In thin film technology, there are basically two different types of vacuum-based coating applications available; one for making all silicon-based thin film photovoltaics and the other for making CIGS (copper, indium, gallium, sulfur, and selenium or copper, indium, gallium and di-sulfur), CIS (copper, indium, di-sulfur) and CdTe (cadmium and telluride) based thin film photovoltaic modules.
The main difference is the substrate onto which the semi-conducting materials are deposited. The second difference is the layer onto which the contacting ribbons are finally applied. Furthermore, for depositing semi-conductive materials, the industry distinguishes between substrate and superstrate.
For making silicon thin film, in principle, the deposition of all semi-conducting layers is done on the front glass. This is the glass that is sunny side up once field-installed or what is commonly also known as the front glass. For producing CIGS, CIS or CdTe however, the deposition of semi-conducting layers is done onto the back glass. In contrast to silicon thin film, the EVA or PVB foil is on the aperture surface. The back contact of the semi-conducting layer system of CIGS, CIS and CdTe is often molybdenum.
When applying the longitudinal contact ribbon onto a silicon-based thin film module, we usually adhere the ribbon to a transparent conductive oxide (TCO) layer. This can be zinc oxide doped with aluminum (ZnO:Al) or, as in the case of CIGS, CIS or CdTe, molybdenum.
As we can see in the graphic Generic artwork – example of CIGS thin film photovoltaic module, each ribbon, the + contact strip and - contact strip are on the sunny side up of this photovoltaic module.
For thin film categories of photovoltaic products that are deposited on to flexible materials like stainless steel, polyimide or other plastic foils, both silicon and CIGS or CdTe follow the same coating/deposition principles.
Longitudinal and transversal
The longitudinal contact is the ribbon applied over the thin film modules length in the edge-area of the aperture surface, which is the semi-conducting active surface, to conduct and transport the anode or the cathode current. These ribbons are often referred to as fingers or larger metal bus bar strips. The purpose of the bus bar is to efficiently gather electrical current from the individual fingers and transport the current to a nearby junction box.
The transversal contact or cross contact is the one that brings the longitudinal contacts to the middle of the thin film module and gets electromechanically connected to the nearby junction box. This is the one-junction-box-strategy of transporting the current outside the photovoltaic module. Such a junction box will have two terminals for contact. Alternatively, a producer can restrict contacting to longitudinal contacts with two single contacts at least and thus apply two single terminal junction boxes at the backside of the thin film module. For designing the process at an equipment level, these different ways of applying the longitudinal and especially transversal contacts, lead to different machine concepts.
In the competition to achieve the best LCOE (leveraged cost for electricity), different cell structures on the photovoltaic thin film module can lead to having up to five longitudinal contact ribbons. In this application, for example, we will find three active aperture areas on one thin film substrate. The transversal contacting, in case the thin film module maker pursues a one junction box strategy, can be in the active area or in the edge insulation area. In this case, where the transversal contact ribbon is being applied to the edge insulation area, usually the ribbon is covered with a black shadowing self-adhesive tape.
One will typically find this in the production of CIGS/CIS thin film products. The reasons for applying such a self-adhesive tape are to make the contacting ribbon invisible to the eye, and to avoid any type of light reflection, as this ribbon is on the sunny side of the thin film module. Prior to applying the cross-contacting ribbons in the edge-deleted area, a double-sided insulating self-adhesive tape must be applied.
Another application of thin film photovoltaics is for building-integrated photovoltaics (BIPV). Here the aperture layout can be lateral to the longitudinal direction. In this thin film product design, the longitudinal contacting ribbons extend out of the thin film module and will get connected with junction boxes outside of the module itself. These become integrated in to frames for insulating windows, for example. There are a variety of applications for longitudinal and transversal contacting ribbons.
Applying the contacting ribbon
In principle there are four application methods to connect a ribbon to a thin film module:
- Ultrasonic welding
- Self-adhesive electrically conductive tapes
- Application of a copper flat wire by using electrically conductive adhesives
Let us first take a closer look at the contacting ribbon this application requires. Similar to the contacting ribbons for stringing silicon cells and bussing these strings to a module, thin film modules use this type of copper flat wire as well. As the laser-structured cells on thin film modules are not that wide, the industry uses either two or four millimeter wide ribbons. The thickness of this material is approximately 100 microns (µm). This contacting ribbon is coated or plated with tin-silver (SnAg) (96.5 percent/ 3.5 percent) in a thickness of approximately twelve to 13 µm. In other words, the electrical squareness for thin film module making using either two or four millimeter wide contacting ribbons accomplishes all technical specifications.
Soldering is well-known in the industry, but at the same time difficult to control. In the early days of thin film, the soldering used by some producers consisted of the contacting ribbons being applied to the module incrementally. The soldered bonds required a certain amount of solderable material – lead (Pb) where possible – as this kept the process temperature beneath 200 degrees Celsius.
This meant that a high level of thermal energy was induced onto the deposited layers and glass by soldering. This application therefore lead to high local thermal stress on the thin film module. It has been demonstrated in tests, that soldered contacting ribbons damage the deposited layers, which are locally separated from the glass. Such a thin film module is damaged and cannot be used.
The principle of ultrasonic welding is based on interlocking materials. Preferably, the materials that are to be interlocked have to be relatively weak. At the same time, a certain thickness is necessary. This is already one principle limitation using ultrasonic welding, as the longitudinal contacting ribbon that is contacted to a TCO layer is often very thin. A TCO like aluminum-doped zinc oxide (ZnO:Al) is no thicker than one µm. Tests have demonstrated that ultrasonic bonded longitudinal contact ribbons, which were incrementally bonded similar to soldering, failed the damp heat test within one hundred hours.
