**1. Introduction**

236 Solar Cells – Thin-Film Technologies

Romeo, N.; Bosio, A.; Romeo, A. & Mazzamuto, S. (2010). A CdTe thin film module factory

Schulz, D.L.; Pehnt, M.; Rose, D.H.; Urgiles, E.; Cahill, A.F.; Niles, D.W.; Jones, K.M.;

Stadler, W.; Hoffmann, D.M.; Alt, H.C.; Muschik, T.; Meyer, B.K.; Weigel, E.; Müller-Vogt,

Yacobi, B.G. & Holt, D.B. (1990) *Cathodoluminescence microscopy of inorganic solids,* Plenum

Yi, X. & Liou, J.J. (1995). Surface oxidation of polycrystalline cadmium telluride thin films

Yoshida, T. (1992). Analysis of photocurrent in screen-printed CdS/CdTe solar cells. *Journal of the Electrochemical Society.* Vol.139, No.8, August 1992, pp. 2353-2357. ISSN: 00134651 Yoshida, T. (1995). Photovoltaic properties of screen-printed CdTe/CdS solar cells on

Wu, X.; Keane, J.C.; Dhere, R.G.; DeHart, C.; Duda, A.; Gessert, T.A.; Asher, S.; Levi, D.H.

Wu, X. (2004). High-efficiency polycrystalline CdTe thin-film solar cells. *Solar Energy.* Vol.77,

Wu, X.; Zhou, J.; Duda, A.; Yan, Y.; Teeter, G.; Asher, S.; Metzger, W.K.; Demtsu, S.; Wei, S.-

Xiaonan Li,. Niles D. W,. Hasoon F. S,. Matson R. J, and Sheldon P. (1999). Effect of nitric-

Zhou, J.; Wu, X.; Duda, A.; Teeter, G. & Demtsu, S.H. (2007). The formation of different

Zoppi, G.; Durose, K.; Irvine, S.J.C. & Barrioz, V. (2006). Grain and crystal texture properties

*and Technology.* Vol.21, No.6, June 2006, pp. 763-770. ISSN: 02681242

cell, *Proceedings of the 17th E-PVSEC,* München, Germany; October 2001. Wu, X.; Asher, S.; Levi, D.H.; King, D.E.; Yan, Y.; Gessert, T.A. & Sheldon, P. (2001 b).

Vol.142, No.9, September 1995, pp. 3232-3237. ISSN: 00134651

Press, ISBN: 0306433141, New York and London.

ISBN: 978-160511138-4, San Francisco, CA, USA; April 13-17, 2009

889-900. ISSN: 08974756

pp.1151-1154, ISSN: 00381101.

2001, pp. 4564-4569. ISSN: 00218979

ISSN: 00406090

9780127521138, London, UK

No6, (December 2004), pp. 803-814, ISSN: 0038092X

515, No.18, June 2007, pp. 7364-7369, ISSN: 00406090

York.

with a novel process. *Proceedings of 2009 MRS Spring Meeting;* Vol.1165, pp. 263-273,

Ellingson, R.J.; Curtis, C.J.; Ginley, D.S. (1997). CdTe Thin Films from Nanoparticle Precursors by Spray Deposition. *Chemistry of Materials.* Vol.9. No.4, April 1997, pp.

G.; Salk, M.; Rupp, E. and Benz, K.W. (1995) Optical investigations of defects in Cd1-xZnxTe, *Physical Review B,* vol.51, No.16 1995, pp. 10619-10630, ISSN: 01631829. Sze, S. (1981). *Physics of Semiconductor Devices* (2nd ed.), Wiley, ISBN:9780471143239, New

for Schottky barrier junction solar cells. *Solid-State Electronics.* Vol.38, No.6, (1995),

indium-tin-oxide coated glass substrates. *Journal of the Electrochemical Society.* 

and Sheldon, P. (2001 a). 16.5% efficient CdS/CdTe polycrystalline thin-film solar

Interdiffusion of CdS and Zn2SnO4 layers and its application in CdS/CdTe polycrystalline thin-film solar cells. *Journal of Applied Physics*, Vol.89, No.8, April

Huai. & Noufi, R. (2007). Phase control of CuxTe film and its effects on CdS/CdTe solar cell. *Thin Solid Films,* Vol.515, No.15 SPEC. ISS., May 2007, pp. 5798-5803,

phosphoric acid etches on material properties and back-contact formation of CdTebased solar cells. *J. Vac. Sci. Technol. A*, vol. 17, No 3, p.p. 805-809 ISSN: 07342101. Zanio, K.; Willardson R.K. & Beer, A.C. (1978). *Cadmium telluride. Volume 13 of* 

*Semiconductors and semimetals. Cadmium telluride*, Academic Press, ISBN 0127521135,

phases of CuxTe and their effects on CdTe/CdS solar cells. *Thin Solid Films,* Vol.

