Performance of heterojunction p+ microcrystalline silicon n crystalline silicon solar cells

We have studied by Raman spectroscopy and electro-optical characterization the properties of thin boron doped microcrystalline silicon layers deposited by plasma enhanced chemical vapor deposition (PECVD) on crystalline silicon wafers and on amorphous silicon buffer layers. Thin 20-30 nm p(+) mu c-Si:H layers with a considerably large crystalline volume fraction (similar to 22%) and good window properties were deposited on crystalline silicon under moderate PECVD conditions, The performance of heterojunction solar cells incorporating such window layers were critically dependent on the interface quality and the type of buffer 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(+) mu c-Si:H layer due to the discontinuity in the conduction band at the amorphous-crystalline silicon interface, thereby reducing the high recombination losses in the microcrystalline layer. At the same time, the discontinuity in the valence band is not limiting the hole exit to the front contact and does not deteriorate the solar cell performance. The defect density inside the crystalline silicon close to the amorphous-crystalline interface has a strong effect on the operation of the cell. dAn extra atomic hydrogen passivation treatment prior to buffer layer deposition, in order to reduce the number of these defects, did further enhance the values of V-oc and fill factor, resulting in an efficiency of 12.2% for a cell without a back surface field and texturization. (C) 1997 American Institute of Physics.