BACK SURFACE FIELD EFFECTS IN THE 17.3-PERCENT EFFICIENT N-TYPE DENDRITIC WEB SILICON SOLAR-CELLS

A solar cell efficiency of 17.3% (4 cm(2) area) has been achieved on 1 1 Omega cm, n-type dendritic web silicon. This is the highest reported efficiency to date on any silicon ribbon material. The minority carri er lifetime in the bulk material was determined to be at least 150 mu s in spite of the fact that dopant diffusions were done at high temper ature in a rapid thermal processing unit (1000 degrees C for 30 s with cooling to 825 degrees C at 50 degrees C/min). Detailed characterizat ion and modeling show that due to the reduced substrate thickness (100 mu m) and long minority carrier diffusion length (>400 mu m), device performance is strongly dependent on the back surface recombination ve locity (S-b). In this study, an n(+) phosphorus back surface field (BS F) was implemented to reduce the effective S-b of a fully metallized r ear surface to approximately 20 cm/s. This effective S-b value was det ermined through measurement and numerical analysis of the BSF doping p rofile. Model calculations reveal that the n(+)-n BSF served to increa se the device efficiency by nearly 4% (absolute) above the case of inf inite S-b. By extending these model calculations to ''mirror'' solar c ells (analogously doped n(+)-p-p(+) and p(+)-n-n(+) devices with equiv alent lifetimes), it is shown that substrate type plays only a minor r ole in determining the overall device efficiency. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.