- Email: [email protected]
Manufacturing of CdTe thin film photovoltaic modules
Thin Solid Films 519 (2011) 7522–7525
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Manufacturing of CdTe thin film photovoltaic modules Alessio Bosio a,⁎, Daniele Menossi a, Samantha Mazzamuto b, Nicola Romeo a a b
University of Parma, G.P. Usberti 7/A, 43124-Parma, Italy SSE: Solar Systems and Equipment, Via S. Maria, 19-56126-Pisa, Italy
a r t i c l e
i n f o
Available online 23 December 2010 Keywords: CdTe Thin film PV modules Close spaced sublimation
a b s t r a c t The technology to fabricate CdTe/CdS thin film solar cells can be considered mature for a large-scale production of CdTe-based modules. Several reasons contribute to demonstrate this assertion: a stable efficiency of 16.5% has been demonstrated for 1 cm2 laboratory cell and it is expected that an efficiency of 12% can be obtained for 0.6 × 1.2 m2 modules; low cost soda lime float glass can be used as a substrate; the amount of source material is at least 100 times less than that used for single crystal modules and is a negligible part of the overall cost. The fabrication process can be completely automated and a production yield of one module every 2 min can be obtained, which implies a production cost substantially less than 1€/WP. A further cost reduction will render this kind of energy production competitive with the energy obtained from fossil fuels by approaching the so-called grid-parity. Some new companies have recently announced the start of production or plan to do so in the near future. Many of these plants are located in Germany, some in the USA. In Italy, a new company has been constituted in 2008, with the aim of building a factory with a capacity of 18 MW/year. In this article, we will describe and compare the basic principles of CdTe solar cells and modules. We will include an overview of the potentials of these technologies and of the R&D issues under investigation. This paper describes how the large-area mass production of CdTe solar modules is realized in the Italian factory and presents a worldwide overview of the current production activities. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaics (PV) continues to be one of the fastest growing industries, with increases beyond 40% annually. This growth is driven not only by advances in materials and technology, but also from market support programs in a growing number of countries around the world. The rising of fossil energy prices in 2008–2009 emphasized the need to diversify the supply for the sake of energy security and stressed the benefits of renewable local energetic sources, such as solar energy. The high growth was achieved primarily by an increase in production capacity based on crystalline silicon technology. However, in recent years, despite the already very high growth rates across industry, thinfilm PV has grown at a faster pace and its market share rose from 5% in 2005 to over 10% in 2009. However the vast majority of PV modules installed today is produced with the consolidated monocrystalline and polycrystalline silicon technology, similar to the technology used to realize electronic chips. The high temperatures involved, the need to work in ultra-high vacuum and the complex operation of cutting and assembly silicon wafer, make this technology inherently complicated and expensive. Other photovoltaic devices based on silicon are produced under the form of “thin-film silicon” and strip modules but these devices are still in the experimental
⁎ Corresponding author. Tel.: +39 0521 905257; fax: +39 0521 905223. E-mail address: [email protected] (A. Bosio). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.137
stage. Those based on amorphous silicon technology have been around for decades. Nevertheless, it remains uncertain when these will reach the performance and production goals initially envisioned for the technology. Moreover, silicon is not as suitable a material for implementation as a thin film, compared to other materials. This is partly due to process constraints (e.g., high temperature) and inherent characteristics of the semiconductor that has a low absorption coefficient in the visible part of the solar radiation (i.e., an indirect energy gap). Because of this, film silicon approaches must employ either thick layers or complex techniques of light trapping. Beyond the use of silicon, thin film technology has an obvious advantage of lending to large-scale productions in which the panel is the final stage of an in-line process that does not require the assembly of discrete smaller cells, as in the case of modules based on crystalline wafers or polycrystalline silicon. The high production rates (in terms of square meters of modules per minute) ascribed to thin-film processes, combined with the necessity of small amount of active material, made people think since '80 that in the future thin-film PV modules have a significant chance to compete with the traditional energy sources. Over the last decade two materials have emerged as potentially suitable for mass production of PV modules: CdTe and CuInGaSe2 (CIGS). For these materials, production processes have been demonstrated that address the typical problems of scalability and module stability. Photovoltaic modules based on CuInGaSe2 (CIGS) and CdTe thin film technology are already being produced with high quality and efficiency, more than 10%, with expected values up to 14% for the foreseeable future.
