Broadband omnidirectional antireflection coating based on subwavelength surface Mie resonators

Reflection is a natural phenomenon that occurs when light passes the interface between materials with different refractive index. In many applications, such as solar cells or photodetectors, reflection is an unwanted loss process. Many ways to reduce reflection from a substrate have been investigated so far, including dielectric interference coatings, surface texturing, adiabatic index matching and scattering from plasmonic nanoparticles. Here we present an entirely new concept that suppresses the reflection of light from a silicon surface over a broad spectral range. A two-dimensional periodic array of subwavelength silicon nanocylinders designed to possess strongly substrate-coupled Mie resonances yields almost zero total reflectance over the entire spectral range from the ultraviolet to the near-infrared. This new antireflection concept relies on the strong forward scattering that occurs when a scattering structure is placed in close proximity to a high-index substrate with a high optical density of states

Zero-charge” SiO2/Al2O3 stacks for the simultaneous passivation of n+ and p+ doped silicon surfaces by atomic layer deposition

To achieve high conversion efficiencies, advanced silicon solar cell architectures such as interdigitated back contact solar cells demand that defects at both the n+ and p+ doped Si surfaces are passivated simultaneously by a single passivation scheme. In this work, corona charging experiments show that the fixed charge density Qf is a key parameter governing the passivation of both surface types. Alternatively, Qf can be controlled from strongly negative to even positive values by carefully tuning the SiO2 interlayer thickness in SiO2/Al2O3 stacks prepared by atomic layer deposition (ALD). This control in Qf allows for a superior passivation of n+ Si surfaces by SiO2/Al2O3 stacks compared to a single layer Al2O3. For instance, for SiO2 interlayer thicknesses of ~3–14 nm, the recombination parameter of an n+ Si surface having a high surface doping concentration Ns of 2×1020 cm−3 was reduced from J0n+=81 fA/cm2 to J0n+=50 fA/cm2. Simulations predict that the SiO2/Al2O3 stacks outperform Al2O3 passivation layers particularly on n+ Si surfaces having a moderate Ns in the range of 1018–1020 cm−3. On p+ Si surfaces, J0p+≤54 fA/cm2 was achieved for all ALD SiO2 interlayer thicknesses investigated (i.e., 1–14 nm). The SiO2/Al2O3 stacks presented in this work are compatible with SiNx capping and subsequent high-temperature firing steps, which are typically used in solar cell processing. Furthermore, the results were successfully reproduced in an industrial ALD batch reactor using a low-temperature process. This makes ALD SiO2/Al2O3 stacks a promising candidate for the simultaneous passivation of n+ and p+ Si surfaces in solar cells