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) / λ
Then 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.