A. Bosio et al. / Thin Solid Films 519 (2011) 7522–7525
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In particular, CdTe is considered to have advantages for a large scale production since scalable techniques such as sputtering and CSS have already been demonstrated in cost-effective production processes. For this reason, several new companies have been constituted during the last years (see Table 1). Among these, only two companies, namely First Solar and Antec Solar are in production. In particular, First Solar has increased its production from 30 MW/year to 600 MW/year during the past few years. They have also announced production costs down to~0.85 USD/ WP. 2. Basic technology CdTe polycrystalline thin film solar cells are made of very thin stacked layers arranged in such a way to form an efficient heterojunction. A particular architecture of the junction benefits of the “window/ absorber” layers so that most charge generation by the light (photovoltaic effect) occurs within the absorber layer, avoiding excessive recombination of carriers at the interface or in the window layer. In this implementation, the window layer has an “energy gap” that is sufficiently large to permit the transmission of sunlight to the absorber layer and a high level of doping to minimize its resistance since it also works as an electrical contact. Since the process light-generation of electron–hole pairs and carrier separation by the electrical field present in the junction region, occurs primarily within the absorber layer, this layer typically identifies the name given to the related technology: CdTe for Cadmium Telluride and CIGS for the whole set of chalcopyrites compounds Cu(In,Ga)(S,Se)2. In Fig. 1 a diagram for the realization of the CdTe/CdS heterojunction is shown. Solar cells based on CdTe are built starting from glass, so that the first layer is a Transparent Conducting Oxide (TCO) that acts as a front electrical contact. This film is followed by the CdS window layer and then by the CdTe absorber layer. An electrical back contact completes the device. The fundamental points in the manufacture process of CdTe thin film solar cells are: the deposition of the CdTe film, the treatment in a chlorine atmosphere and the electrical contact on the p-type CdTe film surface. Different industries and research groups utilize different deposition methods for realizing CdTe/CdS cells. Generally, the technique used to deposit CdTe thin film is taken as a benchmark for the identification of the manufacturer technology. The highest efficiency solar cells were prepared using the closespaced sublimation (CSS) technique. The efficiency record (16.5%) was obtained on a small area cell (1 cm2) at the National Renewable Energy Laboratory (NREL), USA, using a complex process that minimizes the different optical and electrical losses [1]; For the TCO, a complex double layer of zinc stannate (Zn2SnO4 and ZTO) above a low resistivity cadmium stannate (Cd2SnO4 and CTO) film, treated in a vapor of CdS/Ar (at a temperature of 660 °C for 20 min) is applied on a glass substrate. CTO and ZTO layers are both deposited by RF magnetron sputtering. The CdS film is deposited by chemical bath (CBD); on top of the CdS film, the CdTe absorber layer is deposited by CSS at a substrate temperature of 625 °C for a total thickness of 8–10 μm. Before the deposition of the back contact (CuxTe:HgTe and Ag-doped graphite paste), a heat-treatment of CdTe films in the presence of CdCl2 at a temperature of 450 °C and then a chemical etching of the surface are made. Despite the efficiency record
Fig. 1. A CdTe/CdS thin film solar cell structure in the backwall configuration.
was already achieved in 2000, the industrial and commercial possibilities of this process remain questionable since the whole process presents some drawbacks in view of an in-line production such as an expensive glass (Corning 7059-Barium Silicate) as a substrate and the use of large amounts of energy and very high temperatures that require accurate process controls. However, this process demonstrates the importance of a homogeneous TCO and the optimization of CdCl2 heattreatment in order to reduce the loss of optical absorption in CdS/TCO and thus to obtain a better current density (JSC). Unfortunately, no data are available about long-term stability of these high efficiency solar cells. In recent years, taking into account these valuable insights, we went beyond the vision of achieving the maximum efficiency without evaluating the complexity of the production process. For this purpose, the technical realization of the devices has been refined in the sense of a gradual simplification in order to make possible a large-scale PV module production. For example, at the “Thin Film Laboratory” at the Physics Department, University of Parma-Italy, a new method to make the heat-treatment in chlorine atmosphere and a novel electrical back contact were discovered. For the Cl-treatment the CdTe/CdS structure is placed in an ambient containing a non-toxic gas such as HCF2Cl (difluorochloromethane) [2,3]. The system temperature is increased up to 400 °C to allow the HCF2Cl molecule to dissociate resulting in the release of Cl2. After treatment, the morphology of CdTe films has completely changed due to an increasing of smaller grain sizes and a reordering of grain boundaries. The back contact is probably one of the most critical steps in the manufacture of high efficiency solar cells based on CdTe/CdS thin films. This contact is made, in our laboratory, by depositing on the surface of CdTe films a layer of As2Te3, followed by the deposition of a few nanometers of Cu [4]. If the deposition of Cu is done at a suitable temperature, a reaction between Cu and As2Te3 happens forming a CuxTe (1 b x b 1.4) layer through a solid-state reaction in which Cu atoms substitute As atoms [5]. This material forms a good ohmic contact, or a low-resistance electrical contact with the surface of the p-type CdTe film that is stable over time. Including in the production process these two important innovations we have obtained CdTe solar cells with photovoltaic conversion efficiencies above 14% [6]. 3. Future developments
