Screen-printed solar cells require front surface metal contacts to allow current to flow from the generated carriers. The design of the front metal contacts is critical. The metal contact is made of fingers and busbars. The metal contact has 2 or more busbars. The larger number of busbars can allow a reduced height of the screen-printed fingers for a metal resistive loss. The design is optimised based on the shading loss and the metal resistance loss. Electrically, it will either affect JSC or RS, respectively. The typical width of the finger width is 55 – 80 μm. The front contact (silver) transport the current from the peripheral regions of the cell to the busbars, which are typically perpendicular to the fingers. the cells are interconnected to form modules. When cells are connected to make a module, the interconnect ribbon is soldered to the busbars and connects to the p-type contacts on the rear surface of the adjacent cell in a string of cells.
In the video below we show the screen printing process at the Solar Industrial Research Facility (SIRF) at UNSW Sydney.
Front contact
The silver front contact pattern is printed directly over the silicon nitride anti-reflection coating (ARC). Therefore, the silver pattern is required to penetrate through the ARC coating to make an electrical contact with the silicon. The electrical contact is made when the cell is co-fired in an inline firing furnace. The rear contact is also made during the co-firing process. The co-firing process involves a peak firing temperature in a range of 750 to 870 °C for 5 seconds or less. During the process, the paste etches the ARC coating and penetrates through the layer and forms an ohmic contact with the underlying silicon. However, it is important to optimize the firing temperature and time. When the firing process is done at either too high temperature or too long time, the front contact can penetrate deeper into the silicon and make a contact close to the junction. This will effectively increase the contact resistance (so higher RS) as the metal will make a contact with the more resistive region of the wafer. In addition to the binders and solvent required to enable screen-printing (as described for aluminium screen-printing), the silver paste contains silver particles, glass frits (particles) and additives such as lead or bismuth that reduce the melting temperature of the silver and help wet the surface for uniform contacting. A picture of a front screen for a 3 bus bar solar cell is shown in Figure 1.
Rear contact
Most of the rear surface of the solar cell is screen-printed with aluminium paste to form the rear electrode. Additionally, the tabs are also printed with silver paste for the interconnection to other cells by soldering. Optimization of rear contact is not as critical as the front contact, but it is still important to optimise to improve the rear performance. A thick layer of aluminium paste (typically ~ 30 μm) is printed, with deliberated gaps and dried before the silver paste was also printed to form the silver busbar tabs. An undesirably thick layer of aluminium can lead to a wafer bowing during inline firing. The firing through inline furnace involves a rapid heating and cooling, which can build up the stress in the Si wafer due to the difference in the thermal expansion coefficient between Si and Al. The tolerance for the wafer bow is up to 1.5 mm otherwise it will affect the module fabrication process. Currently, most of the industrial solar cells have a full aluminium rear contact (the so-called aluminium back surface field (Al-BSF) solar cell. This technology still has a 70% market share, although it is expected to drop in the next ten years [1]. During the firing process in the firing process and aluminium-silicon eutectic is formed at firing temperatures in excess of 570 oC. During the cooling phase the silicon recrystallises, and an aluminium-doped silicon layer is formed where the aluminium concentration is determined by the temperature at which the crystallisation takes place ruled by the aluminium-silicon phase diagram. This recrystallisation continues until the eutectic temperature is reached and the whole liquid crystallises. This process thus results in a p-type doped region at the rear of the solar cell which assists in the collection of holes. In addition, this reduces rear surface recombination as well.
In the video below we show you the contact firing step, which is the final step in solar cell manufacturing.
Double print
Standard screen printing method for front side metallization of silicon solar cells is a reliable and well-understood process with high throughput rates. The typical line widths that is required to ensure the process stability and sufficiently lower metal resistance is about 120 μm. To achieve higher efficiency of crystalline silicon solar cells, both open circuit voltage VOC and short circuit current density JSC need to be improved. One approach to improve them is to have high sheet resistance emitters. Screen paste has been optimised to contact lowly doped emitters, hence higher sheet resistance. However, higher sheet resistance will lead to higher series resistance Rs from lateral resistance of the cell, which can reduce the fill factor. This can be compensated by the finger spacing, which increases the shading area fraction of the front side structure. Therefore, a reduction of the line width is necessary, to minimise the shading losses. Reducing the width of finger by reducing the width of the line opening in the screen can overcome but this can lead to a smaller cross-section area of the fingers, which can lead to a higher metal resistance. This can be mitigated by performing a double print which can significantly increase the height of the metal fingers. This is enabled by the excellent alignment uniformity of the current generation of screen printers which have an alignment precision of 15 µm or better. An additional benefit is that potential finger interruptions of the first print can be fixed by the second print as it is improbable that the interruptions of two different screen printers would occur at the same position.
[1] International Technology Roadmap for Photovoltaics (ITRPV) 10th Edition, 2019