This application preferably requires aluminum contacting ribbons. Although this material is very weak, the aluminum oxide can become a cause of higher local electrical resistance and therefore is not ideal for PV applications. The application of ultrasonics for thin film PV induces a high level of mechanical stress onto the semi-conducting layers. This can lead to local damage in the form of micro-cracked semi-conducting layers and will lead to bad bonds over time. Self-adhesive electrical conductive tapes are nothing new in the industry. The bulk application seems to be in CdTe. The coated electrically conductive adhesive is an isotropic, conductive, pressure-sensitive tape. Such a tape conducts the electricity through the thickness (Z-axis). During the application, it is important that there is sufficient pressure on the tape in order to ensure the required electrical squareness.
Although this application of contacting ribbons obviously does work, the removal of the liner of self-adhesive tapes is an issue in terms of automation and logistics. Additionally, the limited length of material per reel does not contribute to a low cost of ownership on a tool level. For this application, solutions are available, but in such competitive circumstances, the financial viability of self-adhesives remains to be seen.
The application of electrically conductive adhesive is well known in the electronics industry. In the thin film photovoltaics industry, it is estimated that about 90 percent of all manufacturers work with this class of materials. The intrinsic electrical and mechanical properties are well designed for photovoltaics and, in principle, the module maker can choose from two basic products. They can opt for an epoxy resin-based formulation or an acrylic resin-based one. Both systems are endothermic adhesive systems, meaning they require heat to be cured. Epoxy resin-based adhesives typically require 150 degrees Celsius for approximately 180 seconds. Acrylic resin-based adhesives require 150 degrees Celsius for only five seconds of curing time.
Both adhesives can be fully cured. Another important aspect of electrically conductive adhesives is the percolation factor. This gives the user information about how much filler material has been dispersed into the adhesive system. In thin film, silver (Ag) is used as a filler. A typical percolation factor needed for this application to conduct the current is at least 78 percent. Once the amount of material falls below 78 percent, one observes a level of electrical conductivity but at a significantly higher electrical resistance.
The right amount of silver-filled electrically conductive adhesive also has to be applied onto the module. A sufficient material-securing adhesion is needed on the one hand, and good electrical properties on the other. Such information unfortunately is proprietary and cannot be disclosed in this article. The application of electrically conductive adhesives does have major advantages over other application technologies, as it does not induce any local mechanical or thermal stress. As the material is being microdispensed, controlling at highest reproducibility is enabled.
The process control using microdispensing as the application technology is very reliable. To control the amount of adhesive, for example, one can utilize speed control in relation to power supply of the micro-dispensing valve. The right position of the bead can be controlled by in-line sensor. Such a sensor not only identifies the right position of the bead, but also the pinholes.
The application of the transversal contacting always requires the application of an insulating self-adhesive tape, as one must secure any short-circuiting or leakage current. Furthermore, both ends of the transversal contacting have to be brought onto the back side of the thin film module. Therefore, these ribbon ends are fed through one or more holes in the back glass of the module. This is also the position where the junction box is finally attached to the ribbon end.
The role of the glass
As the semi-conducting layers are deposited on to a glass, like float glass, awareness of glass tolerances is important when designing the process. In this regard, one has to precisely specify not only length and width, but also the rectangularity of the substrate. Furthermore, consideration must be given to the thickness and the waviness of such a substrate. These are highly relevant for a very precise centering of the substrate in the process equipment.
In order to secure a sound application of longitudinal contacting, it is important to specify on such equipment the mechanical coordinates accomplished with the laser landscaping of the aperture surface. This is the cell-making part of thin film production.
If the glass specification in combination with the geometrical specification of the laser landscape the cell structuring have been respected, the positioning and centering of the glass substrate itself will be precise and reproducible at the highest system availability and yield. Any error that occurs will mostly be due to the glass.
The contacting ribbon
Usually made of copper, which is coated or plated with a thin layer of SnAg, a contacting ribbon can be categorized by its material hardness. This value is expressed as RP02. As a copper ribbon is very narrow, it tends to bend over length due to the manufacturing process and the way it is being spooled. For this reason, it is recommended to stretch the contacting ribbon by a factor of one to three percent. Overstretching can lead to damaging the copper crystalline structure and this, among other things, can lead to an increased local electrical resistivity.
Typical equipment requirements
There are a variety of issues and interests that thin film photovoltaic line operators have and the equipment industry has to comply with. Therefore, it might be obvious that such equipment can never be standardized like a stringer for interconnecting crystalline cells. From an automation point of view, it might appear simple to engineer. However, a non-appropriate contacting can have a negative impact on resistance (Rs) and Shunt resistance (Rsh) values. As many thin film back-ends do not operate a second flasher directly after the contacting, this application can lead to higher and hidden production costs. A well engineered and working contacting process, especially for the longitudinal contacting part, can deliver one Watt peak more on every module over time.
This implies that what is needed is very high precision and highest reliability at highest process availability. The process therefore has to be robust and, as is often the case, not based on time pressures when considering microdispensing the electrical conductive adhesive.
In this case, it must be an auger-based microdispensing system that can be controlled and regulated by speed control and power supply. This is simple economics. A reliable, stable interconnect is essential to assuring long-term life at the best LCOE.