Table 1 Companies that are producing or are close to produce CdTe/CdS thin film modules. Company
State
Company
State
First Solar Antec Solar Calyxo USA Willard & Kelsey Solar Nuvo Solar Energy
Ohio — USA Germany USA Ohio — USA Colorado — USA
Arendi S.p.A. Primestar Solar Abound Solar Ascentool Zia Watt Solar
Italy Colorado — USA Colorado — USA California — USA Texas — USA
Having overcome the problems of complexity and long-term stability of thin film modules, there is still a long way to improve the efficiency of the devices. Many of the objectives of R&D are directed to fill the possible gap between the record performance, achieved in the laboratory scale on small area solar cells, and that of photovoltaic modules for commercial use. In particular, concerning the polycrystalline thin film devices, the control of the order and the orientation of crystalline grains and the composition of mixed compounds formed
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A. Bosio et al. / Thin Solid Films 519 (2011) 7522–7525
Fig. 2. Typical interconnect scheme for a CdTe/CdS based solar cell module. P1: Laser scribing of the TCO film. P2: Laser scribing of the active layers (CdS/CdTe). P3: Laser scribing of the back contact/active layers.
in the junction region in order to get an Ordered Defect Compound (ODC) structure must be implemented. One can also better exploit the solar spectrum by using fluorescent dyes (Lumogen, Basf) attached on the transparent contact, which can absorb light (in a range of 300– 500 nm) and re-emit it with wavelengths in the visible part of the solar spectrum not absorbed by the polymer. In addition, it is possible to use multi-junction systems. To maximize the module costefficiency ratio, as well as to act on energy conversion efficiency, one should also work on the quality of the production process: minimizing the thickness of the layers, increasing the stoichiometry uniformity over large areas and strengthening the deposition rate with rotating source sputtering systems that allow the use of the target up to 85% (double in comparison of planar sources). Finding a solution to these issues means that we can achieve in-line production facilities with an annual productivity of 100 MW on a single plant; such a productivity is now seen as the threshold beyond which the cost of PV becomes competitive with traditional energy sources (production cost of about 0.5€/Wp). 4. Industrial production In order to reach a production of thin film PV modules competitive with the more traditional crystalline Si-based modules, it is necessary to develop a technology that exploits on-line fully automated systems suitable for high productivity. This is made possible by the fact that the thin film technology facilitates the construction of a monolithic module on the contrary to what happens with Si-technology. Fig. 2 shows the interconnections scheme between the various cells that make up the CdTe-based module. These interconnections are made using laser scribing on the various layers during the deposition process of the films
Fig. 4. Partial view of the CSS installation, complete with CdS sputtering system and heating tunnel installed in the Arendi factory.
themselves. In other words, the thin film technology offers the possibility of introducing a large glass substrate in a production line and, at the same time, of going out at the line end with the finished module. Thus, it is possible to process one module in a minute and the cost of energy produced by these modules can drop to values comparable with the cost of energy generated from fossil fuels. The production line is schematically represented in Fig. 3. From this diagram we see that the production begins with the cleaning of the soda-lime glass substrate with a special machine (section 1). Section 2 corresponds to the sputtering deposition of the TCO layer. This first film is scribed along parallel lines at an on-center width of 1 cm with a laser beam in section 3. Sections 4, 5 and 6 involve the film deposition and formation of the p–n junction. In this case a single stage process with different areas can cover sputtering deposition of CdS and close-spaced sublimation of CdTe (Fig. 4). Immediately after, in the same chamber there is the CdTe/CdS heat-treatment activation. Section 7 corresponds to the second laser scribing. Back contact is deposited in section 8. This may be a film of Sb2Te3 or As2Te3 covered by a few nanometers of Cu followed by films of Ni–V or Mo all deposited by sputtering (Fig. 5). A
Fig. 3. Flow diagram of the in-line dry process for the production of CdTe/CdS based PV modules used in the Arendi S.p.A. factory.
