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CdTe photovoltaic technology is one of the first being brought into production together with amorphous silicon (already in the mid 90s Solar Cells Inc. in USA, Antec Solar and BP Solar in Europe were producing 60 × 120cm modules) and it is now the largest in production among thin film solar cells (Photovoltaics Report, 2014). CdTe has high chemical stability and a large variety of successful preparation methods available, which makes this technology one of the most suitable for large area module production. Historically there are two large categories of CdTe photovoltaic devices depending on the substrate deposition temperature, typically low temperature processes are considered when substrate temperature is below 450 °C. In this paper we will describe the last progress of CdTe based thin film solar cells, fabricated with low substrate temperature process, and their pros and cons.

For CdTe solar cells copper seems to be necessary to achieve best energy conversion efficiencies, whilst it is known to be the main reason of cell performance degradation due to its tendency to diffuse through the bulk. Some studies have shown a direct connection between defect concentration and copper, but little has been discussed about its relation to the CdTe etching. Within this study many samples with Cu/Au back-contact have been prepared with different etching times and tested applying thermal, luminous and electrical stresses. We have analyzed the aging effects on the cell performance and on the nature and concentration of the defects by means of a variety of characterization techniques, like atomic force microscopy, Raman, current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopies. Results of the accelerated lifetime tests show that different performance degradation is observed for cells with differently etched absorber. Solar cells made with optimized etching have very low degradation while the strongest performance reduction is detected for unetched cells despite their initial efficiency is as high as for the case of etched absorbers.

One of the crucial points in the production of CdTe solar cells is the insertion of copper in the back contact and in the absorber. As demonstrated by the top performing devices presented in literature: copper is necessary for high efficiency devices. However, despite this, copper was found to be a fast diffuser degrading the cell in the long term. Different approaches for limiting this effect have been widely presented from several laboratories, from the preparation of CuxTe by the controlled evaporation of Cu and Te to the application of ZnTe, which incorporates Cu, blocking its diffusion in the bulk. We have developed a wet deposition method for inserting in the CdTe structure a quantity of copper equivalent to a 0.1‐nm‐thick Cu layer. We are able to reach similar efficiencies to the ones of devices with a standard 2‐nm‐thick evaporated copper layer, but with a dramatic improvement in performance stability.

In this paper we present for the first time flexible CdTe solar cells deposited on ultra-thin glass using a low temperature process and compared the results to CdTe cells deposited on polyimides. The effects of the different substrates on the morphology and crystal structure of CdTe absorber layers, as well as on the efficiency of solar cells were investigated by means of different characterization techniques. Atomic force microscopy measurements showed that the CdTe layers deposited on different substrates have a similar morphology. XRD analysis was used to study the structure and the orientation of the crystallite of the CdTe layers on the different substrates. Micro-Raman spectroscopy provided evidence of both the TO phonon of CdTe and the A1 phonon of Te, whose peak position is somehow dependent from the substrate material. CdCl2 treatments change the crystal orientation of the CdTe layers, with the loss of preferential orientation. In the case of the polyimide substrate it has been seen that its transparency decreases after CdCl2 activation treatment, resulting in a parasitic light absorbance and in lower photo-current values of the devices. Moreover strain of the CdTe surface is observed for cells deposited on polymers. Cells on ultra-thin glass do not show these issues, confirming the glass to be an interesting alternative substrate for flexible CdTe solar cells.

Thin film solar cells have reached commercial maturity and extraordinarily high efficiency that make them competitive even with the cheaper Chinese crystalline silicon modules. However, some issues (connected with presence of toxic and/or rare elements) are still limiting their market diffusion. For this reason new thin film materials, such as Cu2ZnSnS4 or SnS, have been introduced so that expensive In and Te, and toxic elements Se and Cd, are substituted, respectively, in CuInGaSe2 and CdTe. To overcome the abundance limitation of Te and In, in recent times new thin film materials, such as Cu2ZnSnS4 or SnS, have been investigated. In this paper we analyze the limitations of SnS deposition in terms of reproducibility and reliability. SnS deposited by thermal evaporation is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. The raw material is also analyzed and a different composition is observed according to the different number of evaporation (runs). The sulfur loss represents one of the major challenges of SnS solar cell technology.

