**4. Results and discussion**

218 Solar Cells – Thin-Film Technologies

typically ranging between a few microns and a few tens of nanometers), equipped with a Gatan MonoCL2 system (Fig. 3). The spectra, as well the panchromatic and monochromatic images, have been acquired using a dispersion system equipped with three diffraction gratings and a system of a Hamamatsu multi-alkali photomultiplier and a couple of liquid nitrogen cooled (Ge and InGaAs) solid state detectors. This experimental set up provides a spectral resolution of 2Å and a detectable 250-2200nm (0.6–4.9eV) wavelength range. By this configuration it is possible to cover a large part of the luminescence emissions of the III-V and II-VI compound semiconductors. In particular, all the possible transitions in CdTe can be detected: from the excitonic lines (around 1.59eV) down to the emissions involving midgap levels (0.8-0.9eV). Additionally, it is possible to change the temperature of the samples in the range 5-300K by a temperature controller interlocked with the sample-holder, thanks

to a refrigerating system operating with liquid Nitrogen and liquid Helium.

Fig. 3. Schematic representation, not in scale, of the CL experimental setup used in this work.

The setup used for acquiring XRD profiles was an X-Ray Diffractometer Thermo arl X'tra, vertical goniometer, theta-theta, operating in an angular range between -8° and 160°, equipped with an X-ray tube, Cu K-alpha and a solid state Si:Li detector. The angular range chosen, between 15° and 80°, assured the detection of all the contributions from the main

Light J-V measurements were performed by an Oriel Corporation Solar Cells Test System model 81160, in order to measure the photovoltaic parameters such as the short-circuit current density (JSC), open circuit voltage (VOC), fill factor (ff) and conversion efficiency ()

Dark measurements were carried out by a Keithley 236 source system in order to measure the diode quality factor (A) of the cells as a function of the HCF2Cl partial pressure during

> *kT J J*

0

(3)

1 ln( 1)

the CdTe treatment. A can be calculated from the diode equation in the dark:

*qV <sup>A</sup>*

Bragg diffractions of CdTe: (111), (220), (311), (400), (331), (422), (511).

**3.2 X-Ray diffraction** 

of the solar cells.

**3.3 Electrical characterization** 

All the CdTe thin films were deposited on SLG/ZnO substrate by CSS; the layer thickness was about 8m. Complete solar cells have been realized by depositing ZnO, CdS and CdTe in the identical conditions and by adding the back contact, as described in paragraphs 2.1 and 2.3. The CdTe films as well as the complete devices were annealed in Ar+HCF2Cl atmosphere (see for details paragraph 2.2), by increasing the HCF2Cl partial pressure from 20mbar to 50mbar and keeping the temperature at 400°C for all samples. The annealing conditions used have been summarized in table 1.


Table 1. Summary of the annealing conditions used to treat the samples studied in this work

#### **4.1 Influence of annealing on the CdTe material properties**

The XRD profiles of all the CdTe films were acquired in the angular range 5°<2q<80°, from this analysis can be deduced that the films have a zinc-blend structure with a preferential orientation along the (111) direction. In all the XRD patterns the peaks related to (220), (311), (400), (331), (422) and (511) reflections are also visible. In addition a peak at 22.77° attributed to the Te2O5 oxide and a peak at 34.34° related to the ZnO (002) reflection are detected. In Fig. 4, only the most representative XRD profiles of the untreated CdTe and of the samples annealed with 40 mbar HCF2Cl partial pressure were shown.

The preferential orientation of each film is analyzed by using the texture coefficient Chkl, calculated by means of the following formula (Barret & Massalski 1980):

$$\mathbf{C}\_{\rm hkl} = \frac{\mathbf{I}\_{\rm hkl} / \mathbf{I}\_{\rm hkl}^{0}}{\frac{1}{N} \sum\_{\rm N} \mathbf{I}\_{\rm hkl} / \mathbf{I}\_{\rm hkl}^{0}},\tag{4}$$

Influence of Post-Deposition Thermal Treatment on the

C111 texture

0,0

and the (111) one for all the studied CdTe thin films

was reduced, producing a thinner distribution of the histogram columns.

0,2

0,4

(hkl)/(111) Intensity ratios

0,6

0,8

1,0 reference

Sample

SEM images.

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 221

UT 2,02 0,52 11.7 0.9 0.72 F30L 1,12 0,29 2.3 3.39 F30H 1,7 0,42 10.8 10.75 F40 1,15 0,31 11.2 14.48 15.47 F50 0,56 0,36 14.74 19.97 Table 2. Summary of the results obtained by processing the XRD profiles, CL spectra and

(111) (220) (311) (400) (331) (422) ZnO Te2

Fig. 5. Plot of intensity ratios among each diffraction (220), (311), (400), (331), (422) and (511)

The loss of preferential orientation due to HCF2Cl annealing results in a slight modification of the CdTe morphology after the thermal treatment. The untreated CdTe films showed already large grains, as visible in the SEM image of Fig. 6 a. The average grain size obtained by processing the images was 11.7m and the largest grains reached 20.4m. The material treated with 40mbar HCF2Cl partial pressure showed grains with dimensions similar (avg = 11.2m) to those of the untreated one (Fig. 6 b). The observed average size confirmed that CSS grown CdTe did not show grain size increase after annealing in presence of chlorine as already described in the literature by several authors (Moutinho et al. 1998). Grain dimensions distribution extracted from the SEM images has been represented in histograms showed in Fig. 7 a and b. It could be observed that the small grains density in the HCF2Cl treated material

On the contrary, all the Freon treated CdTe showed a remarkable grain shape variation with respect to the untreated sample where most of the grains appeared as tetragonal pyramids with the vertex aligned on the growth direction (Fig. 8 a). This shape justified their high preferential orientation along the (111) direction. This grain shape appeared clearly modified in the HCF2Cl annealed films. They were more rounded and the pyramids seem to

Diffraction planes

coefficient Average grain size

XRD results Morphology 1.4 eV/NBE CL intensity

800 mbar Total pressure 30 mbar HCF2

400 mbar Total pressure 30 mbar HCF2

40 mbar HCF2

50 mbar HCF2

untreated CdTe

Cl

Cl

Cl

Cl

(m) 12 keV 25 keV

O5

where Ihkl is the detected intensity of a generic peak in the XRD spectra, I0 hkl is the intensity of the corresponding peak for a completely randomly oriented CdTe powder (values taken from the JCPDS) and N the number of reflections considered in the calculation. Chkl values above the unity represented a preferential orientation along the crystallographic direction indicated by the hkl indices. The texture coefficients C111, calculated by the formula Nr 4 for all the samples, are summarized in table 2, together with the CL intensity ratios. A better comprehension of the orientation of each thin film as a whole can be obtained by the standard deviation of the Chkl coefficients. Each value has been calculated by the following formula:

$$\sigma = \sqrt{\frac{\sum\_{\text{N}} (\mathbf{C}\_{\text{hkl}} - \mathbf{1})^2}{\mathbf{N}}} \tag{5}$$

A complete randomly oriented film is expected to have a value as close as possible to 0. The untreated CdTe thin film shows the highest preferential orientation along the (111) direction with a texture coefficient C111=2.02. The effect of HCF2Cl treatment is highlighted by a decrease of the (111) related intensity and by an increase of the relative intensities of the additional reflections (220), (311), (400), (331), (422) and (511), detected. The calculated value for the untreated CdTe is also the highest one (=0.52) demonstrating the oriented status of that film. This behavior is evidenced in Fig. 5, in which the calculated peak intensity ratios between each (220), (311), (400), (331), (422) and (511) additional reflection and the (111) one are plotted.

