**Influence of Post-Deposition Thermal Treatment on the Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells**

Nicola Armani1, Samantha Mazzamuto2 and Lidice Vaillant-Roca3 *1IMEM-CNR, Parma 2Thifilab, University of Parma, Parma 3Lab. of Semicond. and Solar Cells, Inst. of Sci. and Tech. of Mat., Univ. of Havana, La Habana 1,2Italy 3Cuba* 

### **1. Introduction**

208 Solar Cells – Thin-Film Technologies

This study evaluated a transparent PV module in terms of power generation performance depending on installation conditions such as the inclined slope (incidence angle) and the azimuth angle. The objective of this evaluation was to provide useful data for the replacement of traditional building windows by BIPV system, through the experimental

1. The annual power output of the PV module was measured through the mock-up model. The PV module that was installed at a slope of 30 º exhibited a better performance of 844.4

kWh/kWp annual power output than the vertical PV module with a slope of 90 º. 2. The experimental data was compared with the computed data obtained from the simulation program. The computed data is considered to be reliable with a relative error of 8.5 %. The best performance of annual power output was obtained from the PV module with a slope of 30 º facing south, at an azimuth angle of 0 º. The inclined angle was one of the factors that significantly influenced the power generation performance of the PV module, which varied within a range of 24 % on average and provided a

maximum difference of 63% in the power output at the same azimuth angle. 3. In terms of the computed power output from a slope of 30 º depending on the azimuth angle, the PV module facing south exhibited the most effective performance compared to other azimuth angles. The direction in which the PV module faces can also be a very important factor that can affect the power performance efficiency by 11 % on average

[1] Y. Kuwano, Progress of photovoltaic system for houses and buildings in Japan, Renewable

[2] A. Ja¨ger-Waldau, Photovoltaics and renewable energies in Europe, Renewable and

[3] A. Stoppato, Life cycle assessment of photovoltaic electricity generation, Energy 33 (2008)

[4] A. Hepbasli, A key review on exergetic analysis and assessment of renewable energy resources for a sustainable future, Renewable and Sustainable Energy Reviews 12 (2008)

[5] A. Zahedi, Solar photovoltaic (PV) energy; latest developments in the building integrated and

[6] S. Teske, A. Zervos, O. Schafer, Energy revolution, Greenpeace International, European

[7] R.W. Miles, G. Zoppi, I. Forbes, Inorganic photovoltaic cells, Materials Today 10 (2007) 20–27. [8] S. Guha, Amorphous silicon alloy photovoltaic technology and applications, Renewable

[9] J.H. Song, Y.S. An, S.G. Kim, S,J. Lee, Jong-Ho Yoon, Y.K. Choung, Power output analysis of transparent thin-film module in building integrated photovoltaic system(BIPV), Energy

[10] TRNSYS, A transient system simulation program version 14.2 Manual. Solar Energy

[11] D.L. King, et al., Measuring the solar spectral and angle of incidence effects on photovoltaic modules and irradiance sensors, in: Proceedings of the IEEE Photovoltaic Specialists

and by a maximum of 22 %, depending on the azimuth angle.

Sustainable Energy Reviews 11 (2007) 1414–1437.

Renewable Energy Council (EREC) (2007).

hybrid PV systems, Renewable Energy 31 (2006) 711–718.

and Building, Volume 40, Issue 11, (2008) 2067-2075

Laboratory: University of Wisconsin, Madison, USA, 2000.

**7. Conclusion** 

**8. References** 

224–232.

593–661.

Energy 15 (1998) 535–540.

Energy 15 (1998) 189–194.

Conference, 1994, pp. 1113–1116.

results measured in the full-scale mock-up system.

