By Thomas Netter
Gain in photovoltaic conversion efficiency is one of the most notable measures of progress in the field of photovoltaics. An important factor that motivates applied research teams is to see the efficiency gains demonstrated in the lab find industrial applications within a relatively short time frame. This is why members of the Laboratory for Thin Films and Photovoltaics at the EMPA Swiss Federal Laboratories for Materials Science and Technology collaborate closely with Flisom AG, a spin-off of the ETH Swiss Federal Institute of Technology Zurich.
Flisom is building up a pilot plant in Switzerland to market high-efficiency and ultra-light flexible solar modules based on thin-film Cu(In,Ga)Se2 (CIGS) on polyimide technology. CIGS is known for providing the highest efficiency and intrinsic stability among thin-film semiconductors. The aim of Flisom is to leverage the efficiency gains obtained with CIGS-on-polyimide to market flexible modules that provide a low total cost of ownership and a fast return on investment.
Benefits of Thin Film
Previous articles published in InterPV presented some of the benefits of thin-film technologies and their advantages in conjunction with plastic substrates (The Growing Popularity of Thin Film, July 2010; Next Generation Solar Panels, May 2010). Thin-film solar modules are manufactured by depositing a stack of thin layers, usually a few nanometers- or micrometers-thick, of semiconductors and metals onto a substrate such as glass, metal, or plastic. Thin-film solar modules use far smaller amounts of semiconductor material per watt produced than the first generation of silicon-based solar module technologies which date back to over 50 years. The material savings induced by thin-film technologies, therefore, help reduce production costs and pollution.
Choice of Substrate
Thin-film solar cells are mostly produced on glass substrates. Glass substrates are relatively easy to handle during production as they can sustain high deposition temperatures and will not contaminate the deposited materials. They also exhibit the smoothest surface for higher deposition quality and have a relatively low cost. This is why glass substrates have gained industrial maturity with several key players now manufacturing thin-film modules at annual rates of 10 to 150 MW, with the leader all the way up to 1.2 GW using a CdTe-based process.
Glass, however, is relatively fragile and incurs correspondingly high processing, transportation, and installation costs. The thickness and mass of glass substrates confer a thermal inertia that requires large coating machines which may be several hundred meters long in the case of integrated production lines.
Compared to glass, flexible substrates can be especially advantageous in that they offer the possibility of lower production costs associated with streamlined roll-to-roll processes. Flexible substrates are much less fragile than glass, require less heat to coat, allow rolled-up and compact roll-to-roll production, storage, and transportation. They also open avenues for lightweight and innovative installation solutions. With flexible substrates, one can envision the production of very large solar modules that may be as long as the length of the substrate’s roll.
Several flexible substrate materials are used industrially today and can be divided between metals such as copper, aluminium, or stainless steel, and plastics such as polyimide. A strong advantage of metal foils is that they can be coated at the preferred CIGS-compatible deposition temperature of 550. They also are low-cost but may have a rough surface and cause impurities to diffuse into the photovoltaic active layer, thereby degrading its conversion efficiency. A solution is to add a manufacturing stage where the metal foil is coated with an insulating layer. Possible drawbacks are pinholes and crack formation because of differences in thermal expansion coefficients between layers.
Although metal foil substrates are widely used for CIGS deposition, the Thin Films and Photovoltaics laboratory’s preferred substrate is polyimide. The surface of polyimide is smoother than that of metal substrates and it does not risk diffusing metallic impurities. This preference comes with a challenge, however: vacuum deposition temperature on polyimide must remain at a relatively low 450 to preserve the plastic’s integrity.
Thin-film CIGS Layer
Thin-film solar cells require the deposition of several material layers onto the initial substrate. For CIGS solar cells on polyimide this will involve :
-A back-contact of molybdenum deposited with a vacuum sputtering system
-An active layer, also known as the absorber, made of Copper, Indium, Gallium, and Selenium (CIGS) deposited by thermal evaporation within a vacuum chamber
-A buffer layer, for example, of zinc sulfide or cadmium sulfide, deposited with a chemical bath
-A front-contact of a Transparent Conductive Oxide (TCO) such as zinc oxide deposited with a vacuum sputtering system
Of all parameters, temperature is one of the most important factors to monitor during CIGS deposition. Not only will temperature affect the relative CIGS composition, but it will also tell how well the different material layers adhere to each other. The thin film must withstand stresses associated with different thermal expansion coefficients of both the substrate and the material layers that compose it. Thin-film mechanical endurance is even more important in the context of flexible substrates.
