Photovoltaics

Photovoltaics is a key technology within alternative energies. Solar cells enable generation of electricity for a wide range of applications: large scale utilities, commercial and residential roof top installations and various other on-grid and off-grid application. Improving the cell efficiency and reducing the manufacturing cost of solar cells will even increase this broad range of applications.

Nanoimprint Lithography (NIL) enables architectural innovations to further enhance the performance of solar cells. The efficiency of a PV cell depends on the probability of an incident photon being absorbed, and the subsequent collection of the generated carriers. NIL can be used for wafers (multi- and monocrystalline silicon and compound semiconductor wafers), large area panels as well as for flexible substrates.
Nanoimprint technology has the potential to enhance the efficiency of solar cells at 3 points:

Surface texturing for minimized top reflection:
Starting top down, one of the problems with silicon solar cells is the high refractive index of silicon, which causes more than 30% of incident light to be reflected back from the surface of the silicon crystals. Borrowing from Mother Nature's design book, nano-imprinted anti-reflective coatings like the structures of an eye of a moth directly patterned on the substrate improves the efficiency of the system since less light is lost. This kind of anti-reflective coating enhances the efficiency because the structures are smaller than the wavelength of visible light, so the light sees the surface as having a continuous refractive index gradient between the air and the medium. Furthermore nano-patterned anti-reflective coatings are more stable and durable than multilayer anti-reflective coatings since no foreign materials are involved and imply excellent non wetting properties.

Increased P-N junction area:
Nanoimprint lithography allows texturization of the solar cell surface thereby effectively increasing the p-n junction area. The imprinted etch mask allows the usage of multiple different etching technologies for both, mono- and multicrystalline silicon solar cells. As a result even for multicrystalline Si cells a homogeneous and well defined texture can be achieved.

Light trapping - Increased internal reflection:
Total internal reflection at the backside of the solar cell further increases the power conversion efficiency. The backside of the cell is textured in such a way that reflectivity is maximized. The incident photons are backscattered into the solar cell thereby enhancing the optical path and the absorption layer. The combination of frontside texture, which deflects the incident sunlight, and backside texture, which reflects the photons back into the cell, effectively total internal reflection can be achieved.

Front- and backside texturing are important building blocks for photon management within the solar cell.

Nanoimprinting can structure the boundary on the bottom contact in respect to the surface that internal reflection of sunlight occurs. When a ray of light strikes the boundary at an angle larger than a particular critical angle, then the light will stop crossing the boundary altogether. Total internal reflection back occurs and migrate electrons from n-type to p-type materials.

 

Surface-patterning 
Surface patterning for light management and light trapping created by nanoimprint lithography. Courtesy of EVG, master provided by NILT.