In situ growth of ZnO quantum dots (QDs) on the surface of multi-walled carbon nanotubes (MWCNTs) was presented, and their application in photovoltaic cells by using flexible polyethylene terephthalate substrates was demonstrated. High-resolution transmission electron microscopy images revealed the conjugation of ZnO QDs with MWCNTs. Photoluminescence spectra indicated that the charge transfer efficiency at ZnO QD-MWCNT heterojunctions was above 90%, as confirmed by time-resolved photoluminescence measurements. Current-voltage measurements on the flexible devices fabricated utilizing ZnO QD-MWCNT heterojunctions showed the robust nature of the ZnO QD-MWCNT-based photovoltaic cells and their potential applications as the power source for flexible hand-held consumer electronics.