The combined effect of HCF2Cl partial pressure and the total gas pressure, in the annealing chamber, could be also evidenced by comparing the C111 and values of the CdTe films treated by 30mbar HCF2Cl, but higher total pressure (800mbar), sample F30H in table 2. Its values were higher than the CdTe treated with the same partial pressure and lower total pressure (sample F30L in table 1), but similar to the untreated CdTe.

Fig. 4. XRD profiles of the untreated CdTe thin film compared to the sample annealed with 400 mbar Ar+Freon total pressure in the annealing chamber and 40mbar HCF2Cl partial pressure.

of the corresponding peak for a completely randomly oriented CdTe powder (values taken from the JCPDS) and N the number of reflections considered in the calculation. Chkl values above the unity represented a preferential orientation along the crystallographic direction indicated by the hkl indices. The texture coefficients C111, calculated by the formula Nr 4 for all the samples, are summarized in table 2, together with the CL intensity ratios. A better comprehension of the orientation of each thin film as a whole can be obtained by the standard deviation of the Chkl coefficients. Each value has been calculated by the following

2

(5)

511 422

Cl

<sup>N</sup> hkl (C 1)

N

A complete randomly oriented film is expected to have a value as close as possible to 0. The untreated CdTe thin film shows the highest preferential orientation along the (111) direction with a texture coefficient C111=2.02. The effect of HCF2Cl treatment is highlighted by a decrease of the (111) related intensity and by an increase of the relative intensities of the additional reflections (220), (311), (400), (331), (422) and (511), detected. The calculated value for the untreated CdTe is also the highest one (=0.52) demonstrating the oriented status of that film. This behavior is evidenced in Fig. 5, in which the calculated peak intensity ratios between each (220), (311), (400), (331), (422) and (511) additional reflection

The combined effect of HCF2Cl partial pressure and the total gas pressure, in the annealing chamber, could be also evidenced by comparing the C111 and values of the CdTe films treated by 30mbar HCF2Cl, but higher total pressure (800mbar), sample F30H in table 2. Its values were higher than the CdTe treated with the same partial pressure and lower total

20 30 40 50 60 70 80

angles (Degrees)

Fig. 4. XRD profiles of the untreated CdTe thin film compared to the sample annealed with 400 mbar Ar+Freon total pressure in the annealing chamber and 40mbar HCF2Cl partial

311

220

002 ZnO

400 331

 untreated CdTe 40 mbar HCF2

hkl is the intensity

where Ihkl is the detected intensity of a generic peak in the XRD spectra, I0

σ

pressure (sample F30L in table 1), but similar to the untreated CdTe.

111

0

50

Te2 O5

100

Counts (a.u.)

pressure.

250 300 350

formula:

and the (111) one are plotted.


Table 2. Summary of the results obtained by processing the XRD profiles, CL spectra and SEM images.

Fig. 5. Plot of intensity ratios among each diffraction (220), (311), (400), (331), (422) and (511) and the (111) one for all the studied CdTe thin films

The loss of preferential orientation due to HCF2Cl annealing results in a slight modification of the CdTe morphology after the thermal treatment. The untreated CdTe films showed already large grains, as visible in the SEM image of Fig. 6 a. The average grain size obtained by processing the images was 11.7m and the largest grains reached 20.4m. The material treated with 40mbar HCF2Cl partial pressure showed grains with dimensions similar (avg = 11.2m) to those of the untreated one (Fig. 6 b). The observed average size confirmed that CSS grown CdTe did not show grain size increase after annealing in presence of chlorine as already described in the literature by several authors (Moutinho et al. 1998). Grain dimensions distribution extracted from the SEM images has been represented in histograms showed in Fig. 7 a and b. It could be observed that the small grains density in the HCF2Cl treated material was reduced, producing a thinner distribution of the histogram columns.

On the contrary, all the Freon treated CdTe showed a remarkable grain shape variation with respect to the untreated sample where most of the grains appeared as tetragonal pyramids with the vertex aligned on the growth direction (Fig. 8 a). This shape justified their high preferential orientation along the (111) direction. This grain shape appeared clearly modified in the HCF2Cl annealed films. They were more rounded and the pyramids seem to

Influence of Post-Deposition Thermal Treatment on the

annealed by 40mbar HCF2Cl partial pressure.

al. 2007).

summarized in Fig. 10.

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 223

**(a) (b)**

 Fig. 8. a) SEM image of a typical pyramidal grain oriented along the (111) growth direction of the untreated film; b) SEM image showing pyramidal grains with terraces of the CdTe

emission could be attributed to radiative recombination levels induced by impurities, like Cu, unintentionally incorporated during the CdTe deposition, or diffused from the front contact and buffer layers during the high temperature growth. The 1.47eV peak has been previously observed in polycrystalline CdTe (Cárdenas-García et al. 2005) and ascribed to the dislocation related Y-emission. In our untreated material, a clear dependence of this emission on the dislocations has not been demonstrated, but the disappearance of this peak in the annealed, high crystalline quality CdTe supports this attribution (Armani et

The HCF2Cl annealing effect on the CdTe recombination mechanisms was studied by both CL spectroscopy and monochromatic (monoCL) mapping. CL spectra showed a drastic difference between untreated and HCF2Cl annealed samples, as visible in Fig. 9. All the HCF2Cl treated samples showed, in addition to the NBE emission, a broad CL band centered at 1.4eV which intensity increased by increasing the HCF2Cl partial pressure, suggesting a strong dependence of this emission on the annealing. The literature studies on both single–crystal and polycrystalline CdTe (Consonni et al. 2006; Krustok et al. 1997) showed photoluminescence (PL) and CL bands centered at energies close to 1.4eV; their origin was attributed to a radiative recombination center like the well known A-center, due to a complex between a Cd vacancy (VCd) and a Cl impurity, in Cl-doped CdTe (Meyer et al. 1992; Stadler et al. 1995). The clear correlation between the 1.4eV band and the HCF2Cl treatment supported the attribution of the 1.4eV band observed in our CdTe films to a complex like the A-centre. Either Cl or F impurities could be the origin of the level responsible for this transition. Several impurities, among which Cl and F, created acceptor levels with very similar energy values above the valence band edge as reported by Stadler et al. (Stadler W. et al. 1995). In particular the levels due to Cl and F differ solely by 9meV. The CL spectral resolution, lower than the PL one, did not allow determining the exact energy position of the 1.4 eV band with a precision better than 0.01eV. On this basis a clear attribution, to Cl or F, of the impurity creating the complex together to the VCd was impossible. The 1.4eV/NBE CL intensity ratios represented a tool to study the concentration of the VCd-Cl(F) complex responsible for the 1.4eV band; the comparison among the untreated and the annealed CdTe results obtained at 25keV have been

be made up by a superposition of "terraces" (Fig. 6 b). This morphology change could be correlated to the C111 texture coefficient decrease. Two possible mechanisms related to the HCF2Cl annealing could be invoked: a re-crystallization effect or an "etching-like" erosion of the grain surface. The unmodified grain size and the appearance of the terraces seem to indicate that the latter phenomenon occurred during the Freon treatment.

Fig. 6. SEM image of the polycrystalline CdTe surface morphology: a) untreated film; b) annealed with 40mbar HCF2Cl

Fig. 7. Histograms of the grain size as obtained from the SEM images: a) untreated CdTe; b) CdTe annealed by 40mbar HCF2Cl partial pressure.

