Growth of microcrystalline nip Si solar cells: role of local epitaxy

Compared to microcrystalline silicon (μc-Si:H) films grown under constant plasma conditions in plasma-enhanced chemical vapour deposition, the structural properties of μc-Si:H in solar cells are more complex, since the substrate precondition for the microcrystalline growth changes before the deposition of each functional layer. We present structural investigations on μc-Si:H nip solar cells by X-ray diffraction and transmission electron microscopy (TEM). Special emphasis was attributed to a change from crystalline to amorphous growth conditions for the μc-Si:H absorber layer through a variation of the dilution of SiH4 in H2. In all cases investigated, columnar structures of the intrinsic μc-Si:H layers are observed. The interface between the n-layer and the i-layer cannot be detected by TEM, revealing a local epitaxy of individual crystallites on the nuclei of the n-layer. When approaching amorphous growth conditions for the i-layer, crystalline columns are found, extending through the i-layer, embedded in an amorphous matrix. In contrast, when non-crystalline substrates are used, amorphous layers containing only small crystallites are observed. It is concluded that local epitaxy plays a major role in the preparation of μc-Si:H cells, particularly in the growth regime for high efficiency cells.