Thin film solar cells based on polycrystalline Cadmium Telluride (CdTe) reached a record efficiencies of 16.5% (Wu et al. 2001a) for laboratory scale device and of 10.9% for terrestrial module (Cunningham, 2000) about ten years ago. CdTe-based modules production companies have already made the transition from pilot scale development to large manufacturing facilities. This success is attributable to the peculiar physical properties of CdTe which make it ideal for converting solar energy into useful electricity at an efficiency level comparable to silicon, but by consuming only about 1% of the semiconductor material required by Si solar cells. Because of the easy up-scaling to an industrial production as well as the low cost achieved in the recent years by the manufacturers, the CdTe technology has carved out a remarkable part of the photovoltaic market. Up to now two companies (Antec Solar and First Solar) have a noticeable production of CdTe based modules, which are assessed as the best efficiency/cost ratio among all the photovoltaic technologies.

Since the record efficiency of such type solar cells is considerably lower than the theoretical limit of 28-30% (Sze, 1981), the performance of the modules, through new advances in fundamental material science and engineering, and device processing can be improved. Further studies are required to reveal the physical processes determining the photoelectric characteristics and the factors limiting the efficiency of the devices.

The turning point for obtaining the aforementioned high efficiency values was the application of a Cl-based thermal treatment to the structures after depositing the CdTe layer (Birkmire & Meyers, 1994; McCandless & Birkmire, 1991). The device performance improvement is due to a combined beneficial effect on the materials properties and on the pn junction characteristics. CdTe grain size increase (Enriquez & Mathew, 2004; Luschitz et al., 2009), texture properties variations (Moutinho et al., 1998), grain boundary passivation, as well as strain reduction due to S diffusion from CdS to the CdTe layer and recrystallization mechanism (McCandless et al., 1997) are the common observed effects.

Influence of Post-Deposition Thermal Treatment on the

substituting them with a simple vacuum annealing.

as the treatment carried out in presence of CdCl2.

annealing chamber have been varied.

boundary has been performed.

the lateral luminescence distribution has been observed.

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 211

temperature (about 500°C in air), it cannot be stored in a large quantity, since it is dangerous because it can release Cd in the environment in case of fire. Secondly, CdCl2 is soluble in water and, as a consequence, severe security measures must be taken to preserve environmental pollution and health damage. Another drawback is related to the use of chemical etchings, such as HNO3 and HPO3 or Br-Methanol solution, implying that a proper disposal of the used reagents has to be adopted since the workers safety in the factory must be guaranteed. In order to overcome the aforementioned drawbacks, we substituted the CdCl2 based process with an alternative, completely dry CdTe post-deposition thermal treatment, based on the use of a mixture of Ar and a gas belonging to the Freon family and containing chlorine, such as difluorochloromethane (HCF2Cl)(Bosio et al., 2006, Romeo N. et al., 2005). This gas is stable and inert at room temperature and it has not any toxic action. Moreover, the post-treatment chemical etching procedures have been eliminated by

The only drawback in using a Freon gas could be that it is an ozone depleting agent, but, in an industrial production, it can be completely recovered and reused in a closed loop. In this paper, it will be demonstrated how the CdTe treatment in a Freon atmosphere works as well

This method was successfully applied to Closed Space Sublimation (CSS) CdS/CdTe solar cells, by obtaining high-efficiency up to 15% devices (Romeo N. et al., 2007). This original approach may produce modifications on the material properties, different than the usual CdCl2-based annealing. For this reason, in this work, the efforts are focused on the investigation of the peculiar effects of the treatment conditions on the morphology, structural and luminescence properties of CdTe thin films deposited by CSS on Soda-Lime glass/TCO/CdS. All the samples were deposited by keeping unmodified the growth parameters (temperatures and layer thicknesses), in order to submit as identical as possible materials to the annealing. Only the HCF2Cl partial pressure and the Ar total pressure in the

The aim of the present work is to correlate the effect of this new, all dry post-deposition treatment, on the sub-micrometric electro-optical properties of the CSS deposited CdTe films, with the effect on the device performances. Large area SEM-cathodoluminescence (CL) analyses have allowed us to observe an increase of the overall luminescence efficiency and in particular a clear correlation between the defects related CL band and the HCF2Cl partial pressure in the annealing atmosphere. By the high spatial (lateral as well as in-depth) resolution of CL, a sub-micrometric investigation of the single grain radiative recombination activity and of the segregation of the atomic species, coming from the Freon gas, into grain

