Enhancement effect of biomass-derived carbon quantum dots (CQDs) on the performance of dye-sensitized solar cells (DSSCs)

Corn stover, an agricultural waste, was used to prepare nitrogen self-doped carbon quantum dots (CQDs) through a simple hydrothermal method with only water at near room temperature for the first time. The surface, electrochemical, and photovoltaic characteristics of CQDs doped TiO₂ in dye-sensitized solar cells (DSSCs) were thoroughly and systematically examined. The average diameter of blue-fluorescence CQDs measured by a high-resolution transmission electron microscope (HR-TEM) was 4.63 ± 0.87 nm, which consisted of polar functional groups. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy of the biomass-derived CQDs, determined by the cyclic voltammetry (CV) test, were, −5.48 eV and −3.89 eV, respectively. The negative shift of flat band potential (V_fb) in CQDs incorporated photoanode implies the fermi level shifted upward. Experimental results revealed that the improved performance of DSSCs was due to charge transport enhancement and separation, which resulted in the improved energy level configuration between TiO₂, CQDs, and electrolytes. In this regard, the CQDs serve as a mediator that enables charge carrier transport without hindrance. In this study, CQDs added to TiO₂ + N719, increased short circuit current density (J_SC) and power conversion efficiency (PCE) value by ∼26.00 % (10.13 to 12.69 mA/cm²) and 27.20 % (4.78 % to 6.08 %), respectively.