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 cm/sup 2/ area) has been achieved on 11 /spl Omega/-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 /spl mu/m) and long diffusion length (>400 /spl mu/m), device performance is strongly dependent on the back surface recombination velocity (S/sub b/). In this study, an n/sup +/ phosphorus back surface field was implemented to reduce the effective S/sub b/ to approximately 20 cm/s, and increase the device efficiency by nearly 4% (absolute) above the case of infinite S/sub 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/sup +/-p-p/sup +/ and p/sup +/-n-n/sup +/ devices with equivalent bulk lifetimes), it is shown that substrate type plays only a minor role in determining the overall device efficiency.