Progress in Encapsulant-Integrated Multi-Wire Interconnection

Abstract

In previous contributions, we presented already several multi-wire interconnection approaches, both for back-contact [1,2] and 2-side contacted cells [3]. In this paper we want to report our progress in further investigating and developing the latter approach. We introduce our methodology to allow mapping of individual solder joint quality (that can be extended quantitatively) on commercially available solar cells. Applying the technology on 1-cell samples, we compare different encapsulants, lamination times and stitch patterns, and demonstrate a 2x2-cell module. We subject the fabricated samples to thermal cycling to get preliminary feedback on their performance reliability-wise, and to improve our understanding of the underlying mechanisms. Finally, we made a 9-cell module with busbarless cells and adapted metallization design, to show the full module-level interconnection potential of this technology. 1 DEADLINES AND DELIVERY In the field of c-Si PV interconnection, a growing interest towards multi-wire approaches for 2-side contacted bifacial and busbarless cells is materializing [1,2,3,4]. This trend is in line with the predictions put forward in the ITRPV roadmap [5], and is attracting attention due to the potential it holds for a significantly improved trade-off between optical (finger shading) and electrical (resistive transport) losses, while the potential reduction in cell metallization is promising from a cost perspective. These benefits are valid on the cell-level, but also materialize into the module-level performance. In this context, 2 such technologies are already in an advanced stage of development or even commercially available [1,2]. Though they are both implementing multi-wire interconnection, they are at the same time very distinct in technology: • One approach effectively mimics the standard technology by soldering wires on finger solder pads, replacing the busbar. As in standard production, this step is then followed by a separate encapsulation process [1]. • Another approach applies a contact foil incorporating the interconnect wires directly onto the metallized cell followed by a lamination process [2] in which the wires of the contact foil are soldered directly to the metal fingers of the cell. As a next-generation development, we propose an approach in which we immediately incorporate the wires into the encapsulant material for lamination. More details about the state-of-the-art and our approach are published in [3,4]. In this paper we want to elaborate on the progress we made in developing and characterizing this technology. 2 SAMPLE FABRICATION FOR DETAILED CHARACTERIZATION At present, we fabricate the interconnect/encapsulant foils through manual stitching of encapsulant foils with wires. For the cells we use commercial bifacial cells with 3 busbars and a screenprinted front-and backside metallization. This adds to the industrial relevance of the results, but the presence of overdimensioned fingers and busbars at the same time makes it difficult to determine the quality of individual solder joints as the collected current will be transported to a neighbouring wire with a low-resistive contact, thus blurring the results. To resolve this, we interrupted the fingers between where the wires will end up after lamination through laser scribing, as illustrated in Figure 1. Figure 1: picture and detailed inset of a cell laser grooved at the rearside This way, very little current will be collected from an area with a finger that is not contacted by the wire and will show up as a dark "pixel" in electroluminescence (EL) imaging, as illustrated in Figure 2. Thus it also allows to monitor degradation of the individual contacts, by checking the increase of dark pixels, as illustrated in Figure 3. Figure 2: EL imaging of a multi-wire laminated 1-cell sample without (left) and with (right) laser grooving indicating a method of visualization of the presence or absence of individual wire-finger solder contacts Figure 3: degradation of the individual contacts can be monitored throughout reliability testing: e.g. before (left) 35th European Photovoltaic Solar Energy Conference and Exhibition 71