A. Bosio et al. / Thin Solid Films 519 (2011) 7522–7525
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5. Conclusion
Fig. 5. Back-contact sputtering system completed with heating and cooling tunnel (Arendi S.p.A.).
final laser scribing, in section 9, serves to definitively separate the individual cells inside the module. The removal of the films from the edges, made in section 10 is used to allow complete insulation after encapsulation. In section 11 electrical contacts are applied in the form of metal strips between the first and the last cell and, at the same time (section 12), a preliminary electrical test run before the module is laminated with another glass (section 13). In section 14 metal strips are connected inside the contact-box and, if necessary, an exterior frame is applied. The PV module is completed (sections 15 and 16) with the test under sunlight, classified according to performance and then packed for shipment. Currently there is a real boom in the construction of factories for the production of thin film modules based on polycrystalline compounds. The number of players and their production capacity is constantly increasing. New businesses have been announced by ARENDI (technology developed at the Department of Physics, University of Parma-Italy), Primestar Solar (technology developed at NREL and Applied Thin Films Inc., USA) and Abound Solar (technology developed at Colorado State University-USA). In particular, the Italian project born from the discoveries of physicists from Parma, has been realized with the foundation of a new company, called ARENDI S.p.A., with the aim of producing solar modules using polycrystalline CdTe thin film technology. The social structure is constituted by Marcegaglia Group, IFIS bank of Venice, Alchimia S.p.a. (Marina Salamon), Studio Galli Engineering and Solar Systems and Equipment (SSE). SSE gives, through the University of Parma, scientific and technological support. The Ministry for the Environment, Land and Sea through the Lombardy Region, have contributed to this project with public funds. The factory is located in Lonate Pozzolo (VA) and production is close to start.
The rise of energy price from fossil fuels has increased the market attention towards renewable energy especially in the photovoltaic field. Since 2006, investment and growth of photovoltaic thin films have surpassed the already high growth rate of the whole photovoltaic industry. This is partly due to the difficulty of satisfying the demand in the provision of monocrystalline silicon and partly because of hopes to reduce costs with thin-film technologies. On the other hand, the thin-film technologies are still in need of research and development on a wide range of issues, ranging from improving the understanding of the base-materials properties, production technologies and possible market prospects. To address these problems a long-term vision for the production and research in the field of photovoltaics is necessary. However, there isn't a single “winning technology” and the future can be assured only by extremely valid technological solutions aimed to better understand problems inherent to a PV mass production. In order to achieve high production volumes for PV we look now toward technologies with high productivity and analyze whether and how they can be used in the future. This is particularly important for thin film solar cells that have only limited support from other sectors, such as microelectronics, in the development of production technologies. In addition, there are a number of themes common to all thin-film technologies, which can be solved jointly. Single technology cannot satisfy the demand on a global scale, nor meet all the different desires of consumers for the appearance or performance of photovoltaic systems, but all together can solve the problem of recycling the end of life modules. It is on this common push that an association of photovoltaic industries has formed in Europe with the aim of encouraging the recovery of materials by recycling of modules not only from the end of life standpoint of R&D but also from a regulatory point of view. The consumer should be assured that when his PV system is no longer sufficiently functional, it will be recycled appropriately and the purchase of a new system can be nearly automatic. Acknowledgement This work has been partially supported by the European Commission under the ALPINE project within the Seventh Framework Program. References [1] X. Wu, Solar Energy 77 (2004) 803. [2] S. Mazzamuto, L. Vaillant, A. Bosio, N. Romeo, N. Armani, G. Salviati, Thin Solid Films 516 (2008) 7079. [3] L. Vaillant, N. Armani, L. Nasi, G. Salviati, A. Bosio, S. Mazzamuto, N. Romeo, Thin Solid Films 516 (2008) 7075. [4] N. Romeo, A. Bosio, S. Mazzamuto, A. Romeo, L. Vaillant-Roca, Proceedings of 22nd European Solar Energy Conference, Milan, Italy, 20078 1919. [5] X. Wu, J. Zhou, A. Duda, Y. Yan, G. Teeter, S. Asher, W.K. Metzger, S. Demtsu, Su-Huai Wie, R. Noufi, Thin Solid Films 515 (2007) 5798. [6] N. Romeo, A. Bosio, V. Canevari, A. Podestà, Solar Energy 77 (2004) 795.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Manufacturing of CdTe thin film photovoltaic modules Alessio Bosio a,⁎, Daniele Menossi a, Samantha Mazzamuto b, Nicola Romeo a a b
