A quantitative study of the absorption and photoluminescence (PL) of charged CdSe/CdS nanocrystal films is reported. Unlike CdSe, where the PL was unstable under reducing condition, the CdSe/CdS nanocrystals can be kept under reducing bias in their charged state for tens of minutes, and the PL intensity can be switched quickly and reversibly. Thin films of CdSe/CdS quantum dots are stably reduced to -IV oxidation state without evidence for surface charges or generation of surface nonradiative recombination sites. Upon charging, the quantum dot films exhibit rapid and reversible switching of photoluminescence intensity as well as stable photoluminescence characteristics. The extent of PL quench is not just dependent on the number of carriers but also on their quantum states. The 1P(e) electron quenches the luminescence 8 times more than the IS, electron. The CdSe/CdS nanocrystals also exhibit reduced threshold for stimulated emission in their charged state.