A leaf-inspired photon management scheme using optically tuned bilayer nanoparticles for ultra-thin and highly efficient photovoltaic devices

We present a leaf-inspired biomimetic omnidirectional photon management scheme for ultrathin flexible graphene silicon Schottky junction solar cell. An all-dielectric approach comprising lossless spheroidal silica and titania nanoparticle bilayers is used for mimicking the two essential light trapping mechanisms of a leaf - focusing and waveguiding, and scattering. The ratio of the nanoparticle diameters of the two optically tuned layers plays a crucial role in confining the incident light through whispering gallery modes and subsequent forward scattering into the substrate via strong leaky channels. The scheme does not employ any nanostructuring of the silicon substrate, thereby preventing the optical gain from being offset by recombination losses, completely decoupling the optical and electrical performances of the device. The light-trapping scheme shows ultralow broadband reflection of only 10.3% and causes a 30% increase in efficiency compared to a bare graphene/silicon solar cell. An efficiency of ~9% is obtained for solar cell with 20 µm thick n-silicon absorber and doped bilayer graphene, resulting in highest (1.89) watt/gram utilization of silicon among all graphene/silicon solar cells. The light-trapping nanoparticle-embellished solar cell retains its characteristics for >10 bending cycles for a bend radius as low as 3 mm, demonstrating its flexibility, durability and reliability.