The effect of thermal treatment on the CdTe bulk electro-optical properties has been studied by acquiring CL spectra at electron beam energy (EB) of 25keV, corresponding to a maximum penetration depth of about 2.5m. The CL generation volume dimensions were calculated by means of a numerical approach based on random walk Monte Carlo simulation developed in our laboratory (Grillo et al. 2003). The low temperature (77 K) spectrum of a 240x180 m2 region of the untreated CdTe showed the clear near bend edge (NBE) emission centered at 1.57eV. The temperature is too high to discriminate the acceptor from the donor bound excitonic line, we supposed they were superimposed underneath the NBE band. In addition to the NBE emission, two weak bands, centered at 1.47eV and 1.35eV respectively, were also detected. The 1.35eV and 1.47eV CL peaks were visible only in the untreated CdTe and their origin was not related to the HCF2Cl treatment. The 1.35eV

be made up by a superposition of "terraces" (Fig. 6 b). This morphology change could be correlated to the C111 texture coefficient decrease. Two possible mechanisms related to the HCF2Cl annealing could be invoked: a re-crystallization effect or an "etching-like" erosion of the grain surface. The unmodified grain size and the appearance of the terraces seem to

Fig. 6. SEM image of the polycrystalline CdTe surface morphology: a) untreated film; b)

Fig. 7. Histograms of the grain size as obtained from the SEM images: a) untreated CdTe; b)

The effect of thermal treatment on the CdTe bulk electro-optical properties has been studied by acquiring CL spectra at electron beam energy (EB) of 25keV, corresponding to a maximum penetration depth of about 2.5m. The CL generation volume dimensions were calculated by means of a numerical approach based on random walk Monte Carlo simulation developed in our laboratory (Grillo et al. 2003). The low temperature (77 K) spectrum of a 240x180 m2 region of the untreated CdTe showed the clear near bend edge (NBE) emission centered at 1.57eV. The temperature is too high to discriminate the acceptor from the donor bound excitonic line, we supposed they were superimposed underneath the NBE band. In addition to the NBE emission, two weak bands, centered at 1.47eV and 1.35eV respectively, were also detected. The 1.35eV and 1.47eV CL peaks were visible only in the untreated CdTe and their origin was not related to the HCF2Cl treatment. The 1.35eV

0 4 8 12 16 20 24 28 Grain size (m)

40 mbar HCF2

average size=11.2 m

Cl

 untreated CdTe average size=11.7 m

**(a) (b)**

annealed with 40mbar HCF2Cl

0 4 8 12 16 20 24 28 Grain size (m)

CdTe annealed by 40mbar HCF2Cl partial pressure.

indicate that the latter phenomenon occurred during the Freon treatment.

**(a) (b)**

Fig. 8. a) SEM image of a typical pyramidal grain oriented along the (111) growth direction of the untreated film; b) SEM image showing pyramidal grains with terraces of the CdTe annealed by 40mbar HCF2Cl partial pressure.

emission could be attributed to radiative recombination levels induced by impurities, like Cu, unintentionally incorporated during the CdTe deposition, or diffused from the front contact and buffer layers during the high temperature growth. The 1.47eV peak has been previously observed in polycrystalline CdTe (Cárdenas-García et al. 2005) and ascribed to the dislocation related Y-emission. In our untreated material, a clear dependence of this emission on the dislocations has not been demonstrated, but the disappearance of this peak in the annealed, high crystalline quality CdTe supports this attribution (Armani et al. 2007).

The HCF2Cl annealing effect on the CdTe recombination mechanisms was studied by both CL spectroscopy and monochromatic (monoCL) mapping. CL spectra showed a drastic difference between untreated and HCF2Cl annealed samples, as visible in Fig. 9. All the HCF2Cl treated samples showed, in addition to the NBE emission, a broad CL band centered at 1.4eV which intensity increased by increasing the HCF2Cl partial pressure, suggesting a strong dependence of this emission on the annealing. The literature studies on both single–crystal and polycrystalline CdTe (Consonni et al. 2006; Krustok et al. 1997) showed photoluminescence (PL) and CL bands centered at energies close to 1.4eV; their origin was attributed to a radiative recombination center like the well known A-center, due to a complex between a Cd vacancy (VCd) and a Cl impurity, in Cl-doped CdTe (Meyer et al. 1992; Stadler et al. 1995). The clear correlation between the 1.4eV band and the HCF2Cl treatment supported the attribution of the 1.4eV band observed in our CdTe films to a complex like the A-centre. Either Cl or F impurities could be the origin of the level responsible for this transition. Several impurities, among which Cl and F, created acceptor levels with very similar energy values above the valence band edge as reported by Stadler et al. (Stadler W. et al. 1995). In particular the levels due to Cl and F differ solely by 9meV. The CL spectral resolution, lower than the PL one, did not allow determining the exact energy position of the 1.4 eV band with a precision better than 0.01eV. On this basis a clear attribution, to Cl or F, of the impurity creating the complex together to the VCd was impossible. The 1.4eV/NBE CL intensity ratios represented a tool to study the concentration of the VCd-Cl(F) complex responsible for the 1.4eV band; the comparison among the untreated and the annealed CdTe results obtained at 25keV have been summarized in Fig. 10.

Influence of Post-Deposition Thermal Treatment on the

depth, but not disappeared in the first 4m of the film.

8 keV

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 225

samples showed a dependence of the 1.4eV emission intensity on the HCF2Cl partial pressure similar to the highly depth 25 keV analysis. A more detailed depth-resolved study of the VCd-Cl(F) complex distribution is performed by acquiring CL spectra at different EB from 8 to 36 keV, corresponding to a probing depth between 0,36 and 4,6 m. Fig. 11 showed the CL spectra, normalized to the NBE intensity, in order to better highlight the 1.4eV band intensity variations. The 1.4eV emission intensity decreased till the generation volume of the CL signal extended to about 2m, then kept almost constant for the following 2m. This behavior could be more clearly appreciated in the inset of the figure where the 1.4eV/NBE integrated intensity ratio has been shown. Possible influence of material quality inhomogeneities along the deposition axis could be neglected because the NBE peak position did not change in the studied depth range as well as its intensity showed very small and random variations. The VCd-Cl(F) complex density was the highest close to the CdTe surface, due to the maximum effectiveness of the HCF2Cl treatment however it decreased in

Fig. 11. Depth-dependent CL spectra acquired on the 40 mbar HCF2Cl partial pressure annealed CdTe, by increasing Eb from 8 to 36 keV; in the inset the plot of the 1.4 eV/NBE

1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7

Energy (eV)

**36**

**18**

**12**

**8**

36 keV

CL spectra at T=77K

**24**

Eb

Eb

Eb

Eb

Eb

Eb

=8 keV - Re

=12 keV - Re

=18 keV - Re

=24 keV - Re

=30 keV - Re

=36 keV - Re

=0.38 m

=0.69 m

=1.24 m

=1.9 m

=2.8 m

=3.7 m

**30**

The maximum EB suitable for the SEM used for this work (36keV) was not high enough to investigate the whole CdTe film, limiting the results to about a half of the material thickness. For this reason the samples were etched in order to eliminate a portion of CdTe, leaving the material near the CdTe/CdS interface free. After the etching procedure the decreasing thickness of CdTe film edge showed a slightly sloped surface extending from the upper surface of the front contact, down to about 1m above the CdTe/CdS interface. The SEM

integrated intensity ratios as a function of EB has been shown.

