Anti-reflection coatings on solar cells are similar to those used on other optical equipment such as camera lenses. They consist of a thin layer of dielectric material, with a specially chosen thickness so that interference effects in the coating cause the wave reflected from the anti-reflection coating top surface to be out of phase with the wave reflected from the semiconductor surfaces. These out-of-phase reflected waves destructively interfere with one another, resulting in zero net reflected energy. In addition to anti-reflection coatings, interference effects are also commonly encountered when a thin layer of oil on water produces rainbow-like bands of colour.
Use of a quarter wavelength anti-reflection coating to counter surface reflection.
The thickness of the anti-reflection coating is chosen so that the wavelength in the dielectric material is one quarter the wavelength of the incoming wave. For a quarter wavelength anti-reflection coating of a transparent material with a refractive index n1 and light incident on the coating with a free-space wavelength λ0, the thickness d1 which causes minimum reflection is calculated by:
Reflection is further minimised if the refractive index of the anti-reflection coating is the geometric mean of that of the materials on either side; that is, glass or air and the semiconductor. This is expressed by:
The graph shows the effect of a single layer anti-reflection coating on silicon. Use the sliders to adjust the refractive index and thickness of the layer. For simplicity this simulation assumes a constant refractive index for silicon at 3.5. In reality the refractive index of silicon and the coating is a function of wavelength.
While the reflection for a given thickness, index of refraction, and wavelength can be reduced to zero using the equations above, the index of refraction is dependent on wavelength and so zero reflection occurs only at a single wavelength. For photovoltaic applications, the refractive index, and thickness are chosen in order to minimise reflection for a wavelength of 0.6 µm. This wavelength is chosen since it is close to the peak power of the solar spectrum.
Comparison of surface reflection from a silicon solar cell, with and without a typical anti-reflection coating.
By adding more than one anti-reflection layer, the reflectivity can be reduced over a wide range of wavelengths. However, this is usually too expensive for most commercial solar cells. The equations for multiple antireflection coatings are more complicated than that for a single layer (Wang). The graph below simulates a double layer antireflection coating. By adjusting the refractive index and thickness of the two layers it is possible to produce two minima and a overall reflectance of less than 3%.
Interactive graph showing the effect of thickness and refractive index on a double layer anti-reflection coating (DLARC). Under air, the reflection is minimised by setting the top layer to 105 nm with a refractive index of 1.4 and setting the bottom layer to 55 nm and a reflective index of 2.4.