**2.1 CSS Growth of CdTe layers**

CdTe thin films have been deposited by several deposition techniques such as High Vacuum Evaporation (HVE)(Romeo A. et al., 2000), Electro-Deposition (ED)(Josell et al., 2009; Kosyachenko et al., 2006; Levy-Clement, 2008; Lincot, 2005), Chemical Vapour Deposition (CVD)(Yi & Liou, 1995), Metal-Organic Chemical Vapor Deposition (MOCVD)(Barrioz, 2010; Hartley, 2001; Zoppi, 2006), Spray Pyrolysis (Schultz et al., 1997), Screen Printing (Yoshida, 1992 & 1995) Sputtering (Compaan et al., 1993; Hernández-Contreras et al., 2002; Plotnikov et al., 2011) and Close Spaced Sublimation (CSS)(Chu et al., 1991; Romeo N. et al., 2004; Wu, 2004).

Among these techniques, CdTe deposited by CSS allowed to obtain best results for solar cells (world record photovoltaic solar energy conversion ~16.5%; Wu, 2004).

CSS is a physical technique based on a high temperature process. The apparatus is showed in Fig. 2 and it is composed by a vacuum chamber inside which the substrate and the source are placed at a distance of few millimeters (2-7mm). The difference in temperature between the substrate and the source is kept around 50-150°C. Deposition takes place in presence of an inert gas (Ar) or a reactive one (O2, etc.) with a total pressure of about 1-100mbar. The gas creates a counter-pressure which reduces re-evaporation from the substrate and forces the atoms from the source to be scattered many times by the gas atoms before arriving to the substrate, so that the material to be deposited acts like it has a higher dissociation temperature and higher temperature respect to sublimation under vacuum are necessary.

CSS allows to obtain CdTe film with a very high crystalline quality and grains of about one order of magnitude larger (~10m) than films deposited by other deposition techniques (Sputtering, HVE, etc.) and, for this reason, with a low lattice defect density (Romeo A. et al., 2009).

Influence of Post-Deposition Thermal Treatment on the

al., 2006):

it can be re-used in a closed loop without releasing it in atmosphere.

made in order to remove some CdCl2 residuals from the CdTe surface.

**2.3 Back contact deposition and device processing** 

circuits in the cell because it can segregate in grain boundaries.

**2.4 Etching procedures by a Br – methanol mixture** 

Zhou, 2007). Finally, a Mo layer is deposited on top of the cell by sputtering.

clean CdTe surface ready for the back contact deposition.

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 215

stable and inert at the room temperature; moreover, in the case of an industrial production,

We suppose that the following reaction happens at 400°C during the treatment (Romeo N. et

After that, an annealing is carried out at the same temperature of the treatment for few minutes in vacuum (10-5mbar) in order to let CdCl2 residuals re-evaporate and to obtain a

In this work, the TCO/CdS/CdTe system is placed in an evacuable quartz ampoule. Before each run, the ampoule is evacuated with a turbo-molecular pump up to 10−6mbar. As a source of Cl2, a mixture of Ar+HCF2Cl is used. The samples were prepared by changing the HCF2Cl partial pressure. The first one was an untreated sample, while the other four ones were made by choosing four values of HCF2Cl partial pressure that are 20, 30, 40, and 50 mbar and keeping the total pressure (Ar+HCF2Cl) at 400 mbar. An additional specimen, annealed at 30mbar HCF2Cl partial pressure, but with a larger total Ar+HCF2Cl pressure of 800mbar, has been prepared, in order to study the effect of the total pressure on the recrystallization mechanisms. The Ar and the HCF2Cl partial pressures were independently measured by two different capacitance vacuum gauges and monitored by a Varian Multi-Gauge. The quartz tube is put into an oven where a thermocouple is installed in order to control the furnace temperature, which is set at 400°C. The annealing time is 10min for all samples studied in this work. After the treatment, a vacuum for about 10min, keeping the temperature at 400°C, was

