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

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 prepare the CdTe films for solar cell applications.

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

Chemical Bath Deposited CdS for CdTe and Cu(In,Ga)Se2 Thin Film Solar Cells Processing 247

treatment is not performed, the short circuit current density and the efficiency of the solar cell are very low. This treatment consists in depositing 300–400 nm of CdCl2 on top of CdTe with a subsequent annealing at 400 °C during 15–20 min in air, or in an inert gas atmosphere like Ar. During this process the small CdTe grains are put in vapor phase and re-crystallize, giving a better-organized CdTe matrix. The presence of Cl2 could favor the CdTe grain growth by means of a local vapor phase transport. In this way the small grains disappear

Cu(In,Ga)Se2 and CdTe PV devices are obtained by forming p-n heterojunctions with thin films of CdS. In this type of structure, n-type CdS, which has a band gap of 2.4 eV, not only forms the p-n junction with p-type CuInSe2 or p-type CdTe but also serves as a window layer that lets light through with relatively small absorption. Also, because the carrier density in CdS is much larger than in CuInSe2 or CdTe, the depletion field is entirely in the absorber film where electron-hole pairs are generated (Birkmire and Eser, 1997). After solar cell completion the photovoltaic parameters like Isc, Voc, FF and conversion efficiency were tested by doing the I-V characterization for the two structures; CdTe and Cu(In,Ga)Se2. All

The substrate structure of a Cu(In,Ga)Se2 thin film based solar cell is composed of a soda lime glass substrate, coated with a sputtered 0.7 – 1 m Mo layer as the back contact. After the thermal co-evaporation of Cu(InGa)Se2 deposition, the junction is formed by chemically bath depositing the CdS with thickness 30 - 50 nm. Then a high-resistance (HR) ZnO layer and a doped high-conductivity ZnO:Al layer are subsequently deposited, usually using the sputtering technique. Finally, the deposition of a current-collecting grid of Ni/Al completes the device as shown in figure 12. The total cell area is defined by removing the layers on top

**5. Processing of Cu(In,Ga)Se2 and CdTe thin films into solar cells** 

and the CdS/CdTe interface is reorganized.

**5.1 Cu(In,Ga)Se2/CdS thin film solar cells** 

the parameters were measured under AM1.5 illumination.

of the Mo outside the cell area by mechanical scribing.

Fig. 12. Schematic configuration of a typical Cu(In,Ga)Se2 thin film solar cell

**5.1.1 Discussion on the Cu(In,Ga)Se2 thin film based solar cells results** 

Two Cu(In,Ga)Se2 samples were used to be processed into solar cell devices: sample JS17 had a CdS layer prepared with a recipe based on CdCl2 and sample JS18 with a recipe based on CdSO4 as the Cd source. The *J*-*V* parameters for devices JS17 are: area = 0.47 cm2, *V*oc = 536 mV, *J*sc = 31.70 mA/cm2, fill factor = 64.0 %, and = 10.9 % (see figure 13) and for JS18 are: area

block and the other is the substrate block. Figure 11 shows the block diagram of the CSVT system used to prepare the CdTe thin films. Between the source graphite block "A" and the substrate graphite block "B" is located the graphite boat that contains the material to be sublimated, and on top of this boat the substrate is located, in a very close proximity or close spaced. The material growth is carried out under the presence of an inert atmosphere like argon, nitrogen, etc. The growth rate of the material to be deposited can be controlled by controlling the pressure and gas flow rate. Also this inert gas can be mixed with a reactive gas like oxygen, which benefits the growth of CdTe with the characteristic p-type conductivity. The deposition parameters for this technique are: a) Ts: temperature of the source, b) Tsub: substrate temperature, it has to be lower than the Ts in order to avoid the re-sublimation of the material, c) ds-sub: distance between the material to be deposited and the substrate and d) *Pg:* pressure of the inert gas inside the chamber.

Fig. 11. Schematic diagram of a CSVT system

For the processing of CdTe thin film solar cells, it is necessary to use a *superstrate* structure, so that the CdS is deposited on SnO2:F, in such a way that the growth process allows the film to be deposited over the whole surface, becoming a surface free of holes and caverns without empty spaces among the grains, and with a uniform grain size distribution. It is also required that the CdS layer matches well with the CdTe host, thus favoring a good growing kinetics for CdTe, as well as the formation of the CdSxTe1-x ternary compound in the interface due to the diffusion of S from CdS to CdTe. The high-efficiency CdTe solar cells to date have essentially the same *superstrate* structure. The superstrate structure is composed of a sodalime glass substrate, coated with a SnO2:F; a transparent conductor oxide as the front contact, then a CdS layer is chemically bath deposited, followed by the deposition of a CdTe layer and finally the deposition of two layers of Cu and Au to form the back contact to complete the CdS/CdTe device. In order to achieve solar cells with high conversion efficiencies, the physical and chemical properties of each layer must be optimized (Morales-Acevedo A., 2006). The deposition of CdTe was performed by using CdTe powder 99.999% purity. The deposition atmosphere was a mixture of Ar and O2, with equal partial pressures of O2 and Ar. In all cases the total pressure was 0.1 Torr. Prior to all depositions the system was pumped to 8×10−6 Torr as the base pressure. In the CSVT-HW (hot wall) deposition, the separation between source and substrate was about 1 mm. The deposition time was 3 min for all the samples deposited with substrate and source temperatures of 550 °C and 650 °C, respectively. Under these conditions, CdTe layers of 2 – 4 μm were obtained. The CdTe thin films were also thermally treated with CdCl2. As discussed before, a very important treatment independently of the deposition technique for both CdS and CdTe layers is a thermal annealing after the deposition of CdCl2 on top of the CdTe layer. If the CdCl2

treatment is not performed, the short circuit current density and the efficiency of the solar cell are very low. This treatment consists in depositing 300–400 nm of CdCl2 on top of CdTe with a subsequent annealing at 400 °C during 15–20 min in air, or in an inert gas atmosphere like Ar. During this process the small CdTe grains are put in vapor phase and re-crystallize, giving a better-organized CdTe matrix. The presence of Cl2 could favor the CdTe grain growth by means of a local vapor phase transport. In this way the small grains disappear and the CdS/CdTe interface is reorganized.
