Laser Patterned Electroplated Copper Contacts for Interdigitated Back Contact Silicon Solar Cells

Abstract

Metallization of interdigitated back contacts for Si-based solar cells (IBC), where both the n- and p-contacts are processed on the rear side of the device, currently requires costly and labor-intensive masking, photolithography, and/or screen printing approaches, which are difficult for scale-up to high-throughput high-volume manufacturing. Electrodeposition (ED) offers a low cost, scalable, and selective approach for IBC metallization, but is potentially complicated by the close geometry of the contacts and the need for simultaneous plating to both contacts to achieve high processing throughput. Direct laser patterning of a thin seed metal film offers a fast single-step process for defining contact areas, reducing both device handling and chemical use, and allows simple variation of contact design to implement external electrical connections for ED, as well as rapid testing of alternative pattern dimensions and the subsequent effects on device performance. A simple and flexible direct laser scribing approach for processing of IBCs through patterning of thin Ni seed layers prior to Cu electroplating will be presented.IBC patterns were laser-scribed on blanket evaporated Ni seed layers on a-SiNx:H-coated textured-Si wafers (Ni/a-SiNx:H/Si) prior to plating. Patterning was carried out at optimum laser powers to achieve electrical isolation of the Ni contacts. Pattern designs, with 12 mm long n- and p-fingers of 400 and 1200 µm in width, respectively, were fabricated to include tabs for external contacting for ED (Fig. 1a). Contact patterns were prepared using either double-pass-focused or single-pass-defocused laser conditions during ablation. Samples processed with either single (Fig. 1b) or double (Fig. 1c) laser passes show well-defined ~100 µm cuts, however, those patterned with the latter condition showed a ~20 µm laser-induced trench along the center of all scribe lines, due to overlap of the beam paths, and which was not observed for patterns processed with a single pass. Plating conditions from acidified CuSO4-based baths were optimized for deposit reproducibility, uniformity, and adhesion on substrates of non-patterned thin evaporated Ni seed layers on 150 µm textured-Si wafers. Films of growth rate ~0.1 µm/min were achieved, with contact resistance (RC) ~5 mΩ.cm2, and excellent adhesion, enduring pull tests of >5 N where the Si wafer broke before any peeling of the Cu layer.With plating to laser-patterned IBC substrates, significant Cu features grow from the edges of the laser cuts (Fig. 1d), eventually shorting the fingers and leading to unwanted deposition outside of the contact areas. Shorting was not observed with plating to IBC Ni seed patterns that had been evaporated through a shadow mask, suggesting residual conductive phases in the scribes are responsible for the growth of the shorts. Brief chemical etches were carried out after scribing to remove residues and, following Cu plating, etched samples showed attenuated growths into the laser scribes with the contacts remaining electrically isolated. However, for single laser pass samples, ~40 µm wide parasitically-plated Cu stripes (Fig. 1e) were observed along the center of the laser cuts throughout the scribed pattern. Energy-dispersive x-ray spectroscopy (EDS) mapping confirmed the stripe consists of discrete Cu islands, isolated from the growing layer, that eventually coalesce during plating (Fig. 1f). This Cu-stripe feature was not observed for double laser pass samples, indicating differences in properties of the scribes with the different laser patterning conditions.The formation of the Cu-stripe is postulated to be the result of laser ablation and chemical etch damage to the insulating a-SiNx:H layer that opens pinholes to the more conductive c-Si wafer (Fig. 1g). These higher conductivity points produce localized conduits of enhanced current density, sufficient to nucleate Cu, and allow subsequent rapid growth of islands. In contrast, with plating to double pass processed samples, the more extensive laser damage to the a-SiNx:H layer exposes a significantly larger area of the Si wafer, which results in lower current densities insufficient to nucleate Cu. These observed effects of damage to both underlying layers and to growth habits of subsequent films, highlight the need for careful optimization and control of laser conditions during fabrication of photovoltaic devices.Front-junction Si-based solar cells with Al back contacts were etched and re-contacted with ED Cu to IBC-patterned Ni seed layers that had been evaporated through a shadow mask. Device performance showed little change, maintaining ~18% efficiency, with decreased device series resistance through improved RC. These results confirm ED Cu as a feasible and industrially-relevant approach for processing high quality metal contacts for Si-based solar cells. Figure 1