High efficiency solar cells tailored by biomass-converted graded carbon quantum dots

The solar-to-electric conversion efficiency of mesoscopic solar cells is highly dependent on electron extraction under solar irradiation, which determines the charge recombination processes within devices. Boosting charge transfer via building energy level alignment has been proven a promising route to enhance the power output of solar cells. In the current work, we present here the successful fabrication of functional biomass-converted carbon quantum dots with graded energy levels by doping nitrogen or sulphur atoms from lotus root powders. When employed as light absorbers for mesoscopic solar cells, the light harvest and electron extraction can be markedly increased arising from band alignment of graded carbon quantum dots, yielding the maximal power conversion efficiencies of 0.158% and 0.208% for bi- and tri-cascaded photovoltaics, respectively. The primary results demonstrate employing energy-graded architecture is a promising strategy to optimize the device output. Following this line of thought, we further fabricate a co-sensitized device by integrating graded carbon quantum dots with N719 dyes to enhance the electron extraction ability. The final device yields an efficiency as high as 9.04%, showing the potential application of carbon quantum dots in high-performance solar cells.