  Light management is of great importance to photovoltaic cells, as it
determines the fraction of incident light entering the device. An optimal
pn-junction combined with an optimal light absorption can lead to a solar cell
efficiency above the Shockley-Queisser limit. Here, we show how this is
possible by studying photocurrent generation for a single core-shell p-i-n
junction GaAs nanowire solar cell grown on a silicon substrate. At one sun
illumination a short circuit current of 180 mA/cm^2 is obtained, which is more
than one order of magnitude higher than what would be predicted from
Lambert-Beer law. The enhanced light absorption is shown to be due to a light
concentrating property of the standing nanowire as shown by photocurrent maps
of the device. The results imply new limits for the maximum efficiency
obtainable with III-V based nanowire solar cells under one sun illumination.