Defects in thin-film solar cells such as layer delaminations, cracks, and non-uniformities in the thin film will reduce their efficiency. The consequences may be more damaging when an entire solar module includes one or more defective solar cells. Manufacturing cost savings brought by thin-film technologies are, nevertheless, worth the risk, especially when one considers the benefits of integrated manufacturing of monolithic solar modules.
Monilithic Solar Modules
One of the greatest advantages of thin-film technology is that it enables the design of monolithic solar modules. In monolithic solar modules, cell-to-cell interconnects are established within the thickness of the semiconductive layers as part of the integrated manufacturing process. A monolithic series interconnect links, for example, the back-contact of a first cell to the front-contact of a second cell. Such interconnects are the result of combining successive layer depositions with laser scribing steps (Figure 3).
The result is that very large and highly reliable solar modules, even custom-designed modules, can be created roll-to-roll and as a single piece without incurring the costs usually associated with welding or shingling individual cells. Furthermore, such monolithically interconnected modules enable further weight savings with already ultra-light polyimide foils.
Gains in electrical conversion efficiency of thin-film photovoltaic technologies also help increase their range of applications: the higher the efficiency the more electricity produced per unit area. This is especially important for Building-Integrated and Building-Applied Photovoltaics (BIPV & BAPV) as well as mobile applications where usable area to install solar modules is limited.
Although the overall efficiency of a photovoltaic installation is considerably less than that of the square-centimeter record-breaking solar cells produced in laboratories, such records indicate a given technology’s progress and later potential to reach the goal of electrical grid parity and compete with other energy sources.
Efficiency Gains of CIGS on Plastic
The 17.6% record efficiency was obtained with thin-film CIGS solar cells on polyimide. Thin-film photovoltaic conversion efficiencies have ordinarily been led by solar cells on glass substrates, followed by those on metallic substrates such as stainless steel or copper, in turn followed by those using plastics such as polyimide. CIGS-on-polyimide is now on par with CIGS-on-metal and therefore closer to CIGS-on-glass and polycrystalline silicon-based (poly-Si) cells that dominate the market.
Gains in efficiency will eventually see the runner-ups land in the top ranks. For example, efficiency records of thin-film CIGS solar cells on glass (20.3% in August 2010), have grown 1.5% in absolute efficiency between 2003 and 2010. In comparison, efficiency records of thin-film CIGS solar cells on polyimide, now at 17.6%, have grown 4.8% over the same period (Figure 4). The growth in record efficiencies of CIGS on plastic has not leveled off yet. There is strong confidence that the efficiency of CIGS on plastic will also reach that of CIGS on glass, which is deposited at higher temperatures and, therefore, with a higher thermal budget than CIGS on plastic.
The 17.6% record was achieved under the leadership of Prof. Ayodhya Tiwari who is both head of the Laboratory for Thin Films and Photovoltaics at EMPA and chairman of Flisom AG, Switzerland. Tiwari’s laboratory has produced several record-breaking solar cells over the past decade. The first promising breakthrough materialized in 1999 with a 12.8% efficiency record with CIGS on spin-coated polymer film. In 2005, it was with a 14.1% CIGS cell manufactured by Flisom’s co-founder David Brmaud while working on his Ph.D. that the laboratory realized that polyimide is suitable for industrial production. In June 2010, it was doctoral student Adrian Chirila and his colleagues who claimed the trophy again.
Detailed results of measures certified by the Fraunhofer-Institut fr Solare Energiesysteme, Freiburg, Germany, are shown in Figure 5 and 6. The VOC is remarkably high for CIGS on polyimide as it was previously considered difficult to obtain values greater than 600 mV. This suggests only few carriers are recombining; an indication the CIGS layer is of very high quality and that electrons are not getting trapped in layer defects. This achievement was obtained thanks to careful control of the gradient of gallium across the thickness of the CIGS layer while also taking care to incorporate sodium during the final growth stage. The result is an optimally-graded band gap and larger grain sizes, thereby producing a CIGS semiconductor layer where optical and electronic losses are minimized.
The graph for external quantum efficiency vs. wavelength (Figure 6) reveals several key points:
-The top of the curve is flat and extended. This indicates that photovoltaic conversion extends deep into the absorber layer and that carriers are effectively collected.
-The flat top of the curve is also high up, exhibiting a loss of less than 10%.
-There is a characteristic bump on the short-wavelength side of the graph (between 400 and 500 nm) caused by light absorption at the cadmium-sulfide buffer layer.
The challenge is now to port this high-efficiency know-how to roll-to-roll industrial manufacturing. Flisom AG was founded in 2005 to scale up a laboratory’s technology to an industrial level. The first effort was to master large-scale production of monolithic interconnects on polyimide foils. Today Flisom is completing its first integrated roll-to-roll pilot plant that contains all processing stages to manufacture large, ready-to-use, flexible solar modules.
