Dye-sensitized solar cell (DSSC) coated with energy down shift layer of nitrogen-doped carbon quantum dots (N-CQDs) for enhanced current density and stability

Utilizing ultraviolet light by using Energy Down Shift (EDS) of quantum dots is a recent approach to efficiently utilize a broader spectrum of light, for energy harvesting. Other than higher efficiency, EDS is highly promising for increasing the stability of Dye-Sensitized Solar cells (DSSC); as it decreases the UV degradation of the cell. Previously, heavy metals-based (Cadmium etc) quantum dots are used for the purpose, which are not only toxic but also have less stability and have complicated synthesis methods. To overcome these issues, a thin coating of green emissive Nitrogen-doped Carbon Quantum Dots (N-CQDs) on DSSC, as a stable and efficient EDS layer is proposed here. N-CQDs were synthesized by a scalable, green and facile one-pot hydrothermal pyrolysis of citric acid in presence of nitrogen source, with a production yield of 65%, and quantum yield of 70%. The readily dispersible and highly stable colloid of N-CQDs was coated on the device, and with an optimized thickness, the power conversion efficiency was increased by 10%. This increase was further confirmed by external quantum efficiency (EQE) test, i.e. N-CQDs coated device showed EQE enhancement majorly in the UV spectrum and enhanced stability in UV light was recorded. The transmittance of FTO-glass was increased due to EDS and it also has an antireflection effect, which intern produces a small increase in EQE in the visible spectrum.