Preparation Of Nitrogen-doped Graphene Quantum Dots And Their Applications In Polymer Solar Cells

Graphene quantum dots(GQDs)stand out from the crowd of carbon-based materials because of their unique quantum confinement effects and edge effects.The fluorescence efficiency,optical and chemical stability can be further improved by reducing with reducing agent,surface/edge functionalization or introducing heteroatom doping to change the structure and electronic properties.However,limited by the dialysis method in the existing technology and the pore size of the dialysis membrane itself,it is difficult to obtain uniform GQDs.Recently,the technology of salt extraction and organic extraction can be used to separate and purify GQDs better.The purified GQDs have uniform particle size,high purity and solubility in organic phase,which is expected to accelerate the charge transport in the interface layer,enhance the light absorption of active layer,and improve the performance of photovoltaic devices.This dissertation mainly studied the preparation of nitrogen-doped graphene quantum dots(N-GQDs)with high fluorescence performance,and purifies N-GQDs by salt extraction to improve their application effects in the electron transport layer(ETL)of polymer solar cells.The research content is as follows:(1)Selecting GQDs with high photoluminescence(PL)intensity and high relative quantum yield(QY)by using different kinds of carbon source materials,nitrogen source materials and reaction solvents.On the basis of citric acid(CA)as the carbon source,N-GQDs were synthesized by water/solvothermal method with different nitrogen sources,including ammonia and urea,and water and ethanol used as reaction solvents.By optimizing the reaction time,temperature and molar ratio,A-GQDs with ammonia as nitrogen source and U-GQDs with urea as nitrogen source could obtain QY with45.4%and 54%,respectively.Under different excitation wavelengths,A-GQDs showed excitation wavelength-dependent PL behavior related to the surface state,while U-GQDs showed higher PL intensity and relatively uniform PL emission behavior.Based on the experimental results,the formation mechanism of the hydrothermal/solvothermal synthesis of N-GQDs by CA polycondensation dehydration and introduction of nitrogen was proposed.(2)A miscible organic solvent was added to the water phase GQDs,and then the saturated inorganic salt solution was added gradually to make it salted out,so as to extract the required organic phase GQDs.The structure and optical properties of GQDs before and after purification were characterized by FTIR,XRD,Raman and XPS.The structure of N-GQDs was more ordered and showed more graphitized structure.Ue-GQDs-Os(QY=41.95%)had the least sample loss after purification and the most outstanding performance in optical performance.(3)N-GQDs and purified N-GQDs were introduced into the ZnO electron transport layer of inverted photovoltaic devices with the structure of ITO/ZnO/PTB7:PC₇₁BM/Mo O₃/Al.N-GQDs could enhance the transmittance of the ETL by modifying ZnO and improve the electron transport ability of ETL.When the concentration of Ae-GQDs was 0.5 mg/m L,the PCE of the optimized device reached 6.12%,which was35.10%higher than that of the control device.Ae-GQDs-Os modified photovoltaic devices showed excellent Jsc and PCE,which were 16.29 m A/cm² and 5.98%,respectively.PCE increased by 48.76%compared with the control cell.