Role of n-type seed-layers in microstructural evolution of intrinsic nanocrystalline silicon and solar cell performance

Nanocrystalline silicon (nc-Si:H) thin-film n-i-p solar cells were constructed on flexible stainless steel substrates by plasma-enhanced chemical vapor deposition. Influence of the n-type seed-layer on the microstructural evolution of the subsequent intrinsic nc-Si:H absorbers and the resultant performance of nc-Si:H solar cells was investigated. The crystalline volume fraction of the seed-layer can be effectively controlled by varying the hydrogen (H₂) to silane (SiH₄) gas flow ratio. Defect-dense amorphous regions were observed at the initial growth stage of the i-layers deposited on low crystalline volume fraction (X_cⁿ) n-type seed-layers. Increasing the X_cⁿ reduced the amorphous region at the n/i interface of the i nc-Si:H layers, evidenced by Raman scattering and transmission electron microscopy (TEM) measurements. Elimination of the defect-rich amorphous region within the i-layer by depositing the nc-Si:H solar cells on highly crystalline seed-layer caused significant improvements in the short circuit current density (J_sc) and fill factor (FF). This is mainly due to the enhancement of long-wavelength light response and extraction efficiency of photo-carrier charges. The nc-Si:H solar cells prepared on a highly crystalline seed-layer (X_cⁿ = 73%) exhibited a 65.6% higher conversion efficiency than those on the n-type amorphous layers (X_cⁿ = 0%).