Tin sulfide is an excellent candidate for the mass production of solar cells as it is composed of abundant and not toxic elements. We have prepared SnS polycrystalline films by a novel technique. This technology allows to transfer atoms from a target of SnS to the substrate due to the generation of plasma from the target, giving enough energy to have a good crystallization of the material also at low temperature. Ionized Jet deposition (IJD) is an improved Pulsed Electron Deposition technique in which a pulsed high power electron beam penetrates into the target resulting in a rapid evaporation of the material, and its transformation in plasma state. The non-equilibrium extraction (ablation) facilitates stoichiometric composition of the plasma. This is particularly advantageous in the case of complex, multicomponent materials.

Magnesium-doped Zinc Oxide (MZO) was used as an alternative high resistance transparent layer for CdS/CdTe thin film solar cells. Thin films of MZO were deposited by RF magnetron sputtering and deposited on an Indium Tin Oxide contact (ITO). Thin film CdTe devices including a MZO high resistance transparent layer deposited at above 300 ° C yielded a mean efficiency exceeding 10.5 %. This compares with an efficiency of 8.2 % without the MZO layer. The improvement in efficiency was due to a higher open circuit voltage and fill factor. Lowering the deposition temperature of MZO reduced the performance of the devices.

The CdTe thin film technology for photovoltaics (PV) is attractive because of its potential low cost and good performance. In thin film technology the efficiency of large area cells can be maintained if small segments are interconnected in series to reduce the photocurrent and resistance losses. In respect to this, the scribing process is critical for the performance of the device. Today, fiber lasers represent the most advanced and cheap technology that can be used in PV industry to carry out the cuts, needed for the monolithic integration, at different deposition stages. We will present our results on the scribing of CdTe thin film solar cells by means of fiber lasers, with pulse duration of a few nanoseconds and solid state lasers in the picosecond regime. The quality of the scribing was evaluated by optical and scanning electron microscopy. Finally, mini-modules with a total area of 10 × 10 cm 2 were fabricated, in which the cells were interconnected in series by means of a scribing system, equipped with a fiber laser with the same characteristics of the system mounted on production lines. The mini-modules were characterized by photovoltaic and electrical measurements.

Nowadays the improvement of the photovoltaic building-integration (BIPV) has become very important. In fact, more emphasis has been put on the possibility of producing photovoltaic modules able to be integrated, or even to be directly assembled to form the so-called ventilated walls for buildings. This integration can be improved by using, alternatively to glass, other materials for the substrate or for the final “box”, in which the solar cells are encapsulated. A good candidate as a substrate material that substitutes the glass is ceramic. This kind of non-transparent substrates is useful in the substrate configuration solar cell technology. An example of this technology is represented by the Cu(In,Ga)Se2-based solar cells. The challenge in this field corresponds to directly use, as a substrate large dimension ceramic tiles, which are commercially available on the market. For this reason, the process technology developed at ThiFiLab for glass substrates has been transferred on this kind of new substrates and some solar cells have been produced showing efficiencies in the range of 12–14%.

Cu(In,Ga)Se2 (CIGS)/CdS thin-film solar cells have reached, at laboratory scale, an efficiency higher than 22.3%, which is one of the highest efficiencies ever obtained for thin-film solar cells. The research focus has now shifted onto fabrication processes, which have to be easily scalable at an industrial level. For this reason, a process is highlighted here which uses only the sputtering technique for both the absorber preparation and the deposition of all the other materials that make up the cell. Particular emphasis is placed on the comparison between different precursors obtained with either In2Se3 and Ga2Se3 or InSe and GaSe as starting materials. In both cases, the precursor does not require any heat treatment, and it is immediately ready to be selenized. The selenization is performed in a pure-selenium atmosphere and only lasts a few minutes at a temperature of about 803 K. Energy conversion efficiencies in the range of 15%–16% are reproducibly obtained on soda-lime glass (SLG) substrates. Similar results are achieved if commercial ceramic tiles are used as a substrate instead of glass. This result is especially useful for the so-called building integrated photovoltaic. Cu(In,Ga)Se2-based solar cells grown directly on ceramic tiles are ideal for the fabrication of ventilated facades in low impact buildings.

