Laterally assembled nanowires for ultrathin broadband solar absorbers

We studied optical resonances in laterally oriented Si nanowire arrays by conducting finite-difference time-domain simulations. Localized Fabry-Perot and whispering-gallery modes are supported within the cross section of each nanowire in the array and result in broadband light absorption. Comparison of a nanowire array with a single nanowire shows that the current density (JSC) is preserved for a range of nanowire morphologies. The JSC of a nanowire array depends on the spacing of its constituent nanowires, which indicates that both diffraction and optical antenna effects contribute to light absorption. Furthermore, a vertically stacked nanowire array exhibits significantly enhanced light absorption because of the emergence of coupled cavity-waveguide modes and the mitigation of a screening effect. With the assumption of unity internal quantum efficiency, the JSC of an 800-nm-thick cross-stacked nanowire array is 14.0 mA/cm2, which yields a ~60% enhancement compared with an equivalent bulk film absorber. These numerical results underpin a rational design strategy for ultrathin solar absorbers based on assembled nanowire cavities.