Solvent-engineered performance improvement of graphene quantum dot sensitized solar cells with nitrogen functionalized GQD photosensitizers
By Dey, Tamal; Ghorai, Arup; Das, Soumen; Ray, Samit K.
Published in Solar Energy
2022
Abstract
We report for the first time that the choice of solvent used in the synthesis of nitrogen-doped graphene quantum dots (N-GQDs) has a significant effect on the photovoltaic performance of quantum dot sensitized solar cells (QDSSC) where N-GQDs work as photosensitizers. Switching from protic water (H2O) solvent to aprotic N-N dimethylformamide (DMF) causes significant change in structural, photophysical and electronic properties of N-GQDs working as active materials in QDSSC. Unlike traditional dye-sensitized solar cells, N-GQDs replace toxic and readily degradable organic dyes as an active material, due to enhanced stability, cost-efficient fabrication, and environment-friendly processing. We discussed compositional, optical, and electronic properties of N-GQDs synthesized in different solvents, and their role in the photoconversion efficiency of quantum dots sensitized solar cells. Spectroscopic results reveal that nitrogen-doped graphene quantum dots with green emission (G-GQDs) have relatively shorter effective conjugation length and lower content of pyrrolic nitrogen groups because water is a polar protic solvent, which suppresses the water elimination reaction by donating protons to the reaction medium. On the contrary, DMF, a polar aprotic solvent, cannot donate hydrogen, leading to enhanced nitrogen units in the graphene quantum dots framework and red emission (R-GQDs), thus improving carrier density and transport properties. QDSSCs fabricated from R-GQDs exhibit photovoltaic performance (PCE > 5.0%) superior to other previously reported GQD sensitized solar cells.