The absorption coefficient of CdTe is large enough to assure that all of the visible light is absorbed in a thickness on the order of 1 µm. High efficiency devices are fabricated by using close-spaced sublimation (CSS)-deposited CdTe films with a thickness in the range of 6–8 µm. In order to decrease the thickness of the CdTe film, a novel approach has been used. On top of the CdTe film, whose thickness is reduced to 2–3 μm, another CdTe layer is deposited by RF sputtering, with a thickness of 100–200 nm. The purpose of this approach is to fill up the voids, which tend to form when a low thickness-CdTe film is deposited by close-spaced sublimation. Using this CdTe double layer, solar cells, with an efficiency greater than 15%, were reproducibly obtained. Since the CdTe layer deposited by the CSS technique shows a p-type behavior, whereas the layer deposited by sputtering is n-type, it is supposed that the formation of a p-n junction into the grain boundaries, which makes a mirror for the charge carriers, increases their mean lifetime. In order to also have this system after the essential chlorine treatment of the CdTe layer, a special cadmium-free halogen treatment was developed. This process was especially tuned for very thin (≤3 µm) CdTe film thickness and for not making use of cadmium-based chlorine salt while, producing high efficiency devices, meets a better economic and environmental sustainability.

Tin sulfide is one of the most promising alternative materials for photovoltaic. One main requirement for enhancing device efficiency is high quality SnS thin films. We present our study of recrystallization of SnS thin film by post deposition treatment at high temperature with KI and SnCl 2 . A recrystallization of SnS was achieved with both KI and SnCl 2 , but it has been observed that this phenomenon occurs only when SnS film is deposited at high temperature, whereas for low temperature-SnS film the treatment process produces a mix of secondary phases into the SnS matrix.

In this paper we present a fabrication process of complete CdTe thin-film solar cells where the CdTe absorber layer is made by pulsed electron deposition (PED) method. PED is a high-density pulsed energy thin film deposition system that allows high-energy growth at lower substrate temperatures. In order to analyze and optimize the fabrication process we started from our baseline where CdTe is deposited in a vacuum chamber and then annealed after CdCl 2 deposition (by dipping in methanol solution). Cells with CdTe absorbers made by PED and by vacuum evaporation have been compared in terms of electrical and physical properties. Efficiency for thin CdTe devices demonstrates that PED allows fabricating a very compact and dense film.

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 cm 2 laboratory cell and it is expected that an efficiency of 12% can be obtained for 0.6 × 1.2 m 2 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€/W P . 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.

The state of the art of the second-generation of solar cells based on the CdTe and CuInGaSe2 thin film technology will be described. This type of cells reached, on laboratory scale, photovoltaic conversion efficiencies respectively of 16.5% and 20.3% very close to those obtained with bulk materials. In particular, the materials, the layer sequences and deposition techniques used for the preparation of these two absorbers are described. Particular emphasis will be placed on major innovations that have enabled us to achieve high efficiencies with polycrystalline materials, showing how thin-film technology is mature enough to be transferred to industrial production. Finally, we will discuss about a project of technology-transfer developed at Parma University for the production of photovoltaic modules at an industrial level. In the near future, with our process, Arendi S.p.A. will reach a productivity of 18 MW/year. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

CdTe thin film solar cells have demonstrated high scalability, high efficiency and low cost fabrication process. One of the key factors for the achievements of this technology is the transformation of the absorber layer by an activation treatment where chlorine reacts with CdTe in a controlled atmosphere or in air, improving the electrical properties of the absorber and enhancing the intermixing of the CdS/CdTe layers. With this work we study the activation process by analyzing the CdCl 2 treatment made by wet deposition with different annealing temperatures from 310 °C up to 410 °C in air keeping the same CdCl 2 concentration in methanol solution. In this way the whole dynamic of the chemical reaction from the minimum activation energy is analyzed. Activated CdTe layers have been analyzed by means of X-ray diffraction and atomic force microscopy. Finished devices with efficiencies from 8% for the low temperature annealing up to more than 14% for the high temperature ones have been thoroughly analyzed by current–voltage, capacitance–voltage and drive-level capacitance profiling techniques. The best performance has been achieved with an annealing temperature of 395 °C.

Despite CdTe solar cells have been already industrialized in large scale and modules are already available in the market, still the research for a stable, low cost, high performing back contact is very hot topic. As a matter of fact, it has been shown that the presence of copper in the CdTe device increases efficiency, not only by improving the back contact but also by doping the absorber and enhancing all the electrical properties. However copper is well known to be a degrading element in the device, diffusing down to the front contact. In this paper we present an electrical study and aging tests of a CdTe devices where copper is encapsulated within a Bi2Te3 buffer layer. The solar cells show a remarkable stability and a good efficiency.

The growing demand for high productivity in the thin-film photovoltaic module industry, together with the request for more and more efficient devices, needs high-performance laser-scribing. The results of scribing tests on CdTe and CIGS solar cells samples are here presented. A comparison between the scribes obtained with ns regime fiber lasers, and a ps regime diode pumped solid state laser will be also reported.

