Design, Optimization, and In-Depth Understanding of Front and Rear Junction Double-Side Passivated Contacts Solar Cells

In this article, detailed numerical modeling is performed for front junction (FJ) and rear junction (RJ) n-type Si solar cells with screen-printed double-side poly-Si based tunnel oxide passivated contacts (TOPCon). A roadmap for efficiency projections of commercial-type RJ and FJ topologies reaching 24.8% and 23.3% efficiencies, respectively, has been developed to quantify and explain the impact of various technological innovations on the performance of each design. Understanding of mechanisms governing cell operation is crucial to explore factors that limit the efficiency potential of the two device structures. By investigating several key parameters such as front poly-Si sheet resistance and thickness, bulk material properties, and current transport in our simulation model, we determine and explain why RJ cells outperform FJ cells. Furthermore, our findings reveal that FJ suffers from present technological limitations of p-type poly-Si based passivated contacts-namely, 1) large recombination observed in textured p-TOPCon layers and 2) low boron solid solubility and hole mobility in boron-doped poly-Si which results in very high sheet resistance of the front p-poly-Si emitter that contributes to fill factor degradation, especially when using thin poly-Si layer to reduce absorption losses. RJ on the contrary desensitizes the cell efficiency to front sheet resistance to allow the application of ultra-thin front n-type poly-Si layer and is therefore ideally suited for double-side TOPCon cells.