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

injection density per unit volume constant

Maximum penetration depth (m)

0

2

4

6

1.4 eV / NBE Intensity ratio (a.u.)

CL Intensity (a.u.)

8

10

12

Fig. 9. Comparison among the low temperature (77 K) CL spectra (Eb=25keV) of untreated CdTe and samples annealed at a HCF2Cl partial pressure of 30, 40 and 50 mbar.

Fig. 10. Plot of the 1.4eV/NBE integrated intensity ratios. The experimental points have been calculated from spectra acquired in various regions of each CdTe film at Eb= 25keV.

One of the peculiarities of the CL technique is the possibility to increase the probing depth within the studied materials, by increasing EB, by keeping the injection density in the generation volume constant. The effect of annealing on the CdTe luminescence behavior was expected to be more effective close to the CdTe surface. For this reason, CL spectra at lower beam energy (EB=12keV), corresponding to a maximum penetration depth of 900nm below the CdTe surface, were collected. The comparison among untreated and annealed

**1.4 eV**

**1,1 1,2 1,3 1,4 1,5 1,6**

Fig. 9. Comparison among the low temperature (77 K) CL spectra (Eb=25keV) of untreated

CdTe and samples annealed at a HCF2Cl partial pressure of 30, 40 and 50 mbar.

**HCF2**

untreated **Ar 400 mbar**

calculated from spectra acquired in various regions of each CdTe film at Eb= 25keV.

**Cl 30 mbar Ar 800 mbar**

Energy (eV)

**HCF2**

**Cl 40 mbar Ar 400 mbar**

**HCF2**

**Cl 30 mbar**

0 10 20 30 40 50

Fig. 10. Plot of the 1.4eV/NBE integrated intensity ratios. The experimental points have been

One of the peculiarities of the CL technique is the possibility to increase the probing depth within the studied materials, by increasing EB, by keeping the injection density in the generation volume constant. The effect of annealing on the CdTe luminescence behavior was expected to be more effective close to the CdTe surface. For this reason, CL spectra at lower beam energy (EB=12keV), corresponding to a maximum penetration depth of 900nm below the CdTe surface, were collected. The comparison among untreated and annealed

Cl partial pressure (mbar)

**y band 1.47 eV**

**HCF2**

**Cl 50 mbar Ar 400 mbar**

**NBE 1.57 eV**

0

0

5

10

1.4 eV / NBE ratio

15

20

Eb

25

**untreated**

data calculated from CL spectra acquired at:

=25 keV - depth 2.8 m

HCF2

**30 mbar 40 mbar**

**50 mbar**

20

40

CL Intensity (a.u.)

60

80

**Eb**

**=25 keV T=77 K**

samples showed a dependence of the 1.4eV emission intensity on the HCF2Cl partial pressure similar to the highly depth 25 keV analysis. A more detailed depth-resolved study of the VCd-Cl(F) complex distribution is performed by acquiring CL spectra at different EB from 8 to 36 keV, corresponding to a probing depth between 0,36 and 4,6 m. Fig. 11 showed the CL spectra, normalized to the NBE intensity, in order to better highlight the 1.4eV band intensity variations. The 1.4eV emission intensity decreased till the generation volume of the CL signal extended to about 2m, then kept almost constant for the following 2m. This behavior could be more clearly appreciated in the inset of the figure where the 1.4eV/NBE integrated intensity ratio has been shown. Possible influence of material quality inhomogeneities along the deposition axis could be neglected because the NBE peak position did not change in the studied depth range as well as its intensity showed very small and random variations. The VCd-Cl(F) complex density was the highest close to the CdTe surface, due to the maximum effectiveness of the HCF2Cl treatment however it decreased in depth, but not disappeared in the first 4m of the film.

Fig. 11. Depth-dependent CL spectra acquired on the 40 mbar HCF2Cl partial pressure annealed CdTe, by increasing Eb from 8 to 36 keV; in the inset the plot of the 1.4 eV/NBE integrated intensity ratios as a function of EB has been shown.

The maximum EB suitable for the SEM used for this work (36keV) was not high enough to investigate the whole CdTe film, limiting the results to about a half of the material thickness. For this reason the samples were etched in order to eliminate a portion of CdTe, leaving the material near the CdTe/CdS interface free. After the etching procedure the decreasing thickness of CdTe film edge showed a slightly sloped surface extending from the upper surface of the front contact, down to about 1m above the CdTe/CdS interface. The SEM

Influence of Post-Deposition Thermal Treatment on the

the 40 mbar HCF2Cl partial pressure etched solar cell.

**(a)**

**(b) (c)**

E=1.4eV emission energy.

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 227

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4

Eb=15 keV - depth=1.1 m 3 m far 2 m far 1 m far maximum etched region

**(a) (b)** 

Normalized CL Intensity (a.u.)

Fig. 13. a) SEM image of the etched surface of the solar cell annealed with 40 mbar HCF2Cl partial pressure; b) comparison among the low temperature (77K) CL spectra (Eb=15keV) of

Fig. 14. 40 mbar HCF2Cl partial pressure CdTe CL mapping; a) SEM image of the surface morphology; b) monoCL image at the NBE emission energy (E=1.57eV); c) monoCL at

1,1 1,2 1,3 1,4 1,5 1,6 1,7

Energy (eV)

image of the etched region has been shown in Fig. 13 a. CL analyses have been performed on the beveled CdTe surface, in the region closer to the interface, indicated as the "maximum etched surface" in Fig. 13 a. The comparison among the untreated CdTe and the annealed specimens was shown in Fig. 12. In the untreated and 30mbar treated samples, in addition to the NBE emission only the CL band centered at 1.47 eV has been observed. The 1.4eV band appeared only in cells treated with more than 30 mbar HCF2Cl. By increasing the HCF2Cl partial pressure, the 1.4eV CL intensity also increases confirming the behavior observed on the not-etched CdTe surface. On the other hand, when present, the 1.4eV CL intensities were lower than those observed on the not-etched surfaces and did not exceed the NBE intensity. This behavior was clear in Fig. 13 b, where the comparison among CL spectra acquired on the 40mbar HCF2Cl etched surface at different depths approaching the CdTe/CdS interface was shown. The spectra were acquired from a small area (10m wide) in order to investigate regions at the same depth. The analyzed regions have been indicated by the colored squares in Fig. 13 a. The 1.4eV CL intensity decreased remarkably from the blue to the black curve, which corresponded to regions approaching the "maximum etched region". Only in the spectrum acquired 3m far from this region the 1.4eV band was back the dominant one. The depth-dependent decrease of the 1.4eV CL intensity could be attributed to a not-uniform distribution of the VCd-Cl(F) complex responsible for that transition because of a low diffusion of the Cl (or F) atoms within the CdTe.

Fig. 12. Comparison among the low temperature (77 K) CL spectra (EB=25 keV) of untreated CdTe and samples annealed at a HCF2Cl partial pressure of 30, 40 and 50 mbar.

