Double Layer Anti Reflection Coatings

What are the double layer anti reflection coatings?

Reducing reflectivity is an extremely important criteria in solar cells to ensure maximum efficiency of incident light rays. An increased reduction in reflectivity is achieved by the means of DLARC which is a double layer anti reflection coatings. From the well-known DLARC coatings are ZnS (zinc sulfide) with MgF (magnesium flouride) or silicon nitride layers which have a range of different refractive indices. Yet, this arrangement is costly and for the majority of commercial solar cells it is considered too expensive.The Double layer anti reflection film can be deposited on silicon wafer. The layers are typically deposited on any textured substrate which helps in further decreasing the reflectivity.

Equations for the double layer anti reflection coatings

The equations used for multiple anti reflection coatings are far more complicated than the normal equations used for a single layer. First a series of parameters are defined: r₁, r₂, r₃, θ₁ and θ₂.The surrounding region has the refractive index denoted n₀, the layer after it has a refractive index denoted n₁ with thickness t₁, the layer just before the silicon has a refractive index denoted n₂ and thickness t₂ and finally n₃ is the refractive index of the silicon.r1= (n0−n1) / (n0+n1)r2= (n1−n2) / (n1+n2)r3= (n2−n3) / (n2+n3)θ1= (2πn1t1) / λθ2= (2πn2t2) / λThen the reflectivity can be calculated using the above parameters by inserting them into the following equations:R=∣r²∣= (r²₁ + r²₂ + r²₃ + r²₁r²₂r²₃ + 2 r₁r₂ (1+r²₃) cos 2θ₁ + 2r₂r₃ (1+r²₁) cos 2θ₂ + 2 r₁r₃ cos 2(θ1+θ2) + 2 r₁r²₂r₃ cos 2(θ1−θ2))  /  (1 + r²₁r²₂ + r²₁r²₃ + r²₂r²₃ + 2 r₁r₂ (1+r²₃) co s2θ1 + 2 r₂r₃ (1+r²₁) cos2θ + 2 r₁r₃ cos 2 (θ1+θ2) + 2 r₁r²₂r₃ cos 2(θ₁−θ₂)By adjustment of the thickness and the refractive index of the two layers it is possible to produce a minima for the two layers and an overall reflectance which is less than 3% for optimizing the produced layers for extra efficient Solar cell design that would lead to optimum performance for the produced solar cells.An increased reduction in reflectivity can be further achieved with extra stacks of coatings or by the grading of the index of the layers. Within the optical industry, typically stacks of over 10 layers are used for extremely low reflection losses in the photographic equipment. Yet, any performance benefits of using multiple layers is relevantly marginal. In most of the cases, the actual challenge is incorporating the anti reflection coatings in with the other surface passivation layers.