Wide-bandgap nanocrystalline silicon-carbon alloys for photovoltaic applications

Abstract

p-Type hydrogenated silicon carbide (p-SiC:H) films were prepared using plasma-enhanced chemical vapor deposition by systematically incorporating carbon into nanocrystalline silicon networks. The carbon content of the films was varied by adjusting the methane/silane (CH4/SiH4; RC) ratio. The chemical bonding, microstructural, and electro-optical properties of the films were investigated in detail. With increasing RC, the silicon-carbon bond density in the films increased gradually and the overall microstructure became amorphous. Through sophisticated control of the carbon content of the films during high hydrogen dilution deposition, p-type hydrogenated nanocrystalline SiC:H (p-nc-SiC:H) films with nanometer-scale Si crystallites embedded in an amorphous SiC:H matrix were obtained. The p-nc-SiC:H films had wide bandgaps larger than 2 eV and reduced parasitic light absorption at wavelengths below 550 nm. When using wide-bandgap p-nc-SiC:H films with superior electro-optical properties as window layers in n-i-p flexible and p-i-n semi-transparent nc-Si:H solar cells, in place of conventional p-nc-Si:H layers with narrow bandgaps, enhanced cell performance was achieved because of both high open circuit voltage (VOC) and high quantum efficiency values at short wavelengths of 350–550 nm. The conversion efficiencies of the flexible nc-Si:H solar cells increased from 6.37% (VOC = 0.41 V, JSC = 22.81 mA/cm2, and FF = 68.10%, where JSC is the short-circuit current density and FF is the fill factor, respectively) to 7.89% (VOC = 0.51 V, JSC = 24.04 mA/cm2, and FF = 64.37%). A further increase in the conversion efficiency to 9.18% was obtained by inserting a very thin, highly-doped p-nc-SiC:H buffer layer between a low-doped p-nc-SiC:H window layer and an indium tin oxide front contact. A significant increase in the conversion efficiency from 3.66% (VOC = 0.42 V, JSC = 14.83 mA/cm2, and FF = 58.69%) to 4.33% (VOC = 0.45 V, JSC = 17.74 mA/cm2, and FF = 54.18%) was achieved with the semi-transparent nc-Si:H solar cell, with an average optical transmittance of 17.29% in the visible wavelength region of 500–800 nm.