Plasma Induced Non-Equilibrium Electrochemistry for Synthesis of Nitrogen Doped Carbon Quantum Dots Applied to Third-Generation Solar Cells

Abstract

Highly luminescent nanomaterials with ambient stability, environmental friendliness, low-cost and abundance are in demand for various applications in biological sensing, imaging and optoelectronics [1-6]. Many of the photoluminescent nanomaterials are semiconductors and usually contain toxic elements, heavy and expensive metals which have limited their applications. Carbon nanoparticles are a promising alternative to semiconductor nanocrystals as next generation green nanomaterials due to excellent biocompatibility, low cytotoxicity and solution processability which results in ease of production and incorporation in devices [7]. In the current work, nitrogen doped carbon quantum dots (N-CQDs) are synthesized by one-step atmospheric pressure microplasma process. The direct current microplasmas used in this work serve as a reliable and highly reproducible synthesis method for tuning the optical properties of carbon-based quantum dots in colloid form which result as being highly stable and environmentally-friendly. The outcome of precursors and discharge current affecting the particle morphology and optical properties are studied using various characterization techniques, which have contributed to determine the mechanisms leading to QD formation from the plasma-induced reactions at the interface. The synthesized N-CQDs are crystalline with graphitic core doped with nitrogen and functionalized surface. The particle size and luminescence can be finely controlled to give either excitation dependent or fixed wavelength emission in the visible region. The synthesis conditions can be easily controlled by changing the precursors or the discharge current of the plasma to give rise to required particle size and absorbance in the ultraviolet-visible region. These N-CQDs have potential to be used as an active material in next-generation solar cells or even as down-converters for high energy photons to be absorbed by a lower bandgap transporter [8]. The possible synthesis mechanisms have been analysed and potential chemical pathways leading to the formation of the QDs are described. The particularly high photoluminescence quantum yield (33% to 68%) can be exploited for applications.