University of Parma, G.P. Usberti 7/A, 43124-Parma, Italy SSE: Solar Systems and Equipment, Via S. Maria, 19-56126-Pisa, Italy
a r t i c l e
i n f o
Available online 23 December 2010 Keywords: CdTe Thin film PV modules Close spaced sublimation
a b s t r a c t The technology to fabricate CdTe/CdS thin film solar cells can be considered mature for a large-scale production of CdTe-based modules. Several reasons contribute to demonstrate this assertion: a stable efficiency of 16.5% has been demonstrated for 1 cm2 laboratory cell and it is expected that an efficiency of 12% can be obtained for 0.6 × 1.2 m2 modules; low cost soda lime float glass can be used as a substrate; the amount of source material is at least 100 times less than that used for single crystal modules and is a negligible part of the overall cost. The fabrication process can be completely automated and a production yield of one module every 2 min can be obtained, which implies a production cost substantially less than 1€/WP. A further cost reduction will render this kind of energy production competitive with the energy obtained from fossil fuels by approaching the so-called grid-parity. Some new companies have recently announced the start of production or plan to do so in the near future. Many of these plants are located in Germany, some in the USA. In Italy, a new company has been constituted in 2008, with the aim of building a factory with a capacity of 18 MW/year. In this article, we will describe and compare the basic principles of CdTe solar cells and modules. We will include an overview of the potentials of these technologies and of the R&D issues under investigation. This paper describes how the large-area mass production of CdTe solar modules is realized in the Italian factory and presents a worldwide overview of the current production activities. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Photovoltaics (PV) continues to be one of the fastest growing industries, with increases beyond 40% annually. This growth is driven not only by advances in materials and technology, but also from market support programs in a growing number of countries around the world. The rising of fossil energy prices in 2008–2009 emphasized the need to diversify the supply for the sake of energy security and stressed the benefits of renewable local energetic sources, such as solar energy. The high growth was achieved primarily by an increase in production capacity based on crystalline silicon technology. However, in recent years, despite the already very high growth rates across industry, thinfilm PV has grown at a faster pace and its market share rose from 5% in 2005 to over 10% in 2009. However the vast majority of PV modules installed today is produced with the consolidated monocrystalline and polycrystalline silicon technology, similar to the technology used to realize electronic chips. The high temperatures involved, the need to work in ultra-high vacuum and the complex operation of cutting and assembly silicon wafer, make this technology inherently complicated and expensive. Other photovoltaic devices based on silicon are produced under the form of “thin-film silicon” and strip modules but these devices are still in the experimental
⁎ Corresponding author. Tel.: +39 0521 905257; fax: +39 0521 905223. E-mail address: [email protected] (A. Bosio). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.137
stage. Those based on amorphous silicon technology have been around for decades. Nevertheless, it remains uncertain when these will reach the performance and production goals initially envisioned for the technology. Moreover, silicon is not as suitable a material for implementation as a thin film, compared to other materials. This is partly due to process constraints (e.g., high temperature) and inherent characteristics of the semiconductor that has a low absorption coefficient in the visible part of the solar radiation (i.e., an indirect energy gap). Because of this, film silicon approaches must employ either thick layers or complex techniques of light trapping. Beyond the use of silicon, thin film technology has an obvious advantage of lending to large-scale productions in which the panel is the final stage of an in-line process that does not require the assembly of discrete smaller cells, as in the case of modules based on crystalline wafers or polycrystalline silicon. The high production rates (in terms of square meters of modules per minute) ascribed to thin-film processes, combined with the necessity of small amount of active material, made people think since '80 that in the future thin-film PV modules have a significant chance to compete with the traditional energy sources. Over the last decade two materials have emerged as potentially suitable for mass production of PV modules: CdTe and CuInGaSe2 (CIGS). For these materials, production processes have been demonstrated that address the typical problems of scalability and module stability. Photovoltaic modules based on CuInGaSe2 (CIGS) and CdTe thin film technology are already being produced with high quality and efficiency, more than 10%, with expected values up to 14% for the foreseeable future.