The cell is completed by back contact deposition. The formation of a ohmic and stable back contact with CdTe has always been one the most critical points in order to obtain high efficiency CdS/CdTe solar cells. Normally, CdTe is etched in order to get a Tellurium rich surface. After that, a Cu film (~2nm thick) is deposited in order to form a CuxTe compound that is a good non-rectifying contact for CdTe. This procedure has two disadvantages: chemical etching is not convenient because it is not scalable to an industrial level and it is polluting and the Cu thickness is too small to be controlled. In fact, if a thicker Cu film is deposited it could happen that Cu is free from the CuxTe formation and it could cause short

In our work, back contact is composed by the deposition in sequence of three films. A 150- 200nm thick As2Te3 film and a 10-20nm thick Cu film are deposited in sequence on CdTe surface by RF sputtering in Ar flux. When the deposition temperature of Cu is about 150- 200°C, a substitution reaction occurs between Cu and As2Te3 whose final product material is CuxTe, mainly Cu1.4Te is the most stable compound (Romeo N. et al, 2006; Wu et al. 2006;

The possibility to perform depth-dependent CL analyses, by increasing the energy of the incident electrons of the SEM, allows us to correlate the results obtained on the isolated CdTe to an analysis of the electro-optical properties close to the CdTe/CdS interface region of a complete solar cell. To do this, it is necessary to overcome the problem that summing

CdTe(s) + 2Cl2(g) CdCl2(g)+TeCl2(g) CdTe(s)+2Cl2(g). (2)

Fig. 2. Picture of the CSS setup used for growing the CdTe films studied (left); Detail of the growth region of the CSS chamber (right).

In our work, CdTe was deposited in 1mbar Ar atmosphere, keeping the substrate and source temperatures at 500°C and 600°C respectively. The CdTe thickness was 6-8 m.

The high substrate temperature (~500°C) favors the formation of a mixed compound CdSxTe1-x at the interface between CdS and CdTe directly during CdTe deposition, as shown in the phase diagram (Lane et al., 2000). The mixed compound formation, by means of S diffusion toward CdTe and Te diffusion toward CdS, is advantageous in order to get high efficiency CdS/CdTe solar cells. In fact, its formation is required in order to minimize defect density at the interface acting as traps for majority carriers crossing the junction, caused by the lattice mismatch between CdS and CdTe that is about 10%.

#### **2.2 HCF2Cl post-deposition thermal treatment**

The Cl-treatment on CdTe surface is a key point in order to rise the photocurrent and so the efficiency of the solar cell.

During Cl-treatment CdTe goes in vapor phase as explained by the following reaction (McCandless, 2001):

$$\text{CdTe(s)} + \text{CdCl\_2(s)} \rightarrow 2\text{Cd(g)} + \text{(Te\_2(g)} + \text{Cl\_2(g)} \rightarrow \text{CdCl\_2(s)} + \text{CdTe(s)}\tag{1}$$

where s is the solid phase and g is the vapor phase.

After the treatment small grains disappear from CdTe surface and at the same time an increase in grain dimensions and an improvement in crystal organization can be observed. Also an improvement, in the crystal organization of the mixed compound CdSxTe1-x, at the junction, formed during CdTe deposition, can be observed.

Usually Cl-treatment is carried out by depositing on CdTe surface a CdCl2 (thickness more than 100nm) film by evaporation (Potter et al., 2000; Romeo A. et al. 2000; Romeo N. et al. 1999) or by dipping CdTe in a CdCl2-methanol solution (Cruz et al., 1999), then an annealing at ~380-420°C in an Ar atmosphere or in air is required and finally, an etching in Brmethanol or an annealing in vacuum is carried out in order to remove CdCl2 residuals on CdTe surface. The main drawback of this treatment is that CdCl2, being very hygroscopic, could be dangerous either for people and for the environment since it can release free Cd.