The spatial distribution of luminescence properties of CdTe was studied by acquiring monoCL images at the emission energies of the bands observed in the CL spectra, 1.57eV and 1.4eV respectively. The monoCL image collected at the NBE emission energy (E=1.57eV) showed the maximum intensity contribution from the central part of the grains (Fig. 14 b). The excitonic transitions came mainly from the CdTe grains, meaning that they were of good crystalline quality. On the other hand, in the same image the boundary regions between adjacent grains (*grain boundaries*) showed a dark contrast which corresponded to a very low radiative recombination efficiency. By acquiring the monoCL image at E=1.4eV

image of the etched region has been shown in Fig. 13 a. CL analyses have been performed on the beveled CdTe surface, in the region closer to the interface, indicated as the "maximum etched surface" in Fig. 13 a. The comparison among the untreated CdTe and the annealed specimens was shown in Fig. 12. In the untreated and 30mbar treated samples, in addition to the NBE emission only the CL band centered at 1.47 eV has been observed. The 1.4eV band appeared only in cells treated with more than 30 mbar HCF2Cl. By increasing the HCF2Cl partial pressure, the 1.4eV CL intensity also increases confirming the behavior observed on the not-etched CdTe surface. On the other hand, when present, the 1.4eV CL intensities were lower than those observed on the not-etched surfaces and did not exceed the NBE intensity. This behavior was clear in Fig. 13 b, where the comparison among CL spectra acquired on the 40mbar HCF2Cl etched surface at different depths approaching the CdTe/CdS interface was shown. The spectra were acquired from a small area (10m wide) in order to investigate regions at the same depth. The analyzed regions have been indicated by the colored squares in Fig. 13 a. The 1.4eV CL intensity decreased remarkably from the blue to the black curve, which corresponded to regions approaching the "maximum etched region". Only in the spectrum acquired 3m far from this region the 1.4eV band was back the dominant one. The depth-dependent decrease of the 1.4eV CL intensity could be attributed to a not-uniform distribution of the VCd-Cl(F) complex responsible for that

transition because of a low diffusion of the Cl (or F) atoms within the CdTe.

Cl

Cl

Cl

25 keV - max etching 50 mbar HCF2

40 mbar HCF2

30 mbar HCF2

untreated

0,0

0,2

0,4

0,6

CL Intensity (a.u.)

0,8

1,0

1,2 1,3 1,4 1,5 1,6

Energy (eV)

Fig. 12. Comparison among the low temperature (77 K) CL spectra (EB=25 keV) of untreated

The spatial distribution of luminescence properties of CdTe was studied by acquiring monoCL images at the emission energies of the bands observed in the CL spectra, 1.57eV and 1.4eV respectively. The monoCL image collected at the NBE emission energy (E=1.57eV) showed the maximum intensity contribution from the central part of the grains (Fig. 14 b). The excitonic transitions came mainly from the CdTe grains, meaning that they were of good crystalline quality. On the other hand, in the same image the boundary regions between adjacent grains (*grain boundaries*) showed a dark contrast which corresponded to a very low radiative recombination efficiency. By acquiring the monoCL image at E=1.4eV

CdTe and samples annealed at a HCF2Cl partial pressure of 30, 40 and 50 mbar.

Fig. 13. a) SEM image of the etched surface of the solar cell annealed with 40 mbar HCF2Cl partial pressure; b) comparison among the low temperature (77K) CL spectra (Eb=15keV) of the 40 mbar HCF2Cl partial pressure etched solar cell.

Fig. 14. 40 mbar HCF2Cl partial pressure CdTe CL mapping; a) SEM image of the surface morphology; b) monoCL image at the NBE emission energy (E=1.57eV); c) monoCL at E=1.4eV emission energy.

Influence of Post-Deposition Thermal Treatment on the

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 229

The diode ideality factor (A) has been calculated from those curves and its behavior as function of HCF2Cl was also reported in Fig. 16 b. Specific processes occurring at the junction determined the reverse current and diode factor. In our case, it was observed a decrease of the reverse current when the HCF2Cl partial pressure was increased. This behavior reached a minimum in the most efficient device obtained for this series, corresponding to 40mbar HCF2Cl partial pressure (Jsc=26.2mA/cm2, Voc=820mV, ff=0.69, =14.8%, see Fig. 17). An increase of 10mbar more reactive gas in the annealing chamber yields to a degradation of the reverse current that was increased of various orders of magnitude, showing the high reactivity of the treatment and the impact of an excess annealing on the device electrical performance. At the same time, from the behavior of A, a variation of transport mechanism depending on the treatment conditions could be suggested (Fig. 16 b). For the untreated sample, A=1.8 indicated that recombination current dominated the junction transport mechanism or that high injection conditions were present. An increase of the HCF2Cl partial pressure gave rise to a situation in which diffusion and recombination currents take place together until the case of 40mbar HCF2Cl partial pressure was reached, where the minimum value of A=1.2, appointed to a predominant diffusion current. The cell treated with 50mbar of reactive gas partial pressure showed a sharp modification, by increasing again the diode factor n up to 1.8. The increase of the diode reverse saturation current was responsible for a drastic reduction of ff (Fig16 b), despite the

JSC and VOC did not change appreciably from the others HCF2Cl annealed devices.

Cl partial pressures 20 mbar 30 mbar 40 mbar 50 mbar

Fig. 16. a) Comparison among the dark reverse I-V curves for untreated and, 20, 30, 40 and 50 mbar of HCF2Cl partial pressure treated solar cells; b) Diode ideality factor A as a

The evolution of the J-V light curves (Fig. 17) of all samples showed an increase of the photovoltaic parameters by increasing the Freon partial pressure until 40mbar, while the J-V characteristic of the sample F50 showed a decrease of the fill factor to 0.25. The latter behavior could be related to a very strong intermixing between CdS and CdTe, due to the

A clear roll-over behavior of all the J-V curves was observed in the Fig. 17; mainly for the untreated sample and F20 and F50. This behavior was attributed to an n-p parasitic junction, opposite to the main p-n junction created by the back contact. We assume that this behavior was also strongly related to the incorporation of Cl impurities into CdTe. In our belief, the increment of the photocurrent collection should be essentially due to an increment of the

1,0

1,2

1,4

Diode ideality factor (A)

 untreated total pressure Ar 400 mbar HCF2 Cl 20mbar HCF2 Cl 30mbar HCF2 Cl 40mbar HCF2 Cl 50mbar

HCF2

1,6

1,8

2,0

0 10 20 30 40 50

Cl Partial pressure

**(b)** 

0,0 0,4 0,8 1,2 1,6 2,0

Voltage (V)

treatment, so that a very large p-n junction region was present.

function of the HCF2Cl partial pressure.

untreated

HCF2

1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4

Current (A)

**(a)** 

(Fig.14 c) a complementary CL intensity distribution has been observed. The bright contrast was concentrated in the grain boundaries. For a more clear representation of the correlation between surface morphology and CdTe radiative recombination properties the corresponding SEM image has been shown (Fig. 14 a).

Taking advantage on the possibility of focusing the SEM electron beam in suitable small regions, the spectroscopic behavior of the CL in a single grain has been studied. The adopted investigation conditions were in this case "fixed beam" at EB=12keV, which corresponded to a sub-micrometric generation volume. The CL spectra acquired in the points marked by numbered colored circles on Fig. 15 a have been shown in Fig. 15 b. The curves were normalized to the NBE emission intensity. The spectra collected inside the CdTe grain numbered 2–5 showed a high-intensity NBE emission dominating the spectra. The 1.4eV band intensity, on the contrary, exceeded the NBE one in the spectra 1 and 6, collected close to the grain edges (*grain boundary*). The spatial distribution of the 1.4eV emission intensity could be directly correlated to the non homogenous density of the VCd-Cl(F) complex responsible for that emission. A gettering of the native point defects (VCd), as well as of the incorporated impurities (Cl or F) could be suggested as the origin of this inhomogeneity. A preferential diffusion through the grain boundaries, of those impurities, during the high temperature annealing, has been supposed. The correlation between polycrystalline Cl doped CdTe structural properties and electronic levels created by the dopants in the CdTe gap has been discussed in the literature (Consonni et al. 2006, 2007) and the published results were in good agreement to those presented in this work.

