Effect of TiO2 Antireflection Layer with Various Conductivities and Refractive Indices on Performance of Amorphous Silicon/Amorphous Silicon Germanium Tandem Solar Cells

Abstract

TiO2 films were prepared by RF magnetron sputtering with process variations of substrate temperature and oxygen dilution ratio to investigate the effect of the optoelectronic properties of TiO2 films on antireflection characteristics in solar cells. With an increase in substrate temperature from RT to 350 degrees C, the conductivity and refractive index of TiO2 films increased. However, the absorption coefficient also increased. In the case of oxygen-diluted sputtering (0.5-1%), the conductivity and refractive index decreased with the increase in oxygen dilution. On the other hand, the absorption coefficient decreased simultaneously. To evaluate these optoelectronic property variations of TiO2 films in terms of the antireflection effect in solar cells, amorphous silicon (a-Si:H)/amorphous silicon germanium (a-SiGe:H) tandem solar cells with various optoelectronic properties and thicknesses (20-40 nm) of TiO2 films were fabricated. The TiO2 film deposited at 350 degrees C and 0.5% oxygen dilution showed a high conductivity (similar to 10(-3) Omega(-1) cm(-1)) and refractive index (similar to 2.56 at 550 nm). The fabricated tandem cell with the TiO2 antireflection layer showed an efficiency of 11.22%, whereas the reference cell without TiO2 exhibited an efficiency of 10.97%. (C) 2012 The Japan Society of Applied Physics