The HCF2Cl partial pressure has been changed from 20 to 50 mbar, in order to discriminate the Freon gas effect from the others annealing parameters. A clear correlation between the CL band intensities and the HCF2Cl partial pressure has been found and a dependence on

The results obtained from the material analyses have been correlated to the performances of the solar cells processed starting from the glass/ITO/ZnO/CdS/CdTe structures studied. Electrical measurements in dark and under illumination were carried out, in order to determine the characteristic photovoltaic parameters of the cell and to investigate the transport processes that take place at the junction. In particular the device short circuit current density (JSC), open circuit voltage (VOC) fill factor (ff) and efficiency () have been measured as a function of the HCF2Cl partial pressure. The most efficient device obtained by

In the conventional treatment, based on a solution method, the as-deposited CdTe is coated by a CdCl2 layer and then annealed in air or inert gas atmosphere at high temperature. Afterwards, an etching is usually made to remove some CdCl2 residuals and oxides and to leave a Te-rich CdTe surface ready for the back contact deposition. This etching is usually carried out with a Br-methanol solution or by using a mixture of HNO3 and HPO3. Alternative methodologies avoiding the use of solutions have been developed: the CdTe films are heated in presence of CdCl2 vapor or a mixture made by CdCl2 and Cl2 vapor, or HCl (Paulson & Dutta 2000). Vapor based treatments reduce processing time since combining the exposure to CdCl2 and annealing into one step.

All these post-deposition treatments have been demonstrated to strongly affect the morphological, structural and opto-electronic properties of the structures. The changes induced by the chlorine based treatments depend on how the CdTe and CdS were deposited. For example, in CdTe films having an initial sub micrometer grain size, it promotes a recrystallization mechanism, followed by an increase of the grains. This recrystallization process takes place in all CdTe films having specific initial physical properties, and does not depend on the deposition method used to grow the films. Recrystallization together to grain size increase has been observed in CdTe films deposited by Closed Space Sublimation (CSS), Physical Vapor Deposition (PVD) or Radio Frequency Sputtering. The chlorine based treatment may or may not induce recrystallization of the CdTe films, depending on the initial stress state of the material, and the type and conditions of the treatment. For this reason, the recrystallization process wasn't observed in CSS samples which are deposited at higher temperatures and have an initial large grain size, while, for example CdTe films deposited by Sputtering that are characterized by small grains lower than 1m in size, an increase up to one order of magnitude was obtained (Moutinho et al., 1998, 1999). The driving force for the recrystallization process is the latticestrain energy at the times and temperatures used in the treatment.

Changes in structural properties and preferred orientation are also observed. The untreated CdTe material usually grows in the cubic zincblende structure, with a preferential orientation along the (111) direction. Depending on the deposition method, these texture properties can be lost, in place of a completely disoriented material. The Cl-based annealing induces a lost of the preferential orientation as demonstrated by literature X-Ray Diffraction (XRD) works explaining in terms of value calculation (Moutinho et al. 1998, 1999). However, this treatment is important even in films that do not recrystallize because it decreases the density of deep levels inside the bandgap and changes the defect structure, resulting in better devices.

Maybe the crucial effect of the treatment is related to the p-n junction characteristics. This treatment promotes interdiffusion between CdTe and CdS, resulting in the formation of CdTeS alloys at the CdTe–CdS interface. The CdTe1-xSx and CdS1-yTey alloys form via diffusion across the interface during CdTe deposition and post-deposition treatments and affect photocurrent and junction behavior (McCandless & Sites, 2003).

