A Guide to Anti-Reflective Coatings

The photovoltaic effect is the process by which a solar cell converts photons of sunlight into electricity. When light shines on a solar cell, it may be reflected, absorbed, or transmitted, however, only the absorbed light generates electricity.

The energy of the absorbed photons is transferred to electrons in the solar cell. With their newfound energy, these electrons escape from their normal positions in the semiconductor photovoltaic material and become part of the electrical flow, or current, in an electrical circuit.

Many solar cells are connected together—with a protective glass cover on top—to make one photovoltaic module, which is mounted on a roof or elsewhere to generate electricity.

Conversion efficiency tells us how much of the Sun’s light energy is converted into electrical energy. This is an important measure of success for a photovoltaic module.

Minimizing Reflections Increases Conversion Efficiency

Photovoltaic modules suffer from reduced conversion efficiency even before the sun’s light reaches the solar cell. This is because the solar module's protective glass cover reflects some of the incident sunlight. For typical glass panels, depending on the time of day 4% to 15+% of the incoming light is lost from reflections and thus, is not available to generate electricity. Applying an anti-reflective coating to the cover glass of the module will reduce these reflections and increase the module’s output power. Current commercial PV technologies convert 10%-20% of the incoming light to electricity. The same module with a suitable anti-reflective coating can deliver an additional 0.3%-0.6% power conversion. A product achieving higher conversion efficiency in a cost-effective manner can make solar modules more affordable.

What Makes an Anti-Reflective Coating Effective

An effective anti-reflective coating for solar module cover glass maximizes the light available to be converted to electrical energy. The optimum anti-reflective coating has the following capabilities:

Increases transmission over the entire solar spectrum
Increases transmission over the sun’s incident angles
Applicable for large surface areas at reasonable cost

Increases Transmission over the Entire Solar Spectrum

Anti-reflective coatings create interference between two reflected waves from the top and bottom of a thin film coating. If these waves are out of phase, they will cancel each other and minimize the reflected light. Optimum cancellation occurs when the refractive index of the thin film is tuned for the particular glass used, and the thickness of the thin film is controlled to one-quarter of the targeted wavelength.
Given this, it is relatively simple to design an anti-reflective coating for one wavelength. However, light from the sun has a broad range of wavelengths that are used to generate energy. The traditional solution is to use a multi-layer coating technique, where a combination of many layers produces the desired effect. Besides adding cost, multi-layer coatings will reflect sunlight more than uncoated glass at certain incident angles.

An effective anti-reflective coating must deliver improvements across the broadest range of wavelengths and angles possible to capture as much of the sunlight for conversion to energy.

Increases Transmission over the Sun’s Incident Angles

The light reflected from a surface depends on the angle at which the light is incident on the surface. Uncoated glass reflects more as the incident angle of the light increases. Through the course of a day and through the year, the position of the sun changes. As the sun moves across the sky, the incident angle of the sunlight changes, and the amount of reflection increases in the morning and evening. Most PV modules are fixed in place and do not track the sun as it moves across the sky. Delivering high anti-reflective performance for PV modules requires that the coating reduce reflections through the whole day when the sun’s light is incident from different angles, not just when the sun is overhead. The ideal anti-reflective coating has a graded refractive index to maximize the solar energy available for conversion into electricity transmission over the solar spectrum and broad incident angle. When the refractive index is changed from glass to air gradually the transmission of solar energy is maximized under all conditions of the day.

Applicable to Large Surface Areas at Reasonable Cost

Existing processes for making antireflective coatings, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD), can precisely control the composition and thickness of coatings. However, these methods are complex, expensive, difficult to apply to large areas, and are almost all damaging to the environment. A process that produces an anti-reflective coating where the refractive index is graded to change from glass to air without increasing manufacturing cost is the ideal solution for today’s solar energy technologies.

Conclusion

An anti-reflective coating on the top surface of a solar module’s cover glass delivers more energy to the solar cells, increasing the output power of the module. Single layer anti-reflective coatings with a graded refractive index profile offer the highest performing solution and, when combined with a low cost, high volume manufacturing method, provide the most cost-effective conversion efficiency enhancement for solar energy systems.