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 (CH 4 /SiH 4 ; R C ) ratio. The chemical bonding, microstructural, and electro-optical properties of the films were investigated in detail. With increasing R C , 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 (V OC ) 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% (V OC = 0.41 V, J SC = 22.81 mA/cm 2 , and FF = 68.10%, where J SC is the short-circuit current density and FF is the fill factor, respectively) to 7.89% (V OC = 0.51 V, J SC = 24.04 mA/cm 2 , 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% (V OC = 0.42 V, J SC = 14.83 mA/cm 2 , and FF = 58.69%) to 4.33% (V OC = 0.45 V, J SC = 17.74 mA/cm 2 , 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.