Self-Assembled Nanostructured Rear Reflector Designs for Thin-Film Solar Cells

Abstract

To improve the competitiveness of solar cells, cell efficiency must increase and use of materials must be minimized. Light trapping measures can achieve this by allowing cells to absorb a greater fraction of the incident light. Traditional methods like surface texturing can negatively impact the cell's electrical characteristics and are generally unsuited to thin cell types. A plasmonic light trapping structure that avoids such issues can be formed using self-assembled hexagonal arrays of dielectric nanospheres in a continuous metal layer, at the rear surface of a cell. This can be easily fabricated toward the end of cell production, making it suitable for implementation with various solar cell types. 3D finite-difference time-domain simulations were conducted to investigate the potential of such structures, varying parameters including feature size and spacing, metal and absorber material and thickness, and the impact of random variations in the array. Significant improvements were found for a variety of topographies, with a peak increase in photocurrent from 2 μm silicon of 4.02 mA/cm 2 or 24.4%, relative to the case of a standard rear mirror with a 100 nm SiO 2 spacer layer. This also compares favorably to arrays of rear metal nanoparticles that previously yielded promising experimental results. We also identified critical parameters to control when designing such structures. A particular advantage of this structure is that it can offer light trapping advantages similar to those provided by metal nanoparticle arrays while still being able to serve as the rear contact of the cell due to the continuity of the metal layer.

The Impact of parasitic loss on solar cells with plasmonic nano-textured rear reflectors

“…Non-polarised light absorption into silicon (Si) was integrated across the AM1.5 G spectrum in order to calculate the J ph for each simulated structure as in previous work 15 . For the MNP case, a rear array of hemispherical silver (Ag) MNPs was simulated, coated with a 500 nm thick MgF 2 layer and rear Ag mirror similar to structures that have demonstrated enhancements 13 .…”

“…For the NS case, a rear-located hexagonal array of silica (SiO 2 ) nanospheres was simulated, coated with a continuous silver film. Line-of-sight application of the metal over these NSs gives their resulting texture of the metal film a ‘half-capsule’ shape 8 providing a positive impact on the film’s scattering behavior 15 . For both structures, a 20 nm SiO 2 spacer layer was included at the rear of the Si, selected based on optimization in our previous work 13 .…”

“…The former is an array of metal nanoparticles (MNPs) on a cell’s rear surface, coated with magnesium fluoride (MgF 2 ) and an additional rear mirror 13 , while the latter involves a continuous metal layer textured using an array of Silica nanospheres (NSs) 14 . These metal films incorporating NSs have previously shown the potential to significantly enhance photocurrent ( J ph ) in thin solar cells 15 . A key advantage of these NS-textured rear reflectors is that they offer both the light trapping enhancements of the MNPs, with the continuity of a rear mirror.…”

“…However, heightened absorption was localized over narrow-band peak wavelengths of light, determined by the resonances of the void 22 . Altering the void size and spacing alters the resonant wavelengths and their strength 15 , 22 . These structures can provide both strong absorption into the metal and effective light trapping in the cell, warranting deeper analysis of the balance between these competing characteristics.…”

Nanophotonic resonators for InP solar cells

“…However, a major drawback of thin-film solar cell technology is decreased optical absorption, making compact, high-quality anti-reflection coatings of critical importance to ensure that all available light enters the cell. Various photonic designs have been utilized to raise the efficiency of these cells beyond the enhancement provided by traditional thin-film dielectric antireflection coatings (ARCs), including nano-textured transparent conducting oxides (TCOs) [1,2], nanostructured dielectric reflectors [3][4][5][6], plasmonic structures [7][8][9][10][11], etc. The success and limitations of these strategies (including parasitic absorption and wavelength and angle sensitivity) only further emphasizes the need to take advantage of other optical mechanisms to increase absorption in thin-film solar cells.…”