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Engineering NitrogenâDoped Carbon Quantum Dots: Tailoring Optical and Chemical Properties through Selection of Nitrogen Precursors
The process of Nâdoping is frequently employed to enhance the properties of carbon quantum dots. However, the precise requirements for nitrogen precursors in producing highâquality Nâdoped carbon quantum dots (NCQDs) remain undefined. This research systematically examines the influence of various nitrogen dopants on the morphology, optical features, and band structure of NCQDs. The dots are synthesized using an efficient, ecoâ friendly, and rapid continuous hydrothermal flow technique. This method offers unparalleled control over synthesis and doping, while also eliminating conventionârelated issues. Citric acid is used as the carbon source, and urea, trizma base, betaâalanine, Lâarginine, and EDTA are used as nitrogen sources. Notably, urea and trizma produced NCQDs with excitationâindependent fluorescence, high quantum yields (up to 40%), and uniform dots with narrow particle size distributions. Density functional theory (DFT) and timeâdependent DFT modelling established that defects and substituents within the graphitic structure have a more significant impact on the NCQDsâ electronic structure than nitrogenâcontaining functional groups. Importantly, for the first time, this work demonstrates that the conventional approach of modelling singleâlayer structures is insufficient, but two layers suffice for replicating experimental data. This study, therefore, provides essential guidance on the selection of nitrogen precursors for NCQD customization for diverse applications
Iodine Migration and Degradation of Perovskite Solar Cells Enhanced by Metallic Electrodes
We monitored the
evolution in time of pinhole-free structures based
on FTO/TiO<sub>2</sub>/CH<sub>3</sub>NH<sub>3</sub>PbI<sub>2.6</sub>Cl<sub>0.4</sub> layers, with and without spiro-OMeTAD and counter
electrodes (Ag, Mo/Ag, and Au), aged at 24 °C in a dark nitrogen
atmosphere. In the absence of electrodes, no degradation occurs. While
devices with Au show only a 10% drop in power conversion efficiency,
remaining stable after a further overheating at 70 °C, >90%
is
lost when using Ag, with the process being slower for Mo/Ag. We demonstrate
that iodine is dislocated by the electric field between the electrodes,
and this is an intrinsic cause for electromigration of I<sup>â</sup> from the perovskite until it reaches the anode. The iodine exhaustion
in the perovskite layer is produced when using Ag electrodes, and
AgI is formed. We hypothesize that in the presence of Au the iodine
migration is limited due to the buildup of I<sup>â</sup> negative
space charge accumulated at the perovskiteâOMeTAD interface