N-Type Hydrogenated Microcrystalline Silicon Oxide Films and Their Applications in Micromorph Silicon Solar Cells

ABSTRACTWe have prepared n-type hydrogenated microcrystalline silicon oxide films (n μc-SiO:H) and investigated their structural, electrical and optical properties. Raman spectra shows that, amorphous phase of the n μc-SiO:H films tends to increase when the CO2/SiH4 ratio increases from 0 to 0.28 resulting in a reduction of the crystalline volume fraction (Xc) from 70 to 12%. Optical bandgap (E04) becomes gradually wider while dark conductivity and refractive index (n) continuously drop with increasing CO2/SiH4 ratio. The n μc-SiO:H films have been practically applied as a n layer in top cell of a-SiO:H/μc-Si:H micromorph silicon solar cells. We found that, open circuit voltage (Voc) and fill factor (FF) of the cells gradually increased, while short circuit current density (Jsc) remained almost the same value with increasing CO2/SiH4 ratio for n top layer deposition up to 0.23. The highest initial cell efficiency of 10.7% is achieved at the CO2/SiH4 ratio of 0.23. The enhancement of the Voc is supposed to be due to a reduction of reverse bias at sub cell connection (n top/p bottom interface). An increase of shunt resistance (Rsh) which is caused by a better tunnel recombination junction contributes to the improvement in the FF. Quantum efficiency (QE) results indicate no difference between the cells using n top μc-SiO:H and the cells with n top μc-Si:H layers. These results reveal that, the n μc-SiO:H films in this study do not work as an intermediate reflector to enhance light scattering inside the solar cells, but mainly play a key role to allow ohmic and low resistive electrical connection between the two adjacent cells in the micromorph silicon solar cells.