A. Bosio et al. / Thin Solid Films 519 (2011) 7522–7525
7523
In particular, CdTe is considered to have advantages for a large scale production since scalable techniques such as sputtering and CSS have already been demonstrated in cost-effective production processes. For this reason, several new companies have been constituted during the last years (see Table 1). Among these, only two companies, namely First Solar and Antec Solar are in production. In particular, First Solar has increased its production from 30 MW/year to 600 MW/year during the past few years. They have also announced production costs down to~0.85 USD/ WP. 2. Basic technology CdTe polycrystalline thin film solar cells are made of very thin stacked layers arranged in such a way to form an efficient heterojunction. A particular architecture of the junction benefits of the “window/ absorber” layers so that most charge generation by the light (photovoltaic effect) occurs within the absorber layer, avoiding excessive recombination of carriers at the interface or in the window layer. In this implementation, the window layer has an “energy gap” that is sufficiently large to permit the transmission of sunlight to the absorber layer and a high level of doping to minimize its resistance since it also works as an electrical contact. Since the process light-generation of electron–hole pairs and carrier separation by the electrical field present in the junction region, occurs primarily within the absorber layer, this layer typically identifies the name given to the related technology: CdTe for Cadmium Telluride and CIGS for the whole set of chalcopyrites compounds Cu(In,Ga)(S,Se)2. In Fig. 1 a diagram for the realization of the CdTe/CdS heterojunction is shown. Solar cells based on CdTe are built starting from glass, so that the first layer is a Transparent Conducting Oxide (TCO) that acts as a front electrical contact. This film is followed by the CdS window layer and then by the CdTe absorber layer. An electrical back contact completes the device. The fundamental points in the manufacture process of CdTe thin film solar cells are: the deposition of the CdTe film, the treatment in a chlorine atmosphere and the electrical contact on the p-type CdTe film surface. Different industries and research groups utilize different deposition methods for realizing CdTe/CdS cells. Generally, the technique used to deposit CdTe thin film is taken as a benchmark for the identification of the manufacturer technology. The highest efficiency solar cells were prepared using the closespaced sublimation (CSS) technique. The efficiency record (16.5%) was obtained on a small area cell (1 cm2) at the National Renewable Energy Laboratory (NREL), USA, using a complex process that minimizes the different optical and electrical losses [1]; For the TCO, a complex double layer of zinc stannate (Zn2SnO4 and ZTO) above a low resistivity cadmium stannate (Cd2SnO4 and CTO) film, treated in a vapor of CdS/Ar (at a temperature of 660 °C for 20 min) is applied on a glass substrate. CTO and ZTO layers are both deposited by RF magnetron sputtering. The CdS film is deposited by chemical bath (CBD); on top of the CdS film, the CdTe absorber layer is deposited by CSS at a substrate temperature of 625 °C for a total thickness of 8–10 μm. Before the deposition of the back contact (CuxTe:HgTe and Ag-doped graphite paste), a heat-treatment of CdTe films in the presence of CdCl2 at a temperature of 450 °C and then a chemical etching of the surface are made. Despite the efficiency record
Fig. 1. A CdTe/CdS thin film solar cell structure in the backwall configuration.
was already achieved in 2000, the industrial and commercial possibilities of this process remain questionable since the whole process presents some drawbacks in view of an in-line production such as an expensive glass (Corning 7059-Barium Silicate) as a substrate and the use of large amounts of energy and very high temperatures that require accurate process controls. However, this process demonstrates the importance of a homogeneous TCO and the optimization of CdCl2 heattreatment in order to reduce the loss of optical absorption in CdS/TCO and thus to obtain a better current density (JSC). Unfortunately, no data are available about long-term stability of these high efficiency solar cells. In recent years, taking into account these valuable insights, we went beyond the vision of achieving the maximum efficiency without evaluating the complexity of the production process. For this purpose, the technical realization of the devices has been refined in the sense of a gradual simplification in order to make possible a large-scale PV module production. For example, at the “Thin Film Laboratory” at the Physics Department, University of Parma-Italy, a new method to make the heat-treatment in chlorine atmosphere and a novel electrical back contact were discovered. For the Cl-treatment the CdTe/CdS structure is placed in an ambient containing a non-toxic gas such as HCF2Cl (difluorochloromethane) [2,3]. The system temperature is increased up to 400 °C to allow the HCF2Cl molecule to dissociate resulting in the release of Cl2. After treatment, the morphology of CdTe films has completely changed due to an increasing of smaller grain sizes and a reordering of grain boundaries. The back contact is probably one of the most critical steps in the manufacture of high efficiency solar cells based on CdTe/CdS thin films. This contact is made, in our laboratory, by depositing on the surface of CdTe films a layer of As2Te3, followed by the deposition of a few nanometers of Cu [4]. If the deposition of Cu is done at a suitable temperature, a reaction between Cu and As2Te3 happens forming a CuxTe (1 b x b 1.4) layer through a solid-state reaction in which Cu atoms substitute As atoms [5]. This material forms a good ohmic contact, or a low-resistance electrical contact with the surface of the p-type CdTe film that is stable over time. Including in the production process these two important innovations we have obtained CdTe solar cells with photovoltaic conversion efficiencies above 14% [6]. 3. Future developments
