Optical properties of graphene carbon quantum dots for solar cells applications

In recent years, the need for sustainable clean energy sources has been increasing. Solar cells are a promising alternative to such energy problem. However, high solar cells efficiencies have not been achieved yet. Graphene Carbon Quantum Dots (GCQDs) are a type of nano structures with great potential to improve solar cells efficiencies. GCQDs optical properties are not fully understood. To study the optical properties of GCQDs, a synthesis of GCQDs through a pyrolysis method using Citric Acid as a carbon source had been realized. Changing the pyrolisis exposition time, samples with different GCQD sizes were obtained. The absorption spectra of the samples was measured. In addition, calculations of the absorption spectra and Density of States (DoS) based on Density Functional Theory (DFT) using the software Quantum Espresso were performed. A good agreement between the experimental and theoretical results was obtained. The absorption spectra and the simulations shown two absorption bands. Based on the DoS calculations, the origin of the lower and higher energy absorption bands could be associated to transitions between carbon-oxygen (pi-pi*) and carbon-carbon (pi-pi*) states, respectively.