In the last five years photovoltaic modules production continued to be one of the rapidly growing industrial sectors, with an increase well in excess of 40% per year. This growth is driven not only by the progress in materials and technology, but also by incentives to sup‐ port the market in an increasing number of countries all over the world. Besides, the in‐ crease in the price of fossil fuels in 2008, highlighted the necessity to diversify provisioning for the sake of energy security and to emphasize the benefits of local renewable energy sour‐ ces such as solar energy. The high growth was achieved by an increase in production capaci‐ ty based on the technology of crystalline silicon, but in recent years, despite the already very high industrial growth rates, thin film photovoltaics has grown at an increasingly fast pace and its market share has increased from 6% in 2006 to over 12% in 2010. However, the ma‐ jority of photovoltaic modules installed today are produced by the well-established technol‐ ogy of monocrystalline and polycrystalline silicon, which is very close to the technology used for the creation of electronic chips. The high temperatures involved, the necessity to work in ultra-high vacuum and the complex cutting and assembly of silicon "wafers", make the technology inherently complicated and expensive. In spite of everything, silicon is still dominating the photovoltaic market with 90% of sales. Other photovoltaic devices based on silicon are produced in the form of "thin films" or in silicon ribbons; these devices are still in the experimental stage. Amorphous silicon is a technology that has been on the market for decades and it is by now clear that it does not keep the promises of change and develop‐ ment that were pledged when it was initially launched.

16.2% efficient Cu(In,Ga)Se2/CdS solar cells were prepared with a process which uses only sputtering and selenization. InSe, GaSe and Cu sputtering targets were used for the preparation of precursors. Selenization was done in a vacuum chamber in which pure Se is evaporated from a graphite crucible. CdS was deposited by sputtering in an atmosphere of Ar containing 3% of CHF3. The process is highly reproducible and it is easily scalable to much larger area substrates.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2140-2142, In this paper we propose a new approach for CdTe activation treatment. Starting from a baseline of CdTe devices made with CdCl2 activation process we present a double treatment where CdCl2 and Freon®, namely difluorochloromethane, treatments are applied in sequence. This new activation process delivered solar cells with efficiencies well above 15%. The mechanisms for these improved efficiencies are studied by morphological (high resolution microscopy), structural (X-ray diffraction analysis) and electrical measurements (capacitance-voltage and current–voltage characteristics) of finished devices.

The FP7 EU funded ALPINE project is focused on the processing of new type of thin film CdTe and CIGS photovoltaic modules by means of pulsed fiber laser scribing. The latest results of scribing tests performed for different laser parameters for P1, P2 and P3 steps in both CdTe and CIGS solar cells are discussed showing examples for CdTe module processing.

CuInGaSe 2 /CdS thin film solar cells were prepared by using a novel precursor obtained by depositing in sequence, on top of a Mo covered soda lime glass, In 2 Se 3 , Ga 2 Se 3 and Cu. A subsequent few minutes selenization of the precursor in a selenium vapor is sufficient to obtain a good quality CuInGaSe 2 film.

In the past, we described a dry process for the fabrication of CdTe/CdS thin film solar cells. In this process, most of the layers composing the cell, namely ITO, ZnO, CdS and back contact are deposited by sputtering and CdTe is deposited by close-spaced sublimation. The treatment of CdTe is done at 400°C, for 10-20minutes, without CdCl2, by using a Freon gas, dichlorofluoromethane, as a Cl supplier. Back contact is made by depositing onto a not-etched CdTe film surface a buffer layer of As2Te3, followed by a thin layer of Cu and Mo. Recently, we modified the Cl-treatment and we improved the quality of the back contact. Since dichlorofluoromethane is an ozone depleting agent and its use is now forbidden, we replaced it with a gas, like Ar, containing 4%HCl and mixing it with a Fluorine-containing gas, such as CHF3. Both these gases are not depleting agents and they aren’t forbidden. By adjusting the relative amounts of the chemical species into this mixture, we got results which are very similar to those obtained with dichlorofluoromethane. Concerning the back contact, we discovered that, by using as a buffer layer As2Te3, or Bi2Te3 and by making an annealing in air at 200°C for 15-30minutes, a stable and ohmic contact, without any rollover in the solar cell I-V characteristic, is obtained.

Laser scribing tests on CIGS based thin films solar cells have been performed. The obtained high quality incisions show that laser scribing is a valuable tool for producing low-cost photovoltaic modules.