**1. Introduction**

252 Solar Cells – Thin-Film Technologies

We have found that CBD-CdS thin films grown under different conditions, like monolayers or bi-layers, using a standard bath configuration or a modified configuration, the principle for the deposition process is the same: a common precipitation reaction. Depending of the regime we decide to choose, we must perform an optimization of the deposition parameters in order to get the CdS film with the best physical and chemical properties. The quality of the CdS window partner and the absorber material like CdTe and Cu(In,Ga)Se2 will have a

The authors would like to thank to Bill Shafarman from University of Delaware for device processing and characterization. This work was partially supported by CONACYT, grant

Boyd D, Thompson D, Kirk-Othmer, (1980), Encyclopaedia of Chemical Technology, Vol. 11,

Contreras-Puente G., Tufino-Velazquez M., Calixto M. Estela, Jimenez-Escamilla M., Vigil-

Lane D. W., Painter J. D., Cousins M. A., Conibeer G. L., and Rogers K. D. (2003). Thin Solid

McCandless Brian E. and Sites James R., (2003), Cadmium Telluride Solar Cells, in:

Rocheleau R, Meakin J, Birkmire R, (1987), Proc. 19th IEEE Photovoltaic Specialist Conf., 972–976. Romeo N., Bosio A., Canevari V. and Podestà A. (2004), Solar Energy, Volume 77, Issue 6,

Shafarman W. N. and Stolt L. (2003), Cu(InGa)Se2 Solar Cells, in: Handbook of Photovoltaic

Vigil-Galan O., Arias-Carbajal A., Mendoza-Perez R., Santana-Rodríguez G., Sastre-

Vigil O., Arias-Carbajal A., Cruz F., Contreras-Puente G., and Zelaya-Angel O. (2001). Mats.

Acevedo A. (2005), Semiconductor Science and Technology 20, 819.

Wu X. et al., (2001), Proc. 17th European Photovoltaic Solar Energy Conf. 995.

Science and Engineering, Luque A. and Hegedus S., pp. 567 – 616, John Wiley &

Hernandez J., Alonso J. C., Moreno-Garcia E., Contreras-Puente G. and Morales-

Conference Record of the 2006 IEEE 4th World Conference. Cousins M. A., Lane D. W., and Rogers K. D. (2003). *Thin Solid Films* 431–432, 78.

Galan O., Arias-Carbajal A., Morales-Acevedo A., Aguilar-Hernandez J., Sastre-Hernandez J., Arellano-Guerrero F.N. (2006), Photovoltaic Energy Conversion,

Handbook of Photovoltaic Science and Engineering, Luque A. and Hegedus S., pp. 567 – 616, John Wiley & Sons, Ltd ISBN: 0-471-49196-9, West Sussex, England.

great impact on the conversion efficiencies when applied into thin film solar cells.

47587, ICyT-DF, grant PICS08-54 and PROMEP, grant 103.5/10/4959.

Birkmire R. and Eser E. (1997), Annu. Rev. Mater. Sci., 27:625–53.

Mitchell K. et al., (1990), IEEE Trans. Electron. Devices 37, 410–417.

Repins I. et al., (2008), Prog. in Photov: Research and Applications 16, 235.

Sons, Ltd ISBN: 0-471-49196-9, West Sussex, England. Schon, J. H., Schenker, O. and Batlogg, B. (2001), Thin Solid Films 385, p.271.

3rd Edition, 807–880, John Wiley.

Morales-Acevedo A. (2006). Solar Energy 80, 675.

Films 431–432, 73.

Pages 795-801.

Res. Bull. 36, 521.

**6. Conclusions** 

**7. Acknowledgements** 

**8. References** 

The idea of thin films dates back to the inception of photovoltaics in the early sixties. It is an idea based on achieving truly low-cost photovoltaics appropriate for mass production, where usage of inexpensive active materials is essential. Since the photovoltaic (PV) modules deliver relatively little electric power in comparison with combustion-based energy sources, solar cells must be cheap to produce energy that can be competitive. Thin films are considered to be the answer to that low-cost requirement [1].

