**2. Materials growth and devices preparation**

CdTe is a II-VI semiconductor with a direct energy-gap of 1.45eV at room temperature that, combined with the very high absorption coefficient, 104-105 cm-1 in the visible light range, makes it one of the ideal materials for photovoltaic conversion, because a layer thickness of a few micrometers is sufficient to absorb 90% of incident photons. For thin film solar cells is required a p-type material, which is part of the p-CdTe/n-CdS heterojunction. The electrical properties control was easily developed for single-crystal CdTe, grown from the melt or vapor, at high temperature (above 1000°C), by introducing doping elements during growth. On the contrary, in polycrystalline CdTe, where grain boundaries are present, all metallic dopants tend to diffuse along the grain boundaries, making the doping unable to modify the electrical properties and producing shunts in the device.

CdTe solar cell is composed by four parts (Fig. 1) deposited on a substrate like Soda-Lime Glass (SLG):

1. The *Front Contact* is composed by a Transparent Conducting Oxide (TCO) that is a doped metallic oxide like In2O3:Sn (ITO)(Romeo N. et al., 2010) , ZnO:Al (AZO)(Perrenoud et al., 2011), CdSnO4 (CTO)(Wu, 2004), SnO2:F (FTO) (Ferekides et al., 2000), etc.; and a very thin layer of a resistive metal oxide like SnO2 (Ferekides et al., 2000), ZnO (Perrenoud et al., 2011; Romeo N. et al. 2010), Zn2SnO4 (Wu et al. 2001b). The role of the latter film is to hinder the diffusion of contaminant species from TCO and SLG toward the upper layers of the cell such as the window layer (CdS) or the absorber one (CdTe). Moreover it separates TCO and CdS in order to limit the effects of pinholes that could be present in CdS film.

In our work, TCO is made by 400nm thick ITO film and 300nm thick ZnO both of them deposited by sputtering. ITO showed a sheet resistance of about 5/cm2, while the resistivity of ZnO was on the order of 103 ·cm.


this procedure, corresponding to 40 mbar HCF2Cl partial pressure in the 400mbar Ar total

The solar cells were then submitted to an etching procedure in a Br–methanol mixture at 10% to eliminate the back contacts and part of the CdTe material in some portion of the specimens. On the beveled surface, CL analyses have been performed again in order to extract information as close as possible to the CdTe/CdS interface and to compare the

Finally, a model of the electronic levels present in the CdTe bandgap before and after the HCF2Cl treatment has been proposed as well as a model of the interface region modifications

CdTe is a II-VI semiconductor with a direct energy-gap of 1.45eV at room temperature that, combined with the very high absorption coefficient, 104-105 cm-1 in the visible light range, makes it one of the ideal materials for photovoltaic conversion, because a layer thickness of a few micrometers is sufficient to absorb 90% of incident photons. For thin film solar cells is required a p-type material, which is part of the p-CdTe/n-CdS heterojunction. The electrical properties control was easily developed for single-crystal CdTe, grown from the melt or vapor, at high temperature (above 1000°C), by introducing doping elements during growth. On the contrary, in polycrystalline CdTe, where grain boundaries are present, all metallic dopants tend to diffuse along the grain boundaries, making the doping unable to modify the

CdTe solar cell is composed by four parts (Fig. 1) deposited on a substrate like Soda-Lime

1. The *Front Contact* is composed by a Transparent Conducting Oxide (TCO) that is a doped metallic oxide like In2O3:Sn (ITO)(Romeo N. et al., 2010) , ZnO:Al (AZO)(Perrenoud et al., 2011), CdSnO4 (CTO)(Wu, 2004), SnO2:F (FTO) (Ferekides et al., 2000), etc.; and a very thin layer of a resistive metal oxide like SnO2 (Ferekides et al., 2000), ZnO (Perrenoud et al., 2011; Romeo N. et al. 2010), Zn2SnO4 (Wu et al. 2001b). The role of the latter film is to hinder the diffusion of contaminant species from TCO and SLG toward the upper layers of the cell such as the window layer (CdS) or the absorber one (CdTe). Moreover it separates TCO and CdS in order to limit the effects of

In our work, TCO is made by 400nm thick ITO film and 300nm thick ZnO both of them deposited by sputtering. ITO showed a sheet resistance of about 5/cm2, while the

2. The *Window Layer* is usually an n-type semiconductor; Cadmium Sulphide (CdS) is the most suitable material for CdTe-based solar cells, thanks to its large bandgap (2.4eV at room temperature) and because it grows with n-type conductivity without the introduction of any dopants. Here, CdS film was deposited by reactive RF sputtering in

3. The *Absorber Layer* is a 6-10m thick film. The deposition techniques and the treatment

4. The *Back Contact* is composed by a buffer layer and a Mo or W film. The utility of the

pressure, has =14.8%, JSC=26.2mA/cm2, VOC = 820mV and ff=0.69.

results to the depth-dependent CL analyses.

**2. Materials growth and devices preparation** 

electrical properties and producing shunts in the device.

pinholes that could be present in CdS film.

on CdTe will be explained deeply later.

resistivity of ZnO was on the order of 103 ·cm.

presence of Ar+10%CHF3 flux. Its nominal thickness was 80nm.

buffer layer is to form a low resistive and ohmic contact on CdTe.

due to the annealing.

Glass (SLG):

The cell is completed by a scribing made on the edge of all the cells in order to electrically separate the front contact from the back one.

Fig. 1. Schematic representation of the CdS/CdTe solar cell heterostructure. The layers succession and thicknesses are the ones used in the present work.
