News & Events

Within the IBC4EU project, Imec— a partner in EnergyVille, located in Genk, Belgium—is further developing its 3D multi-ribbon interconnection and module technology for application in interdigitated back contact (IBC) solar cells. This technology uses a 3D interconnection fabric to reduce the number of processing steps, optimize material consumption, and minimize stress buildup in soldered joints.

The 3D multi-ribbon fabric consists of a polymer-based encapsulant foil with integrated solder-coated metal ribbons. A schematic of the fabric design is shown in Figure 1 below. The polymer-based foil acts as a carrier and separates two layers of metal ribbons, providing electrical insulation. The red and blue colors of the metal ribbons indicate their intended opposite electrical polarities.

These vertical and horizontal metal ribbons are referred to as busbar ribbons and cell-to-cell ribbons, respectively, based on their functions. At specific locations in the fabric, the busbar ribbons are stitched through the polymer-based foil and overlap with the cell-to-cell ribbons, establishing a third interconnection dimension. In this way, the busbar ribbons collect charge carriers from the cell metallization and transfer them to the cell-to-cell ribbons.

Figure 1: Top and side view of the 3D interconnection fabric*

Fabricating modules with this interconnection fabric consists of two processing steps: a lay-up and a lamination step. The lay-up stack begins with the placement of a front sheet (glass), followed by the front encapsulant foil on which the cells are placed.The fabric then provides both solder-coated metal interconnection ribbons and rear encapsulant material. In one alignment step, this foil is laid over the cells in such a way that each busbar ribbon is located over fingers or busbars with the same electrical polarity.

During a single vacuum lamination step, the stack is heated under pressure, enabling simultaneous cell encapsulation and soldering of the ribbons. The solder coating on the metal ribbons reflows, resulting in soldered joints at the floating connection points and between the busbar ribbons and the cell metallization. A low-melting-point solder is used, which matches the temperature process window of the encapsulant. This inherently enables a more homogeneous and lower-stress soldering process. No interconnection is created at locations where the encapsulant separates two crossing ribbons.

The 3D interconnection method is an alternative to the more established tabbing/stringing and Conductive Backsheet (CBS) technologies. With a current Technology Readiness Level (TRL) of 4–5, Imec is working to progress the technology toward full-size module applications. Reliability testing of fabricated mini-modules incorporating back-contact cells has been very successful, passing 600 thermal cycles and 1,000 hours of damp heat with negligible degradation.

Imec is also testing the technology for application on Cu busbar as well as busbarless IBC solar cells, directly contacting the metallization fingers in the latter case.

Figure 2: Rear and front view of a 3D interconnected 4-cell IBC mini-module

Project Update

Imec is the work package lead in the IBC4EU project for the development of bifacial IBC modules and is involved in four different tasks: advanced IBC module design, manufacturing equipment, pilot module production and validation, and outdoor testing.

Imec has also taken the lead on an IBC4EU deliverable report comparing the different interconnection options for IBC cells (tabbing-stringing, CBS, and 3D interconnection).

An interconnection experiment between the partners was set up to weigh the differences between the interconnection technologies, using as much as possible the same Bill of Materials (BOM), culminating in comparative reliability testing.

In addition, the different technologies were qualitatively compared for several criteria: performance, bifaciality, sustainability, reliability, design freedom, cost-effectiveness, Technology Readiness Level (TRL), and market segment applicability.

* More information can be found here: R.Van Dyck et al, Progress in Photovoltaics, 2021, vol 29, p 507-215 (DOI: 10.1002/pip.3390)