Effective Passivation of the Low Resistivity Silicon Surface by a Rapid Thermal Oxide/PECVD Silicon Nitride Stack and Its Application to Passivated Rear and Bifacial Si Solar Cells
Ruby, D. S.
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A novel stack passivation scheme, in which plasma silicon nitride (SiN) is stacked on top of a rapid thermal SiO(2) (RTO) layer, is developed to attain a surface recombination velocity (S) approaching 10 em/s at the 1.3 Ω-cm p-type (l00) silicon surface. Such low S is achieved by the stack even when the RTO and SiN films individually yield considerably poorer surface passivation. Critical to achieving low S by the stack is the use of a short, moderae temperature anneal (in this study 730°C for 30 seconds) after film growth and deposition. This anneal is believed to enhance the release and delivery of atomic hydrogen from the SiN film to the Si-Si0(2) interface, thereby reducing the density of interface traps at the surface. Compatibility with this post-deposition anneal makes the stack passivation scheme attractive for cost-effective solar cell production since a similar anneal is required to fire screen-printed contacts. Application of the stack to passivated rear screen-printed solar cells has resulted in V(oc)'s of 641 mV and 633 mV on 0.65 Ω-cm and 1.3 Ω-cm FZ Si substrates, respectively. These V(oc) values are roughly 20 mV higher than for cells with untreated, highly recombinative back surfaces. The stack passivation has also been used to form fully screen-printed bifacial solar cells which exhibit rear-illuminated efficiency as high as 11.6% with a single layer AR coating.