4.4 Antireflective layers

The reflection of light from the surface of the layers making up the cell reduces the amount of light reaching its active layers. In order to reduce the negative effect of this on cell performance, anti-reflective layers (AR or ARC) are applied to the top layer of the cell. These layers are not involved in the photovoltaic conversion. They are usually applied to the top layer of the cell.

Reducing the reflection, while keeping the absorption of the material constant, increases the transmission, allowing more solar radiation to reach the active layer.
The anti-reflective system can be a single layer or a multi-layer system.
A single anti-reflective layer is made of a material with a refractive index value intermediate to that of the layers directly below and above it (e.g., passivation layer and transparent electrode).

In the second case, the operation of anti-reflective systems is based on the phenomenon of electromagnetic wave interference. The choice of thickness and refractive index of the layers making up the system leads to an increase in transmission. Multi-layer anti-reflective systems are based on the LH (low, high) filter theory. This is a set of layers with alternating layers of high and low refractive index. This makes it possible to shape the reflection characteristics in such a way that in the electromagnetic wavelength range of the photovoltaic system operation, the transmission is the highest (highest reflection). An example of an anti-reflective system consists of four bilayers $ TiO_{2} $/ $ SiO_{2} $.

The thickness of the anti-reflective layer $ d $ is related to the refractive index in the layer $ n $ and the wavelength $ \lambda $ of the light from the area of maximum photosensitivity of the cell [1] – equation ( 1 ):

(1)

$ n \cdot d=\frac{\lambda }{4} $

Reflection can be reduced by adjusting the refractive index of the coating [2], $ n_{AR} $ for materials on both sides marked as follows $ n_{surrounding1} $ and $ n_{surrounding2} $ based on the relation ( 2 ):

(2)

$ n_{AR}=\sqrt{{n_{surrounding1}\cdot n_{surrounding2}}} $

The reflectance characteristics of the anti-reflection system are shown in Fig. 1. The red colour indicates the system surrounded by air, the blue the system between the two glass panes.
The characteristics were generated using a simulation program Anti-Reflection Coatings [1].

$Refractive indices for antireflective layer for specified thickness and refractive index a) n =1.4 d=100 nm, b) n =1.8 d=100 nm, c) n =1.4 d=50 nm, d) n =1.8 d=50 nm. Own elaboration.$

Figure 1: Refractive indices for antireflective layer for specified thickness and refractive index a) n =1.4 d=100 nm, b) n =1.8 d=100 nm, c) n =1.4 d=50 nm, d) n =1.8 d=50 nm. Own elaboration.

As mentioned above, depending on the absorption spectral maximum of the absorber in the solar cell, the reflectance characteristics of the anti-reflective layer can be controlled. Anti-reflective films Fig. 2 with maximum transmission/minimum reflection (a) at 500 nm, (b) at 400 nm are shown below.

$Refractive indices for anti-reflective layer for specified thickness and refractive index (a) n =1.94 d=63.4 nm, (b) n =2.47 d=40.6 nm. Own elaboration.$

Figure 2: Refractive indices for anti-reflective layer for specified thickness and refractive index (a) n =1.94 d=63.4 nm, (b) n =2.47 d=40.6 nm. Own elaboration.

Thin-film anti-reflective systems are produced by vacuum methods by sputtering or evaporation.

Bibliography

1. PVEducation: Anti-Reflection Coatings, dostęp:10.10.2020
2. T. Stapiński, K. Marszałek, J. Jaglarz: Właściwości optyczne pokryć antyrefleksyjnych dla zastosowań fotowoltaicznych, Przegląd Elektrotechniczny 2014, R. 90, Nr 9, s. 91-93, dostęp:14.12.2020

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Fotowoltaika

4.4 Antireflective layers

Bibliography