Scanning Acoustic Microscopy (SAM) is shown here for the first time to be suitable for the visualization of defects like electrolyte leakage, faulty electrodes and gas accumulation inside coin and pouch battery cells. These failures are detected through the local atypical reflection of acoustic waves at faulty interfaces. Individual images are produced from the reflected wavefronts obtained at specific time delays allowing additionally information about the depth of the investigated failures. This fast and non-destructive visualization tool can be used for the quality control of battery cells during their production, contributing to a fast and economic screening of new materials or new production steps. SAM also brings a valuable contribution on the assistance in choosing representative spots of the battery for post-mortem analyses. SAM is in its infancy regarding the characterization of batteries. Fields for further development are suggested and discussed here.

One of the main challenges within Si solar cell production is the increase of throughput rates. With throughput rates in the range of 2400 Wafers / hour, metallization lines based on flatbed screen printing are typically the bottleneck of a modern solar cell production. Within this work we present innovative printing technologies allowing for higher throughput rates, e.g. rotational printing technologies as flexographic printing and rotational screen printing and as well multi-nozzle dispensing. In principle, flatbed screen printing processes can be completely replaced by using e.g. rotational screen printing for rear side printing and e.g. flexographic printing or dispensing for front side printing.. Hence, the realization of higher throughput rates and thus lower production costs is addressed. Moreover, new printing technologies allow for Ag paste savings up to 20% and efficiency increases of 1-2% relative by optimizing the finger geometry.

In this study, a new dispensing print head is introduced covering an operational width of 16 cm and therefore allowing for solar cell processing at industrial throughput rates. Like in previous ten nozzle versions, the interior paste flow was designed by computational fluid dynamics (CFD) using rheological parameters of applied Ag-pastes (commercially available pastes), including yield stress, shear thinning and wall slip behaviour. The novel print head features a homogeneous distribution of Ag-paste from an inlet supply with a diameter of Din = 2mm to an outlet slot with a width of ws = 160 mm. A first printing test followed by geometrical characterization of the dispensed contacts revealed a distribution of the contact finger width of wf = 35±1 μm across the whole wafer (156x156mm2) which demonstrates highest precision of the approach. The print head is directly integrated in an inline feasible dispensing platform, developed by ASYS GmbH. Respective nozzle plates can be customized to any desired front side grid regarding nozzle diameter, nozzle pitch and the total number of nozzles. In the actual version, the print head uses two strokes to print 100 contact fingers. Each of the 50 nozzles prints two adjacent contact fingers while moving up and down at a speed of up to vy = 700mm/s. In a first solar cell test sequence with the new 6” print head, a maximum cell efficiency of η = 21.2% on industrially preprocessed Cz-PERC samples was reached which demonstrates a successful launching of the print head.

Realizing narrow contact fingers with low lateral resistance is a major goal for the front-side metallization of silicon solar cells. The formation of screen- or stencil-printed contact fingers is governed by a variety of influencing factors. One of these factors is the surface roughness of the textured silicon wafer. However, only a few investigations have been carried out to investigate this impact in detail. In this study, the influence of arithmetical mean roughness R a of four differently textured wafer surfaces on contact finger geometry and lateral finger resistance, as well as optical and electrical losses, has been investigated. It will be shown that texture roughness has a considerable impact on the properties of the front-side grid. Narrower contact fingers could be realized on the smoothest texture, leading to a current density gain of Δj sc = +0.27 mA/cm 2 . On the other hand, increasing texture roughness has affected the amount of transferred paste and, thus, has led to a lower lateral finger resistance RL. Thus, contact fingers on the roughest texture have benefited from a fill factor gain of ΔFF = +0.24 % abs . A sensitivity analysis of both impacts has shown that the current density gain has overcompensated the fill factor loss. Thus, textures with a small roughness are beneficial with respect to the formation and electrical properties of stencil-printed front-side grids.

Rotational flexographic printing is a promising high-throughput technology for the front side metallization of silicon solar cells. Very low silver consumption and the possibility to realize narrow contact fingers make this technology particularly interesting for multi-busbar solar cells. Within this work, fundamental printing tests have been carried out on a flexographic roll-to-flat machine using an experimental anilox roll and elastomeric laser-engraved printing plates. A double printing process with intermediate drying step has been applied. Contact fingers down to 33μm in width and up to 8μm in height have been realized using this technology. Lateral resistances in the range 500 to 1500Ω/m have been determined by four point measurement method. These results underline the capability of flexographic printing for fine line metallization of multi-busbar solar cells.

