MICROSCALE DIELECTRIC ANTI-REFLECTION COATINGS FOR PHOTOVOLTAICS

Abstract

In order to power our planet for the next century, clean energy technologies

need to be developed and deployed. Photovoltaic solar cells, which convert sunlight

into electricity, are a clear option; however, they currently supply 0.1% of the US

electricity due to the relatively high cost per Watt of generation. Thus, our goal is

to create more power from a photovoltaic device, while simultaneously reducing its

price. To accomplish this goal, we are creating new high efficiency anti-reflection

coatings that allow more of the incident sunlight to be converted to electricity, using

simple and inexpensive coating techniques that enable reduced manufacturing costs.

Traditional anti-reflection coatings (consisting of thin layers of non-absorbing

materials) rely on the destructive interference of the reflected light, causing more

light to enter the device and subsequently get absorbed. While these coatings are

used on nearly all commercial cells, they are wavelength dependent and are deposited

using expensive processes that require elevated temperatures, which increase production cost and can be detrimental to some temperature sensitive solar cell materials.

We are developing two new classes of anti-reflection coatings (ARCs) based on

textured dielectric materials: (i) a transparent, flexible paper technology that relies

on optical scattering and reduced refractive index contrast between the air and semiconductor and (ii) silicon dioxide (SiO2) nanosphere arrays that rely on collective

optical resonances. Both techniques improve solar cell absorption and ultimately

yield high efficiency, low cost devices. For the transparent paper-based ARCs, we

have recently shown that they improve solar cell efficiencies for all angles of incident

illumination reducing the need for costly tracking of the sun’s position. For a

GaAs solar cell, we achieved a 24% improvement in the power conversion efficiency

using this simple coating. Because the transparent paper is made from an earth

abundant material (wood pulp) using an easy, inexpensive and scalable process,

this type of ARC is an excellent candidate for future solar technologies.

The coatings based on arrays of dielectric nanospheres also show excellent potential

for inexpensive, high efficiency solar cells. The fabrication process is based

on a Meyer rod rolling technique, which can be performed at room-temperature and

applied to mass production, yielding a scalable and inexpensive manufacturing process.

The deposited monolayer of SiO2 nanospheres, having a diameter of 500 nm

on a bare Si wafer, leads to a significant increase in light absorption and a higher

expected current density based on initial simulations, on the order of 15-20%. With

application on a Si solar cell containing a traditional anti-reflection coating (Si3N4

thin-film), an additional increase in the spectral current density is observed, 5%

beyond what a typical commercial device would achieve. Due to the coupling between

the spheres originated from Whispering Gallery Modes (WGMs) inside each

nanosphere, the incident light is strongly coupled into the high-index absorbing material,

leading to increased light absorption. Furthermore, the SiO2 nanospheres

scatter and diffract light in such a way that both the optical and electrical properties

of the device have little dependence on incident angle, eliminating the need for

solar tracking. Because the layer can be made with an easy, inexpensive, and scalable

process, this anti-reflection coating is also an excellent candidate for replacing

conventional technologies relying on complicated and expensive processes.