Sealing and Bonding Benefits
Assembly of Thin Film Modules with New Back End Processes
Production costs of thin film modules can be further reduced by introducing new back-end processes. To find the optimum contacting solution, the pros and cons of each technology must be compared together with their suitability for the different connections in a thin film module, keeping the whole current path in mind. It is also important to determine which contacting technology works well in order to gain an meaningful cost comparison of different processes.
Fig. 1: To protect solar cells against humidity the operator can use a double glass structure with a butyl rubber sealing where the sealant is directly dispensed onto the glass
Attainment of grid parity will require a further reduction of production costs. Much effort is therefore being invested in front-end processes to increase efficiency and throughput. However, the production cost of thin-film modules can be
reduced not only by increasing efficiency and throughput, but also by introducing new and improved back-end processes. Two processes in the pre-lamination sector of the back end are contacting and sealing.
Whereas the standard contacting technology for connecting crystalline cells is soldering, there is no standard contacting technique for thin-film modules. The different contacting techniques actually used for such modules are gluing with conductive adhesive, ultrasonic bonding, soldering, and EMI shielding tape.
Fig. 2: One component dispensing head with sensor
Several module manufacturers use an epoxy-based adhesive, highly filled with silver flakes to connect the first and the last cell. The different glues are available either in a premixed and frozen state or in two separated cartridges containing resin and hardener. The premixed and frozen glue needs to be stored at –40 °C. The handling and especially the thawing is delicate because the cartridge containing the glue expands faster than the glue itself and air is sucked in. The air in the thawed cartridge will also be dispensed, especially when the cartridge is nearly empty. This can lead to interruption of the gluing bead or to bad usage of the small cartridges. Since the adhesive glue is highly filled with silver flakes it exhibits thixotropic behaviour. This causes no problems if production proceeds without any interruption. However, if the production flow is interrupted for several hours, the viscosity will change and the dispensing parameters need to be adapted.
Handling of the two-component glue is much easier. It can be dispensed from Semco cartridges (168 ml) or even hobbocks (20 000 ml), whereas the premixed and frozen glue can only be dispensed from small cartridges (30 and 55 ml are most commonly used). In addition, the pot life is much longer. This leads to a simpler handling and lower material cost for the two-component glue. A two-component dispensing head with dynamic mixer must be used for this glue. Cleaning is easily accomplished by rinsing the mixing head with one component and, in the case of volumetric dosing, the dispensing behaviour is unaffected by thixotropy.
Fig. 3: Two-component dispensing head with dynamic mixer
Ultrasonic bonding is a well-proven contacting technology used in the semiconductor industry. It allows solar modules to be reliably contacted without the use of additional materials such as conductive adhesive, solder, or flux. The ribbon (preferably aluminium) is moved back and forth on the back metal at a specific frequency, amplitude, force, and power. Several parameters (frequency, deformation, and current) are constantly monitored and used for integrated process control. The two materials are welded together without any noticeable increase in the temperature of the substrate. The metallic connection between the ribbon and the back metal provides minimal contact resistance and great mechanical strength. Extensive reliability tests have proven the long-term stability of this contacting technique.
Fig. 4: On use of hot-melt sealing the user benefits from better adhesion, increased accuracy, and greater autonomy
Two soldering techniques
Conventional soldering uses a flux to break the surface oxide. However, flux is very aggressive and unused flux is known to attack the different layers of thin-film cells and shorten the lifetime of a thin-film module. In ultrasonic soldering, the flux can be eliminated since the ultrasonic power serves to break the oxide. However, the soldering temperature is still high (up to 250 °C), introducing thermo-mechanical stress into the layers. Unfortunately, lower melting solder either contains lead or is more expensive (indium solder). In addition, integrated process control of soldering is difficult. Another method of connecting the first and the last cell is to use a self-adhesive, conductive tape such as is used for EMI shielding. The conductivity through the adhesive is established by an embossed backing or conductive fillers. Application of this conductive, self-adhesive tape is simple compared to the other options. However, the pressure has to be maintained over the entire lifetime of a thin-film module, which exceeds 25 years.
However, not all of these processes can be used with every back metal. Also the material of the ribbon and the process need to be a good match.
Fig. 5: Lamination allows high throughput due to optimised temperature uniformity, high heat capacity, and high uptime
Sealing technologies
Some thin-film technologies are very sensitive to moisture (such as CI(G)S); other technologies (e.g. a-Si) are less sensitive. Today, all CI(G)S and CdTe modules are protected against humidity by using a double glass structure with butyl rubber seal between the glasses. The butyl rubber is available as tape on a roll, as well as in a barrel for hot melt dispensing.
The butyl rubber tape is formed by extruding butyl rubber onto a liner. The tape is then rolled, packed, and shipped to the module manufacturer. There the tape is applied to the glass. However, the tape rolls only last for a short time and must be replaced every 30 to 45 minutes. In addition, precise and automatic application of the tape is challenging.
Instead of extruding the butyl rubber onto a liner, the it can also be dispensed directly onto the glass. This has advantages over the use of tape:
- More cost efficient solution (saving one process step),
- improved adhesion and humidity protection (since butyl rubber is applied hot),
- higher accuracy (tolerances from liner disappear), and
- improved autonomy (200-l barrel lasts for roughly two days in 24/7 operation).
Material Cost Savings With New
Back End Processes
Compared to the most widley used processes (contacting with premixed and frozen glue and sealing with tape), the new back end processes two component glue, ultrasonic bonding, conductive tape, and hotmelt sealing lead to higher yield and higher uptime, both helping to save cost. In addition, there are savings in material costs. Two component glue for example is much cheaper since the glue is not premixed and not frozen (shipping and storage becomes easier). In addition, bigger cartridges (less waste, larger quantities) can be used. Switching to ultrasonic bonding saves more money because this technique does not need any additional materials such as expensive glue, tape or flux. Changing from tape sealing to hotmelt sealing also provides opportunities save material, since one process is eliminated (production of tape rolls).
Altogether, significant cost reductions can be accomplished by introducing new back-end processes. Material cost savings can reach up to € 1m in a 100-MW production line for contacting as well as for sealing. Hot-melt sealing offers additional benefits such as better adhesion, increased accuracy, and greater autonomy. Ultrasonic bonding also offers additional benefits, such as on-line process control and high reliability.
Images: Komax