Table 1 Companies that are producing or are close to produce CdTe/CdS thin film modules. Company
State
Company
State
First Solar Antec Solar Calyxo USA Willard & Kelsey Solar Nuvo Solar Energy
Ohio — USA Germany USA Ohio — USA Colorado — USA
Arendi S.p.A. Primestar Solar Abound Solar Ascentool Zia Watt Solar
Italy Colorado — USA Colorado — USA California — USA Texas — USA
Having overcome the problems of complexity and long-term stability of thin film modules, there is still a long way to improve the efficiency of the devices. Many of the objectives of R&D are directed to fill the possible gap between the record performance, achieved in the laboratory scale on small area solar cells, and that of photovoltaic modules for commercial use. In particular, concerning the polycrystalline thin film devices, the control of the order and the orientation of crystalline grains and the composition of mixed compounds formed
7524
A. Bosio et al. / Thin Solid Films 519 (2011) 7522–7525
Fig. 2. Typical interconnect scheme for a CdTe/CdS based solar cell module. P1: Laser scribing of the TCO film. P2: Laser scribing of the active layers (CdS/CdTe). P3: Laser scribing of the back contact/active layers.
in the junction region in order to get an Ordered Defect Compound (ODC) structure must be implemented. One can also better exploit the solar spectrum by using fluorescent dyes (Lumogen, Basf) attached on the transparent contact, which can absorb light (in a range of 300– 500 nm) and re-emit it with wavelengths in the visible part of the solar spectrum not absorbed by the polymer. In addition, it is possible to use multi-junction systems. To maximize the module costefficiency ratio, as well as to act on energy conversion efficiency, one should also work on the quality of the production process: minimizing the thickness of the layers, increasing the stoichiometry uniformity over large areas and strengthening the deposition rate with rotating source sputtering systems that allow the use of the target up to 85% (double in comparison of planar sources). Finding a solution to these issues means that we can achieve in-line production facilities with an annual productivity of 100 MW on a single plant; such a productivity is now seen as the threshold beyond which the cost of PV becomes competitive with traditional energy sources (production cost of about 0.5€/Wp). 4. Industrial production In order to reach a production of thin film PV modules competitive with the more traditional crystalline Si-based modules, it is necessary to develop a technology that exploits on-line fully automated systems suitable for high productivity. This is made possible by the fact that the thin film technology facilitates the construction of a monolithic module on the contrary to what happens with Si-technology. Fig. 2 shows the interconnections scheme between the various cells that make up the CdTe-based module. These interconnections are made using laser scribing on the various layers during the deposition process of the films
Fig. 4. Partial view of the CSS installation, complete with CdS sputtering system and heating tunnel installed in the Arendi factory.
themselves. In other words, the thin film technology offers the possibility of introducing a large glass substrate in a production line and, at the same time, of going out at the line end with the finished module. Thus, it is possible to process one module in a minute and the cost of energy produced by these modules can drop to values comparable with the cost of energy generated from fossil fuels. The production line is schematically represented in Fig. 3. From this diagram we see that the production begins with the cleaning of the soda-lime glass substrate with a special machine (section 1). Section 2 corresponds to the sputtering deposition of the TCO layer. This first film is scribed along parallel lines at an on-center width of 1 cm with a laser beam in section 3. Sections 4, 5 and 6 involve the film deposition and formation of the p–n junction. In this case a single stage process with different areas can cover sputtering deposition of CdS and close-spaced sublimation of CdTe (Fig. 4). Immediately after, in the same chamber there is the CdTe/CdS heat-treatment activation. Section 7 corresponds to the second laser scribing. Back contact is deposited in section 8. This may be a film of Sb2Te3 or As2Te3 covered by a few nanometers of Cu followed by films of Ni–V or Mo all deposited by sputtering (Fig. 5). A
Fig. 3. Flow diagram of the in-line dry process for the production of CdTe/CdS based PV modules used in the Arendi S.p.A. factory.
