**3. Cu(In,Ga)Se2 based thin films by co-evaporation technique (PVD)**

Semiconducting CuInSe2 is one of the most promising materials for solar cells applications because of its favorable electronic and optical properties including its direct band gap with high absorption coefficient (105 cm-1) thus layers of only 2 m thickness are required to absorb most of the usable solar radiation and inherent *p*-type conductivity. Besides, the band gap of CuInSe2 can be modified continuously over a wide range from 1.02 to 2.5 eV by substituting Ga for In or S for Se, which means that this material can be prepared with a different chemical composition. Cu(In,Ga)Se2 is a very forgiving material so high efficiency devices can be made with a wide tolerance to variations in Cu(In,Ga)Se2 composition (Rocheleau et al., 1987 and Mitchell K. et al., 1990), grain boundaries are inherently passive so even films with grain sizes less than 1 μm can be used, and device behavior is insensitive to defects at the junction caused by a lattice mismatch or impurities between the Cu(In,Ga)Se2 and CdS. The latter enables high-efficiency devices to be processed despite exposure of the Cu(In,Ga)Se2 to air prior to junction formation. For Cu(In,Ga)Se2 thin film solar cells processing the substrate structure is preferred over the superstrate structure. The substrate structure is composed of a soda lime glass substrate, coated with a Mo layer used as the back contact where the Cu(In,Ga)Se2 film is deposited. The soda lime glass, which is used in conventional windows, is the most common substrate material used to deposit Cu(In,Ga)Se2 since it is available in large quantities at low cost. Besides, it has a thermal expansion coefficient of 9 × 10−6 K-1 (Boyd et al., 1980) which provides a good match to the Cu(In,Ga)Se2 films. The most important effect of the soda lime glass substrate on

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 –

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

Semiconducting CuInSe2 is one of the most promising materials for solar cells applications because of its favorable electronic and optical properties including its direct band gap with high absorption coefficient (105 cm-1) thus layers of only 2 m thickness are required to absorb most of the usable solar radiation and inherent *p*-type conductivity. Besides, the band gap of CuInSe2 can be modified continuously over a wide range from 1.02 to 2.5 eV by substituting Ga for In or S for Se, which means that this material can be prepared with a different chemical composition. Cu(In,Ga)Se2 is a very forgiving material so high efficiency devices can be made with a wide tolerance to variations in Cu(In,Ga)Se2 composition (Rocheleau et al., 1987 and Mitchell K. et al., 1990), grain boundaries are inherently passive so even films with grain sizes less than 1 μm can be used, and device behavior is insensitive to defects at the junction caused by a lattice mismatch or impurities between the Cu(In,Ga)Se2 and CdS. The latter enables high-efficiency devices to be processed despite exposure of the Cu(In,Ga)Se2 to air prior to junction formation. For Cu(In,Ga)Se2 thin film solar cells processing the substrate structure is preferred over the superstrate structure. The substrate structure is composed of a soda lime glass substrate, coated with a Mo layer used as the back contact where the Cu(In,Ga)Se2 film is deposited. The soda lime glass, which is used in conventional windows, is the most common substrate material used to deposit Cu(In,Ga)Se2 since it is available in large quantities at low cost. Besides, it has a thermal expansion coefficient of 9 × 10−6 K-1 (Boyd et al., 1980) which provides a good match to the Cu(In,Ga)Se2 films. The most important effect of the soda lime glass substrate on

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

**3. Cu(In,Ga)Se2 based thin films by co-evaporation technique (PVD)** 

130 nm.

conditions, for this case.

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 into the vacuum chamber during film growth.

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

Chemical Bath Deposited CdS for CdTe and Cu(In,Ga)Se2 Thin Film Solar Cells Processing 245

**(112)**

Fig. 10. XRD pattern for Cu(In,Ga)Se2 thin films thermal co-evaporated

**(101)**

**CuIn0.7Ga0.3Se2 PDF 35-1102** 

**4. CdTe thin films by Close Spaced Vapor Transport (CSVT)** 

prepare the CdTe films for solar cell applications.

**Intensity x1000 (a.u.)**

CdTe is a compound semiconductor of II-VI type that has a cubic zincblende (sphalerite) structure with a lattice constant of 6.481 A°. CdTe at room temperature has a direct band gap of 1.5 eV with a temperature coefficient of 2.3–5.4 x10−4 eV/K. This band gap is an ideal match to the solar spectrum for a photovoltaic absorber. Similarly to the Cu(In,Ga)Se2, the absorption coefficient is large (around 5x104 cm−1) at photon energies of 1.8 eV or higher (Birkmire R. and Eser E., 1997). Up to date the highest conversion efficiency achieved for CdTe solar cells is 16.5% (Wu X. et al., 2001). CdTe solar cells are p-n heterojunction devices in which a thin film of CdS forms the n-type window layer. As in the case of Cu(In,Ga)Se2 based devices the depletion field is mostly in the CdTe. There are several deposition techniques to grow the CdTe like, physical vapor deposition, vapor transport deposition, close spaced sublimation, sputter deposition and electrodeposition (McCandless Brian E. and Sites James R., 2003). In this case, the close spaced sublimation has been selected to