Formation of the CdS1-yTey alloy on the S-rich side of the junction reduces the band gap and increases absorption which reduces photocurrent in the 500–600 nm range. Formation of the CdTe1-xSx alloy on the Te-rich side of the junction reduces the absorber layer bandgap, due to the relatively large optical bowing parameter of the CdTe–CdS alloy system.

Despite the promising results, the transfer to an industrial production of the commonly adopted CdCl2 based annealing may increase the number of process steps and consequently the device final cost (Ferekides et al., 2000). Since CdCl2 has a quite low evaporation

In the conventional treatment, based on a solution method, the as-deposited CdTe is coated by a CdCl2 layer and then annealed in air or inert gas atmosphere at high temperature. Afterwards, an etching is usually made to remove some CdCl2 residuals and oxides and to leave a Te-rich CdTe surface ready for the back contact deposition. This etching is usually carried out with a Br-methanol solution or by using a mixture of HNO3 and HPO3. Alternative methodologies avoiding the use of solutions have been developed: the CdTe films are heated in presence of CdCl2 vapor or a mixture made by CdCl2 and Cl2 vapor, or HCl (Paulson & Dutta 2000). Vapor based treatments reduce processing time since

All these post-deposition treatments have been demonstrated to strongly affect the morphological, structural and opto-electronic properties of the structures. The changes induced by the chlorine based treatments depend on how the CdTe and CdS were deposited. For example, in CdTe films having an initial sub micrometer grain size, it promotes a recrystallization mechanism, followed by an increase of the grains. This recrystallization process takes place in all CdTe films having specific initial physical properties, and does not depend on the deposition method used to grow the films. Recrystallization together to grain size increase has been observed in CdTe films deposited by Closed Space Sublimation (CSS), Physical Vapor Deposition (PVD) or Radio Frequency Sputtering. The chlorine based treatment may or may not induce recrystallization of the CdTe films, depending on the initial stress state of the material, and the type and conditions of the treatment. For this reason, the recrystallization process wasn't observed in CSS samples which are deposited at higher temperatures and have an initial large grain size, while, for example CdTe films deposited by Sputtering that are characterized by small grains lower than 1m in size, an increase up to one order of magnitude was obtained (Moutinho et al., 1998, 1999). The driving force for the recrystallization process is the lattice-

Changes in structural properties and preferred orientation are also observed. The untreated CdTe material usually grows in the cubic zincblende structure, with a preferential orientation along the (111) direction. Depending on the deposition method, these texture properties can be lost, in place of a completely disoriented material. The Cl-based annealing induces a lost of the preferential orientation as demonstrated by literature X-Ray Diffraction (XRD) works explaining in terms of value calculation (Moutinho et al. 1998, 1999). However, this treatment is important even in films that do not recrystallize because it decreases the density of deep levels inside the bandgap and changes the defect structure,

Maybe the crucial effect of the treatment is related to the p-n junction characteristics. This treatment promotes interdiffusion between CdTe and CdS, resulting in the formation of CdTeS alloys at the CdTe–CdS interface. The CdTe1-xSx and CdS1-yTey alloys form via diffusion across the interface during CdTe deposition and post-deposition treatments and

Formation of the CdS1-yTey alloy on the S-rich side of the junction reduces the band gap and increases absorption which reduces photocurrent in the 500–600 nm range. Formation of the CdTe1-xSx alloy on the Te-rich side of the junction reduces the absorber layer bandgap, due

Despite the promising results, the transfer to an industrial production of the commonly adopted CdCl2 based annealing may increase the number of process steps and consequently the device final cost (Ferekides et al., 2000). Since CdCl2 has a quite low evaporation

combining the exposure to CdCl2 and annealing into one step.

strain energy at the times and temperatures used in the treatment.

affect photocurrent and junction behavior (McCandless & Sites, 2003).

to the relatively large optical bowing parameter of the CdTe–CdS alloy system.

resulting in better devices.

