Optical losses lower the solar cell short-circuit current. Typical optical losses include
- shading from the metal on the front-side of the wafer
- Reflection of light from the silicon surface, and
- The light which escapes out of the cell after being reflected off the rear surface.
One way to minimise the surface reflection is to coat the solar cell with an anti-reflection coating (ARC). A silicon surface without an anti-reflection coating will reflect approximately 30 % of all incident light. Texturing will reduce the reflection to around 15 %, and an ARC can further reduce the reflection from the surface of the solar cell to be minimised for a particular range of wavelengths. An ARC is a thin layer of insulating dielectric material, which has a thickness that is selected based to create destructive interference with net zero reflection for a given wavelength. The thickness, d1 of the ARC is calculated based on the refractive index, n1 of the ARC and wavelength λ0 according to the relation,
where n1 is determined based on the refractive index of the substrate n0 and the dielectric layer, n2 (typically silicon nitride, SiNx:H),
Usually, the ARC is optimised to minimise the reflection at a single wavelength of 600 nm, which is the where the peak of spectral intensity AM 1.5 irradiance occurs.
Silicon nitride is a commonly used ARC material. This is because the layer can reduce the carrier recombination at the interface, and hydrogen from the layer can passivate the bulk defects during co-firing steps. Furthermore, the hydrogen that is released can also passivate other defects such as laser damage from laser doping a selective emitter contact, or boron-oxygen defects, which cause light-induced degradation on monocrystalline silicon, when the charge states of the hydrogen is controlled.