Synthesis of carbon quantum dots by chemical vapor deposition approach for use in polymer solar cell as the electrode buffer layer

Fluorescent carbon quantum dots (C-CQDs) with graphitic structure were synthesized by chemical vapor deposition (CVD) for the first time. With this approach, the synthesized C-CQDs exhibit excellent crystaline graphitic nature with an average diameter of 3.5 nm and rich of hydrophobic CH3 terminal groups. Optical band gap of the C-CQDs was determined to be 3.16 eV, whereas low-lying HOMO/LUMO energy levels of −7.00/-3.84 eV were measured for these C-CQDs by cyclic voltammetry. Polymer solar cells using solution-processed C-CQDs as the electron transporting layer (ETL) were fabricated and tested. Results indicate the C-CQDs based devices exhibit similar or slightly improved device performance as the LiF based devices. Lowered series resistance (RS) was found for these C-CQDs based devices, suggesting a better interfacial connection between the Al electrode and polymer with this ETL. More importantly, improved thermal stability was confirmed for the CQD-based devices, which was ascribed to the less diffussion possibility of C-CQDs in solid film. The current work confirmed that the CVD synthesized C-CQDs could serve as a solution processed ETL for use in organic solar cells with improved device stability.