Fig. 15. a) Spatial variation of CL emissions, by acquiring the spectra in spot mode, in different points of a single CdTe grain; b) SEM image of the region where the CL spots are collected.

#### **4.2 Comparison between the material properties and device performances**

The behavior of the electro-optical properties of CdTe as a function of HCF2Cl partial pressure, shown in the previous paragraph, did not allow to obtain information about the role of HCF2Cl on the cell photovoltaics parameters and to investigate the transport processes taking place at the junction. The electrical properties of the annealed solar cells were compared to those of an untreated device. Dark reverse I-V curves were shown in Fig. 16 a, where the evolution of the reverse current (I0) as a function of the HCF2Cl partial pressure can be observed.

(Fig.14 c) a complementary CL intensity distribution has been observed. The bright contrast was concentrated in the grain boundaries. For a more clear representation of the correlation between surface morphology and CdTe radiative recombination properties the

Taking advantage on the possibility of focusing the SEM electron beam in suitable small regions, the spectroscopic behavior of the CL in a single grain has been studied. The adopted investigation conditions were in this case "fixed beam" at EB=12keV, which corresponded to a sub-micrometric generation volume. The CL spectra acquired in the points marked by numbered colored circles on Fig. 15 a have been shown in Fig. 15 b. The curves were normalized to the NBE emission intensity. The spectra collected inside the CdTe grain numbered 2–5 showed a high-intensity NBE emission dominating the spectra. The 1.4eV band intensity, on the contrary, exceeded the NBE one in the spectra 1 and 6, collected close to the grain edges (*grain boundary*). The spatial distribution of the 1.4eV emission intensity could be directly correlated to the non homogenous density of the VCd-Cl(F) complex responsible for that emission. A gettering of the native point defects (VCd), as well as of the incorporated impurities (Cl or F) could be suggested as the origin of this inhomogeneity. A preferential diffusion through the grain boundaries, of those impurities, during the high temperature annealing, has been supposed. The correlation between polycrystalline Cl doped CdTe structural properties and electronic levels created by the dopants in the CdTe gap has been discussed in the literature (Consonni et al. 2006, 2007) and

the published results were in good agreement to those presented in this work.

NBE E=1.57 eV

**(a)**

**4.2 Comparison between the material properties and device performances** 

Fig. 15. a) Spatial variation of CL emissions, by acquiring the spectra in spot mode, in different points of a single CdTe grain; b) SEM image of the region where the CL spots are collected.

**(b)** 

The behavior of the electro-optical properties of CdTe as a function of HCF2Cl partial pressure, shown in the previous paragraph, did not allow to obtain information about the role of HCF2Cl on the cell photovoltaics parameters and to investigate the transport processes taking place at the junction. The electrical properties of the annealed solar cells were compared to those of an untreated device. Dark reverse I-V curves were shown in Fig. 16 a, where the evolution of the reverse current (I0) as a function of the HCF2Cl partial

1.4 eV

corresponding SEM image has been shown (Fig. 14 a).

1,1 1,2 1,3 1,4 1,5 1,6

Energy (eV)

0

pressure can be observed.

10

20

CL Intensity (a.u.)

30

Eb

=12 keV - T=77 K

 1 - lower grain boundary 6 - upper grain boundary 2 - bulk grain 3 - bulk -2m 4 - bulk -4m 5 - bulk -6m

The diode ideality factor (A) has been calculated from those curves and its behavior as function of HCF2Cl was also reported in Fig. 16 b. Specific processes occurring at the junction determined the reverse current and diode factor. In our case, it was observed a decrease of the reverse current when the HCF2Cl partial pressure was increased. This behavior reached a minimum in the most efficient device obtained for this series, corresponding to 40mbar HCF2Cl partial pressure (Jsc=26.2mA/cm2, Voc=820mV, ff=0.69, =14.8%, see Fig. 17). An increase of 10mbar more reactive gas in the annealing chamber yields to a degradation of the reverse current that was increased of various orders of magnitude, showing the high reactivity of the treatment and the impact of an excess annealing on the device electrical performance. At the same time, from the behavior of A, a variation of transport mechanism depending on the treatment conditions could be suggested (Fig. 16 b). For the untreated sample, A=1.8 indicated that recombination current dominated the junction transport mechanism or that high injection conditions were present. An increase of the HCF2Cl partial pressure gave rise to a situation in which diffusion and recombination currents take place together until the case of 40mbar HCF2Cl partial pressure was reached, where the minimum value of A=1.2, appointed to a predominant diffusion current. The cell treated with 50mbar of reactive gas partial pressure showed a sharp modification, by increasing again the diode factor n up to 1.8. The increase of the diode reverse saturation current was responsible for a drastic reduction of ff (Fig16 b), despite the JSC and VOC did not change appreciably from the others HCF2Cl annealed devices.

Fig. 16. a) Comparison among the dark reverse I-V curves for untreated and, 20, 30, 40 and 50 mbar of HCF2Cl partial pressure treated solar cells; b) Diode ideality factor A as a function of the HCF2Cl partial pressure.

The evolution of the J-V light curves (Fig. 17) of all samples showed an increase of the photovoltaic parameters by increasing the Freon partial pressure until 40mbar, while the J-V characteristic of the sample F50 showed a decrease of the fill factor to 0.25. The latter behavior could be related to a very strong intermixing between CdS and CdTe, due to the treatment, so that a very large p-n junction region was present.

A clear roll-over behavior of all the J-V curves was observed in the Fig. 17; mainly for the untreated sample and F20 and F50. This behavior was attributed to an n-p parasitic junction, opposite to the main p-n junction created by the back contact. We assume that this behavior was also strongly related to the incorporation of Cl impurities into CdTe. In our belief, the increment of the photocurrent collection should be essentially due to an increment of the

Influence of Post-Deposition Thermal Treatment on the

1x10-13

40mbar HCF2Cl partial pressures respectively.

distribution and intermixing junction formation

1x10-12

(


)

1x10-11

1x10-10

1x10-9

of the main p-n junction.

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 231

A model of the effect of annealing as a function of HCF2Cl partial pressure, on the bulk CdTe and its grain boundaries as well as on the CdTe-CdS intermixing mechanisms occurring at the interface has been showed in Fig. 19. The Cl (or F) impurities contained in the annealing gas penetrate into the material partially doping the CdTe. The major part was gettered to the grain boundaries, as observed in the monoCL image (Fig. 14 c), passivating them and improving conductivity. Contemporary the interdiffusion of S in the CdTe and of Te in CdS has been promoted by creating an intermixing region, which thickness increased by increasing the HCF2Cl partial pressure, pictured by the orange region between CdTe and CdS. The poor solar cell performances of the 50mbar HCF2Cl partial pressure annealed device have been explained by a complete consumption of the CdS layer and by destruction

40 60 80 100 120 140

30 mbar HCF2

=201 meV 40 mbar HCF2

=142 meV

untreated

=324 meV

Ea

Ea

Ea

Cl

Cl

1/kT (eV-1)

Fig. 18. Temperature dependent I-V curves collected from the untreated, 30mbar and

Fig. 19. Schematic representation of the effect of the HCF2Cl treatment on defects

photogenerated minority carriers lifetime in the CdTe layer which suggested that the passivation of defects in absence of Cl contributed as non radiative recombination centers (Consonni et al. 2006). We considered the 50mbar HCF2Cl cell an overtreated sample where the intermixing process was so strong that all the available CdS was consumed. The presence of shunt paths through the junction can explain the high reverse current and low fill factor values.

