**2.1 CdS by CBD with a modified configuration**

Figure 5 shows the implementation of the new substrate support for the CBD system, from this figure it can be seen that the CBD system is the same as the one shown in figure 1 but with the addition of the substrate holder. It basically holds the substrates vertically and steady, while keeping it free to rotate along with the substrates, in such case the magnetic stirrer is no longer needed. This substrate support can be set to rotate at different speed rates, allowing the growth and kinetics of the reaction of CdS to change and in the best case to improve, improving thus the physical properties of CdS films. The design includes a

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

Figure 5 shows the implementation of the new substrate support for the CBD system, from this figure it can be seen that the CBD system is the same as the one shown in figure 1 but with the addition of the substrate holder. It basically holds the substrates vertically and steady, while keeping it free to rotate along with the substrates, in such case the magnetic stirrer is no longer needed. This substrate support can be set to rotate at different speed rates, allowing the growth and kinetics of the reaction of CdS to change and in the best case to improve, improving thus the physical properties of CdS films. The design includes a

**Wavelength (cm-1**

400 600 800 1000

**)**

 a b c d

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

**2.1 CdS by CBD with a modified configuration** 

**Transmitance (%)**

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 the new substrate holder.

Chemical Bath Deposited CdS for CdTe and Cu(In,Ga)Se2 Thin Film Solar Cells Processing 243

Cu(In,Ga)Se2 film growth is that it is a natural source of sodium for the growing material. So that, the sodium diffuses through the sputtered Mo back contact, which means that is very important to control the properties of the Mo layer. The presence of sodium promotes the growth of larger grains of the Cu(In,Ga)Se2 and with a higher degree of preferred orientation in the (112) direction. After Cu(In,Ga)Se2 deposition, the junction is formed by depositing a CdS layer. Then a high-resistance (HR) ZnO and a doped high-conductivity ZnO:Al layers are subsequently deposited. The ZnO layer reacts with the CdS forming the CdxZn1-xS ternary compound, which is known to have a wider band gap than CdS alone, increasing thus the cell current by increasing the short wavelength (blue) response and at the same time setting the conditions to make a better electric contact. Finally, the deposition of a current-collecting Ni/Al grid completes the device. The highest conversion efficiency for Cu(In,Ga)Se2 thin film solar cells of 20 % has been achieved by (Repins et al., 2008) using a three stages co-evaporation process. The processing of photovoltaic (PV) quality films is generally carried out via high vacuum techniques, like thermal co-evaporation. This was mainly the reason, we have carried out the implementation and characterization of a thermal co-evaporation system with individual Knudsen cells MBE type, to deposit the Cu(In,Ga)Se2 thin films (see figure 8). The deposition conditions for each metal source were established previously by doing a deposition profile of temperature data vs. growth rate. The thermal co-evaporation of Cu(In,Ga)Se2 thin films was carried out using Cu shots 99.999%, Ga ingots 99.9999%, Se shots 99.999% from Alfa Aeser and In wire 99.999% from Kurt J. Lesker, used as received. The depositions were performed on soda lime glass substrates with sputtered Mo with 0.7 m of thickness. The substrate temperature was > 500 C, temperature of source materials was set to ensure a growth rate of 1.4, 2.2 and 0.9 Å/s for Cu, In and Ga, respectively for the metals, while keeping a selenium overpressure

Fig. 8. Thermal co-evaporation system with Knudsen effusion cells to deposit Cu(In,Ga)Se2

into the vacuum chamber during film growth.

thin films

response decreases to 65% compared to the samples prepared without rotation. The deposition time was set to 10 min in all cases, giving thus the growth of CdS films with 120 – 130 nm.

Fig. 7. SEM images of (a) mono and (b) bi-layer of CdS deposited at 35 rpm

Figure 7 shows the SEM images of CdS films prepared using the new substrate holder, according to these images, the morphology of the mono and bi-layers of CdS changes as a function of the rotating speed. Also we can clearly see an increase in the particle size for each case, for the monolayer of CdS the particle size ball- like shape of 0.5 – 1 m, but more uniform and compact compared to the particle size that the bi-layers of CdS exhibit with rotation speed set to 35 rpm, flakes-like shape with size of 1 – 4 m. No devices have been made so far using CdS films grown with this improved CBD system, studies are being performed and research on the subject is ongoing in order to optimize the deposition conditions, for this case.
