Highly conducting phosphorous doped n-type nc-Si:H films by HW-CVD for c-Si heterojunction solar cells

Abstract

Phosphorous doped hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared using the hot wire chemical vapor deposition (HW-CVD) method at a low substrate temperature of 200 °C. The microstructure and opto-electrical properties of these films were systematically studied using Raman spectroscopy, low angle XRD, high resolution transmission electron microscopy (HR-TEM), UV-Visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dark conductivity and its activation energy measurements and Hall measurement as a function of PH₃ gas-phase ratio. It has been found that with an increase in the PH₃ gas-phase ratio, both the volume fraction of the crystallites and its size decrease, signifying that the phosphorous atom favors the growth of amorphization in the nanocrystalline Si network. At the optimized PH₃ gas-phase ratio we have obtained n-type nc-Si:H films with a band gap of ∼1.84 eV, high dark conductivity (∼6.78 S cm⁻¹) with low hydrogen content (∼1.72 at. %), at a reasonably high deposition rate (∼10 Å s⁻¹). Finally, Al/ZnO:Al/n-nc-Si:H/buffer a-Si:H/p-c-Si/Al heterojunction solar cells were fabricated using the optimized n-layer, showing excellent photovoltaic performance with V_oc = 719 mV, J_sc = 9.94 mA cm⁻², FF = 53.8%, and an energy conversion efficiency of 5.2%. These are very encouraging results for the future fabrication of high efficiency silicon heterojunction solar cells and thin film tandem solar cells.