Comparison of dielectric surface passivation of monocrystalline and multicrystalline silicon

Reducing solar cell thickness is an attractive way to reduce material costs. However, model calculations in this paper show that if rear surface recombination velocity (S) is greater than about 1000 cm/s, a 100-μm-thick screen-printed cell on solar-grade material has a lower efficiency than a 300-μm-thick cell. The literature demonstrates that S < 1000 cm/s is readily achievable on monocrystalline materials. However, S on multicrystal line silicon (mc-Si) seems less thoroughly investigated. In this study, string ribbon mc-Si wafers of different resistivities are passivated with a thermal oxide, plasma-enhanced chemical vapor deposition (PECVD) nitride, and an oxide/nitride stack. For comparison, float zone (FZ) and Czochralski (Cz) monocrystalline wafers are passivated identically. By analyzing measured lifetimes under 500 nm and 1000 nm illumination, upper and lower limits on S are determined. For most of the monocrystalline wafers investigated in this study, the upper limit on S is less than 1000 cm/s, while for most of the multicrystalline wafers, 1000 cm/s falls within the error bars. Thus, thinning monocrystalline silicon should improve cell performance; however, it is difficult to conclude from this data that solar cell efficiency will improve when reducing thickness for the specified mc-Si materials and passivation technologies. In fact, results strongly suggest that S on string ribbon mc-Si is higher than S on identically passivated FZ.