Optimization of Al Fire-Through Contacts for AlO$_{\rm X}$–SiN$_{\rm X}$ Rear Passivated Bifacial p-PERC

We present optimizations of rear Al fire-through (FT) contacts for bifacial p -type passivated emitter and rear (AlOX–SiNX) cells. This approach allows omitting the laser contact opening step. Prior to this work, high FT lateral resistance due to its glass frit, and high rear surface recombination due to large passivation damage and back surface field layer (BSF) abscence, decreased the cell performance for cells with Al FT contacts. In this work, a proper BSF has been achieved by adapting the firing process and by printing multilayer fingers. Achievement of increased BSF thickness and reduced rear contact area—by means of dashing or finger narrowing by dispensing (down to 70 μm)—seems to close the gap to the non-FT (NFT) reference level in terms of contact recombination. This is indicated by reaching similar open-circuit voltages for rear-side-only fired (front side plated) cells. For cofired cells, open-circuit voltages were 6 mV below NFT level, stemming most likely from an overfired front side contact due to firing process adaptations. Multilayer printing allows for a decrease of the lateral resistance of the less conductive FT grid due to an increase of the finger cross-sectional area. For best rear-side-only fired FT cells, reduced recombination and resistance closed the efficiency ( η ) gap to the NFT reference. For the best cofired FT cells, η a 21.3% and bifacial power output density of $P_{\beta\_{\rm OUT}}$ a 22.8 mW/cm2 is achieved compared to η a 21.5% and $P_{\beta\_{\rm OUT}}$ a 23.1 mW/cm2 for NFT. The remaining $P_{\beta\_{\rm OUT}}$ gap has the potential to be reduced by the means of mitigating front side contact overfiring by matching thermal tolerance of the front side paste, and reducing rear side finger resistivity by paste optimization.