**2. CdS thin films by chemical bath deposition technique (CBD)**

Chemical bath deposition technique (CBD) has been widely used to deposit films of many different semiconductors. It has proven over the years to be the simplest method available for this purpose, the typical components of a CBD system are a container for the solution bath, the solution itself made up of common chemical reactive salts, the substrate where the deposition of the film is going to take place, a device to control the stirring process and temperature, sometimes a water bath is included to ensure an homogeneous temperature, an schematic diagram of the CBD system is shown in figure 1. The concentrations of the components of the solution bath for CdS can be varied over a working range and each group use its own specific recipe, so there are as many recipes to deposit CdS as research groups working in the subject. The chemical reactive salts are generally of low cost and in general it is necessary to use small quantities. The most important deposition parameters in this technique are the molar concentration, the pH, the deposition temperature, the deposition time, the stirring rate, the complexing agents added to the bath to slowing down the chemical reactions, etc. However, once they have been established these are easy to control. The CdS thin film deposition can be performed over several substrates at a time, and the reproducibility is guaranteed if the deposition parameters are kept the same every time a deposition is done. Substrates can have any area and any configuration, besides they can be of any kind, electrical conductivity is not required.

Fig. 1. Schematic diagram of a CdS chemical bath deposition system

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

Chemical bath deposition technique (CBD) has been widely used to deposit films of many different semiconductors. It has proven over the years to be the simplest method available for this purpose, the typical components of a CBD system are a container for the solution bath, the solution itself made up of common chemical reactive salts, the substrate where the deposition of the film is going to take place, a device to control the stirring process and temperature, sometimes a water bath is included to ensure an homogeneous temperature, an schematic diagram of the CBD system is shown in figure 1. The concentrations of the components of the solution bath for CdS can be varied over a working range and each group use its own specific recipe, so there are as many recipes to deposit CdS as research groups working in the subject. The chemical reactive salts are generally of low cost and in general it is necessary to use small quantities. The most important deposition parameters in this technique are the molar concentration, the pH, the deposition temperature, the deposition time, the stirring rate, the complexing agents added to the bath to slowing down the chemical reactions, etc. However, once they have been established these are easy to control. The CdS thin film deposition can be performed over several substrates at a time, and the reproducibility is guaranteed if the deposition parameters are kept the same every time a deposition is done. Substrates can have any area and any configuration, besides they can be

photoluminescence measurements performed to the CdS films.

of any kind, electrical conductivity is not required.

Fig. 1. Schematic diagram of a CdS chemical bath deposition system

**2. CdS thin films by chemical bath deposition technique (CBD)** 

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

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 small and large area devices.

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

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

Chemical Bath Deposited CdS for CdTe and Cu(In,Ga)Se2 Thin Film Solar Cells Processing 241

direct current motor that has the option to vary the speed rate from 0 to 50 rpm. The motor can move the substrate support made of a Teflon structure that holds up to 4 large area substrates (45 cm2 each). The principal advantage of using this modified structure is the ability to handle 4 substrates at a time, placing them, inside the reactor containing the solution bath and at the same time starting the rotation, by doing this all the CdS films are expected to have a uniform growth and thickness 120 nm. When the substrate holder is set to rotate inside the reactor, the kinetics of the CdS films growth was clearly affected as shown in figure 6, it can be seen that when the rotating speed goes up, the transmission

Fig. 5. Schematic diagram of the new substrate holder for the CBD system

Fig. 6. Transmission response as a function of the rotation rate for CdS films prepared with

300 400 500 600 700 800 900 1000

**Wavelength (cm-1)**

 **w/o rotation 35 rpm**

**52 rpm**

the new substrate holder.

0

20

40

60

**Transmitance (%)**

80

100

complicated. Because of this, to design a better substrate holder/support was imperative. So, a new support was designed and built to facilitate the access and handling of the samples inside the reactor. Figures 4c and 4d shown the transmission response for mono and bi-layers of CdS deposited using the new substrate support, placed in a vertical configuration inside the reactor, for both cases the values were between 85 – 95 %, being the monolayers the ones that exhibit the best response; however its morphology shows a larger surface defect density. The thickness of these samples is in the order of 100 – 120 nm.

Fig. 3. SEM images of a monolayer and a bi-layer of CdS

Fig. 4. Transmission response of CdS films as a function of the position inside the reactor
