Carbon quantum dots improving photovoltaic performance of CdS quantum dot-sensitized solar cells

N-doped carbon quantum dots (CQDs) are introduced into CdS quantum dot-sensitized solar cells (QDSCs) to improve the photovoltaic performance. The fluorescent CQDs with average diameter of 2.97 +/- 0.4 nm are prepared using cotton fiber as carbon precursor and urea as dopant by hydrothermal method. The power conversion efficiency (PCE) of CQDs/CdS based QDSCs is 0.606%, which is 40.9% higher than that of CdS based QDSCs (0.430%). There are three reasons for this improvement: (1) the CQDs broaden the absorption range and enhance the absorption intensity of the sensitized photoanode; (2) the CQDs effectively reduce the charge recombination between the photoanode and electrolyte interface in the QDSCs; (3) the CdS quantum dots (QDs) and CQDs are probably forming a band alignment of type I, which makes the LUMO descend from CdS QDs to TiO2 through CQDs.