Replacement of single crystalline silicon with poly and amorphous films, caused the decline of material requirements, which has led to lower final prices [2]. Furthermore, the thickness of cell layers was reduced several times throughout the usage of materials with higher optical absorption coefficients. Unique, thin film and lightweight, devices of low manufacturing costs and high flexibility, were obtained by applying special materials and production techniques, e.g. CIS, CIGS or CdTe/CdS technologies and organic elements. Taking advantage of those properties, there is a great potential of new, useful applications, such as building integrated photovoltaics (BIPV), portable elastic systems or clothing and smart textiles as well [3].

Low material utilization, mass production and integrated module fabrication are basic advantages of thin film solar cells over their monocrystalline counterparts [4]. Figure 1 (by NREL) shows the development of thin film photovoltaic cells since 1975.

The development of cadmium telluride (CdTe) based thin film solar cells started in 1972 with 6% efficient CdS/CdTe [5] to reach the present peak efficiency of 16.5% obtained by NREL researchers in 2002 [6]. Chalcopyrite based laboratory cells (CIS, CIGS) have recently achieved a record efficiency of 20% [7], which is the highest among thin film PV cells (see Table 1). Solar modules based on chalcopyrites, uniquely combines advantages of thin film technology with the efficiency and stability of conventional crystalline silicon cells [4].


Table 1. Efficiencies of CIGS, CdTe and a-Si thin film solar cells [8].

Innovative Elastic Thin-Film Solar Cell Structures 255

Fig. 3. Total life-cycle Cd emissions by Brookhaven National Laboratory [9].

Sb/Au [12].

**1.2 CIS/ CIGS/ CIGSS structures** 

Low-cost soda-lime glass, foil or polymer film can be used as the substrate of CdTe/CdS solar cell. The best results of 16.5% efficiency are achieved with glass substrate (Table 1). However, Laboratory for Thin Films and Photovoltaics at EMPA, Switzerland obtained 12.7% efficiency of single CdTe solar cell on polymer foil and 7.5% of monolithically interconnected flexible CdTe solar module of 32 cm2 total area [10]. Transparent conductive layers (TCL) are usually thin conductive metal oxides, such as ITO (*Indium Tin Oxide*), Zn2O4, Cd2SnO4, ZnO:Al, CdO, ZnO, In2O3, SnO2 or RuSiO4. However, lastly in flexible solar cells, transparent conductive oxides (TCO) are being replaced by carbon nanotube (CNT) composites [11] or graphene. The CdS film is grown either by chemical bath deposition (CBD), close space sublimation (CSS), chemical vapor deposition (CVD), sputtering or vapor transport deposition (VTD). For the growth of CdTe, three leading methods are used for module fabrication: CSS, electro-deposition (ED) and VTD. Wide variety of metals can be used as back contact for thin film CdTe solar cells, e.g. Cu, Au, Cu/Au, Ni, Ni/Al, Sb/Al,

Several thin-film PV companies are actively involved in commercializing thin-film PV technologies. In the area of CdTe technology major players are or were in the past: First Solar (USA), Primestar Solar (USA), BP Solar (USA), Antec Solar (Germany), Calyxo (Germany), CTF Solar (Germany), Arendi (Italy), Abound Solar (USA), Matsushita Battery (Japan) [8, 12,

Other promising material for thin film solar cell absorber layer is copper indium diselenide CuInSe2. CIS has a direct bandgap of 0.95 eV which can be increased by the addition of gallium to the absorber film. About 30% of Ga added to CIS layer (CIGS cell), changes the bandgap from 0.95 eV to almost 1.2 eV, which improves its match with the AM 1.5 solar spectrum. Higher gallium content (of 40%) has a detrimental effect on the device performance, because of its negative impact on the charge transport properties. The best

13]. This effort lead to 18% share of CdTe cells in global PV marked in 2009 [14].

Fig. 1. Best laboratory PV cell efficiencies for thin films [source: www.nrel.gov]

To comprehend the developmental issues of thin films, it is important to examine each individually. Each has a unique set of advantages and shortcomings in terms of their potential to reach the needed performance, reliability and cost goals [1].
