Resonant and non-resonant dielectric coatings for high efficiency solar cells

Micro-/Nanoscale surface patterning has been extensively applied to solar cells to increase absorptivity and hence photocurrent. However, this approach could also be detrimental to solar cells' performance because of the enhanced surface recombination arising from increased surface area, which can decrease the open-circuit voltage. In this regard, we introduce two new classes of antireflection coatings (ARCs) based on dielectric materials that do not require surface patterning on active materials: a transparent, flexible paper that relies on nonresonant optical scattering and silicon dioxide (SiO
₂
) nanosphere arrays that rely on collective optical resonances. For the transparent paper-based ARCs, there is reduced reflection on a Si cell for all wavelengths due to a combination of index contrast and surface texturing. It is also shown that power conversion efficiencies are improved for all angles of incident illumination, which reduce the need for expensive tracking of the sun's position. For the SiO
₂
nanosphere array-based ARCs, we show that there is a high absorptivity enhancement (> 20%) on a Si substrate. From experiments and finite-difference time-domain (FDTD) calculations, we confirm that the absorptivity enhancement is due to the combinational effects of whispering gallery-like mode excitation within the nanospheres and thin-film interference effects. We also show that the wavelengths for whispering gallery-like resonance excitation can be tuned by either changing the diameter of the nanospheres or using other materials instead of SiO
₂
(e.g., silicon nitride (Si
₃
N
₄
), titanium dioxide (TiO
₂
), etc.). Because these dielectric ARCs can be made with an easy and scalable processes, they are excellent candidates to replace conventional thin-film-based antireflection coatings that require complicated high-temperature vacuum deposition processes.