temperature (about 500°C in air), it cannot be stored in a large quantity, since it is dangerous because it can release Cd in the environment in case of fire. Secondly, CdCl2 is soluble in water and, as a consequence, severe security measures must be taken to preserve environmental pollution and health damage. Another drawback is related to the use of chemical etchings, such as HNO3 and HPO3 or Br-Methanol solution, implying that a proper disposal of the used reagents has to be adopted since the workers safety in the factory must be guaranteed. In order to overcome the aforementioned drawbacks, we substituted the CdCl2 based process with an alternative, completely dry CdTe post-deposition thermal treatment, based on the use of a mixture of Ar and a gas belonging to the Freon family and containing chlorine, such as difluorochloromethane (HCF2Cl)(Bosio et al., 2006, Romeo N. et al., 2005). This gas is stable and inert at room temperature and it has not any toxic action. Moreover, the post-treatment chemical etching procedures have been eliminated by substituting them with a simple vacuum annealing.

The only drawback in using a Freon gas could be that it is an ozone depleting agent, but, in an industrial production, it can be completely recovered and reused in a closed loop. In this paper, it will be demonstrated how the CdTe treatment in a Freon atmosphere works as well as the treatment carried out in presence of CdCl2.

This method was successfully applied to Closed Space Sublimation (CSS) CdS/CdTe solar cells, by obtaining high-efficiency up to 15% devices (Romeo N. et al., 2007). This original approach may produce modifications on the material properties, different than the usual CdCl2-based annealing. For this reason, in this work, the efforts are focused on the investigation of the peculiar effects of the treatment conditions on the morphology, structural and luminescence properties of CdTe thin films deposited by CSS on Soda-Lime glass/TCO/CdS. All the samples were deposited by keeping unmodified the growth parameters (temperatures and layer thicknesses), in order to submit as identical as possible materials to the annealing. Only the HCF2Cl partial pressure and the Ar total pressure in the annealing chamber have been varied.

The aim of the present work is to correlate the effect of this new, all dry post-deposition treatment, on the sub-micrometric electro-optical properties of the CSS deposited CdTe films, with the effect on the device performances. Large area SEM-cathodoluminescence (CL) analyses have allowed us to observe an increase of the overall luminescence efficiency and in particular a clear correlation between the defects related CL band and the HCF2Cl partial pressure in the annealing atmosphere. By the high spatial (lateral as well as in-depth) resolution of CL, a sub-micrometric investigation of the single grain radiative recombination activity and of the segregation of the atomic species, coming from the Freon gas, into grain boundary has been performed.

The HCF2Cl partial pressure has been changed from 20 to 50 mbar, in order to discriminate the Freon gas effect from the others annealing parameters. A clear correlation between the CL band intensities and the HCF2Cl partial pressure has been found and a dependence on the lateral luminescence distribution has been observed.

The results obtained from the material analyses have been correlated to the performances of the solar cells processed starting from the glass/ITO/ZnO/CdS/CdTe structures studied. Electrical measurements in dark and under illumination were carried out, in order to determine the characteristic photovoltaic parameters of the cell and to investigate the transport processes that take place at the junction. In particular the device short circuit current density (JSC), open circuit voltage (VOC) fill factor (ff) and efficiency () have been measured as a function of the HCF2Cl partial pressure. The most efficient device obtained by

Influence of Post-Deposition Thermal Treatment on the

separate the front contact from the back one.

Opto-Electronic Properties of Materials for CdTe/CdS Solar Cells 213

The cell is completed by a scribing made on the edge of all the cells in order to electrically

Fig. 1. Schematic representation of the CdS/CdTe solar cell heterostructure. The 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

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.,

succession and thicknesses are the ones used in the present work.

cells (world record photovoltaic solar energy conversion ~16.5%; Wu, 2004).

**2.1 CSS Growth of CdTe layers** 

2009).

this procedure, corresponding to 40 mbar HCF2Cl partial pressure in the 400mbar Ar total pressure, has =14.8%, JSC=26.2mA/cm2, VOC = 820mV and ff=0.69.

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 results to the depth-dependent CL analyses.

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 due to the annealing.
