Purification of nitrogen-doped graphene quantum dots and its application in polymer solar cells

Nitrogen-doped graphene quantum dots (N-GQDs) have potential application in photovoltaic devices because of their adjustable bandgap and remarkable dispersion in amphiphilic systems. In this study, N-GQDs were synthesized and purified, and changes in their optical properties before and after purification were compared. The introduction of the N-GQDs into the ZnO electron transport layer (ETL) of a bulk heterojunction polymer solar cell based on PTB7:PC71BM enhanced the power conversion efficiency (PCE) significantly. Purified N-GQDs showed stronger N doping, obvious blue shift of photoluminescence (PL) emission, and improved dispersion in organic system. After chloroform extraction and purification, the N-GQDs derived from citric acid-urea ethanol solution had the least loss and the most prominent optical performance (quantum yield =41.95%). The purified N-GQDs showed longer fluorescence lifetime than that before purification and still showed strong PL performance after compounding with ZnO ETL. N-GQDs modified ZnO ETL layer has stronger light transmittance and lower impedance. The stronger light transmittance is capable of improving the light absorption of the active layer while the lower impedance can promote electron transfer. Both properties are conductive to the PCE. [Display omitted]