We have proposed a new treatment by substituting CdCl2, or CdCl2-methanol solution, and the following etching with a treatment at 400°C in a controlled atmosphere containing a gas belonging to the Freon® family which can free Cl at high temperature. This gas is very

Fig. 2. Picture of the CSS setup used for growing the CdTe films studied (left); Detail of the

In our work, CdTe was deposited in 1mbar Ar atmosphere, keeping the substrate and source temperatures at 500°C and 600°C respectively. The CdTe thickness was 6-8 m. The high substrate temperature (~500°C) favors the formation of a mixed compound CdSxTe1-x at the interface between CdS and CdTe directly during CdTe deposition, as shown in the phase diagram (Lane et al., 2000). The mixed compound formation, by means of S diffusion toward CdTe and Te diffusion toward CdS, is advantageous in order to get high efficiency CdS/CdTe solar cells. In fact, its formation is required in order to minimize defect density at the interface acting as traps for majority carriers crossing the junction, caused by

The Cl-treatment on CdTe surface is a key point in order to rise the photocurrent and so the

During Cl-treatment CdTe goes in vapor phase as explained by the following reaction

CdTe(s)+CdCl2(s) 2Cd(g)+½Te2(g)+Cl2(g) CdCl2(s)+CdTe(s), (1)

After the treatment small grains disappear from CdTe surface and at the same time an increase in grain dimensions and an improvement in crystal organization can be observed. Also an improvement, in the crystal organization of the mixed compound CdSxTe1-x, at the

Usually Cl-treatment is carried out by depositing on CdTe surface a CdCl2 (thickness more than 100nm) film by evaporation (Potter et al., 2000; Romeo A. et al. 2000; Romeo N. et al. 1999) or by dipping CdTe in a CdCl2-methanol solution (Cruz et al., 1999), then an annealing at ~380-420°C in an Ar atmosphere or in air is required and finally, an etching in Brmethanol or an annealing in vacuum is carried out in order to remove CdCl2 residuals on CdTe surface. The main drawback of this treatment is that CdCl2, being very hygroscopic, could be dangerous either for people and for the environment since it can release free Cd. We have proposed a new treatment by substituting CdCl2, or CdCl2-methanol solution, and the following etching with a treatment at 400°C in a controlled atmosphere containing a gas belonging to the Freon® family which can free Cl at high temperature. This gas is very

growth region of the CSS chamber (right).

the lattice mismatch between CdS and CdTe that is about 10%.

**2.2 HCF2Cl post-deposition thermal treatment** 

where s is the solid phase and g is the vapor phase.

junction, formed during CdTe deposition, can be observed.

efficiency of the solar cell.

(McCandless, 2001):

stable and inert at the room temperature; moreover, in the case of an industrial production, it can be re-used in a closed loop without releasing it in atmosphere.

We suppose that the following reaction happens at 400°C during the treatment (Romeo N. et al., 2006):

$$\text{CdTe(s)} + 2\text{Cl}\_2(\text{g}) \rightarrow \text{CdCl}\_2(\text{g}) + \text{TeCl}\_2(\text{g}) \rightarrow \text{CdTe(s)} + 2\text{Cl}\_2(\text{g}).\tag{2}$$

After that, an annealing is carried out at the same temperature of the treatment for few minutes in vacuum (10-5mbar) in order to let CdCl2 residuals re-evaporate and to obtain a clean CdTe surface ready for the back contact deposition.

In this work, the TCO/CdS/CdTe system is placed in an evacuable quartz ampoule. Before each run, the ampoule is evacuated with a turbo-molecular pump up to 10−6mbar. As a source of Cl2, a mixture of Ar+HCF2Cl is used. The samples were prepared by changing the HCF2Cl partial pressure. The first one was an untreated sample, while the other four ones were made by choosing four values of HCF2Cl partial pressure that are 20, 30, 40, and 50 mbar and keeping the total pressure (Ar+HCF2Cl) at 400 mbar. An additional specimen, annealed at 30mbar HCF2Cl partial pressure, but with a larger total Ar+HCF2Cl pressure of 800mbar, has been prepared, in order to study the effect of the total pressure on the recrystallization mechanisms. The Ar and the HCF2Cl partial pressures were independently measured by two different capacitance vacuum gauges and monitored by a Varian Multi-Gauge. The quartz tube is put into an oven where a thermocouple is installed in order to control the furnace temperature, which is set at 400°C. The annealing time is 10min for all samples studied in this work. After the treatment, a vacuum for about 10min, keeping the temperature at 400°C, was made in order to remove some CdCl2 residuals from the CdTe surface.
