Microstructure and lateral conductivity control of hydrogenated nanocrystalline silicon oxide and its application in a-Si:H/a-SiGe:H tandem solar cells

Phosphorous-doped hydrogenated nanocrystalline silicon oxide (n-nc-SiO_x:H) films are prepared via radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). Increasing deposition power during n-nc-SiO_x:H film growth process can enhance the formation of nanocrystalline and obtain a uniform microstructure of n-nc-SiO_x:H film. In addition, in 20s interval before increasing the deposition power, high density small grains are formed in amorphous SiO_x matrix with higher crystalline volume fraction (I_c) and have a lower lateral conductivity. This uniform microstructure indicates that the higher I_c can leads to better vertical conductivity, lower refractive index, wider optical band-gap. It improves the back reflection in a-Si:H/a-SiGe:H tandem solar cells acting as an n-nc-SiO_x:H back reflector prepared by the gradient power during deposition. Compared with the sample with SiO_x back reflector, with a constant power used in deposition process, the sample with gradient power SiO_x back reflector can enhance the total short-circuit current density (J_sc) and the initial efficiency of a-Si:H/a-SiGe:H tandem solar cells by 8.3% and 15.5%, respectively.