The luminescence properties observed on the CdTe material showed a continuous increase of the 1.4eV band intensity as a function of HCF2Cl partial pressure; the device electrical characterization showed, on the contrary, a threshold at 40mbar partial pressure. Above this value the solar cell performances collapsed dramatically suggesting a critical correlation between HCF2Cl annealing and junction properties.

Fig. 17. Room temperature I-V characteristics under AM 1.5, 100mW/cm2 illumination conditions of untreated solar cells compared to the 20, 30, 40 and 50 mbar HCF2Cl partial pressures respectively.

The comparison between the diode factor A and the 1.4eV intensity behaviors suggested that the VCd-Cl(F) complex was beneficial for the device performances, but did not explain alone the maximum efficiency value measured for the 40 mbar annealed solar cells. A combined CdTe material doping and grain boundaries passivation effect had to be invoked. The absence of the 1.4eV band in the untreated and low HCF2Cl partial pressure annealed CdTe after etching demonstrated that a non-radiative recombination centre was responsible for the low A values. This centre was then passivated by the Cl (or F) incorporation till the excess, for HCF2Cl partial pressures above 40 mbar, deteriorated the p-n junction.

The complex VCd-Cl(F) formation could also be supported by the temperature dependent I-V analyses carried out on the CdTe thin film. The Arrhenius plot extracted from the CdTe dark conductivity as a function of the inverse of the temperature has been shown in Fig.18. The plot showed that, in the case of untreated CdTe the high calculated activation energy (324meV) has been related to a level due to the presence of occasional impurities like Cu, Ag or Au; the activation energy decreases by increasing the HCF2Cl partial pressure, down to Ea=142meV for the material treated by 40mbar HCF2Cl partial pressure. This value was in good agreement with those obtained in Cl (or F) doped CdTe single-crystals and attributed to the A-centre, due to the complex VCd-Cl(F) acceptor-like (Meyer et al. 1992).

photogenerated minority carriers lifetime in the CdTe layer which suggested that the passivation of defects in absence of Cl contributed as non radiative recombination centers (Consonni et al. 2006). We considered the 50mbar HCF2Cl cell an overtreated sample where the intermixing process was so strong that all the available CdS was consumed. The presence of shunt paths through the junction can explain the high reverse current and low

The luminescence properties observed on the CdTe material showed a continuous increase of the 1.4eV band intensity as a function of HCF2Cl partial pressure; the device electrical characterization showed, on the contrary, a threshold at 40mbar partial pressure. Above this value the solar cell performances collapsed dramatically suggesting a critical correlation

0 200 400 600 800 1000

Voltage (V)

The comparison between the diode factor A and the 1.4eV intensity behaviors suggested that the VCd-Cl(F) complex was beneficial for the device performances, but did not explain alone the maximum efficiency value measured for the 40 mbar annealed solar cells. A combined CdTe material doping and grain boundaries passivation effect had to be invoked. The absence of the 1.4eV band in the untreated and low HCF2Cl partial pressure annealed CdTe after etching demonstrated that a non-radiative recombination centre was responsible for the low A values. This centre was then passivated by the Cl (or F) incorporation till the

The complex VCd-Cl(F) formation could also be supported by the temperature dependent I-V analyses carried out on the CdTe thin film. The Arrhenius plot extracted from the CdTe dark conductivity as a function of the inverse of the temperature has been shown in Fig.18. The plot showed that, in the case of untreated CdTe the high calculated activation energy (324meV) has been related to a level due to the presence of occasional impurities like Cu, Ag or Au; the activation energy decreases by increasing the HCF2Cl partial pressure, down to Ea=142meV for the material treated by 40mbar HCF2Cl partial pressure. This value was in good agreement with those obtained in Cl (or F) doped CdTe single-crystals and attributed

Fig. 17. Room temperature I-V characteristics under AM 1.5, 100mW/cm2 illumination conditions of untreated solar cells compared to the 20, 30, 40 and 50 mbar HCF2Cl partial

excess, for HCF2Cl partial pressures above 40 mbar, deteriorated the p-n junction.

to the A-centre, due to the complex VCd-Cl(F) acceptor-like (Meyer et al. 1992).

fill factor values.

between HCF2Cl annealing and junction properties.

50 untreated

total pressure Ar 400 mbar HCF2

HCF2

HCF2

HCF2

Cl 20mbar

Cl 30mbar

Cl 40mbar

Cl 50mbar

J (mA/cm2

pressures respectively.

)

A model of the effect of annealing as a function of HCF2Cl partial pressure, on the bulk CdTe and its grain boundaries as well as on the CdTe-CdS intermixing mechanisms occurring at the interface has been showed in Fig. 19. The Cl (or F) impurities contained in the annealing gas penetrate into the material partially doping the CdTe. The major part was gettered to the grain boundaries, as observed in the monoCL image (Fig. 14 c), passivating them and improving conductivity. Contemporary the interdiffusion of S in the CdTe and of Te in CdS has been promoted by creating an intermixing region, which thickness increased by increasing the HCF2Cl partial pressure, pictured by the orange region between CdTe and CdS. The poor solar cell performances of the 50mbar HCF2Cl partial pressure annealed device have been explained by a complete consumption of the CdS layer and by destruction of the main p-n junction.

Fig. 18. Temperature dependent I-V curves collected from the untreated, 30mbar and 40mbar HCF2Cl partial pressures respectively.

Fig. 19. Schematic representation of the effect of the HCF2Cl treatment on defects distribution and intermixing junction formation

Influence of Post-Deposition Thermal Treatment on the

Waikoloa, HI, USA, December 1994.

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## **5. Conclusions**

Thin films CdTe deposited by CSS have been submitted to a novel, full dry, post-deposition treatment based on HCF2Cl gas. The annealing demonstrated to affect the structural properties of the materials through the loss of preferential orientation. Texture coefficient of the (111) Bragg reflection decreased from 2, for the untreated CdTe, down to 0.56 for the film treated with the highest HCF2Cl partial pressure. On the contrary, the grain size did not show any change after annealing maintaining an average dimension of about 12m. These results were common for high temperature CSS deposited CdTe films, while a clear dependence on the HCF2Cl partial pressure of the electro-optical properties of the films have been observed through the presence of a 1.4 eV CL band in the annealed specimens. The transition responsible for this emission involved an electronic level in the gap with an energy of about 0.15 eV above the valence band edge, which could be attributed to a complex between cadmium vacancy and an impurity probably identified in Cl or F (VCd-Cl/F) from the annealing gas.