The high efficiency of a laboratory’s solar cell cannot be translated directly into similar gains for an industrial-level solar module. It may, however, contribute to the overall production’s output measured in MW, and, therefore, speed up decreases in cost per MW as the factory scales up its production by adding new machines. This is particularly important in a market where many new entrants have enjoyed subsidies, and where some manufacturers are selling modules at prices which probably do not enable them to recoup their costs.
In a market dominated by rigid and heavy silicon-based solar panels, weight savings provide a selling point to establish thin-film flexible solar modules. At about 2 kg/m2, flexible solar modules are substantially lighter than the 18 kg/m 2 rigid glass- or silicon-based modules. Thanks to their low specific weight, installation on large roofs, such as those over factories and depots, requires no structural reinforcements. The entire installation process then becomes easier, faster, safer, and less expensive. Saving on installation costs is especially beneficial since that segment may represent about 40% of a total residential system cost.
Longevity Ensures Payback
Gains in efficiency contribute to a faster payback. However, apart from integrating all technologies that enable flexible solar modules to be lightweight, low cost, and high efficiency, another challenge is to ensure solar panel longevity. Well-built rigid silicon-based panels encapsulated in glass sheets are known to work effectively for more than 25 years. An essential aspect of flexible module longevity, therefore, lies in the back-end encapsulation process to provide mechanical protection, a water barrier to prevent corrosion, and to ensure stability of the solar modules’s layers. Flexible encapsulation currently uses fixed-size flat plastic laminates and, therefore, limits the final size of monolithic modules.
Unfortunately, flexible encapsulation may also cost more than 30% of the final module’s price. It is, however, the most important ingredient for a lasting payback. This is especially important for BIPV to ensure an installation reliably provides power and continues to pay back competitively despite falling feed-in tariffs, further efficiency improvements of a given technology, and the advent of still newer technologies.
Solar modules on flexible polyimide substrates provide gains across the entire supply chain. From an energy consumption viewpoint, the energy budget to produce the polyimide substrate is lower than that needed to produce stainless steel and temperatures for thin-film deposition on polyimide are relatively low. From a distribution viewpoint, storage and transportation is more compact and lower cost than with conventional rigid modules. From an installation viewpoint, further savings result from the fact that large installations on buildings do not require structural reinforcements.
The lower cost per watt, faster payback, reduced complexity, and light weight of flexible CIGS solar modules open avenues for new applications and greater coverage. Flexible solar modules may be installed cost-effectively at non-optimal angles such as on walls or facades of both permanent and temporary infrastructures. Their flexibility also enables their integration into curved architectural elements, shades, awnings, and umbrellas. They may also be used more reliably on parts subject to vibration such as on cars to power standby ventilation, on sailing boats to power all electrical systems, and for various aerospace applications. Other applications that will benefit from the combined flexibility and high efficiency of CIGS-on-polyimide solar modules include mobile devices, apparels, and various systems for outdoor operations.
Flexible thin-film solar modules will gain market share and eventually a dominant position thanks to an installed cost per watt that is lower than that of established photovoltaic technologies. This is helped by increases in photovoltaic conversion efficiency so as to compete against, for example, polycrystalline silicon.
Another key objective is to deliver electricity at or below grid price so as to eventually compete equitably with more polluting energy sources. The cost of large-scale photovoltaic installations, whatever the technology, is closely related to the size of the area covered by the installation. Here too, how well efficiency gains of CIGS-on-polyimide will transfer to large encapsulated flexible panels will be a key to large-scale adoption.
More and more electric utilities offer their customers the possibility to select a mix of energy sources and this mix includes electricity based on clean or renewable sources. It turns out that the demand for renewables-based electricity is often greater than what the utility can provide. The utility, then, has to purchase ‘clean’ electricity from remote utilities that may even be, as is increasingly the case worldwide, beyond national borders. This stimulates the need to add local solar module installations, often subject to surface constraints. This is probably where CIGS-on-polyimide will be most competitive.
In conclusion, the synergy between the scientific research group that produces record-breaking solar module prototypes and its industrial spin-off is an example of how performance gains can be beneficial to both sides. On the one hand, several innovative techniques that were demonstrated at laboratory scale have had to be scaled up, and on the other hand, new challenges appearing in large-scale production have had to be approached scientifically. The mutual gain in credibility resulting from a bidirectional innovative and applicative channel has fostered the progress characterized by the 17.6% efficiency record. Improvements in the processes that bring laboratory efficiency gains closer to industrial production are essential to increasing competitiveness of new photovoltaic technologies.
Dr. Thomas Netter manages intellectual property at Flisom (http://www.flisom.ch/).
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