Screen printed front side contacts of textured, mono-crystalline p-type silicon solar cells with n-type emitters were investigated. The different pastes (FSP1 and 2) and the different crystallographic orientations of the Si surfaces studied strongly affected the contact resistance. The microstructure of the contacts was analyzed in plan-view and cross-section by combined scanning and analytical transmission electron microscopy. A controlled grinding process rather than a chemical etching process was applied for the plan-view sample preparation. For textured cells processed with different pastes, pronounced differences were seen in the contact resistances (FSP1: efficiency 16.9% and contact resistance 20 mΩ cm2, FSP2: 17.8% and < 5 mΩ cm2). A discontinuous glass layer was found for FSP1 but a continuous glass layer was found for FSP2, yielding a smaller contact resistance. Glass layers contained (Si2Pb)Ox as a main constituent but different mole fractions of Zn, Ti, P, and B as minor constituents, varying for the different pastes. Glass layers were up to 500nm thick and revealed inhomogeneously distributed spherical Ag colloids 5-200nm in size. Planar cells were also studied and served as model systems: planar 〈111〉 oriented Si surfaces yielded specifically lower contact resistance as compared to planar 〈100〉 orientation. Pyramidal Ag crystallites were only observed for 〈100〉 oriented Si surfaces not for 〈111〉 surfaces. Therefore, it is concluded that pyramidal Ag crystallites are not necessary for contacts yielding low contact resistance. Instead, lens shaped Ag precipitates together with a high density of Ag colloids in the glass layer yield low contact resistance, as found for <111> oriented Si surfaces. A percolative current path including charge transport via Ag colloids in the glass layer is proposed. For textured cells, in accordance with these results, pyramidal Ag crystallites were only observed at step edges of {111} faces or at the edges of the Si pyramids.

Back-contact back-junction (BC-BJ) solar cells have been developed to meet the following objectives: process simplification by POCl3 based co-diffusion steps and efficiency improvement independent of the co-diffusion setup. In general, the process sequence is industrially feasible, requires not more than one temperature step (co-diffusion), and implements only industrially available patterning technologies. The process simplification is obtained by introducing a sophisticated co-diffusion process which combines solid diffusion sources with POCl3. Compared with BC-BJ cells featuring solid diffusion sources at the front side, the number of process steps could be decreased. Solar cells based on this process simplification feature conversion efficiencies up to 20.5 %. The efficiency improvement is obtained by integrating an improved B-source, by adjusting the material properties and by changing the pattern of the contact openings. BC-BJ cells based on the latter cell improvements feature conversion efficiencies up to 21.6 %. The efficiency improvement is transferable to cells based on the regarded process simplification featuring POCl3 as gaseous diffusion source.

This study presents a new developed, inline applicable dispensing platform that is equipped with an advanced version of previously introduced parallel dispensing print heads. At process speeds of up to 700 mm·s and a substantially improved process stability, recent cell results on industrial 90 Ω/sq. emitters showed an efficiency increase of up to +0.4%abs. in comparison to standard single screen printing technology. Top values of 19.4% using standard Al-BSF technology were reached in this study. A key improvement of the technology is the new ability to process certain metal pastes originally designed for screen printing applications and thus keep in track with fast emerging paste development. Successfully evaluated screen printing pastes then can be rheologically adapted in order to reach ultrafine contact fingers at high aspect ratios and extract the whole advantage of this non-contacting printing technology.

We present co-diffused bifacial n-type solar cells (CoBiN) with peak efficiencies of 19.9 %. The codiffusion process is based on a high temperature step in an atmosphere containing POCl3 and a BSG/SiOx layer stack deposited by atmospheric pressure chemical vapor deposition. Secondary ion mass spectroscopy measurements show that the reaction of POCl3 with the SiOx layer results in the formation of a PSG layer converting a fraction of the SiOx layer. Electrochemical capacitance voltage measurements do not detect an influence of POCl3 on the boron emitter.