Electrophoretic Deposition of Fluorine–Nitrogen Graphene Quantum Dots with Highly Luminescent Upconversion Properties for Solar Cell Applications

Abstract

Nitrogen–fluorine codoped graphene quantum dots (N, F-GQDs) were synthesized through a one-pot hydrothermal method using glucose and ammonium fluoride (AF). The composition of nitrogen and fluorine with GQDs by AF was optimized to approach the highest light absorption edge in UV–visible spectroscopy and receive upconversion properties in photoluminescence (PL) analysis. Maximum absorption edge of the optimized N, F-GQDs was observed at 580 nm. Transmission electron microscopy analysis revealed the formation of the nanoparticles with an average size of 18 ± 0.9 nm. Raman analysis approved the formation of GQDs with graphitic carbon atom structures. X-ray photoelectron spectroscopy analysis indicated doping of F and N in the GQD structure and photoluminescence analysis revealed excitation-dependent emissions with upconversion properties for the best sample of the N,F GQD synthesized with an AF concentration of 0.07 gmol/dm3 in the primary synthesis solution. For the fabrication of solar cells, a uniform and stable layer of N, F-GQDs was deposited on the TiO2/FTO electrode, as the photoanode, by an additive-free electrophoretic deposition (EPD) method, confirmed by field-emission scanning electron microscopy analysis. Photovoltaic properties of the fabricated solar cells were studied under a simulated solar irradiation of AM 1.5 G. It was revealed that an appropriate ratio of N,F doped in GQDs in addition to the EPD layer formation highly improved the solar cell efficiency. The best cell with N, F-GQD0.07/TiO2/ FTO photoanode (size of 1 × 1 × 0.2 cm3) improved the efficiency, open circuit voltage, and short circuit current density, from 0.07 to 0.51, 370 to 699 mV, and 0.3 to 1.2 mA/cm2, respectively, compared to the cell fabricated by the blank TiO2/FTO photoanode.