**10 15 20 25 30 35 40 45 50 55 60**

**2 Theta (deg.)**

**(213) (301) (211) (103) (312)**

**Mo (110)**

**(220/204)**

 **Reference CIGS\_8 CIGS\_5**

The sublimation technique for the deposition of semiconducting thin films of the II-VI group, particularly CdTe, has proven to be effective to obtain polycrystalline materials with very good optical and electrical properties. There are several steps that involve the formation of the deposited materials, these are listed as follows: 1) synthesis of the material to be deposited through the phase transition from solid or liquid to the vapor phase 2) vapor transport between the evaporation source and the substrate, where the material will be deposited in the form of thin film, and 3) vapor and gas condensation on the substrate, followed by the nucleation and grow of the films. In general, and particularly in our CdTe - case, the vapor transport is regulated by a diffusion gas model. This technique has several advantages over others because is inexpensive, has high growth rates, and it can be scaled up to large areas for mass production. The Close Spaced Vapor Transport technique, named as "CSVT", is a variant of the sublimation technique, it uses two graphite blocks, where independent high electrical currents flow and due to the dissipation effect of the electrical energy by Joule's heat makes the temperature in each graphite block to rise. One of the graphite blocks is named the source

Cu(In,Ga)Se2 thin films were grown with different Ga and Cu ratios (Ga/(In+Ga) = 0.28, 0.34 and 0.35 respectively and Cu/(In+Ga) = 0.85, 0.83 and 0.94). The deposition time was set to 30 min for all cases. All the Cu(In,Ga)Se2 samples were grown to have 2 - 3 m thickness and aiming to obtain a relative low content of gallium 0.30 % (CuIn0.7Ga0.3Se2), while keeping the copper ratio to III < 1 (where III = In+Ga), very important criteria to use them directly for solar cell applications, as shown in table 1. For solar cell devices, samples JS17 and JS18 were used, with a chemical composition similar to that of sample JS13.


Table 1. Results of the chemical composition analysis of the co-evaporated Cu(In,Ga)Se2 thin films

The morphology of the Cu(In,Ga)Se2 samples is very uniform, compact and textured, composed of small particles (see figures 9a - 9c). Figure 9d shows the cross-section SEM image and a film thickness 3.5 m, also notice the details of the textured surface of the film, due to the high temperature processing.

Fig. 9. SEM micrographs of co-evaporated Cu(In,Ga)Se2 thin films (a - c) and (d) cross section image

The XRD patterns of the films show sharp and well defined peaks, indicating a very good crystallization, the films appear to grow with a strong (112) orientation (see figure 10) and with grain sizes ~ 1 µm. The expected shift of the (112) reflection compared to that of the CuInSe2 is also observed, which is consistent with a film stoichiometry of CuIn0.7Ga0.3Se2 (JCPDS 35-1102).

Cu(In,Ga)Se2 thin films were grown with different Ga and Cu ratios (Ga/(In+Ga) = 0.28, 0.34 and 0.35 respectively and Cu/(In+Ga) = 0.85, 0.83 and 0.94). The deposition time was set to 30 min for all cases. All the Cu(In,Ga)Se2 samples were grown to have 2 - 3 m thickness and aiming to obtain a relative low content of gallium 0.30 % (CuIn0.7Ga0.3Se2), while keeping the copper ratio to III < 1 (where III = In+Ga), very important criteria to use them directly for solar cell applications, as shown in table 1. For solar cell devices, samples

Chemical composition (at %) by EDS Sample Cu In Ga Se Ga/III Cu/III Reference 22.09 18.84 7.27 51.80 0.28 0.85 CIGS\_5 21.27 16.73 8.88 53.69 0.35 0.83 CIGS\_8 23.04 16.20 8.24 53.47 0.34 0.94 JS13 24.46 16.87 9.74 48.93 0.37 0.92 Table 1. Results of the chemical composition analysis of the co-evaporated Cu(In,Ga)Se2 thin

The morphology of the Cu(In,Ga)Se2 samples is very uniform, compact and textured, composed of small particles (see figures 9a - 9c). Figure 9d shows the cross-section SEM image and a film thickness 3.5 m, also notice the details of the textured surface of the

Fig. 9. SEM micrographs of co-evaporated Cu(In,Ga)Se2 thin films (a - c) and (d) cross

The XRD patterns of the films show sharp and well defined peaks, indicating a very good crystallization, the films appear to grow with a strong (112) orientation (see figure 10) and with grain sizes ~ 1 µm. The expected shift of the (112) reflection compared to that of the CuInSe2 is also observed, which is consistent with a film stoichiometry of CuIn0.7Ga0.3Se2

JS17 and JS18 were used, with a chemical composition similar to that of sample JS13.

films

section image

(JCPDS 35-1102).

film, due to the high temperature processing.

Fig. 10. XRD pattern for Cu(In,Ga)Se2 thin films thermal co-evaporated
