Core/multishell-structured semiconductor nanocrystals (NCs) are prepared in multigram amounts through a one-pot process without a core-separation step (see figure and cover). The NCs show a maximum quantum efficiency of 85% at a controlled wavelength and strong photostability against UV-light bleaching. The NCs are applied as phosphors on UV-light-emitting diodes (390 nm source), which emit light at 620 nm with a 48% power conversion efficiency.