Back surface field and emitter passivation effects in the record high efficiency n-type dendritic web silicon solar cell

A solar cell efficiency of 17.3% (4 cm² area) has been achieved on 11 Ω-cm, n-type dendritic web silicon. This is the highest reported efficiency to date on any silicon ribbon material. Detailed characterization and modeling show that, due to the reduced substrate thickness (100 μm) and long diffusion length (>400 μm), device performance is strongly dependent on the back surface recombination velocity (S_b). In this study, an n⁺ phosphorus back surface field was implemented to reduce the effective S _b to approximately 20 cm/s, and increase the device efficiency by nearly 4% (absolute) above the case of infinite S_b . Additionally, thermal oxide passivation of the boron emitter was found to improve the cell performance by more than 0.5% (absolute). By extending model calculations to “mirror” solar cells (identically doped n⁺-p-p⁺ and p⁺-n-n ⁺ devices with equivalent bulk lifetimes), it is shown that substrate type plays only a minor role in determining the overall device efficiency