The combined CL mapping and spectroscopy on single CdTe grains showed that the lateral distribution of this complex was not homogeneous in the grain, but it was concentrated close to the grain boundaries. The bulk grain, on the contrary, showed a high optical quality, evidenced by the predominance of the NBE emission. The in-depth effectiveness of the HCF2Cl annealing has been demonstrated by correlating depth-dependent CL analyses to the study of the beveled CdTe surface due to the Br-methanol etching. High density of the VCd-Cl/F complex responsible for the 1.4 eV band has been observed close to the CdTe surface; it decreased by increasing depth in the bulk region of the film about 5m below the surface. By removing several microns of CdTe material and by approaching the CdTe/CdS interface, in the etched specimens, an HCF2Cl partial pressure higher than 30 mbar was necessary to detect the 1.4 eV emission, this means to create the VCd-Cl/F complex. On the other hand electrical characterization determined a threshold in the beneficial role of the HCF2Cl annealing, showing the best solar cell performances for the 40 mbar partial pressure treated device. Temperature dependent I-V analyses showed a remarkable decrease of the electronic level activation energy, from 348meV to 142meV. The last value resulted in good agreement with the energy values of the A-center found in the literature.

The comparison between the diode factor A and the 1.4 eV CL band intensity behaviors evidenced that the VCd-Cl/F complex was beneficial for the device performance, but does not explain alone the maximum efficiency value measured for the 40 mbar annealed solar cells. A tentative schematic model of the mechanisms occurring during post-deposition treatment, in the bulk CdTe and close to the CdTe/CdS interface have been also proposed. A combined CdTe-CdS intermixing and grain boundaries passivation effect has to be invoked.

#### **6. References**

Armani N., Salviati G., Nasi L., Bosio A., Mazzamuto S. and Romeo N., "Role of thermal treatment on the luminescence properties of CdTe thin films for photovoltaic applications", (2007) *Thin Solid Films*, vol. 515, pp. 6184-7, ISSN 00406090

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Barrioz, V.; Lamb, D.A.; Jones; E.W. & Irvine, S.J.C. (2010). Suitability of atmospheric-pressure MOCVD CdTe solar cells for inline production scale. *Materials Research Society Symposium Proceedings;* ISBN: 978-160511138-4; San Francisco, CA; April 2009

Thin films CdTe deposited by CSS have been submitted to a novel, full dry, post-deposition treatment based on HCF2Cl gas. The annealing demonstrated to affect the structural properties of the materials through the loss of preferential orientation. Texture coefficient of the (111) Bragg reflection decreased from 2, for the untreated CdTe, down to 0.56 for the film treated with the highest HCF2Cl partial pressure. On the contrary, the grain size did not show any change after annealing maintaining an average dimension of about 12m. These results were common for high temperature CSS deposited CdTe films, while a clear dependence on the HCF2Cl partial pressure of the electro-optical properties of the films have been observed through the presence of a 1.4 eV CL band in the annealed specimens. The transition responsible for this emission involved an electronic level in the gap with an energy of about 0.15 eV above the valence band edge, which could be attributed to a complex between cadmium vacancy and an impurity probably identified in Cl or F (VCd-

The combined CL mapping and spectroscopy on single CdTe grains showed that the lateral distribution of this complex was not homogeneous in the grain, but it was concentrated close to the grain boundaries. The bulk grain, on the contrary, showed a high optical quality, evidenced by the predominance of the NBE emission. The in-depth effectiveness of the HCF2Cl annealing has been demonstrated by correlating depth-dependent CL analyses to the study of the beveled CdTe surface due to the Br-methanol etching. High density of the VCd-Cl/F complex responsible for the 1.4 eV band has been observed close to the CdTe surface; it decreased by increasing depth in the bulk region of the film about 5m below the surface. By removing several microns of CdTe material and by approaching the CdTe/CdS interface, in the etched specimens, an HCF2Cl partial pressure higher than 30 mbar was necessary to detect the 1.4 eV emission, this means to create the VCd-Cl/F complex. On the other hand electrical characterization determined a threshold in the beneficial role of the HCF2Cl annealing, showing the best solar cell performances for the 40 mbar partial pressure treated device. Temperature dependent I-V analyses showed a remarkable decrease of the electronic level activation energy, from 348meV to 142meV. The last value resulted in good

The comparison between the diode factor A and the 1.4 eV CL band intensity behaviors evidenced that the VCd-Cl/F complex was beneficial for the device performance, but does not explain alone the maximum efficiency value measured for the 40 mbar annealed solar cells. A tentative schematic model of the mechanisms occurring during post-deposition treatment, in the bulk CdTe and close to the CdTe/CdS interface have been also proposed. A combined

Armani N., Salviati G., Nasi L., Bosio A., Mazzamuto S. and Romeo N., "Role of thermal

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*Symposium Proceedings;* ISBN: 978-160511138-4; San Francisco, CA; April 2009

treatment on the luminescence properties of CdTe thin films for photovoltaic

MOCVD CdTe solar cells for inline production scale. *Materials Research Society* 

agreement with the energy values of the A-center found in the literature.

CdTe-CdS intermixing and grain boundaries passivation effect has to be invoked.

**5. Conclusions** 

Cl/F) from the annealing gas.

**6. References** 


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**11** 

*México* 

**Chemical Bath Deposited CdS for CdTe and** 

M. Estela Calixto1, M. L. Albor-Aguilera2, M. Tufiño-Velázquez2*,*

*2Escuela Superior de Física y Matemáticas, Instituto Politécnico Nacional, México,* 

Extensive research has been done during the last two decades on cadmium sulfide (CdS) thin films, mainly due to their application to large area electronic devices such as thin film field-effect transistors (Schon et al., 2001) and solar cells (Romeo et al., 2004). For the latter case, chemical bath deposited (CBD) CdS thin films have been used extensively in the processing of CdTe and Cu(In,Ga)Se2 solar cells, because it is a very simple and inexpensive technique to scale up to deposit CdS thin films for mass production processes and because among other n-type semiconductor materials, it has been found that CdS is the most promising heterojunction partner for these well-known polycrystalline photovoltaic materials. Semiconducting n-type CdS thin films have been widely used as a window layer in solar cells; the quality of the CdS-partner plays an important role into the PV device performance. Usually the deposition of the CdS thin films by CBD is carried out using an alkaline aqueous solution (high pH) composed mainly of some sort of Cd compounds (chloride, nitrate, sulfate salts, etc), thiourea as the sulfide source and ammonia as the complexing agent, which helps to prevent the undesirable homogeneous precipitation by forming complexes with Cd ions, slowing down thus the surface reaction on the substrate. CdS films have to fulfill some important criteria to be used for solar cell applications; they have to be adherent to the substrate and free of pinholes or other physical imperfections. Moreover, due to the requirements imposed to the thickness of the CdS films for the solar cells, it seems to be a function of the relative physical perfection of the film. The better structured CdS films and the fewer flaws present, the thinner the film can be, requirement very important for the processing of Cu(In,Ga)Se2 based thin film solar cells, thickness ~ 30 - 50 nm. In such case, the growth of the thin CdS film is known to occur via ion by ion reaction, resulting thus into the growth of dense and homogeneous films with mixed

The reason to choose the CBD method to prepare the CdS layers was due to the fact that CBD forms a very compact film that covers the TCO layer, in the case of the CdTe devices and the Cu(In,Ga)Se2 layer without pinholes. Moreover, the CdS layer in a hetero-junction solar cell must also be highly transparent and form a chemical stable interface with the

cubic/hexagonal lattice structure (Shafarman and Stolt, 2003).

**1. Introduction** 

*1Instituto de Física, Benemérita Universidad Autónoma de Puebla, Puebla,* 

*3CINVESTAV-IPN, Departamento de Ingeniería Eléctrica, México,* 

G. Contreras-Puente2 and A. Morales-Acevedo3

**Cu(In,Ga)Se2 Thin Film Solar Cells Processing** 