of absorber layers in MOCVD-grown CdTe/CdS solar cells. *Semiconductor Science* 

Extensive research has been done during the last two decades on cadmium sulfide (CdS) thin films, mainly due to their application to large area electronic devices such as thin film field-effect transistors (Schon et al., 2001) and solar cells (Romeo et al., 2004). For the latter case, chemical bath deposited (CBD) CdS thin films have been used extensively in the processing of CdTe and Cu(In,Ga)Se2 solar cells, because it is a very simple and inexpensive technique to scale up to deposit CdS thin films for mass production processes and because among other n-type semiconductor materials, it has been found that CdS is the most promising heterojunction partner for these well-known polycrystalline photovoltaic materials. Semiconducting n-type CdS thin films have been widely used as a window layer in solar cells; the quality of the CdS-partner plays an important role into the PV device performance. Usually the deposition of the CdS thin films by CBD is carried out using an alkaline aqueous solution (high pH) composed mainly of some sort of Cd compounds (chloride, nitrate, sulfate salts, etc), thiourea as the sulfide source and ammonia as the complexing agent, which helps to prevent the undesirable homogeneous precipitation by forming complexes with Cd ions, slowing down thus the surface reaction on the substrate. CdS films have to fulfill some important criteria to be used for solar cell applications; they have to be adherent to the substrate and free of pinholes or other physical imperfections. Moreover, due to the requirements imposed to the thickness of the CdS films for the solar cells, it seems to be a function of the relative physical perfection of the film. The better structured CdS films and the fewer flaws present, the thinner the film can be, requirement very important for the processing of Cu(In,Ga)Se2 based thin film solar cells, thickness ~ 30 - 50 nm. In such case, the growth of the thin CdS film is known to occur via ion by ion reaction, resulting thus into the growth of dense and homogeneous films with mixed cubic/hexagonal lattice structure (Shafarman and Stolt, 2003).

The reason to choose the CBD method to prepare the CdS layers was due to the fact that CBD forms a very compact film that covers the TCO layer, in the case of the CdTe devices and the Cu(In,Ga)Se2 layer without pinholes. Moreover, the CdS layer in a hetero-junction solar cell must also be highly transparent and form a chemical stable interface with the

Chemical Bath Deposited CdS for CdTe and Cu(In,Ga)Se2 Thin Film Solar Cells Processing 239

Previously we have reported the preparation of monolayers and bi-layers of CdS deposited by chemical bath deposition technique using a solution bath based on CdCl2 (0.1 M), NH4Cl (0.2 M), NH3 (2 M) and thiourea (0.3 M), maintaining fixed deposition time and temperature conditions and varying the order of application of the CdCl2 treatment (Contreras-Puente et al., 2006). Initially, the solution is preheated during 5 min prior to add the thiourea, after that the deposition was carried out during 10 min at 75 C, then the second layer (the bilayer) was deposited at a lower deposition temperature, thus allowing us to control the growth rate of the CdS layer. This was aimed to obtain films with sub-micron and nanometric particle size that could help to solve problems such as partial grain coverage, inter-granular caverns and pinholes. In this way, CdS thin films have been deposited onto

Figure 2 shows the typical X-ray diffraction pattern obtained with a glancing incidence Xray diffractometer, for CdS samples prepared in small and large area, respectively. CdS films grow with preferential orientation in the (002), (112) y (004) directions, which correspond to the CdS hexagonal structure (JCPDS 41-049). Small traces of SnO2:F are observed (\*) in the X-ray patterns. Figure 3 shows the morphology for both mono and bilayers of CdS films, respectively. It can be observed that bi-layer films present lower pinhole density and caverns. This is a critical parameter because it gives us the possibility to improve the efficiency of solar cell devices. Several sets of CdTe devices were made and their photovoltaic parameters analyzed, giving conversion efficiencies of 6.5 % for both

SnO2: F substrates of 4 cm2 and 40 cm2, respectively.

Fig. 2. X-ray diffraction patterns of mono and bi-layers of CdS

(002)

**Intensity (a.u.)**

**\***

**\***

Also, we have found that the position of the substrate inside the reactor is an important factor because the kinetics of the growth changes. Figure 4 shows how the transmission response changes with substrate position inside the reactor. The deposition time for all samples was 10 min. According to figure 4a when the substrates are placed horizontally at the bottom of the reactor the CdS film grows a thickness of 150 nm, but the transmission response is poor, when the substrates are placed vertically and suspended with a pair of tweezers inside the reactor the CdS film grows a thickness of 110 nm and the transmission response is 83% (see figure 4b), however in this configuration handling the substrate is

(112)

(004)

20 30 40 50 60 70 80 90

**2 Degrees)**

**\* \***

**b**

**a**

Bi-layer of CdS

Monolayer of CdS

small and large area devices.

Cu(In,Ga)Se2 and CdTe absorbing layers. The micro-crystalline quality of the film may also be related to the formation of the CdZnS ternary layer in the case of the Cu(In,Ga)Se2 and CdS1-xTex ternary layer for the case of CdTe, at the interface helping to reduce the effects associated to the carrier traps in it. Hence, the deposition conditions and characteristics of the CdS layer may affect strongly the efficiency of the solar cells. We have worked with this assumption in mind for making several experiments that will be described in the following paragraphs. As it will be shown, we have been able to prepare optimum CdS layers by CBD in order to be used in solar cells, and have found that the best performance of CdS/CdTe solar cells is related to the CdS layer with better micro-crystalline quality as revealed by photoluminescence measurements performed to the CdS films.
