Reduction of silver content in Electrically Conductive Adhesives for low-temperature interconnection of solar cells

The interconnection of solar cells with electrically conductive adhesives (ECAs) is a promising lead-free, low-temperature, low-stress interconnection technology, with proven reliability in ageing tests. However, ECAs contain large fractions of silver particles, contributing to a high consumption of silver in the photovoltaic industry. At the same time, most of the ECAs used are commercial products with poorly known composition, and appropriate methods to characterise their fillers are lacking.

In this work, we investigated the link between ECAs’ resistivity and the content and morphology of their fillers. Chemical dosages and observations of cured ECAs with electronic microscopy were supplemented with the development of a new method allowing to wash away the resin of ECAs and observe the particles themselves. This last method allows simultaneously a precise determination of the particles’ morphology and an overall reliable estimation of ECAs’ silver content, which is not possible with other reported means.

We showed that some ECAs can contain silver particles with tortuous shapes, while others contain optimised particles with flaky or core–shell morphologies, often under the form of a mix of particles with various sizes and shapes. Resistivities down to (8.18 ± 4.91) × 10^-5 Ω cm were measured for ECAs containing more than 50 wt% silver. Conductive behaviours can be obtained along with reduced silver contents as low as (25.6 ± 1.3) wt% when optimised fillers are used to make ECAs.