Additional Author: Brenna Tryon

What is a solar panel?

**Figure 1:** Dust, dirt, and pollen on solar panels can block incoming sunlight. Any light that is blocked or reflected cannot be absorbed by a solar panel and converted into electrical power.

Solar panels capture sunlight and convert it into electricity that can power our houses and cars. The more light a panel absorbs, the more electrical power it can produce. Ideally, a solar panel will absorb all incident sunlight, rendering it completely black. To do this, all incoming light must enter the panel and be trapped there until it is completely absorbed. In reality, some light is not absorbed because it reflects off the panel, is blocked by particles like dirt, or escapes the panel before being absorbed (Figure 1).

Nanoscale features such as moth-eye coatings, light-trapping features, and plasmonic structures can reduce reflections and confine light inside the solar panel so that more light is absorbed and converted into electricity. Many of these nanopatterns also act as self-cleaning or dust-mitigating surfaces that prevent the buildup of dust, dirt, and pollen that can block incoming light and decrease efficiency.

Moth-eye features decrease reflections

Moth-eye features are modeled after the nanofeatures that make moths’ eyes anti-reflective. Moth-eye coatings have nanofeatures that are smaller than the wavelength of light. Their small size makes moth-eye features anti-reflective by tricking light into believing that the transition from air into the solar panel is a gradual change rather than an abrupt interface.

Moth-eye features reduce the amount of light reflected from the surface of the solar panel, as illustrated in Figure 2. Theoretically, perfect moth-eye features can completely eliminate the Fresnel reflection off the top surface, allowing 100% of the incident light to enter the panel. Unfortunately, this same phenomenon will also allow light that has traveled through the panel to escape more easily. As a result, anti-reflective coatings such as moth-eye nanofeatures are often combined with light-trapping microfeatures that confine light inside the solar panel.

**Figure 2:** Illustration of light interacting with a solar panel without (A) and with (B) a moth-eye coating. Incoming and reflected light are shown in yellow and orange, respectively.

Light-trapping features increase absorption

Light-trapping features redirect light to increase its chance of being absorbed by a solar panel. These microscale features are larger than moth-eye nanofeatures and redirect light according to ray optics – the same physics that describes how a camera lens focuses light. Properly designed light-trapping microfeatures increase the amount of light a solar panel absorbs by increasing its path length inside the panel and giving reflected light a second chance at being absorbed.

Optical path length is the distance light travels in a material, in this case the light-absorbing material of a solar panel. Figure 3A shows a solar panel without light-trapping features where the optical path length is slightly more than twice the panel thickness. Light-trapping features like those illustrated in Figure 3B can bend, scatter, or diffract light to increase its optical path length and thus increase its chance of being absorbed by the solar panel. Although Figure 3 shows light-trapping features on top of the solar panel, similar features on the reflective back surface can also scatter light to increase its optical path length.

If light is scattered to a shallow enough angle, it can undergo total internal reflection so that all the light inside the panel is internally reflected. This enables the light to make multiple passes back and forth inside the panel and creates an optical path length that is many multiples of the panel thickness. For a random texture on the top surface, the Yablonovitch limit predicts a maximum optical path length of about 50 times the panel thickness for silicon solar panels (maximum optical path length for random structures = 4n²(panel thickness) where n is the refractive index of the solar panel). Periodically arranged structures like diffraction gratings and photonic crystals can overcome this limit and achieve even higher optical path lengths due to the excitation of resonances.

Light-trapping features can also increase absorption by capturing reflected light. For example, inverted pyramid features like those illustrated in Figure 3B enable light that is initially reflected to enter the solar cell through an adjacent inverted micropyramid, thus decreasing losses from reflection (indicated by the orange arrows).

**Figure 3:** Illustration of optical path length in solar cells without (A) and with (B) light-trapping microfeatures. Incoming and reflected light are illustrated in yellow and orange, respectively.

Plasmonic structures redirect and confine light

Plasmonic structures can also scatter light and confine it inside a solar panel, although they function very differently from the light-trapping microfeatures discussed above. Plasmonic structures consist of nanoscale metal structures, often in the form of patterned metal films or metal nanoparticles. These structures manipulate light by causing the electron cloud inside the nanoscale metal to oscillate. These plasmonic structures can be added on the top, middle, or back reflector of a solar panel to scatter light, increase its optical path length, promote total internal reflection, and act as antennas that concentrate light inside the panel. Well-designed plasmonic structures therefore make it easy for light to enter and hard for it to leave a solar panel.

Nanostructures have anti-fouling properties

The buildup of dirt, dust, grime, and sand that blocks incoming light is a big problem for solar panels. In fact, the buildup of dust on the solar panels of Mars rovers often reduces available power or temporarily shuts them down until Martian windstorms blow away the dust, reviving the rovers’ solar panels. On Earth, cleaning is often needed, especially in desert environments that lack rainfall.

**Figure 4:** Video showing how a nanopatterned surface (right) exhibits significantly less dust adhesion than a smooth surface of the same material (left).

Fortunately, many moth-eye and light-trapping nano- and microfeatures also make surfaces dust mitigating and anti fouling. The lotus effect, named for the nanoscale features that make lotus leaves self cleaning, decreases particle adhesion to a surface by decreasing the contact area. Nanostructures can also make surfaces superhydrophobic, making it easier for rain to wash away any particles that do buildup.

Smart Material Solutions, Inc. and Professor Chih-Hao Chang’s group at UT Austin recently created nanopatterns that significantly decrease the adhesion of Lunar dust for a NASA-funded project (Figure 4). This is great news for solar panels both in space and on Earth!

Solar research at Smart Material Solutions

Smart Material Solutions, Inc. (SMS) recently won the Army XTech Clean Tech Competition and a Phase I Small Business Innovation Research (SBIR) grant to use nanocoining and R2R NIL to manufacture light-trapping, self-cleaning coatings that increase the efficiency and reliability of thin-film solar panels. Thin-film solar panels can be lightweight and flexible, making them easy to transport and deploy. SMS will increase the performance of these solar panels by adding nanopatterns like those shown in Figure 5 to solar panels and demonstrating their light-trapping and anti-fouling properties. After the creation of prototypes in Phase I, SMS plans to partner with a solar company to roll-to-roll manufacture panels with self-cleaning, light-trapping coatings during Phase II of the project.