternative substrate was bonded onto this

Development of Flexible Cu(In,Ga)Se2 Thin Film Solar Cell by Lift-Off Process 407

[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

Material PI PTFE Polyester

Thickness (m) 55 120 25

Thermal tolerance temperature (˚C) 450 260 120

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

Current density-voltage (*J-V*) measurements were performed under standard air mass 1.5 global conditions (100 mW/cm2) at 25C. External quantum efficiency (EQE) measurements of the AC mode were performed at 25C under white light bias (~0.3 sun) conditions. The laser-beam-induced current (LBIC) method using the laser diode (: 783 nm, laser power: 0.3 mW) was performed to investigate a spatial distribution of an EQE (Minemoto et al., 2005). In LBIC measurements, a nominal spot size is less than 50 m and a scan step is 53 m. The surfaces of the fabricated flexible solar cells were observed with an optical microscope. The

*J-V*, EQE, and LBIC measurements were performed after light soaking.

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

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

cells using the PI film.

study are summarized.

**2.2 Characterization methods** 

**3. Results and discussion** 

**process** 

surface with epoxy glue. The single crystalline thin film was transferred to the alternative substrate by applying tensile strain. The CLEFT process is theorefore defined as a mechanical lift-off process.

Unfortunately, the conversion efficiency of the GaAs thin film solar cell using the lift-off process was lower than that of the GaAs bulk solar cell (Schermer et al., 2006). Recently, comparable conversion efficiencies have been demonstrated (Bauhuis et al., 2009).

On the other hand, the energy:weight ratio of the photovoltaic module is a very important index for space applications. Integration of high-efficiency III-V solar cells with light weight substrates is required. Schermer et al. developed high-efficiency III-V solar cells with lightweight by the ELO process using the GaAs substrate (Schermer et al., 2005b).

In addition, the lift-off process was applied to reuse Si substrates (Bergmann et al., 2002; Brendel, 2001). Moreover, the lift-off process was applied to fabricate flexible solar cells in the developments of II-VI and I-III-VI2 semiconductor thin film solar cells (Marrón et al., 2005; Minemoto et al., 2010; Romeo et al., 2006; Tiwari et al., 1999).

Here, we focus on advantages of the lift-off process in fabrication of flexible Cu(In,Ga)Se2 (CIGS) thin film solar cells. For example, for the fabrication process where CIGS layers were directly grown on flexible substrates, Ti foils (Hartmann et al., 2000; Herz et al., 2003; Ishizuka et al., 2009a; Kapur et al., 2002; Kessler et al., 2005; Yagioka & Nakada, 2009), Cu steel sheets (Herz et al., 2003), Mo foils (Kapur et al., 2002, 2003), stainless steel sheets (Britt et al., 2008; Gedhill et al., 2011; Hashimoto et al., 2003 ; Kessler et al., 2005; Khelifi et al., 2010; Pinarbasi et al., 2010; Satoh et al., 2000, 2003; Shi et al., 2009; Wuerz et al., 2009), Al foils (Brémaud et al., 2007), Fe/Ni alloy foils (Hartmann et al., 2000), ZrO2 sheets (Ishizuka et al., 2008a, 2008b, 2009b, 2010), and polyimide (PI) films (Brémaud et al., 2005; Caballero et al., 2009; Hartmann et al., 2000; Ishizuka et al., 2008c; Kapur et al., 2003; Kessler et al., 2005; Rudmann et al., 2005; Zachmann et al., 2009;), are used as flexible substrates. Since these materials do not include Na, other processes to introduce Na are required (Caballero et al., 2009; Ishizuka et al., 2008a; Keyes et al., 1997). Since the thermal tolerance temperature of a PI film is ~450C, the low temperature growth of a CIGS layer is required. The first of the advantages of the lift-off process is to enable to use a high quality CIGS layer grown on a conventional Mo/soda-lime glass (SLG) substrate in the flexible solar cell fabrication. Consequently, the low temperature growth technology for high quality CIGS layer formation and novel processes for a Na source are not required. The second is to enable to use low thermal tolerance films as the flexible substrate of a CIGS solar cell, because in the CIGS solar cell fabrication process, the highest temperature process is the growth of a CIGS layer and the process temperature after the growth of a CIGS layer is less than 100C.