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5. Conclusion
Fig. 5. Back-contact sputtering system completed with heating and cooling tunnel (Arendi S.p.A.).
final laser scribing, in section 9, serves to definitively separate the individual cells inside the module. The removal of the films from the edges, made in section 10 is used to allow complete insulation after encapsulation. In section 11 electrical contacts are applied in the form of metal strips between the first and the last cell and, at the same time (section 12), a preliminary electrical test run before the module is laminated with another glass (section 13). In section 14 metal strips are connected inside the contact-box and, if necessary, an exterior frame is applied. The PV module is completed (sections 15 and 16) with the test under sunlight, classified according to performance and then packed for shipment. Currently there is a real boom in the construction of factories for the production of thin film modules based on polycrystalline compounds. The number of players and their production capacity is constantly increasing. New businesses have been announced by ARENDI (technology developed at the Department of Physics, University of Parma-Italy), Primestar Solar (technology developed at NREL and Applied Thin Films Inc., USA) and Abound Solar (technology developed at Colorado State University-USA). In particular, the Italian project born from the discoveries of physicists from Parma, has been realized with the foundation of a new company, called ARENDI S.p.A., with the aim of producing solar modules using polycrystalline CdTe thin film technology. The social structure is constituted by Marcegaglia Group, IFIS bank of Venice, Alchimia S.p.a. (Marina Salamon), Studio Galli Engineering and Solar Systems and Equipment (SSE). SSE gives, through the University of Parma, scientific and technological support. The Ministry for the Environment, Land and Sea through the Lombardy Region, have contributed to this project with public funds. The factory is located in Lonate Pozzolo (VA) and production is close to start.
The rise of energy price from fossil fuels has increased the market attention towards renewable energy especially in the photovoltaic field. Since 2006, investment and growth of photovoltaic thin films have surpassed the already high growth rate of the whole photovoltaic industry. This is partly due to the difficulty of satisfying the demand in the provision of monocrystalline silicon and partly because of hopes to reduce costs with thin-film technologies. On the other hand, the thin-film technologies are still in need of research and development on a wide range of issues, ranging from improving the understanding of the base-materials properties, production technologies and possible market prospects. To address these problems a long-term vision for the production and research in the field of photovoltaics is necessary. However, there isn't a single “winning technology” and the future can be assured only by extremely valid technological solutions aimed to better understand problems inherent to a PV mass production. In order to achieve high production volumes for PV we look now toward technologies with high productivity and analyze whether and how they can be used in the future. This is particularly important for thin film solar cells that have only limited support from other sectors, such as microelectronics, in the development of production technologies. In addition, there are a number of themes common to all thin-film technologies, which can be solved jointly. Single technology cannot satisfy the demand on a global scale, nor meet all the different desires of consumers for the appearance or performance of photovoltaic systems, but all together can solve the problem of recycling the end of life modules. It is on this common push that an association of photovoltaic industries has formed in Europe with the aim of encouraging the recovery of materials by recycling of modules not only from the end of life standpoint of R&D but also from a regulatory point of view. The consumer should be assured that when his PV system is no longer sufficiently functional, it will be recycled appropriately and the purchase of a new system can be nearly automatic. Acknowledgement This work has been partially supported by the European Commission under the ALPINE project within the Seventh Framework Program. References [1] X. Wu, Solar Energy 77 (2004) 803. [2] S. Mazzamuto, L. Vaillant, A. Bosio, N. Romeo, N. Armani, G. Salviati, Thin Solid Films 516 (2008) 7079. [3] L. Vaillant, N. Armani, L. Nasi, G. Salviati, A. Bosio, S. Mazzamuto, N. Romeo, Thin Solid Films 516 (2008) 7075. [4] N. Romeo, A. Bosio, S. Mazzamuto, A. Romeo, L. Vaillant-Roca, Proceedings of 22nd European Solar Energy Conference, Milan, Italy, 20078 1919. [5] X. Wu, J. Zhou, A. Duda, Y. Yan, G. Teeter, S. Asher, W.K. Metzger, S. Demtsu, Su-Huai Wie, R. Noufi, Thin Solid Films 515 (2007) 5798. [6] N. Romeo, A. Bosio, V. Canevari, A. Podestà, Solar Energy 77 (2004) 795.