Reliable Interconnection of the Front Side Grid Fingers Using Silver-reduced Conductive Adhesives

AbstractElectrically conductive adhesives as an alternative interconnection technology can potentially avoid the need for busbars on crystalline silicon solar cells. The adhesive is applied to the grid fingers and the ribbons for module integration can be directly attached to them. We analyze the interconnection related power losses by establishing an electrical model and validating the model with experimental I-V curve data. The maximum error is 7% for one-cell-minimodules. In the following, we select silver-reduced adhesives and tin-coated ribbons to build minimodules and perform environmental chamber tests. The interconnection related cell-to-module losses are higher by 0.5% compared to standard soldering on busbars. The minimodules with silver-reduced glues and tin-coated ribbbons are stable in 1000h damp heat and degrade by a maximum of 3% after 200 thermal cycles. Only the highly Ag-filled acrylate failed the thermal cycling test

A Comprehensive Study of Intermetallic Compounds in Solar Cell Interconnections and their Growth Kinetics

AbstractIntermetallic compounds (IMC) in soldered interconnections influence the reliability of PV modules. Thus, the microstructure of solar cell interconnections and the growth of IMCs are to be investigated in this paper. Sn60Pb40 and Sn41Bi57Ag2 are chosen as alloy coatings of copper interconnectors and semi-automatically soldered to screen-printed front Ag-busbars of industrial mono-crystalline solar cells. The microstructure of the solder bonds is characterized with metallographic cross sections and confocal laser microscopy, as well as scanning electron microscopy and electron dispersive x-ray spectroscopy. The cross section samples are isothermally aged between 85°C to 150°C and for 15 hour to 155 hour to obtain the kinetic parameters of a diffusion-based growth model of the IMCs. The model is used to estimate the IMC thickness after 3000h at 85°C, and after 600 thermal cycles as well as after 25 years in the outdoor location Freiburg, Germany. It is found that extensive microstructural changes take place within the solder bonds during thermal aging. Grain coarsening within the solder matrix, in particular for Sn41Bi57Ag2 solders, is observed, which can lead to an entire Sn depletion of the solder matrix. Moreover, non-uniform Sn penetration and IMC growth at cavities and lead-glass particles of the busbar are observed for both solders, which is discussed in terms of its effect on metallization adhesion. Eventually, simulating the IMC growth for 3000h at 85°C forecasts a 3.7μm thick Ag3Sn IMC at the busbar for the Sn41Bi57Ag2 solder compared to 2.6μm for Sn60Pb40. The prognosis of the IMC thickness after 25 years in Freiburg yields an Ag3Sn thickness of 1.3μm for Sn41Bi57Ag2

Reduction of Thermomechanical Stress Using Electrically Conductive Adhesives

We compare the thermomechanical stresses in solar cell interconnections based on electrically conductive adhesives (ECA) with soldered joints by using bending experiments and finite element analysis (FEA). Additionally, the influence of an increasing number of busbars is studied. The FEA is validated by measuring the bending of cell strips after cooling down from a single-sided interconnection process. The material parameters are determined by tensile tests, microscopy and nanoindentation. The comparison of ECA and soldering shows that an elastomer with a Young's modulus of below 0.5 GPa is capable of reducing the thermomechanical stress effectively resulting in, approximately, a mean tensile stress in the ECA of 5 MPa, 110 MPa in the ribbon, and a maximum compressive stress in the silicon of 75 MPa. Increasing the number of busbars from three to five leads to a reduction in compressive stresses in the silicon and a slight increase of the peak tensile stress in the busbars

Electrically conductive adhesives for photovoltaic modules

Electrically conductive adhesives (ECAs) are used for the interconnection of crystalline silicon solar cells during the photovoltaic module integration as an alternative to soldering. These materials are lead-free, are processed at a temperature below 200 °C and are particularly suited to interconnect sensitive solar cells due to their mechanical elasticity. In this work ECAs are characterized and assessed with regard to their appropriateness for PV module integration. Electrical characteristics such as volume and contact resistivity are analyzed as well as mechanical properties (e.g. Young's modulus) or thermal stability. Besides characterization of the materials the effect of their proper-ties is studied by simulation. The relation to characteristics of the PV module such as maximum power and fill factor is obtained. Significant results of this work are the definition of assessment criteria for ECAs, the demonstration of a 60-cell photovoltaic module based on a new solar cell metallization pattern and ECA interconnection technology

Lead-free Solders for Ribbon Interconnection of Crystalline Silicon PERC Solar Cells with Infrared Soldering

We report about the analysis of Pb-free, low-temperature solders for the ribbon-interconnection of PERC solar cells with an industrial infrared stringer. Five solders (SnPb, SnBi-A, SnBi-B, SnBiAg and a proprietary lead-free, low-temperature (PLFLT) alloy) are characterized with differential scanning calorimetry to determine the melting and solidification temperature. It is found that SnBi-B, SnBiAg, and the PLFLT composition melt in a temperature range between 137 C to 171 C instead of a single temperature. Solidification occurs at a 3K to 11K lower temperature (undercooling). Mono-crystalline silicon PERC cells are contacted using an industrial stringer. The microstructure of the solder bonds is investigated with scanning electron microscopy. For the SnBi-A-solder, large and brittle Bi-phases are identified. The SnBi-B, SnBiAg and PLFLT solder show a finer grain structure. The added Ag in SnBiAg forms an intermetallic compound of Ag3Sn close to the Cu-core of the ribbon. The peel strength of the connected solar cells with the Pb-free solders is on average 1Nmm\u1000001 or slightly higher. Some bonds show low adhesion. The observed fracture mode is mainly failure at the busbar metallization to solar cell irrespective of the solder type. However, the occasionally observed solder residues on the metallization clearly reveal brittle fracture for the Pb-free solders, which is not observed for SnPb. First reliability tests show similar degradation of 1% to 2% for all solders