Application of plasma deposited nanocomposite silicon suboxides and microcrystalline silicon alloys to heterojunction solar cells

In this work, a novel approach for heterojunction solar cell fabrication, featuring plasma deposited amorphous silicon sub-oxides (a-SiO_x:H) for high quality surface passivation combined with plasma deposited doped microcrystalline silicon (?c-Si(n⁺/p⁺)) is used to form the heterojunction. Special attention is paid (i) to the front and back surface passivation of the bulk material by high-quality wide-gap amorphous silicon sub-oxides (a-SiO_x:H), and (ii) to the influence of wide-gap high-quality ?c-Si:H at the front side for use as emitter to suppress absorption losses. Heterojunction solar cells fabricated by combining the excellent surface passivation properties of the intrinsic a-SiO_x:H and the doped high-transparent ?c-Si layers show cell performance up to 19.3% efficiency (certified) so far. By incorporating a-SiO_x:H(i) to the heterojunction structure a drastic increase of the open circuit voltage (up to 695 mV for n-type substrates) is found, and accordingly, a conversion efficiency higher than obtained with standard a-Si:H(i). These high open-circuit voltages can be consistently ascribed to the adequate surface passivation by a-SiO_x:H preventing surface recombination at the hetero-interface and to the decrease of the optical absorption in the blue light region due to an enhanced optical bandgap of 1.95 eV. Heterojunction solar cells using textured substrates exhibit an expected gain of short circuit current of 5 mA/cm², when transferring the optimized process parameters of cells using polished substrates to cells using textured substrates. For cell devices using textured substrates, efficiencies exceeding 19% have been obtained, so far limited by a relatively low fill factor of 77%, and a loss in the open circuit voltage compared to cells based on flat substrates. This deterioration can be attributed to process related issues, such as photolithography steps damaging the ti- ps of the textured upside pyramids.