Dielectric antireflection fiber arrays for absorption enhancement in thin-film organic tandem solar cells

Effort to improve the bulk heterojunction organic solar cells' absorption efficiency utilizes tandem structures combining low- and high-bandgap absorbing polymers so that a broader spectrum of sunlight's energy can be used for the photon-to-electron conversion and the thermalization loss of photon energy can be reduced. However, at the intersection of the upper energy level of the high-bandgap polymer and the lower energy level of the low-bandgap polymer, photon absorption efficiency remains low due to the destructive interference of these energy states at the band edges. In order to circumvent this issue, we theoretically propose using dielectric fiber arrays to generate lensing effects atop of a thin-film polymer/organic tandem solar cell (OTSC), aiming to enhance its optical absorption efficiency. Dielectric fibers have been calibrated to generate the optimal antireflection coating resulting in an integrated absorption enhancement up to 11% compared to its flat-OTSC counterpart. The overall integrated AM1.5 G (air mass 1.5 global simulated solar spectra) absorption yields absorption efficiencies of 83% in an optimal configuration, with a cell size of 600 nm and fiber radius of 150 nm. By using fibers as focusing lenses/antireflectors, we demonstrate that there exists zero reflection at multiple visible frequencies, leading to a relative broadband absorption of an ultrathin substrate.