PERFORMANCE OF HETEROJUNCTION P(-CELLS() MICROCRYSTALLINE SILICON N CRYSTALLINE SILICON SOLAR)

We have studied by Raman spectroscopy and electro-optical characteriza tion the properties of thin boron doped microcrystalline silicon layer s deposited by plasma enhanced chemical vapor deposition (PECVD) on cr ystalline silicon wafers and on amorphous silicon buffer layers. Thin 20-30 nm p(+) mu c-Si:H layers with a considerably large crystalline v olume fraction (similar to 22%) and good window properties were deposi ted on crystalline silicon under moderate PECVD conditions, The perfor mance of heterojunction solar cells incorporating such window layers w ere critically dependent on the interface quality and the type of buff er layer used. A large improvement of open circuit voltage is observed in these solar cells when a thin 2-3 nm wide band-gap buffer layer of intrinsic a-Si:H deposited at low temperature (similar to 100 degrees C) is inserted between the microcrystalline and crystalline silicon [ complete solar cell configuration: A1/(n)c-Si/buffer/p(+)mu c-Si:H/ITO /Ag)]. Detailed modeling studies showed that the wide band-gap a-Si:H buffer layer is able to prevent electron backdiffusion into the p(+) m u c-Si:H layer due to the discontinuity in the conduction band at the amorphous-crystalline silicon interface, thereby reducing the high rec ombination losses in the microcrystalline layer. At the same time, the discontinuity in the valence band is not limiting the hole exit to th e front contact and does not deteriorate the solar cell performance. T he defect density inside the crystalline silicon close to the amorphou s-crystalline interface has a strong effect on the operation of the ce ll. dAn extra atomic hydrogen passivation treatment prior to buffer la yer deposition, in order to reduce the number of these defects, did fu rther enhance the values of V-oc and fill factor, resulting in an effi ciency of 12.2% for a cell without a back surface field and texturizat ion. (C) 1997 American Institute of Physics.