Photocurrent Enhancement with Arrays of Silicon Light Nanotowers for Photovoltaic Applications

Light management for the trapping of solar power over a wide spectral range and a wide range of angle of incidence (AOI) is important to various renewable energies and particularly to photovoltaic applications. We report on arrays of subwavelength silicon light nanotower (LNT) arrays for photocurrent enhancement under broadband and omnidirectional illumination. A silicon LNT is a composite composed of a lower photoactive silicon nanotower and an upper SiO2 part. The presence of the top SiO2 part entails a significant reflection decrement mainly in the near-infrared with up to 40% decrease in broadband reflection for both normal and oblique illumination. The presence of the top SiO2 part triggers two light-trapping mechanisms. The first is light trapping by light concentration, where the top SiO2 part concentrates the impinging illumination into the lower silicon nanotower and silicon substrate. The second mechanism is light trapping by refraction where the top SiO2 part induces the formation of lateral components and, hence, permits excitation of the lateral modes inside silicon in such a way as to increase the interaction length and absorption. Photocurrent calculations show that the presence of the top SiO2 part induces an increase of 15% under normal broadband solar illumination. The numerical investigation is followed by experimental measurements. LNT arrays are realized using conventional top-down fabrication techniques. Far-field optical spectroscopy and diffuse reflection are reported.