IBC-SHJ cell interconnection with multi-wire reaches major milestones
The challenges of interconnecting interdigitated back-contact and heterojunction cells is connecting opposite cell polarities in the same plane with selective isolation and the necessity to use low-temperature processing. To develop an industrially relevant technology, low-cost and simplicity are essential but unmet requirements by current solutions.
The concept of using a multi-wire approach for contacting IBC cells has strong advantages: it will lead to a large reduction of power losses in the fingers and will have a huge potential in cost decrease due to lower material usage and the possibility of passing from thick copper electro-plating to ultra-fine-line screen-printing. The application of SmartWireConnectionTechnology (from Meyer Burger) for back-contacted heterojunction solar cells is based on a multi-level metallization scheme of the cells. The first step is the screen-printing of fine-line silver fingers on both electron and hole collecting areas in the same printing-step, followed by screen-printing of conductive pads and finally by application of insulation pads, to be able to contact only one polarity for one wire over two. The thickness of the conductive pads is about three to four times the one from the fine silver finger enabling the possibility of isolated lateral conductivity in the wire and transverse conductivity in the finger.
a) Scheme of SWCT interconnection of IBC and b) 3D confocal image of contacting pad and finger
At CSEM further effort focused on careful optimization of the process parameter and on improving the reliability of the multi-wire technology. This enabled to pass the IEC standard 61215 for thermo-cycling between -40°C and +85°C and damp-heat at 85°C and 85% relative humidity with less than 5% degradation.
Finally, CSEM and Meyer Burger Research could demonstrate a 23.2% efficient laminate on 90.2 cm2 based on a 23.9% efficient solar cell. This samples reaches an impressive 99.3% CTM measured on the final FF of the record module, demonstrating the high-performance module integration achieved with the developed scheme.
Current Voltage properties of record laminate
As a next generation technology to further lower the cost of interconnection, imec has introduced the 3D woven interconnection concept based on standard module materials. Polymer encapsulant is interwoven with low-temperature solder-coated metal ribbons to form a three-dimensional woven fabric. The encapsulant provides the electrical insulation between ribbons of opposite polarity. Part of the ribbons electrically interconnect neighboring cells while ribbons in a perpendicular direction connect the cell metallization to the cell-to-cell ribbon, hence creating a two-level metallization. Additionally, the co-development of the materials and the lamination process enables a combined soldering and module lamination in one process step and within the limits of industrial laminators. We will demonstrate the adaptation of this technology to bifacial IBC cells as well as to busbarless cell design. We report on the properties of the interconnect in single-cell and mini-modules demonstrating < rel. 2% FF loss, Rsh> 7MΩ in isolation tests and showcase its stability in over 400 thermal cycles following IEC61215 requirements.